Section 3 - Understanding & Analyzing Overpressure Scenarios Training 1.0.5.2.pdf

iPRSM Technical TrainingUnderstanding and Analyzing Overpressure Scenarios Version 1.0.5.2 iPRSM ® Copyright © Curtiss-Wright Flow Control Service Corp., 2000-2009. 2 Contents Course overview Intended use 5 6 Module 1: Understanding protected systems for pressure vessels Objectives Lesson 1: Defining and evaluating a protected system 7 7 8 8 12 13 13 13 13 15 Defining protected system boundaries Evaluating a protected system Lesson 2: Analyzing a protected system Pressure variable analysis Using set pressure and MAWP Using overpressure for piping losses Module review Module 2: Analyzing, calculating & evaluating relief contingencies Objectives Lesson 1: Rules for analyzing relief contingencies 16 16 17 17 17 18 18 19 20 20 23 24 26 30 About contingency relief analysis Upstream pressure Double jeopardy Operator intervention Control response Lesson 2: Evaluating blocked outlet About blocked outlet Blocked outlet vaporization Liquid overfill Pump deadhead Upstream piping and fittings 3 Lesson 3: Evaluating the effects of control valves 32 32 35 36 37 38 38 40 40 43 43 43 43 45 46 49 50 52 53 58 58 58 60 61 62 62 62 62 63 63 64 Automatic control failure Multiple control valves in combination Abnormal heat input Instrument air failure Lesson 4: Evaluating inadvertent valve operation Inadvertent valve opening Lesson 5: Evaluating heat exchanger tube rupture About heat exchanger tube rupture Lesson 6: Evaluating fire About fire scenarios Applicability of fire cases Fire case physical properties Fire boiling liquid with vapor generation Insulation credit Fire vapor expansion Fire supercritical Fire high boiling point liquid Fire on liquid full equipment Lesson 7: Evaluating other scenarios About other scenarios Liquid thermal expansion Cooling failure Power failure Mechanical equipment failure Chemical reaction Steam out Check valve failure Series fractionation. reflux failure. & loss of quench Other Module review 4 . Course overview COURSE OUTLINE Module 1: Understanding Protected Systems for Pressure Vessels Module 2: Analyzing. Calculating & Evaluating Relief Contingencies IN MORE DETAIL… Module 1: Understanding protected systems for pressure vessels This module describes how to define and evaluate a protected system. 5 . calculating & evaluating relief contingencies This module describes how to decide which relief scenarios to apply to a protected system. documenting and evaluating orifice sizing and inlet and outlet piping losses for relief scenarios in iPRSM. and presents methods for calculating. Module 2: Analyzing. These guidelines are generic. Application of the guidelines in this document is entirely at the discretion of the user. These guidelines apply to the evaluation of relief requirements and overpressure protection of pressure vessels designed for ! 15 psig and other equipment. Any liability associated with the application of these guidelines is solely that of the user. and it is understood that client/site-specific guidelines must take precedence over the guidelines presented here. Farris Engineering Services makes no claim as to the completeness or accuracy of the material presented in this document. It is the responsibility of the user to review and consider client/site-specific guidelines in addition to those provided in this document. in the protected system documentation in iPRSM. Deviation from these standard procedures may be appropriate in specific situations. as well as related calculations. there are many possible approaches to a solution. It is essential in any case to include complete notes clearly describing your reasoning. and are not relevant for atmospheric tanks and similar low-pressure equipment.Intended use This document provides guidelines for determining required relief rates for contingency scenarios and for the documentation of those scenarios. Each client or plant may have specific guidelines that will supersede these generic guidelines. 6 . For any given engineering problem. you will be able to ! ! ! define the boundaries of a protected system describe the steps involved in evaluating a protected system use pressure variable analysis to decide how overpressure will be applied to a protected system during evaluation 7 . Objectives At the end of this module.Module 1 Understanding Protected Systems for Pressure Vessels This module describes how to define and evaluate a protected system. Lesson 1: Defining and evaluating a protected system Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. For context-sensitive information on any of these topics, pick Help on any iPRSM page. Defining protected system boundaries A protected system is a grouping of interconnected types of process equipment. The grouping of process equipment is broken down into smaller units designated as ! ! ! ! protected equipment pressure vessels, heat exchangers, piping, etc. protecting equipment relief valves, rupture discs, pilot valves, etc. overpressure sources pumps, upstream equipment operating at a higher pressure, etc. ancillary equipment control valves, other relief valves, etc. All protected systems have a specific boundary, with a starting point and ending point. Typically, the upstream and downstream boundary is some type of a valve, like a block valve or a control valve. Everything within the boundary must be protected by the protecting equipment. 8 EXAMPLE DISCUSSION SKETCH 1 ! ! The overpressure source is an upstream vessel. The control valve downstream of the upstream vessel is ancillary equipment, and is the starting point of the downstream protected system. The relief valve on the upstream equipment is also ancillary equipment. The pressure vessel is protected equipment, and the relief valve is the protecting equipment. The end points of this system are the two control valves on the discharge side of the protected pressure vessel. Notice that the end-point control valves are not designated as ancillary equipment; their size and flow capacity have no influence on the required relief rate for the protected system. These control valves would become ancillary equipment in the downstream protected system. ! ! ! 