GCV SRV CALIB..

SERVO VALVE NULL Bias CurrentPosted by saha on 24 February, 2009 - 11:06 am For GE Gas turbine Frame 5 controlled by Mark 5 TMR system, what can make the Null Bias current of a Servo Valve incorrect? In other words if the Null Bias is far from -2.667mA, What should we check to make the null bias current close to -0.267mA? Reply to this post... Posted by CSA on 25 February, 2009 - 12:40 am I'm going to split some hairs here (be very, very detailed) and make some corrections to what you're describing. This topic can't really be covered completely in a forum like this; it's complicated. But, we're going to take a short swing at the high points. In the Speedtronic turbine control panel, the error between a servo-valve output's regulator feedback and its reference is converted into servo current. When the feedback of a regulator is equal to the reference the error is zero, so zero error would mean zero current. But, if zero current is applied to the servo-valve, the fail-safe spring in the servo-valve will make the hydraulic actuator move to shut off the flow of fuel or air or steam. Null bias current is the value of current that is added to the servo-valve output current to overcome the tension of the fail-safe spring in the servo-valve. So, some amount of current must be added to the output when the regulator error is zero (when the regulator feedback is equal to the reference) to provide sufficient current to overcome the fail-safe spring to keep the device in position to maintain a steady flow of fuel or air or steam. In the Mark V, null bias current is a *fixed* value of current, defined in the I/O Configurator, that is *added* to the output to overcome fail-safe spring tension. In the Mark V, servo current is expressed as a percent of full-scale servo current. 100% servo current is equal to 10.0 mA, so 0.1 mA equals 1.0%. The servo current values you see when the unit is running or when you are manually positioning a device are almost never the null bias currents. The servo current values you see when the unit is running are the total servo current being put out by the control processor, *including* the null bias current value. You can't really see the null bias current portion of the total current that's being applied to the servo coils *unless the feedback is nearly exactly equal to the reference.* Then and only then is the servo current value being displayed equal to the the null bias current and only the null bias current. And this is done by each control processor independent of the others in a TMR control panel. So, if one control processor thinks the feedback for some device is different than the reference and different than another control processor's or processors', the total amount of current from the control processor will be different than the current from the other control processor(s). Each control processor will have the *same amount* of null bias current added to its output, but each control processor's output can be different if each control processor thinks its feedback is different than the others'. When the regulator feedback is different from the reference, then the control processor will adjust its total current output *which includes the fixed null bias current value* to try to make its feedback value equal to the reference value. And each control processor is doing this for every servo-valve output. Again, the null bias current is a fixed value, defined in the I/O Configurator, which is always added to the total current output of each control processor. When something like what you are asking about happens, you need to find out what the feedback values are for all three control processors for the servo output and you will likely find that one or two of them are very different from the other(s). If all three control processors don't think the feedback is the same and equal to the reference (the reference should be the same for all three control processors), then each control processor will adjust its servo output current to try to make its feedback equal to the reference. For example, consider the GCV servo output. The GCV regulator feedback is the high-selected value of LVDT feedback from the two LVDTs on the GCV. Let's say that <R> thought the GCV position was 57.8% and <S> thought the GCV position was 55.4% and <T> thought the GCV position was 54.8%, and the reference position for the GCV was 55.2%, then the servo currents would likely be unbalanced. And probably by a fairly large amount. <R> might be putting out -3.9% servo current, and <S> might be putting out - 1.9%, and <T> might be putting out -2.5%. Those values are *not* null bias currents, but each one includes the fixed value of null bias current which is defined in the I/O Configurator. In this example, the problem is *not* the fixed null bias current value. The problem is that the three processors have fairly different ideas about the position of the GCV and each one is trying to move the valve to the reference position, and they all have to work together (and that means that one or two are trying to overcome the other) to keep the valve at a steady state position. The bigger the discrepancy in what each control processor believes the feedback to be, the bigger the discrepancy in the servo output currents (which include the fixed null bias current value). Now, let's talk specifically about the null bias current value. Let's say that the value of null bias current defined in the I/O Configurator and that was downloaded to and being used by all three control processors was 2.667 % (the Mark V automatically inverts the value in the I/O Configurator!). Further, let's say all the control processors were indicating a GCV position of 49.7%, the measured position was approximately 49.8%, and the reference was 50.0% and the three servo currrents were all indicating about -2.9% per control processor, or thereabouts. If you changed the null bias current value in the I/O Configurator to approximately 3.0 (which would correspond to -3.0%; remember: the Mark V automatically inverts the value from the I/O Configurator!), downloaded that value to all three control processors, and re-booted all three control processors, you'd probably find that the indicated valve position feedback for all three processors was nearly 50.0%, the measured position would be about 50.0%, and the servo currents would be almost exactly -3.0% per control processor. In this case, the amount of current being displayed for each control processor would be nearly equal to the null bias current amount, because each control processor thought the feedback was nearly identical to the reference *AND* because the amount of null bias current was exactly equal to what was required to overcome the fail-safe spring tension. But it should be clear that unless all three control processors believe their regulator feedback values to be nearly identical to each other, the servo currents being put out by each control processor will not be the same. And it has nothing to do with the fixed value of null bias current being applied to the servo-valve output. The value of current that is displayed when the unit is running is not just the null bias current unless all three control processors are using nearly the same value of feedback for the device and the feedback is very nearly identical to the reference. The amount of null bias current required to overcome the fail-safe spring is actually a range: -0.267 mA, +/- 0.133 mA, or, -0.133 mA to -0.400 mA (-1.33% to -4.00%). So, the actual amount of null bias current required for a particular servo may be anywhere between -0.133 mA to -0.400 mA and still be within spec. The value of *null bias current* doesn't have to be exactly equal to -2.667%, but 2.67% is a fairly good value and works for the majority of servo-valves in use on the majority of GE-design heavy duty gas turbines. About the only time that null bias servo currents need to be adjusted is for some DLN valves, and even then, it's questionable whether or not it's really required. The regulator feedback is compared to the reference 128 times per second, and the total servo current output is adjusted as necessary to try to make the feedback equal to the reference. *BUT* the value that's shown on any display or in any VIEW tool capture or output is only updated four times per second. In other words, the value of servo current written into the control signal database is only updated 4 times per second, even though it could be changing at the rate of 128 times per second. (I think that's different for Mark V LM panels, by the way.) Lastly, the LFBV uses Liquid Fuel Flow Divider Feedback as its primary control feedback and the SRV uses P2 pressure feedback as its primary control feedback. So, feedback is not always position. Some LFBVs have LVDTs as another stabilizing element of the control loop. Reply to this post... Posted by Jeff Ladwig on 9 February, 2010 - 9:28 am I am in the process of checking the calibration of PM1, 2, 3 and SRV control valves, and have run in to exactly the difficulty you describe in trying to measure null current. Since we are currently offline, I tried disconnecting two of the three servo outputs and driving the servo to 50% stroke, to eliminate the competition between the controlers. It seems that this would allow the one controller to match reference and feedback, and then go to zero output (except for the null current). For example, with one output active at a time, these are the VDC and calculated servo currents for PM1: R active: 0.290 vdc, 0.280 mA S active: 0.280 vdc, 0.275 mA T active: 0.300 vdc, 0.283 mA This calculates to a null bias of 2.79%, the I/O configurator value is 2.67%. Should I put the new null bias value in the I/O configurator? Valve position is within 1% of reference when operating. Reply to this post... Posted by CSA on 9 February, 2010 - 12:23 pm Did you record any as-found conditions, or are these the as-found conditions? Did you, or based on the as-found conditions do you, feel it necessary to re-calibrate LVDT position feedback? Valve position feedback calibration changes can impact emissions, since fuel splits are converted to valve positions. If you will be DLN tuning after the outage it might not be so critical. But if you won't be DLN tuning after the outage it might be crucial not to change the LVDT feedback calibration. When two of three controller outputs are disconnected, then two of the three null bias currents are not being applied to the servo-valve, and it will not be able to make the actual position equal to the reference position. All this does (operating the servo-valve with a single controller output) is to verify that the polarity of the current being applied to the coil that is still connected is correct, and does not cause the device the servo is controlling to reduce the flow of fuel (or air or steam) to the unit. Remember, the servovalves used on GE-design heavy duty gas turbines are bipolar devices and the polarity of the current applied to each individual coil is very, very important. The three null bias currents are summed in the servo-valve to produce the total null bias current applied to the servo-valve. The summing is accomplished by the magnetic force developed as the result of current flowing through the coils (amp-turns). 2.79% vs. 2.67% is a negligible difference, in my personal opinion, but changing it might make the actual position more closely match the reference position; a 1% error is approaching the limit of tolerance (and that's a personal limit, not a GE-defined limit, since GE never defines one), especially for a unit with DLN combustors. Reply to this post... Posted by Jeff Ladwig on 10 February, 2010 - 9:25 am The values given for PM1 are as found. In addition, the actual position value as shown on the valve calibration screen while at 50% stroke ranged from a low of 51.15% to a high of 52.48%. I followed the same procedure for PM2, and found that with only one core active, the valve would only most people don't have any idea how to change individual servo current values for a TMR Mark V and end up just making things worse in the end.. Posted by CSA on 12 February. is it recommended to change the null bias current value in the I/O configurator to the value supplied by MOOG in the attached specs. realizing that all three cores are needed to supply the required null current.48.49. I don't have the tool that attaches to the side of the cylinder.0.01% calculated null bias.. I've seen it cause more trouble and lost time than it's worth. Speedtronic panels are good.4:12 pm I have not measured the actual position. the feedback percentages are: R . for a TMR) then there's something wrong with the servo-valve. PM2 is also about 2% off from reference online.136 mA Reply to this post... At 50% stroke. no matter how hard people make it out to be.67% value (negative) in the Speedtronic for all three processors and then tell PM1 to go to 50% stroke? Again.0.achieve 44. And worse. which is outside the range allowed by the procedure.160 mA T .. and came up with 1.48. and provides a small platform to rest the end of the dial indicator on. If the valve (or the IGVs) doesn't behave the way they think it should.5% to 45..17% (LVDT 1) 48. it would seem to make sense to use them as starting points at a minimum--as long as they are in the range specified by GE. The calculated servo currents: R .79% (LVDT 1) 48.8 mA.6:21 pm And what happens when you put the 2. 2010 . This now makes sense.007 mA S . But it does prove a major point: Gas turbine control IS NOT rocket science.13% (LVDT 1) 48.1:00 am It would sound as if there's something amiss with the null bias current value(s) for PM1? What is the actual physical position of the valve with respect to LVDT feedback? I am not a fan of the null bias calculation procedure in the control Specification. Posted by Jeff FLadwig on 12 February.92% (LVDT 2) If I replace a servo.0% stroke reference. 