Manual on Installation of RefineryInstruments and Control Systems Part I-Process Instrumentation and Control Section 3-Temperature Refining Department API RECOMMENDED PRACTICE 550 FOURTH EDITION, JANUARY 1985 American Petroleum Institute API recommended practices are published as an aid to standardization of methods and procedures. These recommended practices are not intended to inhibit the use of practices other than those of API nor to inhibit the purchase or production of products made to s~ecificationsother than those of API. Nothing contained in any API recommended practice is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use in connection with any method, apparatus, or product covered by letters patent, nor as insuring anyone against liability for infringement of letters patent. API recommended practices may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in con- nection with the publication of API recommended practices. The Institute hereby expressly disclaims any liability or responsibility for loss or damage resulting from their use; for the violation of any federal, state, or municipal regulation with which an API specifica- tion may conflict; or for the infringement of any patent resulting from the use of an API recommended practice. Copyright D 1985 American Petroleum Institute Certain systems are not covered in this section because of their very specialized nature and limited use. invention. and installation I-eaches a fair degree of standardization. the various components must be accessible for effi- cient maintenance and certain of these elements must be readable for good operation. much of it is applicable without change in chemical plants. no publication of this type can be complete. Although the information contained herein has been prepared primarily for petroleum refineries.W. observation.C. this section will be revised to incorporate such addiiional information. as well as analyzer sample points. and efficient operation with minimum maintenance. gasoline plants. 1220 L Street. continuous. sketches. When one of these systems gains general usage. and local controllers. or fixed ladders. Acknowledgement is made of the work of all the engineers and operating and maintenance personnel who. . thermocouples. anc' analytical instruments and related accessories to achieve safe. per- manent platforms. and other data should be provided to enable the constructor to install the equipment in the desired manner. Users of this manual are reminded that in the rapidly advancing field of instrumenta- tion. Orifices. American Petroleum Institute. D. drawings. through years of study. control. 20005. FOREWORD This recommended practice is based on the accumulated knowledge and experience of engineers in the petroleum industry. Successful instrumentation depends upon a workable arrangement that incorporates the simplest systems and devices that will satisfy specified requirements. In this manual. The various industry codes and standards and laws and rulings of regulating bodies should be followed where applicable. and sometimes trial and error. Sufficient schedules. Its purpose is to aid in the installation of the more generally used measuring. Washington. accessibility. special consideration is given to the location. Suggested revisions are invited and should be submitted to the director of the Refining Department. In the installation of an instrument. level gages. accurate. N. have contributed to the technology of instmmentation. transmitters.. nor can any written document be substituted for qualified engineering analysis. control valves. generally should be readily accessible from grade. and similar installations. and readability of the elements. . and flexibility of revision. Users of this recommended practice are cautioned to obtain a complete set of sections in order to accomplish efficiently any cross-referencing that is required for a full under- standing of the subject matter. RP 550 is composed of four parts: Part I-Process Instrumentation and Control Part 11-Process Stream Analyzers Part HI-Fired Heaters and Inert Gas Generators Part IV-Steam Generators Part I analyzes the installation of the more commonly used measuring and control in- struments. Manual on Installation of Refinery Instruments and Control Systems. con- venience of reference. is designed to facilitate continuity of presentation. with cross-referencing done only where very extensive repetition would have been required. An attempt has been made to make each section as complete as possible. Part III covers installation requirements for in- struments for fired heaters and inert gas generators. as well as protective devices and related accessories. it was necessary to decide on a logical method of presentation: should each point be explained as fully as possible. Part I1 presents a detailed discussion of process stream analyzers. or should extensive cross-referencing be done between sections? The publications contain a combination of these methods of presentation. PREFACE This section is one of a series which make up Recommended Practice (RP) 550. Part I. These discussions are supported by detailed information and illustrations to facilitate application of the recommendations. Each section is published individually as follows: Section 1-Flow Section 2-Level Section 3-Temperature Section 4-Pressure Section 5-Automatic Controllers Section 6-Control Valves and Accessories Section 7-Transmission Systems Section 8-Seals. and Part IV covers instrument in- stallation requirements for steam generators. and Winterizing Section 9-Air Supply Systems Section 10-Hydraulic Systems Section 11-Electrical Power Supply Section 12-Control Centers Section 13-Alarms and Protective Devices Section 14-Process Computer Systems In the preparation of these documents. Purges. The format of RP 550. D.C.Manual on Installation of Refinery Instruments and Control Systems Part I-Process Instrumentation and Control Section 3-Temperature API RECOMMENDED PRACTICE 550 FOURTH EDITION. JANUARY 1985 American Petroleum Institute 1220 L Street. Northwest Washington.20005 fi . . .3. . . . . . . . . . . . . . . .4. . . . . . . . . .5 Extension Wires . . . . . . . . . . .5. .4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 General . . . . .6 Thermometers for Local Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. . . . . . . . . 9 3-12-Temperature Difference Measurement with Thermocouples . . . . . . . . . . . . . . . . . . 