9 ! Designating each piece of equipment in a plant as protected, protecting, overpressure, or ancillary allows you to easily manage process changes for their potential impacts to an entire process unit’s relief systems. Any process changes made to the system or any of its pieces of equipment, like changing the set pressure of the upstream pressure vessel relief valve, will uncheck this system as well as the upstream pressure vessel’s protected system, generating a message in iPRSM noting that your change may have impacted both systems and prompting you to evaluate the change and determine if the relief protection of both systems is still adequate. DISCUSSION SKETCH 2 ! 10 DISCUSSION SKETCH 3 11 . the tube rupture scenario may not be applicable. what is a potential overpressure source.Evaluating a protected system Successful evaluation of relief systems is based on a thorough understanding of the relief system and how it interacts with the process. 3. like specification sheets for pumps. EXAMPLE What are the high-side operating conditions of an exchanger in which you are evaluating the low-side relief protection? What are the upstream operating conditions of a control valve that is supplying a downstream pressure vessel? 5. isometrics of inlet and outlet piping for relief devices. 4. Decide what pieces of equipment are being protected by a given relief device. 2. Accurate and complete operating data is essential in determining whether a particular scenario may or may not be applicable. consider the high-pressure side as a possible overpressure during tube rupture. 1. Collect all relevant equipment data. Apply engineering principles to determine if a given failure can actually cause relief. Collect all relevant process-specific data. and how the system interacts. heat exchangers and control valves. EXAMPLE If the high side of a heat exchanger operates at a pressure that is less than the low-side hydrostatic test pressure. U-1s and dimensional drawings for pressure vessels. and which overpressure scenarios can result in a relieving event. 12 . ! ! Identify what is being protected. Begin by reviewing the P&IDs associated with the system. Look at upstream equipment and downstream equipment for the effects of control failures as possible sources of overpressure to gain a full understanding of how the process operates. EXAMPLE When evaluating the low-pressure side of a heat exchanger’s relief protection. what equipment failures can possibly cause overpressure. the capacity at which pressure drops should be computed is the scenario-required capacity for liquid relief and the capacity of the device for vapor relief scenarios calculated at the overpressure used in the scenario calculation. 2. USING OVERPRESSURE FOR PIPING LOSSES ! Piping losses are computed for vapor and steam relief cases at the full valve capacity. pick Help on any iPRSM page. 13 ! . Equipment Notes and in the Protected System Notes. iPRSM will not calculate scenarios with the Pset > MAWP. USING SET PRESSURE AND MAWP 1. If the RV Set Pressure is equal to or less than the lowest MAWP of the protected system equipment. For context-sensitive information on any of these topics. Enter the Pset on the relief valve equipment worksheet equal to the lowest MAWP. As a general rule. not at the required flow rate. all calculations and evaluations of the protected system should be done based on the RV Set Pressure. record the deficiency in the relief device Findings and Deficiencies Notes. ! Evaluating protection with set pressure at MAWP yields the most useful information. ! ! Set the system to mitigate when calculations are completed. for existing installations. Pressure variable analysis The following considerations help you to decide the appropriate pressure to use in determining the relief contingencies that should be applied to a protected system. If the RV Set Pressure is greater than the lowest MAWP of the protected system equipment.Lesson 2: Analyzing a protected system Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. and record the actual set pressure of the device in the RV Equipment Worksheet Notes to explain that calculations are done based on the limiting MAWP. The options are ! 10% for ASME Section VIII and Section III. This may be useful in cases where the valve capacity is far less than the required capacity.! For new installations. 14 . This is the more conservative approach. 3% for ASME Section I scenario OVP for all ! The selection can be overridden on the individual piping pressure drop calculations by picking the OVP Select checkbox. the pressure drops in the inlet and outlet piping should be computed at the valve capacity at a maximum of 10% OVP for all scenarios. iPRSM has a plant-level default that lets you specify which OVP is ! the default setting for piping loss calculations. record it in the Protected System Notes. ! It is possible to have iPRSM compute the pressure drops at the maximum valve capacity instead of the required flow rate for liquid relief cases by picking the Relief Flow Select checkbox in the scenario piping losses view. including multiple valve applications and fire cases. If you make this change. Module review Have you met the objectives of this module? Can you ! ! ! define the boundaries of a protected system? describe the steps involved in evaluating a protected system? use pressure variable analysis to decide how overpressure will be applied to a protected system during evaluation? 15 . Objectives At the end of this module. and presents methods for calculating. Calculating and Evaluating Contingency Scenarios This module describes how to decide which relief scenarios to apply to a protected system.Module 2 Analyzing. documenting and evaluating orifice sizing and inlet and outlet piping losses for relief scenarios in iPRSM. you will be able to ! ! define the upstream pressure for a system evaluation describe rules and limits of using certain credits to reduce relief rates use a variety of methods to calculate relief contingencies apply calculation methods to evaluate relief systems in iPRSM ! ! 16 . etc. an overpressure of 16% on the downstream relief device is allowed for this evaluation even if the equipment in question is in different protected systems. 90% of the upstream set pressure can be used. If the case is not considered as a result of this condition. Fire. Inst Air Failure. 