2010 . following the GE procedure.5% of the 50. It rides the actuator spring bushing up & down. If the null bias current doesn't fall within the range that GE specifies ( 0. I measured PM1 servo currents at 50%. If Moog is providing null bias currents with their servo-valves. I don't know if they're that good.87% (LVDT 2) T . but.99% (LVDT 2) S . 2010 .0. Posted by CSA on 11 February. Reply to this post. I've seen people . Reply to this post. or is the procedure that GE gives in the control spec better? Thanks. then they start changing the null bias current(s) after they spent hours trying to calculate one based on the Control Specification value. And many people try to use the null bias current for something it was not intended for. +/-0.4 mA total. I see more and More and MORE people never bothering to measure the stroke or angle. Usually. (Some documentation refers to electro-hydraulic servovalves as "torque motors. Before GE started using Woodward valves the GE-designed valves used to have scales on them which were very handy. Adjusting the spring tension is not an easy task even in a factory or lab. wrong calibration of the null bias (maybe you have refurbished servo valves from none OEM supplier).. Reply to this post. but they are less expensive. Every time I've seen people try to adjust null bias spring tension on a servo-valve which is in service. the difference between the reference and the feedback will also usually be a little greater than normal. when I get this question it's related to one single coil that has a servo current that is out of balance with the others. Posted by CSA on 25 February. One cannot *really* fix the problem with a single control processor's servo current being greatly different from the other two by changing the null bias current value. That is. also). .. (I still find it simply amazing that GE uses Woodward valves. Oztas brings up a point I neglected to address. it has resulted in having to replace the servo-valve (that has been said in previous posts here on control. then all three control processors will likely be trying to overcome some problem with the servo-valve: clogging or varnishing of internal components leading to sticky or sluggish operation. it might lessen the differential between the three control processor's servo currents. Also. degraded lube oil (clogging of servo ports).. 2009 . so changing the null bias value and downloading it to all three control processors will only mask the problem with the one control processor. the the fail-safe spring tension usually is not adjusted per GE-design specifications after refurbishment. a single open-circuit in one of the three coils of a servo-valve will cause the output currents from the other two control processors to increase to try to supply the "missing" torque which would have been produced by the lack of current in the one coil. though they do require more maintenance and repair when in service. worn internal passages or o-rings. Oztas brings up another issue which has been reported by many sites which have tried to use rebuilt or refurbished servo-valves. literally. Guess another old adage is proven true: You get what you pay for.. A. They're certainly not as good as Fisher valves. If all three control processors have basically the same feedback and it's not very equal to to the reference. There is only *ONE* fail-safe spring..11:33 am In general: Internal mechanical failure in the servo will lead to deviation of null bias current.00 and mag bases for around GBP10. Posted by A. Good Luck.700 VAC RMS.. I've seen three-inch stroke dial indicators for about GBP50.) Reply to this post.00 recently. my advice is to replace it with new one from OEM.. 2009 . OZTAS on 25 February.") Usually. in this case. One needs special equipment to monitor oil flow-rates and volumes in addition to the currents applied to the three coils. Yes.4:23 pm A. trying to get the zero stroke reading for LVDTs to be exactly 0.com..spend tens of hours. but it's not addressing the problem of why the one control processor's servo current is out of balance.Tempus Fugit. Possible causes are. That is usually related to the feedback for that control processor being out of balance with the other two control processors. there is a way to calibrate the null bias. as if that improved the accuracy of the operation of the turbine. I've even used dial calipers in a pinch with a couple of zip ties to hold one portion stationary and manually moving the other portion. and the likely major reason is cost. Although. 667mA is actually shown as -0. 100% servo current is equal to 10. servo current is expressed as a percent of full-scale servo current. -0.And usually when they're trying to adjust the spring tension. servo current is expressed as a percent of full-scale servo current.0 mA. I'm saving my pennies to buy Moog. Which mean that -2.. 100% servo current is equal to 10. it's an attempt to bring the servo current of one control processor into line with the others. the once perfectly good servos get dirty and are not handled very well. Would you please open that file on you operator interface and copy one of the lines which carries the words "null bias" and paste it into a response? My suspicion is that someone found the TCQA RAM address for the null bias current value that gets downloaded from the I/O Configurator. I just looked through several ACALIB. For instance. or someone has re-labeled the servo current line to read "null bias".267%. Which mean that -2..1 mA equals 1. the value for null bias current is shown in percentage and not in amperes..67%. Well.0%.1 mA equals 1.DAT is an ASCII text file t hat ACALIB. As I've said before.267%.DAT files.. ACALIB.) In AutoCalib page.667mA is actually shown as -0.. Reply to this post. .EXE uses to configure the displays seen in AutoCalibrate is running.. the value for null bias current is shown in percentage and not in amperes.) Reply to this post.. None of them had lines to display null bias current. 2009 .) In AutoCalib page. I think you observed is actually some optical illusion. Posted by sd on 25 February. I think you observed is actually some optical illusion. and are basically useless after that. Posted by CSA on 26 February.2:26 pm In the Mark V. so 0. 2009 .. so 0.10:07 pm Wonder why my earlier reply is not shown here.0%. Cha-CHING! Reply to this post. . ... In the process. NVM. Don't worry too much as your servo is still fine. from early Mark Vs (with <I>s) and from very late Mark Vs (with GE Mark V HMIs). From the fourth paragraph of the first response: In the Mark V. These things are so misunderstood and people replace them so quickly without understanding how they work or what they're capable of that Moog must be making a fortune because I know of sites that have replaced a single servo-valve several times before fixing the real cause of the problem.0 mA... According to this explination we tried to compare the current servo values on the Mark V screen with the ones measured directly on the screw of the board.267 mA would be represented as -2. 2009 . And that simply can't be done with the adjustment on a single fail-safe spring. Posted by Walter on 13 March.11:15 am >Well. let me rephrase it again now. or -0. Thanks very much in advance. In TC2KREPT. remember that the value in the I/O Configurator is *inverted* (negative) in the Mark V. The reference for the SRV is usually signal name FPRG and the feedback for the SRV is usually FPG2.TXT. I think the second thing that's wrong with this post is that we don't know if you were looking at the required position for SVO1 and the feedback for SVO1.667. if there is that much difference between the setpoint and the actual.dat.DAT. Was the unit running when you were observing these values and taking these voltages? QTBA-27 & -29 are for servo-valve output #1. REALLY wrong with the servo -valve output. 2009 .. There are eight servo-valve outputs from a Mark V. means that that on the screen is not taking account of the null bias? or we are measuring in a wrong place? How can i get in the demand desplay the signal's name of the requiered position since I can not find it in the Longname.For the SRV (as seen on interface) <R> Required Position 15.53% and the actual position is 99%.) Whatever value of current you see or measure is the total current that's being applied to the servo coil and includes the value of null bias current that's being applied to the output. really. From the data you provided. So.87 % Then checking in the file TC2kReport and we found the screw number and board to measure the output tension (?) to the servo.53 Actual Position 99. the reference and the actual are *far* from equal. Because. If you want to see what value of null bias current is being applied to a particular servo-valve output. That was <R> QTBA screw number 27 and 29. really. Posted by CSA on 15 March.. unless the reference and the actual values for the output are nearly identical the value of current you see on the display will not be the value of null bias current. the column labeled "Signal Name" is the value you need to look up in the CSP and work "backwards" from that to find the reference signal name.39 Servo Current -2. And.21 <T> Required Position 15.2667 mA. then the control ain't working. Reply to this post.37 % Servo Current -2. the actual value will be 2. If you've been reading this post.43 Servo Current -2. then I would say that something is really.4mA). you need to look in the I/O Configurator for that particular servo-valve output.1:36 am The first thing I see wrong with this post is that if the required position is 15. (If you see 2.667%. is that what should be? That is far different from the value we get on the screen (-0.53 Actual Position 99. You won't find this in LONGNAME.53 % Actual Position 99.25 mA). And. . We got -0. the signal name for the SRV servo current is usually FAGR.50 <S> Required Position 15. You will find it in the CSP. SVO1 is usually assigned to the Stop/Ratio Valve of a gas fuel system.4v in <R> ( bearing in mind that the resistance is 1Kohm would be 0. and since this is a DC output. But.97% As we understand in these posts. ( servo null bias + output current to adjust the error).85% TNH .50% Actual Position: 99.4 mA ( 4%). here the output SVO1 servo current is 1. Posted by CSA on 17 March.. Reply to this post.1 psi.4 mA. And measuring on the QTBA SVO1 output currrent we get -0. So. which would be more logical ( null bias (-0. any measurement you make would only be an assumption unless you know the exact value the coil resistance. at the moment we took all that data the turbine was runing at base load (MS 6001B MKV TMR).. the servo current we see in the screen is the TOTAL output current to the servo. it can have positive and negative values of voltage (tension) and current.57 % Servo Current: -2. Greetings from Argentina.2667mA) + output current to the servo).1 psi at 99. 2009 . and our actual P2 is approx 223 psi at the same TNH. Is that possible? Other doubt is why we get the signals FPRG and FPG2 in psi and not in % as is should be to set the position? Stop Ratio Valve. So.50 % Actual Position: 99.. then our maths don't work out. The typical resistance of a servo coil used for a GE-design heavy duty gas turbine is approximately 1000 ohms..667%. one of the futbol powerhouses of the world! . 2009 .50 % Actual Position: 99. First thing.3:43 am Hello! Here we go again.28 % <T> Required Position: 15.79 % Servo Current: -1.Tension is another name for voltage.87 % <S> Required Position: 15. as FPRGOUT is not reached ( because of the lack of fuel) the valve goes practicaly to 99 % of its position in order to provide 248. <R> Required Position: 15. Posted by walter/roberto on 17 March.55% Servo Current: -1. Reply to this post. since the servo null bias current is already -2.3:29 pm Hello to Argentina.87 % for <R> .0 V DC would equate to a servo current of approximately -0. Any clue where are we going wrong? Could be something wrong with the configuration of SRV Autocalibrate display? Thanks very much again and sorry for our insistence. Since CSA warned us about the wide difference between SRV required and actual position we went into the old files ( I'd dare to say ever since commissioning) and found out that that error was as wide as is currently. so -4. Could be that problem related to the inadequate fuel pression supply? According to CSP FPRGOUT must be 248. be prepared to supply the flow characteristics from the vendor or the sizes of the orifices in the nozzle tips. But. be prepared to supply all the start-up. and if it's from the AutoCalibrate display then you might be right: There might be a problem with the configuration of ACALIB. If your gas fuel supply pressure can never be greater than approximately 260 psig. Did you put the signals FPRG. warm-up. When the filter canister was opened. and acceleration FSR Control Constants. have they been checked? I was at one site where they said the d-p gauge had never registered any d-p. Turns out the gas fuel filter was one of those sock-type filters made of wound stringy material. Some times they are coalescing filters or just "sock" filters. Someone also commented that they had had exhaust temperature spread problems a couple of years earlier and that the gas nozzle tips were found to be plugged with some kind of stringy material. and if you can read the pressure directly upstream of the y-strainer.4 mA (or 4%) would be more likely what one would expect to see if the Mark V were driving the SRV to be more open than the valve could physically travel such as what you are describing. If they are present. When the Speedtronic panel is putting out excessive current to try to open the SRV it can go into what's deemed "wind-up". but aren't able to put them all together. Since there's approximately a 0. again. as well. and most units have a y-strainer just upstream of the SRV. If the gas fuel supply pressure is less than the P2 pressure reference.7 barg pressure drop across the SRV (typically) that would mean that. This information will be necessary to be able to determine if a lower P2 pressure might be possible. the source of which was never identified. What is the supply pressure? I have seen clogged y-strainers cause high pressure drops. I want to warn you: That is a bad condition to be operating the unit in if you ever experience gas fuel supply pressure spikes or sudden load decreases. 