5 3-9-Knife-Edge Tube Surface Thermocouple for Heater Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 vii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3-2-Temperature Bulb Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3-8-Thermocouple-to-Conduit Connections . . . . . . . . . . . . . . 11 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Filled-System Temperature Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. . . . . . . . . . . .6 Temperature Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3-10-Typical Firebox Thermocouple Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Tube Temperature Measurement . . . . . . . . . . . . . . . . . . .3 Extension Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Thermowells .1 Application . . .1 General . . . . . . . . . . . . . . . . . . . . .7 Averaging Thermocouples . . . . . . 3 3. . . . 11 . . . . . . . . . .9 Optical Pyrometers . . . . . . . . . . . . .5 Thermowell Construction . .4. 11 3. . . . . . . . . . . 4 3-7-Installation of Thermocouples Without Wells . . . . . . . . . . . . . . . . . . . . 11 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. . . . . . . . . . .4 Resistance Converters (Transmitters) . . . . . . 8 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3. . . . . . . . . . . . . .3 Self-Acting Temperature Controllers . 1 3. . . . 1 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7. . 1 3. . . . . .5 Precautions . . . . . . . . . . . . .7. 11 3. . . . . Mineral-Insulated Thermocouple Assemblies . . . . . . . . . . . . . . . . . . . . . . . .6. . .4. . . . . . . . . . . . . . . . . . 3 3-5-Metal-Sheathed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3. . . . . . . . . . . . . . . . . . . .3. . 10 3. . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Temperature Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. . . . . . . . .3. . . .2 Applications . . . . . . . . . . . . . . . 3 3-4-Typical Flanged Thermowell Installation . . . . . .8 Radiation-Type Pyrometers . . . . . . . . . . .4 Themowell Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 General . . 10 3. . . . . . . . 10 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. 8 3. . . . . . . . .4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6. .5. . . . . . . . . . . . .4. . . . . . . . . . . . . . . . . . . . . . . . . . 10 3. . . . . . . . . . . . . . . . . . . . . . . . . . . .7. . . . . . 7 3-1 1-Average Temperature Measurement with Thermocouples . . . . . . .4 Firebox Temperature Measurement . 11 3. . . . . . . . . . . . . . . . . . . 11 B @ 3. . . . . . . . . . . . . . . . . . . . . . . . 13 Figures 3-1-Screwed Themowell . . . . . . . . .4. . . . . . . . . . . . . . . . . . . 5 3. . . . . . . . . . . . . . . . 3 3. . . . . . . . . .2 Themowell Insertion Length . . . . . . . . . . . . . . . . . . . . . . . . .5 Resistance-Type Temperature Measurement . . . . . . . . . 11 3. . . .2 Referenced Publications . . . .2 Dial Thermometers . . . . . . . . . . . . . . . . . . .7. . . . . . . . . . . . . . . . . . . . . . . . . . CONTENTS SECTION 3-TEMPERATURE PAGE 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. . . . . . . . .4 Thermocouple Temperature Instruments . . . . .3 Thermowell Immersion Length . . .. . .3. . . . . . . . .2 Resistance Temperature Devices . . . . . . . . . . . . . . . . . . . . . 9 3-13-Standard Configuration for Resistance Temperature Devices . . . . 3 3. . . . . . . 2 3-3-Typical Screwed Thermowell Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Applications . . . . . . . . . . . . . . . . . . . . . . . . . 11 3. . . . 3 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3-6-Sheathed-Type Thermocouple and Head Assembly . . . . . . . . . . . . . . . . . . . . .4 Temperature Transmitters . . . . . . . . .2 General . . . . . 5 3. . . . . . . . . . . . . 11 3. . . . . . . . . . . . . . . 1 3. . . . .7. . . . . . . . . . . . . . . . . 10 3. . . . . . . . . . . . . . . . . 11 3. . . . . . . . . . . .9 Reference Junctions . . . . . . . . . . . . . .3. . . . . .1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 viii .Tables 3-1-Thermocouple Materials and Temperature Ranges . . . . . . . . . . . . 6 3-3-Application Data for Filled Measurement Systems . . . . . . . . . . . . . . . . . . . . . . . . 4 3-2-Thermocouple Extension Wire Materials . . . . . . . . . . . . . . . 12 3-4-Classes of Thermal Systems in General Use . . . . . . . . . . . . . . . . . . . A detailed review of most of these factors PTC 19. usually impractical. (Normally. New York. 345 E. from the free end of the temperature-sensing element or well tions. R (see Figure 3-I).2 THERMOWELL INSERTION LENGTH pyrometers. 'American Society of Mechanical Engineers. 1430 Broadway. to 30 inches (76 centimeters) in diameter. multiplexers. 3. Part 3.1 Temperature Measurement: Thermocouples This section presents common practices for installation of devices for measuring temperature in refinery process ser- vices and for: 3. 3-4) are employed in temperature measurement in spite of ) dicating. Displaying the temperature at the point of measurement.) The immersion length required to obtain optimum accuracy and response time is a function of factors such as type of sensing The latest edition or revision of the following publications element. 47th Street. Section 5. and to-but not including-the external threads or other means digital-type instruments are also included insofar as their use of attachment to a vessel or pipe. filled-system in- struments.1 Scope ANSI' MC 96.3. and radiation-type 3. depends upon heat transfer considerations as determined by API the physical properties of the measured fluid.2 Referenced Publications coincide with the inner wall of the vessel or pipe. Thermowells (see Figures 3-1 through 3. 'ASTM. Section 7. temperature-sensing elements and the boaom of their wells. the tip of the thermowell 'American National Standards Institute. mechanical connection design. 