17 . ! ! ! Use the upstream high operating pressure as the upstream source pressure for analysis if: ! the upstream source equipment would not likely increase in pressure as a result of overpressure in the downstream protected equipment ! ! Use upstream operating pressure from mass balance information or operating records.Lesson 1: Rules for analyzing relief contingencies Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. For context-sensitive information on any of these topics. Use the upstream relief pressure for analysis of potential sources of overpressure for the system in these cases: ! ! if the upstream equipment and the protected equipment have the same possible occurrence of overpressure (ex. be sure to record it in the Scenario Notes. If plant data on the normal high operating pressure is not available.) if an increase in pressure of the protected system (downstream) would result in an increase in pressure from the source vessel if the upstream relief device and the protecting devices being evaluated are set within 5% of the lowest MAWP. pick Help on any iPRSM page. About contingency relief analysis Contingency relief analysis is guided by the following general rules. UPSTREAM PRESSURE ! Many scenario evaluations are dependent on the upstream or source pressure of a feed stream. Selection of the source pressure to determine if relief is applicable can vary depending on the system. it cannot be considered to respond on the failure of the second regulator in series. If the operator can respond to the possible overfill of equipment within a reasonable time. or if it is adjusted in such as way that it is normally full open. That action would be considered a single failure. a single failure could be considered if the operator inadvertently lined up the incorrect line to the system. EXAMPLES ! Double regulators in series are fairly common. and double jeopardy would not apply. OPERATOR INTERVENTION ! Operator intervention is commonly used as a viable prevention of overpressure for liquid overfill cases. If double block valves are present in a line. and should not be considered as double jeopardy. The operator cannot respond if he is unaware of the problem. If there is no indication that the first regulator may have failed open. Latent failures are those that can occur without being identified. Caution in declaring double jeopardy is advised. but it is normally considered to be anywhere from 15 minutes to 30 minutes within an operating unit. Operator response time should be determined by persons familiar with the operations of the facility being reviewed. and up to two hours for a storage area. Operator response requires an alarm of the problem. The failures considered must be truly independent of and unrelated to one another. The inadvertent opening of two independent block valves would normally be considered as double jeopardy. and double jeopardy would not apply. the case is not considered viable. The time required for response can vary. ! ! ! 18 . Full flow through both valves would be appropriate.DOUBLE JEOPARDY ! Overpressure that would require more than one independent failure is considered double jeopardy and is not considered viable. The transmitter or bridle could have become plugged. If the designed response of a control valve would act to increase the relief requirement. an alarm using the same level bridle or transmitter cannot be considered as independent notification of the rising level. CONTROL RESPONSE ! The proper response of a control system or valve cannot be credited in eliminating an overpressure scenario from consideration. causing the valve failure simultaneous with failure to activate the alarm. Control valve response times are not considered rapid enough to provide protection. or with reducing the amount of relief flow required. but care needs to be used to ensure that the upstream pressure will not also increase in response.! The alarm must be independent of the possible cause of failure. ! ! ! ! 19 . it must be assumed to respond in its designed manner. Credit can be taken for reduced flow through the control valve in its normal position due to the increase in downstream pressure. EXAMPLE If overfill can occur when an outlet level control valve on a vessel fails closed. Use engineering judgment. If the response would serve to reduce or eliminate the overpressure it is assumed to maintain its normal position. This is the basis from which several of the contingencies are determined to be applicable or not applicable. EXAMPLE ! ! ! The incoming streams <X> and <Y> do not have sufficient pressure <list the maximum operating or relief pressure and the source of the information> to cause relief <list the system set pressure>. ! Do not state relief can occur but overpressure will not occur because the relief valve is set below MAWP. The case should be calculated if upstream pressure exceeds the set pressure by more than the allowable overpressure.Lesson 2: Evaluating blocked outlet Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. even if by less than 10% above minimum MAWP. and is crucial to a comprehensive understanding of the protected system. The streams should be documented through listing in the Scenario Notes. Each stream should be named something easily identifiable from the P&ID or system sketch. The supply pressure may be from operating data or based on an upstream PSV set point. pick Help on any iPRSM page. For context-sensitive information on any of these topics. About blocked outlet ! A list of all incoming streams with the maximum expected pressure or its pressure source should be recorded in the Blocked Outlet Contingency Notes. Calculations are not required if the source pressure cannot exceed the set pressure + the allowable overpressure . This scenario assumes that all outlets that can be blocked are blocked. ! ! 20 .by 10% for single valve installations or 16% for multiple valve installations. For the streams to be combined. they should also be entering the system simultaneously during normal operation. dryers with regeneration cycles.! Any inlets with streams that cannot provide sufficient pressure to cause relief are also assumed closed. The vessel is assumed to contain its normal high operating liquid level.batch processes. The full flow rather than the normal flow would be appropriate for the relief flow rate. Review the operation of any control valve on inlet streams. operations with varying piping lineups . All pumps. ! ! ! ! ! ! ! 