0. Wind-up can take a split-second to recover from if there is a sudden supply pressure increase or a sudden load decrease ("load rejection") and the unit can trip on exhaust overtemperature if it's being operated at or near Base Load when the disturbance occurs because the current has to be reduced at a rate and it might not reduce the current sufficiently to prevent a "burst" of fuel from being admitted to the combustors.DAT.DAT for the SRV (SVO1 or SVO01) and post it to this thread ? You have never told us where you were reading the servo currents from. Also."Here we go again. . what is it versus the gas fuel supply pressure upstream of the SRV? Some units have some kind of fuel filters upstream of the y-strainer. They may also ask for P/Ns (part numbers) of the gas fuel nozzle tips installed in the machine. per the information you provided. and if they're not OES equipment. to calculate a lower P2 pressure reference curve. Can you find the section of ACALIB. you haven't told us). and FAGR on a Demand Display or on the Logic Forcing Display and observe them versus the readings you are seeing on the AutoCalibrate Display? I would submit that you aren't doing anything wrong with the measurements you're taking. the filter element had been so dirty at some point that it had ruptured and was effectively non-existent. please? You seem to have pieces of the puzzle and the answers. then the SRV is going to go wide open to try to get the P2 to be equal to the reference. the supply pressure would need to be approximately 258 psig to keep the SRV from opening fully. Please put the above signals on a Demand Display or the Logic Forcing Display and tell us what the differences are between them and the values you are seeing on the AutoCalibrate display. FPRGOUT. if that's where you're observing the values from (which. but it's something with the display values or the display that you're reading the values from. and if so. Has it been checked for cleanliness recently? There's not usually a d-p gauge across the y-strainer. be prepared to supply them with a recent gas analysis and some details of the configuration of the gas fuel supply system. you may be able to ask GE or the OES to recalculate the P2 pressure reference curve to allow the SRV to operate in a controlling region and fashion." Do you mind explaining that comment. You did not provide the gas fuel supply pressure reading upstream of the SRV. But I'd really like to understand the.0 2 '% ') MAN_SCALE (F2 128. then no change to valve position is to be made. and if the summing junction output is zero. Ref. of the Mark V Application Manual. Reply to this post. If the two are equal. just that.. you are invited to see a football match whenever you want! First of all let us explain to you that we did not mean to be impolite writing "here we go again". and that error will be compared to the current LVDT position feedback and the servo current will be changed to make the necessary position change to make the P2 pressure feedback equal to the reference.0 0.5:07 am Hello CSA!.. After the primary regulator summing junction.If you will look at Section 7.0 2 '%') LINE01 BTEXT 'DESCRIPTION <R> <S> <T> UNITS' line 02 data 'SVO number' <D0A 09> (C1) line 03 data 'Regulator type' <D0A 19> (H1) LINE 05 DATA "LVDT #1 0% cal. Talking about gas fuel supply pressure upstream the SRV we have 260 psi what would be pressure enough.. 2009 . you will see that the SRV regulator uses FPRGOUT as the primary reference and FPG2 for the feedback of the regulator summing junction. It is more likely to be a problem on the Autocalibrate display. the SRV LVDT feedback is compared to the output of the summing junction.667 0 3 "V rms") .667 0 3 "V rms") LINE 08 DATA "LVDT #2 100% cal.LINE 09 DATA "LVDT #3 0% cal." <D0A 62> (F2 6." <D0A 64> (F2 6. PROC Q SOCKET 1 SVO 1 IOP 21 CARD "TCQA" TITLE "GAS STOP RATIO VALVE" PERMISSIVE L3ADJ POSITION_NEG_SAT 100 POSITION_POS_SAT -0. Fuel Flow Divider. we just wanted to express that we are here trying with the same topic again. which means the SRV position is equal to what it needs to be to make the P2 pressure equal to the P2 pressure reference. Posted by walter/roberto on 19 March. If there is an error between FPRGOUT and FPG2. Here goes the ACALIB. The LFBV (Liq. then the output of the summing junction is zero.667 0 3 "V rms") LINE 07 DATA "LVDT #2 0% cal. Ref." <D0A 66> (F2 6. Ref.. Ref." <D0A 60> (F2 6. After reading your explanation it seems that is not a gas fuel supply problem.0 0.667 0 3 "V rms") LINE 06 DATA "LVDT #1 100% cal." <D0A 68> (F2 6. then that difference will be treated as a requiring a change to the valve position. again from the land of football (no soccer) no offence. Fuel Bypass Valve) reference is a liq fuel flow rate and the feedback is from the magnetic speed pick-ups on the Liq.667 0 3 "V rms") .) . of GEH-6195. The GCV and IGV regulators are "straight" position regulators and the feedback is from the LVDTs mounted on the device actuators.DAT from the SRV. Ref. I believe. "Here we go again!" comment.1 POSITION_SCALE (F2 256. The servo currents we provided you were taken from the Autocalibrate Display. Ref.LINE 10 DATA "LVDT #3 100% cal.. Is it maybe a wrong signal reference we are seeing in the Autocalibrate Display as FSGR? And last but not least In our second turbine (TG 2) happens exactly the same with the SRV autocalibrate display. FPG2: 233. FPG2( actual pos): 99. FSGR. This is not football.95 % In Autocalibrate Display: FPRGOUT (required pos): 15.667 0 3 "V rms") LINE 14 DATA "LVDT #1 Voltage" <D0A 20> (F2 6.65 %. Quick look to your ACALIB data shows that is standard configuration for a FR5.667 0 3 "V rms") . as you saw above 99.667 0 3 "V rms") LINE 23 DATA "Position at POS Cur Sat. Reply to this post. Cross check also the IO_CFG SVOx configuration (stroke 100 % or 128 %) .667 0 3 "V rms") . more closed to the value. However this should not give the problems that you describe. 2) Verify the ACALIB data for your GT and the revisions of the PROMs.2:23 pm Hombre. FAGR:-1." <D0A 56> (F2 100 0 2 "%") LINE 31 STATUS 1 R "R STATUS:" <D0A 16> LINE 32 STATUS 1 S "S STATUS:" <D0A 16> LINE 33 STATUS 1 T "T STATUS:" <D0A 16> LINE 34 DATA "Permissive: L3ADJ" L3ADJ LINE 35 DATA "Permissive: SPEED <28% (TNH)" TNH Then we compared the signals in the Logic Forcing Dispay to the same ones in the Autocalibrate Display. definitely the upstream FG pressure is below the specifications as given by the OEM. Again thanks very much in advance. it look like that we are ping ponging Now get back reality.59 % and in the Autocalibrate is. Ref. Use the logic forcing display and/or prevote data display. 2009 ." <D0A 12> (F2 256 0 1 "%") LINE 25 DATA "Manual control position" <D0A 14> (F2 128 0 1 "%") LINE 27 DATA "Required Position" <D0A 50> (F2 128 0 2 "%") LINE 28 DATA "Actual Position" <D0A 58> (F2 128 0 2 "%") LINE 29 DATA "Servo Current.LINE 12 DATA "LVDT #4 100% cal.99 % all values in <R> I found a signal which it seems to be the reference for us. Posted by Ore Rotundo on 19 March. and this is what we get: In Logic Forcing Display: FPRGOUT :248.667 0 3 "V rms") LINE 15 DATA "LVDT #2 Voltage" <D0A 26> (F2 6." <D0A 10> (F2 256 0 1 "%") LINE 24 DATA "Position at NEG Cur Sat.5 psi. which in Logic Forcing Display is 99. FAGR: -2.LINE 17 DATA "LVDT #4 Voltage" <D0A 60> (F2 6.667 0 3 "V rms") .LINE 16 DATA "LVDT #3 Voltage" <D0A 32> (F2 6." <D0A 74 > (F2 6.LINE 11 DATA "LVDT #4 0% cal.5 psi.667 0 3 "V rms") .52 %." <D0A 72> (F2 6.." <D0A 70> (F2 6. There are two options for your problems: 1) DONT use ACALIB for calibration and monitoring purposes. I wonder whether your system is HMI or I ? Or is it may be in the past upgraded to HMI? How about the PROM revisions? If your SRV is operating at 99%.65%.. Ref. will drive the device to shut off the flow of fuel or air or steam. With zero current. when the error between the reference and the feedback is zero. We're not asking questions to be making you run around needlessly. Not being able to look at your CSP and card jumpers. and the null bias spring will act just like the application of a positive current. I assumed that you have verified pressure transducer readings against reasonably accurate gauges and didn't ask you to confirm that. I might also suspect that someone has done something with the TCQC card configuration jumpers or even something "unique" in the CSP to try to rectify this SRV situation. Please don't arbitrarily choose what information you are going to provide or what information you deem to be relevant or necessary. The only time that the current being applied to the servo-valve's coils is equal to the null bias current value is when the feedback is exactly equal to the reference and no additional current is required to keep the device in a steady-state position to make the feedback equal to the reference. the displayed value of servo current is always the total amount of current being supplied. but which we are asking you to visually confirm). only when the error is zero will the amount of current being supplied to the servo be equal to the null bias value. Just use the basic calibration procedure. To overcome the spring and to keep the device in a steady-state position such that the feedback is equal to the reference. I've assumed the SRV LVDTs have been calibrated properly and that the valve is physically at or near full open (something we haven't asked. there is no force (torque) developed by the torque 'motor' in the servovalve. can you please provide the URL link to the post? My bad for making some assumptions (I really try not to do that. So. but I'm willing to take that chance at this point. and I just might get a great big surprise. I cannot explain why the SRV servo current is less than what the expected null bias current should be. we're asking because we aren't on site and can't . 2009 . I am hoping that by answering the GCV questions that we can establish that one servo output is operating as expected.12:14 pm If you've asked this question before on control. we don't yet (!) know what that value is. based on the information you have provided and what you have chosen to provide. And. I assumed the P2 pressure transducer(s) (by the way. You have *NOT* provided all the information asked of you. I suspect that in an attempt to try to get the SRV into a controlling position that someone has changed the null bias current value in the I/O Configurator. the servo-valve has a spring which.. I'm going to try to explain this again: When the servo-valve regulator feedback is equal to the servo-valve regulator reference. depending on the application). in the absence of current (zero mA). That's what a bias value is: something that's continually added to something (or subtracted. the only current being supplied is the null bias current.. which includes the null bias current. Posted by CSA on 20 March. has anyone tried to adjust the null bias spring tension of the SRV servo-valve? At this time. how many transducers does the unit have: 1 or 3?) are reasonably well calibrated and that the feedback (input) is properly scaled in the Mark V. the output is zero mA. which would be to shut off the flow of fuel or air or steam. When the error is zero. When the regulator error is zero. meaning that the polarity of the applied DC voltage affects the flow of hydraulic oil through the servo-valve. we can'tell that. but I'm asking a question that I don't know the answer to. However. Under normal conditions. a small amount of current is continually added to the servo-valve output (at all times!). Remember: The Moog servo-valves used for GE-design heavy duty gas turbines are polarity-sensitive devices.com. Which brings up another question I haven't thought to ask: To your knowledge. but I failed on this one!). so the null bias current value defined in the I/O Configurator is continually added to the output.Remember that you dont necessarily AUTOCLIB display to perform calibration. the servo-valve regulator error is zero. the servo-valve output current would be equal to zero mA. but. in my experience with the Mark V. Docendo Discumus Reply to this post. 8) Sometimes the pressure drop across the SRV is a little higher. without saving anything. and if so. Alarms in the list) 6) Please tell us exactly where you're measuring this 260 psi. exit the I/O Configurator. we presume that you are interested in learning something and providing the information requested to help you resolve your issue. And because you wrote here asking for help with an issue. and is it psig or could it be psia?. But 0. where are you measuring this 260 psi supply pressure? At the SRV inlet/supply pressure gauge in the Gas Fuel Compartment. FPG3).93% and the P2 pressure reference is 248. which would be downstream of the y-strainer. If you don't want to fully participate in the exercise of solving your issue. then there's no point in continuing this thread. is it a rotary valve or a plug valve? The fact remains.8 bar to 1. usually signal name FSG (Fuel StrokeGas))? 3) What is the value of FSR? 4) What is the value of servo current being applied to the GCV (usually signal name FAG)? 5) What Diagnostic Alarms are active when the unit is running? (Include any locked-out Diag. if you're reading a static pressure transducer for the supply pressure (and this is usually located upstream of the gas fuel y-strainer). and the static pressure transducer is usually calibrated in psia. not psig. or some place in the gas fuel supply piping upstream of the two units' y-strainers? From a pressure transducer on the gas fuel supply? From a pressure transducer on a metering tube and orifice in the gas fuel supply piping upstream of the unit's y-strainers? 3) Has anyone visually checked the y-strainer recently? 