1916 Race Street. and shall. form a part of this recommended well strength requirements. is the distance quirements as a part of temperature systems on most applica. to the extent specified.3. alarming. flow velocity.! e 3. Part I . the temperature C Petroleum Measurement Standards. U (see Figure 3-I). Self-acting temperature controllers.3. Custody transfer temperature from the free end of the temperature-sensing element or well measurement is covered in Chapter 7 of API's Manual of to the point of immersion into the medium. a 45-degree angle to the pipe wall. Because transmission systems are discussed in RP 550. of which is being measured. Pennsylvania 19103.3 THERMOWELL IMMERSION LENGTH I. thermistors. Transmitting the temperature to a remote location for in. and material and mass distribution of the well and the ASME' sensing element. to permit removal of these elements during plant operation. Manual of Petroleum Measurement Standards temperature difference between fluid measured and well Chapter 7. When the thermowell is installed perpendicular to or at New York IM)17. New should be located in the center third of the pipe on lines up York 10018. "Temperature Determination" head. STP 470 B Manml on the Use of Thermocouples in Insufficient immersion can result in errors because heat will Temperature Measurement be conducted to or away from the sensitive end of the thermowell. Included in the discussion are the more common types of It is important to maintain good contact between all measuring devices and accessories: thermocouples. available space.3 Thermowells 1. Measurement The thermowell should extend sufficiently deep into the medium for the temperature-sensitive portion of the measur- ASTM2 ing element to be subject to the medium's actual temperature. temperature transmission is only The immersion length. Optimum immersion depth also practice. Thermowells are discussed because of their re- The insertion length. New York. dial thermometers. recording. Part I-Process Instrumentation and Control SECTION 3-TEMPERATURE 3.3 Performance Test Code-Temperature is rarely justified for petroleum refinery applications. resistance-type temperature detectors. andlor controlling at that the thermal lag introduced to protect thermal elements and point. this point would .1 GENERAL 2 . Philadelphia. The installation of automatic control equipment is discussed m RP 550. in temperature measurement is concerned. If the thermowell 1 . is the distance discussed briefly in this section. Using the temperature for local control of the process Exposing temperature-sensing devices to process fluids is $ variable. Tapered stems and U lengths established from a s ~ s analysis s are recommended for hjgh velocity lines (see ASME PTC 19. Thermowells installed in lines having high velocities may be subject to vibration. 3. 2 API RP 550. the tip should point toward the flow in the process line. The wake frequency (commonly referred to as "Strouhal" or as "Von Kaman Trail") shall not ex- ceed 80 percent of the natural frequency of the themowell. and fluid catalytic crack- ing units require special engineering attention when select- ing materials of construction. chlorides. hydrocracking. . i.3. For general services where carbon steel piping is normally used. Thermowells shall be suitable for stresses due to stream velocity conditions. which may cause a mpture. Themowells for use in hydrofluoric alkylation reforming (hydrogen service). Thennowell manufacturers include material selection guides in their handbooks and catalogs. En:) STANDARD PIPE THREAD (USUALLY DEPENDING ON BU Figure 3-1-Screwed Thermowell is installed at an angle or in an elbow. TEE.3). Figure 32-Temperature Bulb Installation .4 THERMOWELL MATERIALS The materials selected must be suitable for the temperature and corrosion environment encountered. which serve as useful references. and heavy organic acids) re- quire well materials suitable for the specific corrosive media. ETC. UNION OPTIONAL Where the 90 percent requirement results in nonstandard thermowell configuration. all data and parameters affecting the design should be reviewed for alternatives. the minimum quality material usually specified is Type 304 or Type 3 16 stainless steel. Thennowells in certain corrosive services (such as dilute acids.of the well below the mounting.1 THERMOWELL 1 I IPS THREADED CONNECTION (BOSS. TEMPEAATURE BULB @Urn-lYPETEMPEBANRE RECORDER OR CONTROLLER INDUSTRIM OR CONTROLLER DIAL THERMOMETER. CAUTION:Where atomic hydrogen may permeate the thermowell. BUND FLANGE DRILL AND TW bination can be installed.2. Figure 3-4-Typical Flanged Therrnowell Installation . glass coating).1 Materials end Ranges The thermocouple materials most commonly used in the refining industry are listed in Table 3-1. the couple is usually selected for the temperature range. Where FULL PENETRATION WELD. NOTE:Material used for thennowell installation shoula cornply witin pip- ing specification material and rating. Thermowells may be screw-mounted as shown in Figures 3-1. REDUCER OR SWAGE (TYPICAL) perience indicates that rapid temperature response is ELBOW INSTALLATION USE FOR 4" AND SMALLER LINES necessary. 2" AND SMALLER AS SHOWN W T T E D tions should be constructed with wall thicknesses as thin as operating conditions will permit. They are applicable for a wide range of temperatures 90' INSTALLATION 4" x LINE SIZE with acceptable accuracy and repeatability.4. where frequent inspection. INSTAUTION USE FOR 4" AND SMALLER UNES 2" AND SMALLER AS SHOWN DOlTED Figure 3-3-Typical Screwed Thermowell Installation USE FOR ALL SIZES 4" MINIMUM UNE S E E 3. When ex. AND AS SHOWN DO-D Newer thermocouple materials may require different types FULL PENE. 3-2. 3. themowells for temperature controller installa.4. 3. or pressure and temperature limitations require. temperature cycling..4. flange-mounted wells (such 3" x LINE SIZE as shown in Figure 3-4) are commonly installed. USE WELD ROD COMPATIBLE WlTH of themowell materials.1 APPLICATIONS Temperature instruments utiiizing themocouples are now the most generally applied of all temperature-measuring devices.5 THERMOWELL CONSTRUCTION Typical thermowell construction and installation details are shown in Figures 3-1 through 3-4. it should be vented to the atmosphere.3. and the well material is selected to suit the measured media. 3.RAION WELD.2 GENERAL 3. such as newly developed ceramics. -w However. Additional in- USE FOR 4" AND LARGER LINES REDUCER OR sw/(GE 3" AND SMALLER AS SHOWN DOTTED WPICAL) formation on applications may be found in ASTM STP 470B. and 3-3. special materials (for 3" MINIMUM UNE SlZE example.4 Thermocouple Temperature Instruments 3. USE WELD ROD BLIND FLANGE COMPATIBLE WlTH THERMOWELL DRILL AND TAP MATERIAL 4" TO LlNE SlZE ELBOW INSTALLATION REDUCER OR SWAGE REDUCER OR SWAGE USE FOR 4" AND SMALLER LINES WPlCAL) (TYPICAL) 2" AND SMALLER A S SHOWN DOfTED 45. Because thermo- couples are normally instalied in thennowells. THERMOWELL MATERIAL Suppliers should be consulted so that the most optimum com. 5.4. use of thermowells. usually to obtain better speed general use: of response.Male hlbing fining for thermocouple to pass through. The correct type of extension wires high purity. SEE KlTE THERMOCOUPLE \ HEAD FOR SINGLE ELEMENT THERMOCOUPLE 3.2. Out- sometimes installed with the thermocouple head separated side sheathing material is available in a variety of stainless from the thermowell.1. and allows the junction or sheath to be welded to a surface. The sheathed thermo- couple provides increased physical and chemical protection Figure 3-5-Metal-Sheathed. Type A is the standard construction with grounded tip. example.3 Installation \I THERMOWELL BUSHING Thennocouples are generally installed in thermowells as POTTED discussed in 3. mineral. titanium. 