21 . A control valve set to open on flow may respond to reduced flow rate caused by increased down stream pressure by going to its full open position. Generally.additional blocked outlet contingency scenarios may be needed to address all possible relief cases. upstream equipment. For equipment with multiple operational modes . Record all assumptions in the Scenario Notes. The relief device should pass all fluid entering the system at relief conditions and any change in volume generated by energy entering the system. and headers should be linked to the protected system as overpressure sources even if they are determined to not be able to cause overpressure or relief. the sum of all entering streams should be considered when calculating relief rate. All upstream relief valves that are used to demonstrate the inability to cause relief should be linked to the protected system as ancillary equipment. DISCUSSION SKETCH 22 . The design duty of heat exchangers using a clean coefficient of heat transfer should be used. the design LMTD. Change in volume due to vaporization of a trapped liquid is normally considered under blocked outlet. The duty may be adjusted for the relief Log Mean Temperature Differential (LMTD) vs. Create a spreadsheet showing the adjustment to the LMTD.BLOCKED OUTLET VAPORIZATION ! Change in volume due to liquid thermal expansion should be considered under the thermal expansion contingency. Annotate with the source of the information and a demonstration of the calculations used to determine the clean heat transfer coefficient. Attach the spreadsheet calculations in the Protected System Documents in iPRSM. EXAMPLE: SPREADSHEET CALCULATIONS ! ! 23 . Vapor 2 Phase or 2 Phase (DI). Enter the determined flow rate in iPRSM using the hazard type Given Flow and an appropriate flow type . and enter the case as a given flow rate 2 phase using the appropriate direct integration or Omega method calculations. or that the control valve flow is not greater than the pump capacity. EXAMPLE: MERR 1 SPREADSHEET ! Set property flash linked to scenario at the relief pressure and relief temperature calculated in the MERR 1 spreadsheet to use the appropriate properties in the iPRSM scenario calculations. use the MERR 1 spreadsheet to determine the relief rate. LIQUID OVERFILL ! Review the source vessel to ensure that sufficient volume in the source equipment exists to overfill the protected equipment. ! 24 . as appropriate. Include calculations for all except the very obvious cases.! If the vapor generated at relief pressure exceeds the thermodynamic critical point. The fill time is based on the high (alarm) liquid level on the equipment design criteria. Blocked outlet in these cases is typically not considered for a liquid overfill case which meets the operator response time requirements. You cannot expect response without a method of notifying the operator of the problem. ! ! 25 . VESSEL EQUIPMENT DESIGN CRITERIA ! If the protected system contains a predefined amount of residence time above the high liquid level alarm. a fully independent alarm is needed.! iPRSM calculates the vessel fill time on the vessel equipment worksheet. This time frame will vary based on the facility and site-specific guidance. In order to credit operator response. operator response may be credited. Evaluate the pump deadhead at the installed impeller size. Include high liquid level and assume vessel at high operating pressure. Use of installed or maximum impeller size is project or site specific. including the source pressure above the liquid in the upstream of the pump. VESSEL EQUIPMENT CALCULATION REQUIREMENTS PUMP DEADHEAD ! Evaluate pumps that feed the system to determine if the deadhead pressure plus the effects of any hydrostatic head exceed relief. For design of new systems. If the installed impeller is unknown. Do not include the hydrostatic head in the suction pressure as these are added by iPRSM. use the maximum possible impeller size. Obtain the height of the suction equipment from the field inspection or design data for that equipment. and input the required data into iPRSM on the pump equipment worksheet. determine the deadhead pressure. From the pump curve or pump specification sheet. 26 ! ! ! ! .! Verify site or plant-specific guidelines time limits and document in Scenario Notes why the case is not applicable. use the maximum impeller if practical. EQUIPMENT VIEW .PUMP EQUIPMENT PARAMETERS PANEL 27 . assume 90% of the set pressure of the suction equipment’s relief device.! If the operating pressure of the suction is unknown. Using the Pump Curve and Pump Head At Relief calculated by iPRSM. determine relief rate for Pump with Installed Impeller. This can be estimated from the isometric.PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW ! In the blocked outlet scenario view. Enter the height of the relief device in the pump pressure scenario Worksheet. Record any cases that cannot cause relief in the Protected System Notes. this pump must be linked to the protected system as OVPSource or protected equipment. ! ! ! ! 28 . Select the overpressure pump from the Overpressure Pump dropdown menu. select the Hazard Type: Pump Pressure and the Flow Type: Liquid. To be available for selection. Note that this value is not displayed if you are using input mode. SCENARIO VIEW – PUMP PRESSURE WORKSHEET – FULL VIEW – NOT INPUT MODE 29 . If the normal action of the control valve would act to increase the flow then assume that it would go to its full open position . ! Flow reduction due to resistance from the piping is normally ignored. assume it stays in its normal position. refer to the two-phase calculation methodologies for determining the area calculation and piping losses. The correct response of any control valve that would act to reduce the flow to the system cannot be credited. a flow control valve on the inlet line that would see a reduction in flow as the downstream pressure increases would be calculated at the full flow through the wide open control valve. EXAMPLE If there is an upstream pressure control valve that would normally close to reduce the flow rate. The Gas Pipe Flow or Liquid Pipe Flow spreadsheets can be used to calculate the maximum flow through a section of piping.that is. If the relieving material flashes either across the relief valve or in the outlet piping.! This discussion assumes all liquid relief to the point of discharge. ! 