4) Are there any filters upstream of the y-strainers. sometimes a little lower. eight new questions: 1) Please confirm the actual. can you please tell us what the P2 pressure gauge reading is?) then there is *not* sufficient flow capacity from your gas fuel supply to achieve required P2 pressure.5 psig and the actual P2 pressure is 232.5 psig (and I presume you are reading this from the P2 pressure transducer feedback on the Mark V operator interface display. physical position of the SRV is at or near full open. and one should GCV discharge pressure. please confirm the calibration and scaling of the input (the signal name is usually FPG1. sometimes. Many GEdesign heavy duty gas turbines use a metering tube and orifice flow-meter to measure gas fuel flow-rate. and tell us what is listed in the null bias current field. 2) What is the GCV position indication (from the LVDT feedback. and click on the TCQA card. 7) What are the pressures on the three gas fuel pressure gauges in the Gas Fuel Compartment? One should be SRV inlet ("supply" pressure. what is the d-p across the filters and have they been visually checked recently? Now. one should be Gas Fuel Valve Intervalve Pressure (P2 pressure). Exit the card. can you describe the SRV? Is it in a combined casting with the GCV or is it a separate valve from the GCV? If it's a separate valve. settings.) 2) Precisely. There would generally be no other reason for the SRV to be open so high and the P2 pressure to be . and scroll to the screen for SVO1. So. and you won't disturb any of the I/O Cfgr.3 par is a typical range. if the SRV is at 99. Specifically: 1) What is the value of null bias current listed in the I/O configurator for SVO1? (Open the I/O Configurator. or gas fuel manifold pressure.get the information for ourselves. It's that simple. Also. again. Please provide all the information requested. but I would expect that reasonably accurate gauges would have alerted you to this issue much sooner. Perhaps the answers you haven't provided and the answers to the new questions will lead us in a different direction. and it's not listed in the ACALIB. FPG2 is not the SRV position. and to see what the value of servo current is that is being applied to that valve when operating in a properly controlled fashion.DAT section you posted. please.lower than the P2 pressure reference. is the actual feedback. even if the SRV LVDTs are calibrated properly. Lines 27. or if it's only "active" when the valve is being manually positioned using AutoCalib. .5 psig. Posted by servo_2001 on 18 March. FSGR (Fuel Stroke-Gas Ratio) is the typical signal name for the SRV LVDT feedback.4:39 pm I would like to clarify that the answer is not just the OEM as the solution to the problem. The reason we're asking about the GCV information is to try to establish that at least one valve is operating in a properly controlled fashion. 2009 . is the P2 pressure transducer calibrated properly? Is the feedback scaled properly in the Mark V? If the gauges in the Gas Fuel Compartment are relatively accurate. which should be calibrated in psig (gauge pressure). Our company has specialized in electro-hydraulic servovalves since 1969. Now. respectively. the you can look at the signal name FPG2 in the Prevote Data Display to see the individual. and servo current. Lastly. Do not let the OEM make you think they are the only game in town. the regulator for the SRV is a pressure control loop and it will put the SRV at whatever position it needs to be at to make the actual P2 pressure equal to the P2 pressure reference. which. and then report them to us. The only other thing I could think of to cause the SRV to behave as it's being described is that if the P2 pressure transducer was not properly calibrated or the feedback (input) was not properly scaled. then there's just not enough supply pressure and flow capacity to allow the SRV to operate in a controlling range (which should be something less than 99. I agree with Ore Rotundo. We were schooled at Moog on the repair of their valves.93% and less than full open). actual position. To null the servovalve is the easiest calibration you can perform to a servovalve. the section of ACALIB. it has nothing to do with null bias current. 28.DAT that you have provided seems to be fairly generic. If you have more than one P2 pressure transducer. for two turbines). and we can try to get to the bottom of your problem. it's not a reference. Do your homework and you will see that there are reputable repair facilities that have been repairing these valves for some time. and 29 in the section you copied displays the valve's reference position. pre-voted values of the three feedbacks. provided there is sufficient pressure and flow capacity upstream of the SRV to allow the SRV to control the P2 pressure without going full open (and that just doesn't seem to be the case in this instance. but it's not really likely. this would be a good indication of whether or not the transducer(s) is(are) calibrated properly and the feedback is scaled properly. And. But it can't because the upstream supply pressure and flow capacity isn't sufficient to be able to do that... (The answers to the GCV question may help with that!) FPG2 is the scaled feedback (in psi) from the P2 pressure transducer(s). I can't recall if the reference position is "active" when the unit is running. That's what the data you have provided to date tells us. should not be related to the null bias current value. Because. If the P2 pressure gauge in the Gas Fuel Compartment is reasonably accurate and is indicating roughly the same pressure as the transducer feedback. the amount of null bias current being applied to the SRV as understood in this thread has nothing whatsoever to do with the fact that the SRV appears to be at or near full open in an attempt to control P2 pressure at 248. Reply to this post. but rather the methods of applying a null bias current and why they are necessary and how to "measure" them. centre position. For this talk reference. if the spool is not coming back to centre position. Now look at the drawing of Servo. amount of current applied. which is done by providing null bias current by controllers Similarly the mechanical null bias key has been provided to adjust the armature at centre positon. with controlled flow-rates and measuring equipment. once again armature is required to be at to null position i.. And thus flapper (Feedback Sleeve) should be at centre and so the armature should be at the centre. both side of Piston must be equal pressured. Ultimately Piston will move the IGV to open. 2009 . Posted by Mehul on 21 March. Posted by CSA on 23 March. The movement of the spool will be such that one port will be connected to pressure source and at the same time. By viewing the scheme of IGV actuating Piston & Cylinder. We can use the drawing of Servo along with schematic of IGV or GCV-SRV to understand the concept.com which listed firms which refurbish Moog servo-valves.. It's easy under those circumstances. Nowhere in this thread did anyone claim that the OEM was the only game in town.. The Controller will change the current to servo such that the armature will forces spool to move.. Consider what would happen. Thanks in advance Reply to this post. the IGV is required to be open to maintain the Exhaust temperature.. There is good talk on the Null Bias Current for the servo. let us take IGV scheme. on both side of piston.Reply to this post. You are requested to confirm the same. as CSRGVOUT is CSRGVBAK(for Mark_IV) to keep IGV at 57 deg STABLE.e. And how the currents being applied (including the null bias current) can be out of balance. There have been other posts here on control. Definitely! it will drive the IGV in either extreme position.12:19 am What's missing from your reply? Contact info for your company.6:50 am Dear CSA. Posted by CSA on 19 March. And to keep the armature at physical centre position. . is null bias current. At the end. when CSRGVOUT=CSRGVBAK(command=feedback). Here I have concept of NULL Bias. 2009 . the spool should be at the middle of the bushing(such that it neither allows any port{connected to one side of piston} to drain nor gives extra pressure to Port{Other side of Piston}). Can you add yours to the group? Reply to this post. This will create the pressure diff. opens the other port to the drain. the IGV is stable at one position 57 deg.8:10 pm I will confirm all but the last statement about the "null bias key". What's also missing from your reply? The fact that adjusting servo null bias springs is best done in a "laboratory" environment. 2009 . The problems being discussed here are not generally related to null bias spring adjustment.. to maintain the both sides of piston equally pressurized. Now consider. such as in a TMR control panel. cleaning the moog servo or replacing the filter. PLEASE write and let me know. understandable terms.4:59 am Thanks once again CSA. then serviceing the servo. Reply to this post. then. Hands of personals and envoirement at which the servicing is being carried out. and each control processor can output a different current and each servo-valve output includes a null bias current (the same amount of current) to overcome a single spring. absolutely no! No one should be adjusting the null bias spring tension of a three-coil electro-hydraulic servo-valve. So if possible try to replce the servo.. and I've yet to find the right words. with the exception of a facility with the appropriate means to do so and verify proper results. I find this to be one of the most difficult concepts to try to explain to people. If the error is greater than zero. I am almost desperately searching for the right words and the right means to explain this to people. If it would help to try to relate it to some other control system concept or input or output.If by null bias key you mean a tool to adjust the null bias spring tension. spares of moog servo requires the extream extream exteram(03 Times)care to be taken for the cleanliness of the tool tackels. But. with or without pictures. plus a null bias current. I really appreciate your questions. I mentioned it here to confirm the null bias adjustment device available with the servo. I often wonder if Moog doesn't include the little hex key/wrench for the same reason.claening. this one is proportional to the error between the reference and the feedback. one should prefer to replace servo & maitaining clean Pall filter other than service. that would be great. So. Mehul. Instead of an output that's proportional to desired position such as a 420 mA output being at 12 mA for a reference of 50%. oils. . Regarding Servo Valve Maintenance. then the output is greater than zero plus a null bias current.. plus a null bias current. "Another thing.2:55 pm Many electronic manufacturers used to give away small screwdrivers to use to adjust the potentiometers on printed circuit cards. 2009 . with or without the ability to use diagrams or pictures. one MUST NOT adjust spring tension to set null bias current without proper FACILITY . 2009 . Posted by Mehul on 26 March. I think that one of the things that most people misunderstand about servo-valves as used on GE-design heavy duty gas turbines is that they are not like the overwhelming majority of most actuator outputs on most other types of control systems. There used to be a joke that they gave away those "tweakers" (as they were affectionately called) so that technicians and supervisors would misadjust the pots and have to call the manufacturer in to readjust the pots. I've been trying for years to put this in simple. to anyone reading this thread. if you have some idea about how to make this any easier to understand. Definetly .. which doesn't seem to make much difference. and anyone else's who's reading this thread and has some interpretation of this concept they wish to confirm. And if the error is zero then the output is zero.. It gets even more complicated when there's more than one coil in the servo-valve." Reply to this post. replacement of spares for servo. Otherwise. Posted by CSA on 26 March. one of my earlier Comment in "IGV TEMP CONTROL TRIP" topic. . professional companies will purchase anyway OEM servos. I know some of these GT users buying reconditioned servos who are complaining after having the servo valve in service less then a year. positive Null Bias. I have found something odd troubleshooting the moog of one of the machines.9:45 am I would like to add some more interesting facts and kindly ask for explanation. Posted by Ore Rotundo on 19 March..speedyshare. 2009 . then downloading the VSVO. One of our machines (G4) also have out of spec. changing moogs didn't help.) Any explanation on the moog servo current changes? Why all units affected? Reply to this post.temporary solution we use(unit online) is to recalculate new Null Bias measuring coils voltage and having coil resistance measured before.. in order to safeguard the reliability of their asset. There are plenty of other GT users who are purchasing repaired parts such as the servo valves. We have 4 Frame 5 non-dln turbines running in the island mode.com community our opinions and suggestions in order to help them out with their technical issues. Although it happens on all four units. GCV and IGV servo currents change at the same time from one stable value to another one (usually by the step of 3. we are not fundamentally promoting the OEM's and one thing is sure that we are also not getting paid for it to do it! Our intention is to give the control.. The servo valve users are plenty and indeed healthy competition is welcome. Posted by CSA on 18 July. Also polarity has been checked many times so far by confiming that MKVI is able to drive the valve by each servo coil separetely. Also no sign on P125DC and N125DC. 2009 . This way it's fine for some time.It's interesting to note that the originator of this thread hasn't replied to the questions asked to try to help with their issue (which likely isn't related to null bias current. Finally its all about money. I have trended hydraulic pressure 96HQ but there is no sign of the event.com/476835562.. usually until next start-up. since commisioning by GE (MKVI config value -12. 