0.2. Install femle R Platinum. mineral- or any workable material. These assemblies time). An example of this type of installa- the sheath) with slower response.w THERMOCOUPLE A 3. and durable material are usually attached as a warning to 2.04 inches to thermocouple extension wires. tantalum. platinum.4. Type B has an ungrounded tip (electrically isolated from maintenance personnel. 36 gage up to No. insulated thermocouples are sometimes installed as bare Two types of measuring junctions (see Figure 3-5) are in elements without thermowells. Metal-sheathed thermocouples provide longer life and im- The choice as to which type to install is an engineering con- proved long-term accuracy when compared to bare wire sideration beyond the scope of this document. To minimize temperature lags (response Table 3-1-Thermocouple Materials and Temperature Range Normal Temperature -TH !LIEF BUSHING RUBBER GROMMET ANSI Thermocouple 2' LONG FLEXIBLE ARMORED Symbol Materials "F "C SHIELD FURNISHED INTEGRAL WITH COUPLE E Chromel-Constantan -300to 1600 -200to 900 C 40"-Constantan 30 to 1400 0 to 750 K Chromel-Alumel -300 to 2300 -200 to 1250 NOTE. An example of this type of installa- steels. 13 percent 30 ta 2700 0 to 1450 and nut and push couple to bottom of well then tighten tubing nut to secure rhod~urn-platinum couple in well.84 inches (1 millimeter to 21 millimeters) and thermo- Metal-sheathed. CROSSSECTION Increased use has been made of metal-sheathed. it is essential that the thermocouple be incontact with are made by insulating the thermocouple conductors with a the bottom of the well.4 API RP 550-PART I thermocouples are installed without protective thermowells. Plant prac- tices and manufacturers' literature are good sources of information. tion is shown in Figure 3-7. alloys of nickel-chromium-iron or nickel-copper (for tion is shown in Figure 3-6. Where thermocouples are installed without the 1. Mineral-Insulated to the thermocouple wires. Outside diameters range from 0. S Platinum. provides the ability to bend or Thermocouple Assemblies shape the thermocouple to a necessary form. see 3. -s insulated thermocouple assemblies.4. densely packed ceramic insulation (usually for the particular thermocouple must be used in connecting magnesium oxide) and enclosing the assembly in an outer the thermocouple to the instrument. special wiring tags of a distinct color welded or silver-soldered for fast response.2 Fabrication Fabrication details for thermocouples are covered in ANSI / / lNSUUTION MC 96. 10 percent 30 to 2700 0 to 1450 rhodium-platinum Figure 3-6--Sheathed-Type Thermocouple and Head T Copper-Constantan -300 to 650 -20 to 350 Assembly . 8 gage. There are applications where metal-sheathed. mineral-insulated thermocouples are couple wire sizes from No. SHEATH . For information on metal sheath. Inconel and Monel). it is common practice to use Iron-Constantan (or equivalent materials) in reducing atmospheres and Chromel-Alumel (or equivalent materials) in oxidizing atmospheres.2. For typical installation. are complex. Gaps between the tube wall and the thermocouple junction MINEFUL. transfer the reference junction to the end away from Wells the thermocouple to a point where the temperature is . . installation lengths. METALSHEATHED. A seal-off with 1 drain at the thermocouple end of the conduit and seal-off with U. This will. such as in reactors. Section 7. refer to RP 550.5 EXTENSION WIRES U Thermocouple extension wires must have the same elec- tromotive force (em0 temperature characteristics as the thermocouple to which they are connected. JACKET (SEE NOTE) NIPPLE ungrounded thermocouples is dictated by the application.4. refer to Figure 3-10 and for further in- mERMmUPIE formation on this measurement. Metal-sheathed thermocouples have gen. INSULATED should be minimized. Part III.4. OR PIPE W A U 3. ~t should be vented to relieve the pressure tn case of thermocouple well fatlure. For field installation where the thermocouple cannot be directly connected to a rigid conduit. These installations can be costly. grounding FLEXIBLE CONDUIT I should only be done at one mint. II 3. One design for attaching this type of thermocouple to heater e tubes is shown in Figure 3-9. c. Care must be exercised to avoid adding mass at the point of measurement. NOTE Where "TW" jacketed flexlble steel condrut 1s used. The choice of grounded or STEEL .3 TUBE SURFACE TEMPERATURE STUFFING BOX MEASUREMENT A special application of thermocouples is the measurement GATE VALVE of skin-point or tube-metal surface temperature of furnace tubes. Such installations require careful attention to ensure L that the thermocouple is properly attached to the tube and 2 is shielded from furnace radiation. They require the same care as the furnace tube surface temperature installations. Thermocouples of this type are also used to measure external wall surface temperatures of reactors or F1rrlNG other vessels. WEEP MALE erally been more satisfactory in applications requiring long THERMOCOUPLE -. The ad- dition of mass may result in a temperature different from THERMOCOUPLE that of the relatively cool tube wall to which it is attached. and may not be entirely reliable. For additional information on installation of thermo- r' couples. in ef- Figure 3-7-Installation of Thermocouples Without fect. Many companies have their own stan- THERMOCOVW &rds for this application. a flexible conduit may be used as shown in Figures 3-6 and 3-8. refer to RP 550. Other designs give satisfac- tory service. I @ Figure 3-8-Thermocouple-to-Conduit Connections 3. Part I.4 FIREBOX TEMPERATURE MEASUREMENT The application of thermocouples in fireboxes requires some special handling because of the wall construction. ) thermocouples.FLEXIBLE METAL CONDUIT drain at the control room entry provides a double block and II bleed in case the thermowell should fail and process fluid U" FLEXIBLE or gas enter the conduit. Regardless of the type used for measurement.4. For limits of error associated with extension wire. 3.4. - 6 API RP 550-PART I . Where possible use strength for pulling. spurious themocouples at the thermocouple terminal block or in the instrument. 