30 . UPSTREAM PIPING AND FITTINGS ! The normal flow rate might be used to evaluate the system with flow from an upstream system at a higher pressure under pressure control. Be sure to document any assumptions used such as length of piping. but can be considered if a more thorough evaluation is required. If you want to determine the maximum flow through a section of piping a spreadsheet should be used. field isometric inspection data is not normally requested. GAS PIPE FLOW SPREADSHEET 31 .! In a first-pass analysis. Lesson 3: Evaluating the effects of control valves Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. use the Installed Cv for the control valve at 100% open. When using a Cv from Table 1. give the scenario a unique extension name to help identify which control valve has failed. Automatic control failure ! For each automatic control failure case. Control valves that don’t result in relieving cases may be listed together in the general automatic control failure scenario. Wide open Cv from the manufacturer’s data should used and may be obtained from the control valve spec sheet. This will be shown as the valve Cv. provided represent a conservative assumption for globe style valves. not as the Cv at maximum flow. For this scenario. If the manufacturer’s data is not available. clear the Apply Scenario checkbox. record in the Equipment Data Notes that specific valve Cv data was not available. Use the Cv for the port diameter if known and the pipe size if unknown. and that. and may be used if the specific valve data cannot be located. ! ! ! ! ! 32 . Be sure to include appropriate comments in the Scenario Notes if assumptions are used. the values in Table 1. Each control valve that requires calculations should have its own scenario. pick Help on any iPRSM page. For context-sensitive information on any of these topics. Do not use these values until after you have attempted to obtain specific data on the valve. When calculating flow through a control valve. conservatively. Be sure to include appropriate Scenario Notes to document cases that don’t apply. the Cv for an <X> inch full port globe valve is used. 022 0.812 22.013 0.18 8.9 8905.012 0.78 5.5 7.278 1.48 7.068 3.9 1484.08 4.25" 80 0.68 113.90 17618.3 193.546 0.021 0.012 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 340 9.013 0.93 12664.4 262.0 18686.012 0.95 5.35 7576.92 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 100 192 333 607 856 1504 2505 3868 5246 6854 11102 16559 29680 46689 68913 83286 108798 143337 178111 227268 257608 320880 373726 430598 472002 535666 599043 5.42 4.08 4.012 0.TABLE 1: FULL PORT GLOBE VALUE CV DATA This table lists the calculated Cv (for liquid) and Cg (for gas) values for globe valves.58 326.80 5238.5 1.124 15 16.01 11.5 33 .957 1.14 6.5" 3" 3.3 21529.624 34.08 4.25 40 40 40 30.8 75. For other types of valves such as ball and gate.6 16.79 17.012 0.18 44.018 0.013 0.08 4. Full Port Globe Valve Cv Per Crane Nom.76 4.1 26783.1 4.939 2.21 2026.23 73.08 4.938 13.4 12880.7 555.25 STD 27.08 4.64 13882.742 0.08 4.3 5439.42 4.9 = f (L/D) d^2/sqrt(K) Crane A-31 Assumed Globe Valve Fisher Catalog Friction factor L/D 1/2" 3/4" 1" 1.014 0.5 3445.94 1373.58 3199.25 STD 29.4 16044.012 0.9 7166.3 342.016 0.9 23600.25 STD 25.026 5.5 11363.012 0.018 0.015 0.08 4.5" 2" 2.55 6684.94 5.44 5. Size Sch. use the equations in Crane Technical Paper 410.76 154.876 18.012 0.02 872.0175 340 STD 21. ID Crane A-27 Full Port Globe Crane K Cv C1 Cg Cs Steam below 1000 psig Cg/20 A-26 =29.95 4215.65 9.12 5.65 10991.012 0.027 0.7 4164.3 29952.46 6.6 30.624 32.86 25.30 201.41 15754.76 4.70 9437.86 2449.023 0.4 42.548 4.12 6.019 0.2 1.025 0.0 2334.08 4.5" 4" 5" 6" 8" 10" 12" 14" 16" 18" 20" 22" 24" 26" 28" 30" 32" 34" 36" 80 80 80 80 80 40 40 40 40 40 40 40 40 40 40 40 40 40 40 0.012 0.469 3.08 2.1 827.624 0.981 10.065 7.2 125.82 7.047 6.014 0.42 4.0 9.017 0.54 487. A control valve failure simultaneous with a blocked outlet is normally considered a double jeopardy. the control valve must be linked to the protected system as ancillary equipment. The equations assume that the entire pressure drop is taken across the control valve.PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW ! When entering scenario data in iPRSM: ! ! ! ! use a unique name for the scenario to help identify which control valve has failed Hazard Type: Control Valve Failure Flow Type: Vapor or Liquid Select the control valve for evaluation by picking its checkbox in the Selected Control Valves list. To be available for selection. 34 . The required relief rate for a failed control valve is typically the flow rate through the failed open valve less the normal flow rate. ! Use W adjust in iPRSM to increase or reduce the control valve flow as needed to account for normal flows. ! iPRSM has equations to calculate the flow rates for automatic control failure for liquids that do not flash and vapors. MULTIPLE CONTROL VALVES IN COMBINATION Remember that you cannot take credit for the correct response of a control valve. like spec sheet for equipment or control valve.) ! The Cg is computed based on the combined Cv calculated above. but can also be used to increase the flow rate for relief when needed. often one of the two control valves in series is full open. ! A more rigorous approach is to iterate the flows through the two valves until they balance. There is some error in this method of calculating Cg. W adjust ! ! To be conservative. as well as the source of data for the normal flow rate used. Parallel For two or more control valves in parallel the Kv or Cv can be calculated as: C = Cv1 + Cv2 + … 35 . use the Low Normal flow rate for the credit. The easiest method to use to determine the flow is to combine the Cv’s of the two valves using the following formula. ! There are two possible methods for determining the maximum flow through two control valves in series. Record in the Scenario Notes that the required relief rate is the difference. PFD. etc. Series ! Depending on their set pressures. Cv combined = 1/ SQRT (1/CV1^2 + 1/CV2^2 + 1/CV3^2 ….! is normally used to subtract the normal flow rate through the control valve being considered. Value must be negative if subtraction is desired. etc. and C1 = 34 for a globe valve. so even though one of the two valves is not considered to fail. This is not normally required as a first-pass calculation. it is not double jeopardy to assume that one does not respond. The heat transfer of the equipment may be adjusted based on the temperature difference of the hot side supply vs. the cold side fluid bubble point temperature at relief pressure. a supply limitation in the fuel or heating medium. The spreadsheet referenced for blocked outlet vaporization can be used to predict the relief flow rate.ABNORMAL HEAT INPUT ! Abnormal heat input is a special case of control failure involving the flow of fuel or heating medium to process heat transfer equipment. assume there is an infinite supply of the heating medium so the hot side temperature is the supply temperature. the more rigorous and time consuming calculations need not be performed. ! ! ! ! ! ! 