2009 . such as after market parts. I have to mention. I'm running out of ideas what to trend. The same time error doesn't change. believe or not! Docendo Discumus Reply to this post.1:25 pm Dear Servo 2001 Maybe its good to highlight that. the event doesn't occure at the same time looking from unit to unit (see my trends). Reply to this post. but may be related to a misadjustment of the null bias spring!). 4 %). Posted by minister on 17 July. The odd thing is that each unit SRV. Please check the data collected I uploded below: http://www. that randomly changes its bias causing 3% position error .. Back to the moog story.2:14 am . anyway machine is controlled correctly.html Do not be surpirsed looking at these trends. that we have already tried three different Moogs with the same result. you know that the null bias current is the current required to overcome the null bias spring tension. There is only flow when a valve or the IGVs are being commanded to move. There's just not much more to them than that. possibly even and. a unit without the problem? What is the dither set to on the unit with the problems vs. The hydraulic system of a GE-design heavy duty gas turbine is basically a static system. especially if all of them on one unit are behaving similarly. orifice-like passages and clearances which are very susceptible to dirt and varnishing. So. They're nothing more that electromagnets which produce small amounts of torque. Or at least that's what it sounds like. Another couple of months and I should be in a position to make my bid for Moog. Servo valves are about the most misunderstood device I've ever run across. a unit which seems to be running well (not the one with the whacko null bias value). the polarity and magnitude of the current causes the direction and amount of torque to change. This causes excessive flow through the hydraulic pump. meaning that under steady-state operating conditions there is no flow. It just doesn't seem like the servos can be the problem here. This also causes the flow through the hydraulic pump to be excessive which could be a cause of high oil temperature. it would seem that either something is making the spring tension change or something is causing the coil resistance to change. Is there possibly a problem with oil temperature? I have seen units with the relief valves set improperly. The overwhelming cause of "failure" is oil cleanliness and oil temperature. You've changed the servos. Do these units have IS barriers? If you've been reading this thread. Or. Very tiny. and I'll be paying my business loan off very early--I'm sure of that. they must be raking in the cash! I just want to buy the GE-design heavy duty gas turbine line. I've also seen units with the hydraulic accumulator not properly in service. if the current required to overcome the spring tension changes. But. But. They're just coils of wire through which DC current is applied.You say you have four turbines and you're only having a problem with one. There's something different about the hydraulic system of that unit. . there is some configuration and/or jumper settings that are not correct. You haven't told us what Diagnostic Alarms are present when this happens. Have you compared all the hardware ("Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with the problem vs. The way these things get changed on a whim. There's something else that's common to the servos that the problem. Actually. you say the problem happens with all the servos. It wouldn't seem likely that a problem with 125 VDC battery voltage would manifest itself in the servo outputs. How can they be so complicated? They're not. The 125 VDC battery supply voltage is converted to voltages by the rack power supplies which are then used by the VSVOs to drive the servos. I can't really tell how many units are having the problem now that I re-read the message. here goes anyway. used as the "pressure regulators" for the hydraulic pump. for a Moog servo to be changing its null bias spring tension requiring on-line null bias current changes? That's just pretty not right. or before this happens. >Have you compared all the hardware ("Berg") jumpers on the TSVOs? And all the configuration settings on the VSVOs? Of the unit with the problem vs. but hardware. . to avoid trip exceeding 5% value we just downloaded recalculated null bias. This really drops the error to zero. This Moog creates error >3%. I' won't be contributing to threads like this on control. used as the "pressure regulators" for the hydraulic pump. Again. I will check remaining units. a unit without the problem? < All TSVO the same (jumpers). To address what I thought the problem was. what do you think? Especially. none for IGV. >I have seen units with the relief valves set improperly. >Is there possibly a problem with oil temperature? < I'm going to trend all servo currents with oil temperature starting this afternoon. 2. One thing's for sure though: When I own that piece of Moog. >You say you have four turbines and you're only having a problem with one. and it's not the polarity. This causes excessive flow through the hydraulic pump. all MKVI hardware configs the same (to be sure. Now.com. I have also compared m6b with the MKVI).11:42 am Thank you CSA for your reply. >I've also seen units with the hydraulic accumulator not properly in service. >Do these units have IS barriers? < Yes we have the barriers on SRV and GCV (some MTL-7765ac). Posted by minister on 18 July. (I'm aware about on-line software download. On one machine the SRV changes it's Null Bias mostly after restart (I have greased the stem already and stroke after many times. outside ambient 46 deg C (air oil cooler). it's not the machine with positive NullBias value. I'm not so sure..Anyway. 2009 . < The only alarm we have is the increased position error that can lead to turbine trip. that two other barriers for seismics on the same machine had failed. that's about all I can think of. Right now (2:42PM) the header temperatures are 57 to 58 deg C. Investigating these two issues I trended all machines and found interesting fact that I shared on previous post. < No accumulators here. < That's another good hint you gave me.. I found our technician resetting hydraulic pressure on the PSV instead of using pump PCV. Or at least that's what it sounds like. or before this happens. I have it planned to run the machine bypassing them. >You haven't told us what Diagnostic Alarms are present when this happens. Reply to this post. Another machine GCV has positive Null Bias (MKVI RegNullBias =-11. single VSVO I tried many times with no bad luck).96). so it's probably not the friction). < Actually we have problem with two servos: 1. Because of production I can't just stop the machine to do it off-line. result the same. I know better than to kill the goose that lays golden eggs. < Thanks for reminding that. >Would you list the servo gains and null biases for all of the servos? For all of the turbines? A simple chart: GT1 GT2 GT3 GT4 Null Bias/Gain Null Bias/Gain Null Bias/Gain Null Bias/Gain . I have seen barriers cause all manner of strange problems over the years.. Although it's pressure control loop our concern is not to be tripped by position error. THAT'S a problem. the pressure loop is going to put the valve at the position required to make the actual P2 pressure equal to the P2 pressure reference. I'll bet any amount of money the differential is related to the calibration method of that combined SRV/GCV assembly. The SRV/GCV we have is the old design. if so.snip ---As long as the actual P2 pressure is equal to the P2 pressure reference.10:59 am Dear CSA.4% and it thinks it should be at 28. on all units.0.. It doesn't make any difference if the valve thinks it's at 35. When it's not perfectly okay is when the unit is a TMR panel and the three servo currents are not fairly equal.4% stroke (physical measurement) and it thinks it should be at 37.1% and the LVDT feedback says it's at 35.4% or 44.. Also. As far as dither goes. Would you list the servo gains and null biases for all of the servos? For all of the turbines? A simple chart: GT1 Null Bias/Gain SRV GCV IGV GT2 Null Bias/Gain GT3 Null Bias/Gain GT4 Null Bias/Gain I don't believe you told us if these are TMR or SIMPLEX Mark VI panels.4%. Freq 100hz on all Moogs (including IGV). or if it's actually at 31. containing both valves in one body.9%. Even if the LVDT calibration is WAY off. Posted by minister on 19 July. but they're not. some seem to be temperature and current sensitive when they start "failing. don't think we have to disable the dither as someone posted dither disabling is recommended for new types of valves. everything is perfectly okay.4:39 pm One more time: The SRV regulator is a pressure loop. As long as the actual P2 pressure is equal to the P2 pressure reference. everything is perfectly okay. When it's not perfectly okay is when the unit is a TMR panel and the three servo currents are not fairly equal. The above was also mentioned many times on this forum.. 2009 . 2009 .Dither Amplitude 2. ---. They appear to be fine. please remind me." Reply to this post. Posted by CSA on 18 July. Regards! Reply to this post. THAT'S a problem. my personal belief is that none is needed with most of these legacy-style actuators. Thank you this time for your reply! > One more time: The SRV regulator is a pressure loop. do you think it makes sense to use the same settings for the SRV as for.69/1..5% difference from its rest position.8 2.8 2. there might be a problem with one of the processors thinking that the P2 pressure isn't the same as the other processors.61/6. Can you please list the null bias for each processor for each servo of each turbine? One thing all manufacturers do is to "copy" functions to similar applications. Posted by CSA on 19 July. and if you have three P2 pressure transducers and one of them drifts or has a leak or a wiring problem. the frequency of jumps seems to be the same) Can anyone trend servo currents for at least 12hours to observe if these jump like it happens here? I do not understand why three servos jump at the same time and it happen on all machines but at different times." < Can I bypass these barriers? I know it's for EEX zone but I do not recall (I'm not 100% sure) seeing them on 9E turbine in Europe for example.4/1.2/1. This happens a LOT. > I don't believe you told us if these are TMR or SIMPLEX Mark VI panels. like the LVDT position error check. say. < TMR. some seem to be temperature and current sensitive when they start "failing.8 -11.8 The way we calibrate these valves is to lift the stem and insert and leave filler gauge to get rid of the gap between the actuator and valve stem.8 3.8 2.8:09 pm You have a TMR control system. that can cause a problem like this. but they're not. should have mention that at the beginning. 2009 . They appear to be fine.1/6.8 3.8 3. the IGVs or the GCV? I'm still confused about how the position error can start out at one value (even if it's zero) and then change when the unit is running.3/1.67/6.8 3. How many P2 pressure transducers does each turbine have? One or three or ???? Have you trended the P2 pressures for all three processors to see what they're doing before and after this problem? . if so. and that would cause that processor to change its current output and the other processors would have to change theirs to counteract the other. After trending the servo currents jumps together with Oil Temperature no relation could be observed (during night when temperature decreases.82/6.8 2.1/1.. one at a time? As MIKEVI says.8 3. Do you download a new value to one processor? Or to all three processors. I wonder if these jumps happen anywhere else? Regards! Reply to this post. Also. Hope this is the way.9/1.8 3. >I have seen barriers cause all manner of strange problems over the years. That means that each control processor has it's own gain and null bias for each coil of the servo valve. We have running hrs varying from 6000 hrs to 12000 hrs and we have never change the lube oil.SRV GCV IGV 2.65/1. Since the position isn't the problem for the SRV (the inability to control P2 pressure is the problem). And I'm confused when you say you download new null bias value(s). please remind me.3/1. This way the valve is still closed and LVDT indicates 2. sorry. There are thousands of units operating in all manner of applications around the world which do not and have never had IS barriers installed. but I've never heard of this problem... and the servo current outputs from each processor. You say you've replaced this servo and the new one still behaves the same way???? And you've tested the servo current . that's to PUT them in their true zero position. I really don't understand this whole thing. I really think MIKEVI has suggested a good course of action. Some places it's called "closed end overtavel". again. a lot of times that is a part of the problem: Someone attributes an occurrence to this reason or that reason without any real evidence that this reason or that reason is causing the problem. in that position the valve stem is not touching the bottom of the valve plug. OR the policy of the company which is operating the unit. and that you need to also trend the P2 pressures of all the processors to try to understand what's happening. I do not understand why three servos jump at the same time and it happen on all machines but at different times. And. but then you say it happens on three servos on all machines at different time. I would really like to help solve this problem. The use of barriers is entirely a function of the code and requirements of the country or regulatory agency where the unit is installed.. or a problem described like this. You're trending the oil temperature of the oil at the inlet to the hydraulic pump. just when I think I'm getting a handle on it. but. I'm also keenly interested to try to solve the problem with the outrageous null bias current. We don't know the application or the site conditions so we can't tell you to bypass them." By "three" servos do you mean all the coils of a single servo valve? Or do you mean all the servos on all the machines? You say it's only on the SRV. The true zero stroke position for these valves is NOT with the valve stem fully down when the