10 percen1.1. 3. alloy platinum this protection lowers input impedance and is not compati- TX Copper-Constantan Copper-Constantan ble with parallel instrument applications. The wire sues normally used for must be considered when using this type of instrument: extension wire either singly or in pairs are 14. the in- Table 39-Thermocouple Extension Wire put impedance of the receiving instrument must be high (no Materials less than 0. SX Platinum.4. Recently designed instruments use solid state digital elec- tronic amplifiers or microprocessors to linearize or EX Chromel-Constantan Chromel-Constantan JX Iron-Constantan Iron-Constantan characterize the thermocouple inputs to match the calibra- KX Chromel-Alumel Chrornel-Alumel tion tables.01 Copper-copper nickel . The following points refer to ANSI MC 96. Differential or isolated input is necessary for use with American wire gage (AWG). dual thermocouples or two separate thermowells and thermo- couples. and 20 1. designed to meet system requirements.4. Older instruments usually used a bridge-type input circuit. common size used. the bridge and amplifier must be carefully Table 3-2. Part I. Section 7. When bundled and reinforced to provide 2. The use of incorrect extension Measurement data can be collected and displayed in two wire will cause errors in the temperature readings by creating bqsic forms. with 16 gage being the most grounded thermocouples. Since the system deals with a low millivolt input from the Materials for thermocouple extension wires are liSted in thermocouple. 20 gage and smaller may be used. I L-l DETAIL HEATER TUBE FULL TANGENTIAL AlTACHMENT WE FULL LENGTH ON SIDES OF KNIFEEDG Figure 3-9-Knife Edge Tube Surface Thermocouple for Heater Tube reasonably stable and where the effect of temperature varia.6. Thermocouple burnout protection can be provided. scribed in RP 550.6 TEMPERATURE INSTRUMENTS tions can be compensated. ANSI Thermocouple Wire Symbol Materials Materials 3.1 Input Circuit Thermocouple extension wires (available either in pairs or bundles with multiple pairs) should be installed as de. Parallel systems should be avoided. HEATER TUBE ELEMENT SHEATH ELEMENT EXPANSION LOOP OF TWO TUBE DIAMETERS 7 HEATER TUBE 7 ELEMENT 6tiEATH -\ \\ I I // HEATER WALL HOLBDOWN CLIP EXIT OPENING LOCATION OF HOT JUNCTION .5 megohm) to reduce interaction with other Extension parallel devices. but 13 percent rhodium. analog or digital. Refer to item 6 . Where parallel systems cannot be avoided. 16. Input impedance is usually very high. Transmitters 6. SECTION 3-TEMPERATURE 7 REFRACTORY AND STEEL SHEATHING TUBE COUPLING OR APPROXIMATELY SCREWED FLOOR FLANGE FLANGE 3" APPROXIMATELY 4t Now: I . This is extremely important in high-speed measur. Material external of firebox may be other than specified in Table 1-1. Some designs of vantages in overcoming these difficulties. Thermocouple should be O. One is mounted directly voltage.4. Figure 3-10-Typical Firebox Thermocouple Installations below for additional information on thermocouple burnout 3. Palt 111. and grounding. The head end of the thermocouple should have 2 inches of exposed wire. The mineral insulation shall be removed to a depth of at least 'A-inch and potted with compound. 2. thermowell.120-inch wall MgO insulated 14-gage nickel (90 percent)shromium (10 percent) thermo- couple wire with 446 stainless sheath. electromagnetic interference. on the thennowell and one is mounted remotely from the ing circuits and should not be neglected in slow-speed cir. 3" APPROXIMATELY 3. . Two types of field- tion to prevent conversion of common mode to normal mode mounted transmitters are available. installation. Table 1-1. digital systems provide continuity checking by generating Manufacturers should be consulted to assure a satisfactory pulses. The amplifier should have a high common-mode rejec. See RP 550. The transmitter output signal is then transmitted cuits since the normal mode alternating current (AC) voltage over standard copper wire to the remote receiver. vibration. REFRACTORY AND INSULATION STEEL SHUlTHlNG NOTES: 1. which are sent down the thermocouple lines. with regard to ambient temperature. Table 1-1 for materials.5Winch outside diameter by 0.2 Thermocouple Transmitters protection. could possibly be rectified by a nonlinear component in the Field-mounted thermocouple transmitters have limitations measuring circuit with a resulting millivolt error. transmitter can be located in the field.6. Part GI. or material listed in RP 550. Thermocouple burnout (open circuit protection) is usually mounted separately from the thermowell offer greater ad- provided as a pan of the amplifier design. the 5. To avoid the use of thermocouple extension wire. The 24-inch maximum imrnenion does n a apply to topentering installations. 2. accessibility. and remote loca. or other processor for the averaging function are becoming more devices.4. As in the case plexing of thermocouples for temperature control has had of parallel thermocouple connections.4 Multiplexing tial temperatures between two points. and scaling. 3. or 0 millivolts at the end of the scale. Serial.7 AVERAGING THERMOCOUPLES Switching devices can be obtained to select one of several To measure average temperature. should be-the same. the temperature/electromotive force (emf) curve and to enable random selection or scanning for alarm or logging purposes. The con- mounted units.6. Solid state switches. junction compensation that is normally furnished when tion of control devices can readily be accomplished. 3. 