36 . or condensing temperature at supply pressure for steam systems. For screening calculations. the column feed composition should be used. The heat input is limited for these cases by either a limit in the heat transfer capacity of the equipment. If the relieving capacity of the system is adequate based on the screening calculations. or a combination of both. Distillation reboiler: If the normal bottoms composition bubble point temperature exceeds the hot side supply temperature. EXAMPLE ! ! It is likely that plant-wide instrument air failure might shut down the process gas compressor in an ethylene unit. Often this results in no relieving case. Also look at other overpressure sources such as compressors that might cause a relief if they were to fail. evaluate each of your automatic control failure scenarios to see if any cause relief situations with the valve in the design failure position. 37 .DISCUSSION SKETCH INSTRUMENT AIR FAILURE ! For the global instrument air failure scenario. Nonetheless. ! Instrument air failure is a global failure in which the high variable back pressure should be used. evaluate the effect of all of the control valves in a system going to their failure position. This assumes that control valve bypasses are not used to supplement the control valve capacity. so the presence of multiple block valves does not rule out the case. A single failure can also be an operator making an incorrect line up involving two block valves. Note that valves that are part of a CSC/CSO inspection program are typically assumed to be in their given position and are not subject to inadvertent valve operation. Consult the specific plant or client guidance. then that line would be subject to the single failure criteria. Calculate the required relief as the maximum flow through a valve and piping segment. The maximum calculated flow through the bypass at relief pressure is the relief flow. Review any feed streams that can cause overpressure . Include control valve bypasses as susceptible to inadvertent valve opening.Lesson 4: Evaluating inadvertent valve operation Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook.these should all be included in the Blocked Outlet Scenario Notes . For context-sensitive information on any of these topics. pick Help on any iPRSM page. Assume that the control valve stays in its normal position and the normal forward flow continues. Consult plant-specific guidelines and operating instructions. If there are two valves in a line but one is typically left open. ! ! ! ! ! ! ! 38 . Some facilities have procedures that make the consideration of the control valve bypasses not applicable for relief sizing. Inadvertent valve opening ! Inadvertent valve operation is the scenario that reviews the possible overpressure created by the opening or closing of any valve.and determine if they have a single valve that can be opened or closed that would result in overpressure. assume it is a full port valve. a more detailed review of the piping configuration from the pressure source to the protected system may be needed. Clearly record all assumptions. If the relief valve is inadequate based on these simplifying assumptions. You can also determine the flow with the control valve failure calculations and enter the Cv for the type of block valve being reviewed. For the first-pass calculations. Often an ISO is not needed and a fitting count and estimate of the length of pipe is sufficient to calculate the flow resulting from the inadvertent valve opening using the gas flow or liquid pipe flow spreadsheets. For control valve bypasses shown as globe valves. and enter the flow as given flow rate into iPRSM.! Calculate the flow for the inadvertent valve opening scenario using the gas flow or liquid pipe flow spreadsheets as appropriate. include the obvious fittings from the P&ID to determine the K’s and equivalent lengths. DISCUSSION SKETCH ! ! ! ! 39 . 40 . Use caution when declaring that a tube rupture scenario is not applicable. Unless site specific guidance applies. If low side test pressure is greater than or equal to the high side MAWP. not just the exchanger. use an Orifice Coefficient: 0. For existing installations.60 and 2 full tube areas. or 90% of the set pressure of the pressure relief device. pick Help on any iPRSM page. should a tube rupture occur. For context-sensitive information on any of these topics. no relieving case is required to be evaluated. Remember to compare all of the low pressure equipment in the system.Lesson 5: Evaluating heat exchanger tube rupture Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. ! ! ! iPRSM can be used to calculate the flow through the broken tube. that it is not exceeded. you can compare the low side test pressure against the maximum operating pressure on the highpressure side. About heat exchanger tube rupture ! The criteria to determine if tube rupture is a valid scenario are: ! Compare low pressure side test pressure with the high pressure side MAWP. or 90% of the set pressure of the low set valve for multiple valve applications on the high-pressure side. Be sure to review the test pressure of other equipment in the system to ensure that. Verify that there are no control valves or other possible restrictions that would limit the low pressure side flow.! Be sure to link the high pressure side of the exchanger to the protected system as an overpressure source. then enter the volumetric flow on the low pressure side as the CapCredit liq or CapCredit vap on the Tube Rupture Worksheet. Be sure to record in the Scenario Notes that you are taking credit for the normal volumetric forward flow rate on the low pressure side as well as where you got the flow rate information for the LP side.TUBE RUPTURE SCENARIO WORKSHEET PARAMETERS ! The flow required for relief is the difference between the flow through the two tubes and the normal volumetric flow on the low pressure side. SCENARIO VIEW . ! ! 41 . Several models exist that can be used to perform this calculation. The fill time should be determined based on the liquid inflow from the rupture minus the normal liquid outflow of the low pressure side. for flashing liquid. 