valve is closed.. that's what's SUPPOSED to happen. and that's what the feeler gauges do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the calibration procedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performs when run). All you know is that a position error is changing (increasing) and that you can change (reduce) the position error by changing the null bias current value downloaded to "the Mark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at a time. You should be trending the P2 pressure of each processor. So. Is this happening on the SRV of all machines at different times? Again. Heat would seem to me to be the most likely cause for either. The purpose for the gap is to ensure that the valves aren't held open by the actuator bottoming out. I can't imagine why the null bias spring tension would change during operation. We don't know if the IS barriers are installed on the LVDTs or the servo-valve outputs. You don't know if the coil resistance is changing or if the null bias spring tension is changing. or why the coil resistance would change during operation.. you're not trending the temperature of the hydraulic pump discharge. only you can decide if you can operate without the barriers. you say ". The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug. When you're trending oil temperature.When you put the feeler gauges into the gap prior to performing the calibration of the SRV and GCV. When you remove the feeler gauges and the valve stems drop and the indication goes negative. every time I think I'm understanding what's happening then I re-read the posts and I get confused even more. the LVDT feedback from both LVDTs as read by each processor. header temperature value of 90 deg C isn't good.O.polarity under the individual control of each processor for this device (GCV or SRV or ???) and it will closely maintain the position with only one processor???? I think there's a typo in your chart. From what we can understand.09). Sure. Using Toolbox I have (v11. the value last hour or yesterday or two days ago or one week ago or one month ago or six weeks ago. a L. And we're only talking about position feedback vs. I have been to more sites that have incorrectly calibrated LVDT feedback so that the actual physical position is way off from the indicated LVDT feedback. thanks for your post. GE would have been out of business a long time ago because if every servo output had to operate with zero error the units wouldn't run. I'm both perplexed and intrigued." And he was very correct.02. Reply to this post. But. position reference on one or more servo outputs. It's the value today vs. and whether or not that value has changed in that time and how fast it has been changing. You should know this: On every GE-design heavy duty gas turbine operating anywhere in the world. To . Posted by minister on 20 July. position reference. And. Are you sure there's not an oil leak somewhere in the actuator of the device which this servo is installed on? I apologize if my response seems a little "random" but I'm really baffled by this and at the same time very interested to solve these problems. in a perfect world we'd all like the position error to be zero. > Can you please list the null bias for each processor for each servo of each turbine? The NullBias values I gave are the same in each processor for each servo coil (RST). I heard a former colleague say once a long time ago about GE-design heavy duty gas turbine control. because you've previously said that the GCV of one unit is the one with the outrageous null bias. I appreciate your time spent at the computer. Sorry. And the turbines still run. Some more than others. Header temperature of 90 deg C. Sure. What counts is: Is the position error increasing or decreasing? This has been said many times before on control.com: The value of something today isn't really informative. with no possibility of selecting destination processor (R. would you say it's been running incorrectly? Or would you go to find another way to verify that reading? In your case. You confused me a little with this question. there is likely some error in position feedback vs. the position reference error seems to be changing relatively quickly and then remaining relatively constant. S or T). "This ain't rocket science. And run well. they are all running. We really need to know what the running servo currents for each processor are for the servo with the outrageous null bias value. But they do run. Which changes with every reading. and they run very well. If it were rocket science. It could be only possible if all three VSVOs are in Simplex config like for example Thermocouple VTCC cards..11:49 am CSA. I'm a very literal person (but you couldn't tell that could you?).. but if the unit has been running for four years with an indicated L. it goes directly to all three VSVO cards. when I select Download Configuration. it doesn't have to be zero.O. and it should be zero. But. 2009 . the only way that comes to my mind is to remove Ethernet cables during download. > do you think it makes sense to use the same settings for the SRV as for. . in my post I wanted to confirm if anyone has ever observed these step changes. See the trend I have uploaded below: http://www. on each machine. To have different NullBias values in each R/S/T card.0. What I found using TSM checking regulators (command A*pplications -> R*egultors) and then servos (commands S1 for SRV. I'm not sure what was the reason I can only speculate as above (as I said I greased the SRV/GCV stems with the same result after restart). It may be that someone here started the calibration and haven't fix Min/Max value .100). More on that. but as it is know that SRV is fuel gas pressure controlled valve.html The whole story started there You can see that one day we started having the error.100. lets say on GT1. Can only check on stopped unit. S3 for GCV and S4 for IGV) : G1/G2/G4 have parameter FIX 1 and FIX 2 equal to zero (for all three GCV/SRV/IGV servos) and MKVI LVDT Min/Max (0/100% position) values at LIMIT HI VOLTS and LIMIT LO VOLTS. So. Am I close? > Do you download a new value to one processor? Or to all three processors. Please see the trend I included in my first post.have no idea. gain and null bias values are the same in all three processors.com/470864238. I can't think of the situation that this protection is needed. so loosing the LVDT the card probably doesn't know how to adjust servo current (what direction to go). The G3. Suddenly either SRV position increased by overcoming the friction or the moog current increased for some reason and then MKVI reduced SRV command and so we got both value (position and command) at the same level. the IGVs or the GCV? < For some reason we have it (5% position error trip). Again. Servo command had to be higher then the position required to drive SRV to desired position. not three separate fields for R/S/T. the one that must have had SRV Null Bias corrected after two last restarts has MKVI values of SRV and GCV LVDT min/max in FIX 1/FI X 2 fields and LIMIT HI VOLTS and LIMIT LO VOLTS are different by around +-0. I'm not sure if you are familiar with this part of TSM. I found it today looking for any unusual thing. Will include it in GT3 trend at next start-up. say. one at a time? < OK. all three moogs SRV/GCV/IGV indicates the step change at the same time. FIX 2 . If you help me with all my problems I will be very grateful.set-up hardware config you only have one common field. I had mention GT3 servo problem as an origin. IGV is the same as G1/G2/G4. its LVDT is used as a feedback for VSVO card (SRV VSVO regulator type is 2_LVPosMax).speedyshare.100 (FIX 1 + 0. Have you heard about MKVI having different NullBias values for coils on the same servo? What is your way to achieve that? Gains are of course the same for R/S/T as NullBias and the rest of the HW config. >As MIKEVI says ----snip--->How many P2 pressure transducers does each turbine have? One or three or ???? >Have you trended the P2 pressures for all three processors to see what they're doing before and after this problem? < I have just trended P2 (R/S/T) and see nothing (flat lines during the step change that just happened on GT1 and GT3). Then I started my research and found such Moog current step changes on all machines (but again. on other machines the do not cause an position error). I highlighted each GT set of moogs with different color. Also TSM has a lot of stuff that can be checked. Once again. I confirmed the config with TSM. or why the coil resistance would change during operation. < . that's what's SUPPOSED to happen. as stated the thing above (it's really difficult to me to explain it as English is not my native language). you're not trending the temperature of the hydraulic pump discharge. >The purpose for the gap is to ensure that the valves aren't held open by the actuator bottoming out. Please see again the trend and color marked GT's. or a problem described like this. just when I think I'm getting a handle on it. < No doubt about it. > So... I will feel more confident doing it. I do not understand why three servos jump at the same time and it happen on all machines but at different times. a lot of times that is a part of the problem: Someone attributes an occurrence to this reason or that reason without any real evidence that this reason or that reason is causing the problem. >And. >The use of barriers is entirely a ----snip----. you say ".. > I really don't understand this whole thing.> When you remove the feeler gauges and the valve stems drop and the indication goes negative. I can't imagine why the null bias spring tension would change during operation. >You don't know if the coil resistance is changing or if the null bias spring tension is changing. that's right (the value (negative) is from 3% to 2. is jump/step change up/down/up/down (and so on) of the total current of each Moog. Also the drawing say to check that gap is within desired limits and if no grind the piston rod to obtain it. > The true zero stroke position is when the valve is closed and the valve stem is touching the bottom of the plug. 3x IGV). 3xGCV.. but I've never heard of this problem.. So. You're trending the oil temperature of the oil at the inlet to the hydraulic pump. on one single turbine. Heat would seem to me to be the most likely cause for either. the change happens at the same time on all 9 coils (3xSRV. and that's what the feeler gauges do: They keep the valve stem in contact with the bottom of the valve plug when the valve is closed and ensure that at the end of the calibration procedure that the valve will return to the same position as when it started (which is one of the checks that AutoCalibrate performs when run). < No other option for me right now. < Yes. < That is exactly how I understand the valve." By "three" servos do you mean all the coils of a single servo valve? Or do you mean all the servos on all the machines? You say it's only on the SRV.5% depending on machine). We don't know if the IS barriers are installed on the LVDTs or the servo-valve outputs. and you can observe the same on all machines but from machine to machine it's not synchronized. < Thanks for advice. We don't know the application or the site conditions so we can't tell you to bypass them. only you can decide if you can operate without the barriers. >When you're trending oil temperature. >Again. but then you say it happens on three servos on all machines at different time. you can call it static observation). Is this happening on the SRV of all machines at different times? < Again. I'm assuming the gap is for safety if the seat is worn and no gap the valve will leak. What I can trend. But I observed looking at card points ServoIOutxNVR/S/T that it happens also in the same direction on all three coils (second screenshot I attached. > Are you sure there's not an oil leak somewhere in the actuator of the device which this servo is installed on? < I will check. What I found after stop that the current is different each time (I register in Excel sheet all NullBias calculations ). Sorry if I messed something again. when I have seen friction on a gas valve (SRV or GCV) the effect has been that the servo current increases with no change in position until the device "jumps" usually to a position past the reference and then the current changes to try to drive device back to the reference with no change in position until it "jumps" to a position past the reference and this continues and in some cases gets worse. But where the shaft was worn and scored (on a couple of turbines) or where the . I measure the voltage for each coil separately.And one minor that is servos current step changes on all machines that I accidentally discovered. You say you've replaced this servo and the new one still behaves the same way???? < Actually It has been replaced by GE TA.. I think I checked that a year ago). < I wrote G4 .on the trend you will find G4\signal . My initial reaction is that there is something about/with the IS barriers. < All three at one time. We really need to know what the running servo currents for each processor are for the servo with the outrageous null bias value. Affected valve is SRV.1300ohm on all machines (barriers give additional resistance.. . I haven't seen a Mark VI in a couple of years.12:42 am In the early days of Toolbox. As for the friction comment.position error that disappear after start-up or we have to help it disappears changing NullBias value (we did it last time instead of waiting). Regards Reply to this post. It took all day to think about answers also have a headache. It stays on all machines 1200. it was possible to download to individual VSVOs. the action was normal. I have to re-read your posts guys and prepare a checklist) Finalizing the story: you can see that we have two major problems . 