3. The selected input is converted at the field unit to ferent temperatures exist.6. One wiring method is the use of an external connection 4. run to the instrument terminal panel. Signal conditioning is connection box to which the thermocouple or extension wires readily accomplished. so that minor differences in resistance of the thermocouple extension wires have a negligible effect. Wiring from the field to developed oppose each other. For digital output. including of the thermocouple.11. and must be connected by the correct extension wires. Two similar thermo- Multiplexing systems generally gather many thermo. The instrument should not have the reference readout. An alternative method is to incorporate a noninductive. Multi. For accurate Such a device should always break or disconnect both leads measurement. and a common pair of extension wires specified. all extension wires between the thermocouples temperature data collected by these multiple input systems and connection box should be the same length with the ex- can be presented: cess leads coiled up to the nearer couples. low temperature 1. the configuration of the should not be grounded and the resistance of both thermo- temperature systems is a design function and beyond the couple circuits should be the same. Generdly. one input to be measured. reference junction compensation.4. thermocouples are con- thermocouple inputs ahead of the input circuit (see 3. Drum switches. A multipoint indicator or recorder can give analog values. either by random or programmed the net electromotive force (emf) will be zero.1). the resistances of all thermocouples. The master unit selects both thermocouples are equal.6. depending upon whether thermocouples are operated at high or low temperature with 3r4. performs respond to this temperature difference and the actual dif- linearization. 8 API RP 550-PART I 3. random access or point select are brought.4.4. . Scanning for alarm conditions can be nected in common. the voltage generated will cor- a digital signal. Additional options on digital output can be achieved by using a temperature instrument with an integral micro. the information can be presented on thermocouple. The advent of the microprocessor has allowed for greater flexibility in designing and applying thermocouple-driven Copper leads are used to connect the instrument with the multiple-input temperature systems. When dif- access. To prevent the flow forms. ference may be determined from the proper calibration curve. cathode ray tubes. box placed in a convenient location. In the a connection box. point alarm or sequential alarm. Toggle or push-button switches. coefficient. It can take any one of the following extension wires. 2. All thermocouples must be of the same type 2.8 TEMPERATURE DIFFERENCE Two thermocouples may be used for measuring differen- 3. Reference junction temperature is measured nections are made in such a way that the electromotive forces * at the field in the multiplexer unit. A suggested method is scope of this manual. Installations using separate transmitters and a micro- processor to drive typewriters.6. couples are connected together with extension wire of the couples (or other sensors) together into one or more field. nominal 1500 ohm resistor in series with the These devices can be made to scan points sequentially and sound an alarm if a measurement exceeds a desired value. Relays of various forms. regardless of the magnitude. not be grounded. if the temperatures of the master unit is a common cable. To obtain equal The following forms are examples of how the output resistances. and displays the temperature or provides a digital output for An instrument-either calibrated for 0 millivolts at midscale computers or other devices. shown schematically in Figure 3-12. Thus. measuring temperature with a thermocouple. the thermocouples limited application. depending on system requirements: of current through a ground loop. The master unit receives the signal. The microprocessor can also be used to calculate common. same materials as those used in the thermocouples. all terminals of each polarity should be con- digital indicators.5 Microprocessors respect to each other-can be furnished and used to read temperature differences.4. nected in parallel as shown in Figure 3. the thermocouples should 1.3 Multiple Input Systems 3. = To + AT).To). sometimes called the cold junc- DEVICE READOUT DEVICE tion. is the junction of a thermocouple circuit which is held at a known stable temperature. Duplex extension wires to be of equal resistance-same lengths. (T. Figure 3-11-Average Temperature Measurement with Thermocouples BLACK r \ f 3. makes it possible to determine the temperature of the other junction Figure 3-12-Temperature Difference Measurement by algebraic addition of ATand To.4. . 2. Connenion head with 15OO-ohm resistor-prmits use of different lengths of extension wires. such as that of melting ice. POWER NORMALLY If the temperature of one of the junctions is at a known tem- 18 TO 20 VOLTS DC perature To. the measurement of electromotive force. . SECTION 3--TEMPERATURE 9 METHOD A METHOD B INSTRUMENT INSTRUMENT TERMINALS TERMINALS EXTENSION WIRES UNGROUNDED THERMOCOUPLES Nom: 1. The with Thermocouples reference junction is usually located in the transmitter. and hence of AT = (T.9 REFERENCE JUNCTIONS RESISTANCE TEMPERATURE The reference junction. it should be noted that the accuracy curves are based on a sensing element resistance of 100ohms of temperature measurements is no better than the constancy at 0 degree Celsius. To obtain the greater accuracy and sensitivity in- herent in a resistance system and to minimize thermal lag. used.00392 ohmslohmldegree Celsius. the ranges of -450 to 1800 F (-270 to 980 C) (extreme). The Resistance-type temperature measurement can be used in wire normally used-is 18 AWG stranded copper.4. Generally. multiplexers. European R = 0. Generally. A third type. a connection head is employed and the reason. Two types of wire are generally used kilometers). Special attention needs to be directed at main- in resistance elements. The 3. device to the sensor. the reference junction is external. The use of transmitters.6. with the most common type being a tip-sensitive construction (see Figure 3-13). so a reference junction is not is located at the termination point of the conventional exten. as outlined in 3. Applications have been in thermal more accurate measurement of temperature than'is possible conductivity analyzers and laboratory measurements. nickel for ranges up to 600 F (315 taining a minimal number of junctions or terminations in the C) and platinum for ranges up to 1500 F (800 C). or receiver instrument. needed. no problem exists up to 5000 feet (1. Accordingly. a noncompensating for resistance and temperature change in the lead wire.2 RESISTANCE TEMPERATURE bEVlOES Resistance temperature devices (RTDs) operate on the the control panel.5. cable is used and a reference junction compensation device 2. is also applicable to HEADS resistance temperature devices. In any event.2.4. Figure 3-13-Standard Configuration Resistance perature devices with two exceptions: Temperature Devices .5. in an enclosure where the temperature is thermostatically con. The wire in the multiconductor cable may be 20 or 22 AWG. or American R = 0. For this ing unit. Individual extension wires (usually three) from the it is important that optimum thermowell dimensions (for the resistance element may terminate in a connection head or particular resistance element) be employed to maintain good in a quick disconnect fitting or extend directly to the measur- contact between the resistance bulb and the well. of the reference junction temperature or its compensation in the instrument. wells for resistance bulbs frequently are provided wires are frequently run in a three-wire cable to the board- with the resistance bulbs as matched units. 