1998) contains software tools that are widely used to document this calculation. Credit for operator response should not be taken unless there is a level alarm to initiate operator response.! If two-phase or flashing flow through the tube. which yields a conservative result as a first pass evaluation. Include surge time calculation as documentation if credit is taken. This can be done by inspection. the relief can be evaluated as the flashed vapor portion of the tube rupture flow. Optionally. flow calculations will have to be performed outside of iPRSM. review the system to determine if there is adequate disengagement for the liquid. If disengagement seems practical. ! ! ! 42 . ! The Guidelines for Pressure Relief and Effluent Handling Systems (CCPS. Then convert the liquid tube rupture rate into a vapor rate for the relief rate. you can calculate the tube rupture rate as a liquid. Calculate the surge time to verify. for instance disengagement may be possible in the shell side of a kettle type exchanger but not in the tube side of any exchanger. ! ! If the liquid flashes at relieving pressure. pick Help on any iPRSM page. APPLICABILITY OF FIRE CASES ! Unless specifically directed otherwise. plant procedures require cooling water to be drained when not in service and will relieve to the cooling tower when in service. Fire is not considered an applicable overpressure scenario for any direct fired equipment. Fire is not considered applicable to steam systems unless they typically have a condensate level. About fire scenarios Calculation and evaluation of fire scenarios takes the following approaches into account. Thermal expansion relief remains a viable contingency on the cooling water side of heat exchangers. Check your specific plant or client guidelines to determine if fire calculations apply to cooling water (CW) side of heat exchangers. assume that all equipment is in a potential fire zone. select pressure/bubble point with the pressure set at relief pressure. For context-sensitive information on any of these topics. If the relief valve is mounted under the liquid level. Verify the plant specific guidelines.Lesson 6: Evaluating fire Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. Verify that there are no control devices on the cooling water return line that might become closed. ! ! ! ! FIRE CASE PHYSICAL PROPERTIES ! If you are working with a pure component: ! ! obtain physical properties by selecting the relief pressure and dew point to obtain the vapor properties. Record in Scenario Notes that fire calculations do not apply to CW systems. 43 . DISCUSSION SKETCH ! ! ! 44 . pick the View Graph link. This is used to determine the vapor composition and effective latent heat for a multicomponent fluid stream through a series of successive flashes and calculations. Additional details on the analysis are available from the Export S/S link on the LH flash maximization plot view. To see the details in the calculations. Input the maximum percentage of the stream to be vaporized for the analysis. temperature. and specific heat ratio of the vapor generated. compressibility. The system will determine the point at which the maximum vapor relief requirement is reached as a factor of the effective latent heat. select the latent heat flash type in the stream flash view. The vapor phase reported from this flash is the relief fluid.! For multicomponent streams. ! ! Input the pressure for the flash. The point of the maximum relief rate is predicted based on the effective latent heat and volume of the relief fluid. molecular weight. and select whether the latent heat is to be corrected for the specific heat of the liquid by selecting no-fire from the dropdown menu. iPRSM calculates the wetted surface area for the liquid holdup in the tower. adjust by setting the He Select dropdown menu. iPRSM will make these computations for you. Record what was assumed in the Scenario Notes. Record in the Equipment Notes what liquid level is used and other pertinent data elements. All fire calculations on equipment within the unit boundaries are subject to a maximum fire height of 25’. As a worst case assumption. Based on the distillation tower entries. Equipment in tank farms are subject to a 30’ fire height. Verify any plant specific guidelines in case your plant has adopted the more stringent 30’ NFPA limit instead of the 25’ API standard. ! ! ! ! ! ! ! ! 45 .FIRE BOILING LIQUID WITH VAPOR GENERATION Determine heat load ! iPRSM will calculate the wetted area for the fire case on the equipment worksheet. This may exceed the normal fire height limit. If the normal liquid level is to be used. use the liquid level corresponding to 100% of the field level transmitter. For a sphere. More stringent default fire heights can be entered as a plant-level parameter and option. or operating data. equipment drawings. Unless otherwise directed. These can be overridden for any specific fire evaluation if needed. ! Liquid heights are evaluated from the bottom of the vessel. All data required for wetted area calculation must be entered on the equipment worksheet. specifications. Be sure to add the height of the bottom head if the liquid level is known from the tangent line. The plant level defaults should be set to the fire height used at the facility you are working on. the fire level includes all wetted area up to the maximum vessel diameter. if data is not available. For the liquid level in a distillation column. enter the normal and high liquid levels as done for other types of equipment. and elevation data entered in the vessel data worksheet. The wetted area will be based on either the Normal Liquid or the High Liquid level as selected. use the high operating liquid level for the fire case. The liquid level may be available from the P&ID. When insulation credit is taken for any fire case. propylene. EQUIPMENT VIEW – WORKSHEET PARAMETERS ! On the equipment worksheet enter the appropriate heat absorption rate. IPRSM wetted area calculations allow you to include or exclude the area of the bottom head. the area of a vessel head that is enclosed by a skirt may be excluded from fire calculations. record in the Protected System Notes that insulation credit is needed to ensure ! 