2009 .that was the reason I used it instead of GT4. divide V/R. On either side of the sticky portion of the valve stems. not resistance. This is the GE calculation way given by the ControlSpec and it works (offline :).I agree. have all three resistance noted before the start last month. for me also online) >I'm also keenly interested to try to solve the problem with the outrageous null bias current. the valve behavior was jumpy and erratic.. > All you know is that a position error is changing (increasing) and that you can change (reduce) the position error by changing the null bias current value downloaded to "the Mark VI" and we don't know if you're downloading the change to one VSVO or to all three VSVOs one at a time. that left it saying it's fine.. because you've previously said that the GCV of one unit is the one with the outrageous null bias. Maybe that's not the case any longer. Simply calculation. Posted by CSA on 21 July. summarize all three results and finally divide by three to get average (negative result must be placed as positive in MKVI config and so on). So. They just wanted to leave the place :)))) > I think there's a typo in your chart. but that's just because I don't have a lot of experience with them and the experiences I have had have not been very good. I don't want to risk and disconnect servo coils one by one online.very high positive null bias on one machine running relatively good . I have never seen trends like this. http://www. I'd really like to know what happens to the position feedback at the time the current jumps. It seems that "jumps" are not caused by the hydraulic part of the system as these also exist during cool down and after when Auxiliaries are off.html .. I had opportunity to shutdown GT1 and found something interesting. Have you had your Lube Oil tested for contaminants? And agglomeration (I think that's what it's called)? Some of the newer formulations of turbine lube oils seem to be causing lots of problems for servos. I don't believe the null bias spring tension is changing. does the turbine power output change? Does the unit continue to run normally or does an operator have to make a change to keep the unit running as desired? If the AHJ (Agency Having Jurisdiction (thanks for that Phil Corso!)) would permit a test with the servo IS barriers disconnected it would be very interesting to see what happens. Reply to this post. it's difficult to say what's happening. 2009 .5:27 am CSA. I know they are slow to respond. it seems that the current just "jumps" and without seeing the valve position feedback at the time the current jumps. I did some checking on the MTL site. which seem much higher than those listed in the tables in Sect. but when they do get involved they are pretty helpful. Reply to this post. 2009 ... A change in voltage would be indicative of a change in circuit resistance.com/748565606.10:48 pm The more I think about this the more I think if you're seeing it on all the servos on all the units at your site. the one thing that's common to the gas valves is the IS barriers. I also wonder if they're also affecting the other servo outputs on the VSVO.cylinder walls were scored (on a couple of turbines) the behavior was jumpy and erratic. When these jumps in current take place. such as the IGVs. It would be very interesting to monitor the voltages of each servo coil and see what happens. Trend with some description added (trip occured :) ). Hmmmm.. Posted by CSA on 21 July. That's about all I can think of. From what you describe. I have seen Moog provide some very good troubleshooting assistance when asked to get involved. and it says the MTL7765ac is primarily for high frequency low voltage applications.. Posted by minister on 22 July.. 8 of the manual. Would it be possible to monitor the voltage across the IS barrier and the voltage across the servo coil? I do NOT believe the servo coil resistance is changing..speedyshare. I also wonder about the resistance readings you mentioned. but I've been wrong before and I'll be wrong again. Have you reviewed the manufacturer's manuals/documents to see if these are properly applied? I would like to know how the *circuit* resistance changes when this phenomenon occurs. I think BP has a formulation that has been used successfully by a lot of heavy duty gas turbine users.. I don't recognize the IS barriers you cited. Will try today to bypass barriers and trend again. this I will provide soon. I had usual average value of around -0. I think it's very telling that when these "jumps" occur when the turbines are running that there is no change in power output. but the changing LVDT feedback. > I do NOT believe the servo coil resistance is changing. Could it be changing MKVI cabinet inside temperature affecting them? I wonder if it's sync'ed with the cabinet fan switching on/off (I can simply check it by keeping all fans on for few hours. < I hope barriers is the reason. You say that the position error changes. > When these jumps in current take place. Even when the unit is not running. let's see next time.490V). but we are trying to make it more often (monthly).and humidity controlled environment. > test with the servo IS barriers disconnected it would be very interesting to see what happens. Right? Please trend the LVDT feedback as well as the servo current. It should be located in a temperature. > Have you had your Lube Oil tested for contaminants? < Yes we have it tested. If so. does the turbine power output change? < Nothing unusual I have trended DWATT as well. 2009 .2:17 pm This is troubleshooting: Understanding the possible causes of a problem and working to eliminate them one at a time or to narrow the field of possibilities down to a very few.. and the LVDT feedback from each LVDT would be the best to trend. then when I started the machine and found the position error. The Mark VI is fast enough to maintain steady and stable power output in the face of whatever is causing the servo current to have to change. You have not provided any information on specifically what's happening to the LVDT feedback when these jumps are occurring. < The voltage changes for sure. not the high-selected value. It would be unusual for the Mark VI enclosure temperature to be changing very much. And. and when that happens the servo current would change to try to maintain the error closer to zero. the emphasis is on control. and if the LVDT excitation and/or feedback also have IS barriers. what is the manufacturer/part number? And here's where I'm going to suggest that the problem is not the changing servo currents. Cheers! Reply to this post. I have to find some extra terminals for barriers bypassing. > It would be very interesting to monitor the voltages of each servo coil and see what happens.320V after last calibration.. even if the position error increases (for whatever reason) the Mark VI is automatically "compensating" and keeping the turbine operation stable and steady. but I've been wrong before and I'll be wrong again. < Yep. Nothing unusual last time. I made another measurement of coil voltages and as described before recalculated the NullBias and downloaded new VSVO config (average voltage this time 0. Posted by CSA on 22 July. Are you saying the . Right? < Right. not unheard of.) If the current still changes. It can be possible that we loose one LVDT by vibrations while running unit (e. or wherever the IS barriers are located. then I would suggest removing the IS barriers from the LVDT circuits and monitoring the individual LVDT feedbacks and the servo currents. You have addressed the hydraulic possibility pretty conclusively... loose wiring) or we are getting the LVDT back by the same reason. So let's concentrate on servo current jumps. but not typical. by how much during the course of the day? Are there fans controlled by temperature? If so. but let's not lose sight of the other possible contributing factors. We can exclude IS barriers from servo current change phenomena.g. (position error and high positive null bias).O. which experiences large temperature swings during starting. changes? If so. the output. If so. that's unusual. The IGV servo is usually located in the turbine compartment. in the tank is hotter than the L. (This would have to be done when the unit is not running. The whole servo regulator loop needs to be considered: the reference.O. but the changing LVDT feedback. Posted by minister on 23 July. and if the LVDT excitation and/or feedback also have IS barriers. the feedback. but still have to take them into account for two running units. I think the barriers you listed are considered "passive" devices and don't require bus power. and shutdown. that the offline unit is the one controlled "perfectly" (only servo current jumps" are observed). . servos do get very warm in the environments they are placed in. The L. I have used terminal block to bypass the barriers and during last 12 hours "jumps" occurred again. Let's keep in mind. Reply to this post. what is the manufacturer/part number? < No barriers at LVDT at all.temperature in the Mark VI panel. reservoir/tank. > And here's where I'm going to suggest that the problem is not the changing servo currents. But. Let's not drop the theory that the servo barriers may be the problem. and when that happens the servo current would change to try to maintain the error closer to zero. 2009 . If they are powered. h eader (which is cooled).O. This gave in another suggestion as the error appears some time after restart. operation. You say that the position error changes. Sometimes. Summarizing we have only 6 barriers installed for SRV and GCV.. where do they receive their power from? What IS barriers are used for the LVDTs? Do all the LVDTs have IS barriers? Another possible test would be to take suitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in place of the barriers and then trending the servo current. where the vapors collect. This also happens sometimes in troubleshooting: The focus gets placed on the effect rather than the "affect" (the cause). but the manufacturer instruction book is really unclear on these particular barriers.12:26 pm Thanks CSA for you input: Post update: >You have not provided any information on specifically what's happening to the LVDT feedback when these jumps are occurring. the gas valve servos are located at the top of the L. ) If the current still changes.O. As the unit will stay some days.html http://www. < Trend set-up and running. temperature should come down and we will see possible effect on the trend. but the manufacturer instruction book is really unclear on these particular barriers. that's unusual.. < That's the case we have. also fans are on MKVI terminal board side only) > I think the barriers you listed are considered "passive" devices and don't require bus power. It should be located in a temperature.com/727988916. and you have to separate them using two Monitors.O.trn file. and shutdown. Resistance stays always the same): http://www. the gas valve servos are located at the top of the L. Keep in mind that Regulators use usually two LVDT's. < We have HVAC unit for the MKVI room so we have quite good environment. operation. fill in Mn and MxLVDT1_Vrms with data you have in your Regulator.. this time also one toolbox . (This would have to be done when the unit is not running.html 2) GCV/SRV/IGV jump (see signal description in first file. we have fans controlled by thermostats. < Will add to the plan. scale for some signals is different. not the high-selected value. but not typical. The L. The IGV servo is usually located in the turbine compartment. which experiences large temperature swings during starting. but I met the feature used on some sites).> Please trend the LVDT feedback as well as the servo current.MON12 (depending of your set-up) > It would be unusual for the Mark VI enclosure temperature to be changing very much. select LVDT you want to monitor (see your Regulator). After the config is downloaded you will see each LVDT value in VSVO Card Point section under MON1. where the vapours collect. the emphasis is on control. If they are powered. > Sometimes. IGV in turbine compartment. I was aware of this as very often we have to clean cabinet ventilation inlet filters and fan outlet filters from accumulated fine desert sand. Are you saying the temperature in the Mark VI panel.. And yes. 1) change observed only on GCV/SRV (I was measuring coil resistance a while before it happened. color the same) . Even when the unit is not running. or wherever the IS barriers are located. changes? If so. The average temperature measured using TBTC cold junction is 30 deg C (IS barriers are mounted on the back side of the cabinet where TBTC are mounted. > Another possible test would be to take suitably sized 1000 ohm resistors (heat dissipation) and connecting them directly to the TSVO in place of the barriers and then trending the servo current. header (whi ch is cooled). You have to turn Monitors on (we have it unused.and humidity controlled environment. where do they receive their power from? < Passive barriers.speedyshare. valve servos are mounted inside the tank underneath the valve. Select 1_LVposition as monitor type. After all day trending: Please find another trends (40ms sample time). and the LVDT feedback from each LVDT would be the best to trend. not unheard of. in the tank is hotter than the L. reservoir/tank.com/314203559.O. then I would suggest removing the IS barriers from the LVDT circuits and monitoring the individual LVDT feedbacks and the servo currents. A hint for people that want to trend LVDT's. After trending servo currents yesterday afternoon with all of fans running the servo current phenomena still exists. by how much during the course of the day? Are there fans controlled by temperature? If so.speedyshare. MON2. 