10 API RP 550-PART I multiplexer.1 APPLICATION upper operating temperature is determined by physical changes in the semiconductor material and is typically 200 Resistance-type temperature measurement can provide F to 750 F (93 C to 398 C). is used in large motor windings up to 300 F (150 C). copper.00385 ohmslohmldegree Celsius. the wires can be run to a field terminal strip.5 Resistance-Type Temperature negative) temperature coefficient of resistance. Such external reference junctions may be installed curves. permitting full-scale ranges of less than 1 F. The reading is absolute. EXTENSION CONNECTON WIRES processors. CONNECTION Resistance temperature elements are available in many con. a check should be made with the manufacturer on allowable wire e principle of change in electrical resistance on the wire as a resistance. when connection to the other end of the sensor. A mutticonductor cable is then used to bring the signals into 3. such as in low-differential temperature mea. Both trolled.2 Thermistor A thermistor is a semiconductor exhibiting a large (usually : 1 3. are used in many installations where their greater accuracy is warranted. Elements are available conforming to one of two sion wire. 3. Where multiple installations of resistance elements are and -420 to 1500 F (250 to 800 C) (practical).They are very Measurement sensitive.6 function of temperature. mounted resistance temperature measuring instrument. 3. Precautions and practices en- countered using thermocouples also apply to resistance tem. however. In some instances where I . Also. Ordinary copper wire is used to connect the readout especially accurate temperature measurements are required. The most commonly used configura- or where the temperature instrument is subjected to varying tion provides one wire connection to one end and a two-wire temperatures. / m+ BOX I figurations.5. the elements are designed to be used bare when very fast (5-6 second) response times are required. and micro. This compensates a number of very long leads are required. Even though most resistance UNGROUNDED elements used in the petroleum industry are mounted in a THERMOCOUPLES thermowell. resistance units dustrial applications are increasing.5.3 EXTENSION WIRES surement. for long distances. but in- with thermocouple installations. Some form of tem- Dial thermometers are the most common thermometers in perature indication is always desirable with these self- industrial use.5. The capillary tubing should not be cut.4 for the use of multiplexers with 3. There is no easy way to calibrate the units.4 RESISTANCE CONVERTERS In operation. It is usually desirable to use armored capillary Nbing and to support the tubing run between the capsule. or The use of filled-system devices is limited by the capillary pinched in any manner. the bulb scales and styles. Because of the The use of converters with resistance bulbs has become many different fluids being used. Instruments 0 3. and a capillary tube operatively connecting the two.6 Dial Thermometers for Local Temperature Measurement Temperature indication in the form of a dial mounted on the top of the valve and operated by the same thermal system is available from some manufacturers. to convert the measured 3. mechanical damage. The nor- that are in general use are shown in Table 3-4.3 SELF-ACTING TEMPERATURE transmission systems. The ability pending on the manufacturer. Dimensional.2 APPLICATIONS curacy of about 2 percent.5 PRECAUTIONS 8 A filled thermal system is a closal system containing a fluid fill (the temperature sensitive medium) and composed In all installations of filled-system temperature instruments.6. bulb and controller or transmitter to protect it from accidental damage.4. Dial-type thermometers that use filled and capillary should be protected from any damage. They are frequently of the bimetallic type with contained devices. diaphragm.7. Care should be taken to ensure the installation should be made so that the valve can be removed readability of the dial from a convenient location. Refer to 3. See Table 3-3 for limitations.7. which systems are also available (see 3. are summarized in Table 3-3. This self- adjustment assures accurate measuremen: of the temperature Radiation-type pyrometers are special instruments with where the bulb is located. bulb sizes and filling fluids more common both for pneumatic and electronic systems. REGULATORS Installation practices as outlined in RP 550. and physical data vary de. together with pneumatic or electronic transmitting and amplifying devices. temperature regulators are frequently used. Also. The most common type may cause fluid leaks or impede the flow of fluid.7). Temperature transmitters may use any one of several types of filled systems. an expansible device (Bourdon tube. See Figure 3-4 for installation. Systems having com- pensation are built to self-adjust for changes in temperature 3. Valve operators are in bellows form. bellows. which con- ". The classes of thermal systems limited use in refineries or synthetic fuel plants.4 for multiplexer applications. should be followed. As with other temperature- Such converters allow the use of standard transmission sensing instruments. manufacturers offer a type that can be adjusted to various angles. Some for inspection or service without damage to the capillary. to accomplish this is based on the fact that every object emits . opened. the has a fixed orientation. it is necessary to protect the bulb and capillary tubing from of a bulb.7. They detect high temperatures and offer the ad- Applications comparisons of various filled thermal systems vantage of fast response and noncontact measurement. These devices use thermal expansion systems and direct-operated valves.4 TEMPERATURE TRANSMITTERS C). For low-temperatureapplications below -22 F (-30 3.6. functional. mal range of use falls between -20 F to 7000 F (-30 C to 3900 C). SECTION 3-TEMPERATURE 11 f lead wire.4. The bellows may operate either a valve (directly) or a pilot valve. trols line fluid for actuating power to operate the main valve.8 Radiation-Type Pyrometers either on the case or on the capillary and the case. of an object without requiring physical contact. an increase in temperature expands the liquid (TRANSMITTERS) in the system and thereby operates the valve. 