2% (by 0. I think I wrote the value before).3:22 am Update: After switching the dither off I haven't observed any more jumps last night.com/810365505. after 4 hrs of trending servos with removed dither I can confirm no jumps on offline GT so far (have to finish my shift). not to confuse you guys. I observed that all current signals (SRV R/S/T.5Hz and 25Hz dither trend. Tomorrow morning I will updated whether jump occurred or not.html I'm trying try do the same with dither off.speedyshare.."Jump" UP http://www.com/340410574.speedyshare. can it induct something in the circuit?): After switching dither off. I'm going to do another experiment.0 to 0. these are trend taken on the offline machine.. the machine that has no problems with position error. Posted by minister on 23 July.speedyshare. Again.6:12 pm I was thinking in my bed about this changes of the servo current. It's moved down by 2% from the high value or 2% up from the low value. and observed servo current travelling from -7 % to -5% without load or position change. At the end of the post. 2009 . IGV R/R/T) have moved in the middle of the "jump high" and "jump low" servo current values. Cheers! Reply to this post. (As dither is an AC on the DC command signal. Is it coincidence. put all three servos on different dither frequency and amplitude trend. 2009 . is it whole circuit loop resistance I mean inside MKVI hardware? The dither seems to be the one not explored :))) Another experiment: On running unit I have gradually changed dither amplitude for GCV from 2. GCV R/S/T. I will troubleshoot affected unit ASAP (read when available after shutdown software modification needed to trend LVDT's and servo outputs separately).. when the dither amplitude is also 2%? Please see dither off trend: http://www. It's not the resistance that changes. I . please share your thoughts and suggestions for next checks. Ok have to go to sleep Reply to this post..2 % step) . (I recall having sometime unusual higher voltage across coils during NullBias verification. but the voltage and then current had to compensate the change for the servo to do the same work (Power = U*I = const).html These 4% current switches seems odd when I have checked servo currents loading the machine by 1MW (around +2% GCV and SRV) and couldn't even indicate the difference on the trend. CSA. As MKVI fastest sample time is 40ms I should see oscillation at least on 12. If it's not the coil resistance that changes.html "Jump" DOWN: http://www.com/860219617. Posted by minister on 24 July. the GE Trending tools are pretty good because the execution rate is not generally higher than the fastest trending rate. I suspect that these jumps are just recorded extrema of the oscillation signal when its frequency is higher than the sample rate. and this is one of them. though I've recently been made aware of some "lore" (because it's not documented anywhere) that there was non-configurable dither built into the Mark V.1:08 pm This explanation would make more sense. An old-fashioned pen-and-ink chart recorder had its limits as well. I will write more when I have a chance to make more tests on these machines Regards Reply to this post. Posted by minister on 24 July...5 Hz and 33Hz frequency dither. Posted by CSA on 24 July. which I haven't really seen a good trend of. But what isn't explained is the position error that you reduce by changing the null bias. is still puzzling. Reply to this post. 2009 .12:32 pm Update 2: I can observe full curve oscillation for servo currents with 12. But..4:23 am > But. this could be an exception because of the rate issue.. In general. Reply to this post. Posted by minister on 25 July. 2009 .. The nature of operation doesn't really cause the control valves to require dither from my experience. Very highspeed toggling of discrete inputs is also hard to catch on GE Trending tools. GE Trending tools can be misleading. which I haven't really seen a good trend of. as can any manufacturer's. especially because there is no upset in the turbine output when these "jumps" occur. 2009 . The trending tool gets periodically unsynchronized with the servo current oscillation generator. the position error. < . digital technology has its limits.com/660758165. They never seem to increase or decrease by more than the same amount. at least from the trends you've sent. is still puzzling. The 25Hz (40ms) dither as being the same as trend sample could be catched only as shown on attached the screenshot: http://www. again. Technology is great. But.html Well. But there wasn't any dither in the Mark IV or earlier Speedtronic panels because the servo outputs were all analog circuits and there wasn't any dither that I was aware of.want to check if there is a relation between jumps and dither frequency. This explanation also makes more sense because of the repeatable nature of the "jumps". but. now I will focus on the other two issues I have here.speedyshare. and this case is probably a great exception because of the variable frequency rates for the dither option. the position error. Catch you later.. I'm not a fan of dither for GE-design heavy duty gas turbine applications. speedyshare. What I need for my tests is to set-up servo currents and LVDT's to be trended all separately. next day the position and the feedback after the restart were correct.4% is may cause a fired trip although the turbine is running fine on load as the position is not considered in the process control loop (as we know the SRV reference will try to maintain the interstage pressure proportional to the turbine speed regardless the LVDT feedback. What has happened here was the valve being open during shutdown by the given above 1. at least on my machine where it's only used by VSVO hardware and HMI). negative as a value for MKVI config). 2) The other machine with jumping error that could be corrected adjusting the Null Bias online.swapping the MKVI hardware. The dither on this machine has been disabled yesterday (machine online).Please see another trend I found in my collection for the position error.4% while the command was 0% from the MKVI. (I only changed NullBias having the alarm above 3%).barrier bypass . 2009 .html) Looking at the trend and step changes. SRV and its LVDT's connected in place of GCV equipment .no change . it could be that what caused the error created also a delay (momentary desync) for trending tool (???) (what happens during that moment can be seen on this zoomed trend: http://www.that's why I wanted to raised the step changes issue. Regards Reply to this post. Posted by minister on 27 November. .no change This gives me a hint to look at the field hardware. Odd is.6:25 am An update (sorry for not updating for so long) 1) The turbine with reversed (positive null bias. http://www. This is just an information (useful I think) what may happened with an error on the valve. I have the LVDT's and moog currents setup in the MKVI to be able to trend as separate signals for each coil and LVDT.html The error wasn't so high that time and went back to normal by itself.com/585332895. as I haven't witnessed the previous MOOG replacement I'm not sure if it's true there was a replacement. the overall picture is very confusing .. I managed to do some tests: . I noticed that error on SRV position (positive one) of the value +1. I will have to work on this valve later. This resulted in gas leak through SRV (Exhaust went up to 500 deg C!) with GCV being still op en at about 6% and the speed at about 20%.speedyshare. the step change is recorded at the same moment. Nothing unusual trending LVDT's or the moog servo current. but the turbine is running fine since.. Then the protection tripped the turbine (shutdown exceeded the given period of time) Funny.com/900781673. Lastly this assumes that your MKVI is a TMR system. I offer this suggestion as something to trend.. it would try to close the valve. Look at your . I then trended the individual servo currents from each MKVI core. and FAGR_T. 2009 . 2009 . I don't know if this will help.I will update the thread with any news I find. We were intermittently getting an alarm for low P2 interstage pressure. ServoOut#NVS. Then "T" would decide to go back to normal. If there is no point associated with it.. and "R" and "S" would have to catch up.. at the same time we noted the SRV valve position was erratic. Trending the position of the SRV in the MKVI confirmed that the SRV valve was not holding a steady position. I have this it in my plans but have to wait for scheduled shutdown to create new signal PINs. it will say "not used". minister Reply to this post. if that is the case you will need to create a point for each card point and download before you can trend the currents individually. Posted by minister on 19 July. and not a simplex. Also note that I have seen some sites where the signals were never added at the card level to be able to trend the servo currents individually. but it would be something else for you to check.m6b file. I realized I have one unused VSVO (R/S/T) and two terminal boards. Again this may not be the problem. "S" and "T" would respond by opening the valve. although from your post it is difficult to tell if this is happening on one gas control valve only or multiple valves. in my unit these are called FAGR_R. 40ms. and ServoOut#NVT. And if the problem is on one unit or multiple units. but it would be something else you can check. I have read your post about the servo current trend 40ms before. under the VSVO card points.3:04 pm Dear Minister.. Good idea with testing another VSVO. under constant load and constant main inlet fuel gas pressure. Regards! Reply to this post.9:21 am Dear MIKEVI Thank your for reply.. FAGR_S. Posted by MIKEVI on 18 July.. I trended the SRV servo current known as "fagr" and found it was erratic during the event. The other 2 cores. . was that intermittently the "T" core would decide to fight the other 2 cores. What I found by running a high speed trend. Please also note that replacing the VSVO card repaired the problem. Reply to this post. for the given regulator you are having trouble with look at the "ServOut#NVR. Anyway I had an issue recently with a frame 7ea machine. 2009 . some don't etc. I agree with you. But no matter. The downside is that all the same tools are not given to all the customers (some . The valve went fully open instead close. after a shutdown when we were recalibrating the SRV. Unfortunately I have no detail drawing for the system and have to compare to another machine when possible. and hopefully we all learn something in the process. The WD40 wash helped.. And best of all we have a participant such as yourself that provides the information we need to try and help solve a problem. and look forward to you resolving the problem. sorry) update: 1) The turbine with reversed (positive null bias. I think. before the trip solenoid valve was closing but being sticky was not providing enough oil to the valve actuator (still draining some amount) when energized. Posted by minister on 25 November. either during commissioning or straight from Salem. 2) The other machine with jumping error that could be corrected adjusting the Null Bias online. . I think I have finally solved this one. either the Moog is installed incorrectly supplying the oil from the opposite port or the hydraulic block between the Moog and the actuator is installed incorrectly (if possible).10:38 am Latest (very late. Surprisingly GE left us the machine with servo working in such an unsafe setup. 2010 . We found that 20HD solenoid didn't work correctly . the sticky trip solenoid made us the same condition (draining some oil to the tank) so we got the wrong null bias again. 2009 .m6b have pins. thanks guys. that before the cleaning of the trip solenoid valve. Then after like two hours running on-line the situation changed..Posted by MIKEVI on 20 July. I wish you continued luck with your issue. the trip solenoid valve fully closed providing all the oil to the actuator (pressure changed) and we had to change the NullBias to correct the new (described above in previous posts) situation. was looking for this information for a very long time. negative as a value for the MKVI config): Should have done that test before: I decided to check the behavior of the SRV (I think I made a mistake stating it was the GCV in previous posts) having all three coils disconnected from the MKVI. Posted by minister on 21 July.stuck. One day we couldn't restart the machine. SRV didn't open. Reply to this post. I suspect.. we have this forum full of knowledgeable people to take advantage of.10:55 am Minister. Posted by RVT on 6 June. the great thing about GE's control systems are that they have lots of great tools to use for diagnosing problems. or something else inside the Moog.) I am not sure where the ball gets dropped.. So.. 2010 .. Reply to this post.9:52 am Great reading. Reply to this post. I will update my post if I get any new information.3:37 am MIKEVI The forum is great thing. don't fix it!" Anyway. having similar result in operation after. Regards to all of you guys and Merry Christmass! Reply to this post. well.. that he replaced it with another one taken from the spares. then when they finally get around to replacing the servo (either because it fails or because of valve refurbishment) everyone associated with the initial effort develops amnesia and a lot of blame gets misdirected. and while some are.. forcing some rash things to be done to make them work.9:35 am Thanks CSA. I haven't checked the part number as I believed the TA during commissioning. many are not. I've seen incorrect servos been used in dire circumstances.. thanks for the feedback! Reply to this post. The best intentions are to send the right part back to site. "It's working! If it's not broke. 2010 . I've also seen them left in service for so long because. Posted by CSA on 25 November. There is a popular sentiment amongst some TAs. 2010 . Posted by minister on 7 December. but I've seen similar things happen during commissioning as well. I will try to schedule the check and inform you. but sometimes others in the decision-making chain have this same mentality..Regards minister Reply to this post. .. especially when a job is over schedule and/or over budget to do whatever it takes to get finished and get off site. they're working! And.7:40 pm Thanks for the feedback! Have you checked the servo valve part number against the appropriate parts list for the gas valve assembly? Are you sure the servo is the correct one for the application? A lot of people think that all the servos on some GE-design heavy duty gas turbines are interchangeable.. I'm *NOT* saying that's what happened at your site.