3.7 Filled-System Temperature temperature into an air or electrical signal. Part I. e 3. and so forth).7. vary with the temperature range. self-acting tion 7. circular dials and are available in a wide range of temperature As with all capillary types of thermal systems. bulbs should be protected by signals and offer more flexibility in the use of receiver equip thermowells. Sec- Where precise control is not essential. tubing that may be employed and by the maximum tempera- ture to which the bulb may be exposed. it may be desirable to use a filled thermal system type. They are nonlinear in output and have an ac- 3. Refer to 3.7. This information is a guide The radiation-type pyrometer measures the temperature only.1 GENERAL 3. ment. and 6. gas pressure. 10. and high. Short Nbing lengths 9. Minimum gas and mercury system span for force balance pneumatic 4. Time for temperature to reach 63 percent recovery constant of a step transminers is 50 C (90 F). 254x21 6x3 152x16 2-8 Note 8 Note 9 Note 10 Mercury expansion V-A . temperature of -225 C (-425 F) is possible with special construction. particularly very low temperatures. medium-high. temperature compensation usually result. respectively. . 3. The other three SyShmS are transmitters. and D -225-315 -425-660 40 70 220 400 28 50 Note 6 Note 7 -425-660 40 70 220 400 50 or 100 120 or 212 Note 6 Gas pressure 111-B -270-760 -450-1400 110 200 550 lo00 760 1400 30 100 IOXj/.Maximum Overrange Length (Note 3) Maximum Minimum Constant and (Note 4) SAMA Class (Note 2) C F C F C F C F meters feet inches mm inches mm seconds Vapor pressure (Note 5) 11-A. 8. but unwieldy bulb shes or poor ambient 7 . change for bulbs immersed in well-agitated liquid baths. Standard spans as narrow as 10 C (20 F) are possible under cenain ap- expansion liquid $re low. Above top scale temperature. Scale for vapor pressure is nonuniform. medium-low. mercury expansion. plication conditions. Scientific Apparatus Manufacturer's Association. Lowest value generally attainable only with force balance pneumatic 5.185-315 100% of span NOTES: I. Uniform motion or output with temperature may be accomplished systems) and 3 x 3 inches (mercury systems). Table 3-3-Application Data for Filled Measurement Systems Span Bulb Size Temperature Limits of Maximum Tubing 63 % Time System Type (Note 1) Limits Minimum . These instruments have bulbs as small as 6Xj( inches (gas uniform. C. Reduce to 122 C (250 F) for narrowest spans. Longer lengths possible. Minimum 2. Relative costs for vapor pressure.38-650 -38-1200 55 100 550 IMX) 200% of span 30 100 6XK 152x16 3xY1 76x13 2-6 Note 8 Note 10 V-B -38-650 -38-1200 55 100 315 600 200% of span 15 50 6x3 152x16 3xH 76x13 2-6 Note 10 Expansion liquid I-A -185-315 100% of span I-B . and minimum bulb diameters are required to obtain these minimum figures. for cenain vapor pressure ranges by mechanical means. 822-55003 .Order No. Northwest Washington. 20005 .C.American Petroleum Institute 1220 L Street. D. Class 11-A Vapor pressure. these background conditions. The subject surface must be hot enough ing the radiation laws of physics or experimentally by char. Since they are somewhat special in application. tromagnetic spectrum cannot be completely eliminated Class 111-A Gas-filled. or glass. or absorbing materials such as fumes. Class Description In industrial temperature measurement using a properly Class I-A Liquid-filled.I - Table 34-Classes of Thermal Systems in General Use SECTION 3--TEMPERATURE a basic product change all represent cases in which an ab- 13 solute change in effective emissivity is exhibited. instrument. fully compensated. and below case and tubing temperature. . the nonhomogeneous nature of some materials. increasing scale above. typically above 1400 F. increasing scale. uniform scale. This can be determined indirectly by apply. Error acterizing the material at a known temperature. with measured spurious effects of radiant energy from sources other than temperature above and below case and tubing the target. but energy overlays in the elec- Class 11-D Vapor pressure. user work closely with the manufacturer. increasing scale. uniform scale. with measured are not a detrimental factor as long as the infrared field of temperature above case and tubing temperature. Class V-A Mercuryfilled.9 Optical Pyrometers used in some instances. They usually rely on material in the spectral range of the radiation pyrometer) must the human eye comparing a filtered view of the target with be determined. case compensated only. If the radiation pyrometer is to measure absolute temper.and the intensitv of this radiation is a func. case compensated only. The reflection Class 11-B Vapor pressure. uniform scale. background radiation effects Class 1-8 Liquid-filled. to give off visible radiation. uniform scale. the effective emissivity (the emissivity of the target operate within the visible spectrum. smoke. increasing scale. Optical pyrometers are radiation-type pyrometers that ature. without impairing the accuracy and performance of the Class 111-B Gas-filled. Most applications use infrared radia- f> tion as the measurement source. view is aimed exclusively on the target area. ultraviolet is also 3. uniform scale. Class V-B Mercury-filled. however. an internal reference. uniform scale. it is recommended that the \ radiant energy-. fully compensated. at. Most radiation-type pyrometers used in refinery applica- tions are in the high-temperature range. fully compensated. Class 11-C Vapor pressure. Infrared instruments are designed to minimize temperature. designed industrial instrument. case compensated only. Such target may be introduced if the target surface reflects radiation from nonuniformities as significant temperature changes in the other hotter surfaces or if the path is partially obstructed by material. tion of its temperature. with measured from the background area should be minimized to reduce temperature below case and tubing temperature.