46 . The options are listed as: ! ! API 521 Adequate for good drainage for poor drainage or NPFA equipment propane. LPG storage API 521 Inadequate INSULATION CREDIT ! The environmental factor on the equipment worksheet in iPRSM is used to define the insulation credit.! In many instances. adequate relief protection in the event of fire. ! If insulation credit is not needed. Heat may also be transferred internally across the tubes.0. If the boiling point of the fluid on the side opposite to the side you are working on has a bubble point temperature at its relief pressure that is higher than the bubble point temperature at the set pressure of the fluid on the protected side.0 for no insulation or non-fire proof 1. The vapor rate from the protected vessel and the W area fire vapor are additive.03 EQUIPMENT VIEW . heat will enter a heat exchanger through all surfaces exposed to the fire.0 for cold insulation (normally non-fire proof) calculated value if insulation configuration is known and vessel has SS jacketing and banding If the Environment Factor Selector is set to Fire Proof Insulation. then the entire heat exchanger external surface area should be used. . the wetted area can be entered in iPRSM as User-Supplied Wetted Area or the vapor rate can be entered as W Area Fire Vapor if it is to be added to the rate calculated by iPRSM for a vessel on the fire scenario worksheet. however this will increase the flare header loading for the global fire case. Insulation on protected equipment must be maintained to ensure adequate protection.05 for jacketed vessels 1. calculations can be performed with F: 1. iPRSM will calculate the environmental factor based on the values provided. During a fire. Typical environment factors to be used per API: ! ! ! ! ! ! ! for full insulation with SS banding and jacketing . ! ! ! 47 .ENVIRONMENT FACTOR SELECTOR ! If the wetted area is determined outside of iPRSM. select Hazard Type: Fire Vapor Generation and Flow Type: Vapor. The flows and streams will need to be added outside of iPRSM and re-entered in a different scenario. Select the equipment that is participating in the scenario. you must attach the involved equipment to the system as either protected or overpressure sources. Fire should be calculated at 21% overpressure. Use multiple fire scenarios for varying physical properties if needed. Evaluate relief ! In the fire scenario.! If the bubble point temperature at relief pressure of the opposite side is less than the bubble point temperature at set pressure of the side you are working on. only the exposed surface area of the protected side need be used. All equipment involved in a given scenario is evaluated based on the single latent heat and fluid properties attached to that scenario. Note that the comparison is with the temperature at set and not at full relief. To add their wetted area to the calculation. ! ! ! ! 48 . You may have multiple pieces of equipment involved in the fire. During a fire the walls of a vapor filled vessel quickly reach elevated temperatures. Failure of the walls due to the elevated temperatures can occur before relief pressure is reached.! Multiple fire scenarios may also be needed in cases where some equipment in the system is diked and separated from other pieces of equipment. PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW FIRE VAPOR EXPANSION ! Protected systems that contain no liquids and are vapor filled may require fire calculations. ! ! 49 . An assumed wall temperature (carbon steel vessel) for a fire case is 1100 deg F based on API 521. and record in the Scenario Notes and Protected System Notes that relief will not occur in a fire case. ! ! ! FIRE SUPERCRITICAL ! In the event that the fluid is above the critical temperature or pressure at fire relief conditions.8. 50 .3 calculation in iPRSM. These may include systems such as water curtains. vessel wall temperature will exceed allowable temperature prior to reaching the relief pressure. Select the fire relief flash and phase in the scenario and reevaluate to have iPRSM calculate the required relief rate and orifice area. etc. calculate iPRSM as vapor. ! Obtain the physical properties needed by flashing the fluid at relief pressure and the relief temperature predicted by iPRSM. If Z<0. use two phase direct integration or the Omega D. ! Evaluate relief ! To calculate the relief requirements. select the Hazard Type: Fire Gas Expansion in the fire scenario worksheet. iPRSM will calculate the relief temperature based on the operating ! ! temperature from the relief valve equipment data worksheet and high operating pressure of the protected equipment. Pick Evaluate Scenario to have iPRSM calculate the relief temperature for the gas expansion. The higher the operating pressure and the lower the operating temperature. If the relief temperature is predicted to exceed the wall temperature. If Z>0. Note that this value will not be shown if you are using input mode. the fluid will be similar to a vapor. set the scenario to not apply. fire monitors. See discussion on supercritical fire cases for specific instructions related to Omega and piping pressure drops when using Omega.8. Evaluate a flash of the relief stream at the relief pressure and the fire relief temperature. other methods of cooling the vessel in the event of fire should be evaluated.! Select the vapor expansion hazard type in iPRSM to calculate the relief temperature and relief flow rate. the more conservative the results will be. Enter required information in iPRSM using Given Flow 2 Phase (DI) or 2 Phase to use API omega calculations for supercritical vapors entering the relief valve. MERR 1 MODEL SPREADSHEET ! Determine heat load ! Set up a fire scenario using the fire vapor generation hazard type to have iPRSM calculate the heat input during the fire case. Set liquid height of the protected vessel to 100% full. ! Piping pressure drops ! Piping pressure drops are not calculated in iPRSM when using the 2 phase Omega calculation methods. The MERR 1 Model identifies the point at which the supercritical expansion and resultant factor of relief parameters is maximized. Set this scenario to not apply. Input this relief case as a hazard type of Given Flow/Two Phase or Given Flow Rate/2 Phase (DI). Use the MERR 1 Model spreadsheet to identify the point at which this function is maximized.! The relief rate is calculated at the point where the density change and the physical properties maximize the required relief area. ! Evaluate relief ! Complete the MERR 1 spreadsheet to determine the relief point and flow rate. 51 . ! ! ! ! 52 . but it will not boil. use MERR 1 to determine relieving rates and direct integration or the Omega method to calculate the relief valve orifice area. ! ! ! ! If the outlet fluid does not condense.! For the supercritical fluid. Enter to Outlet Loss as User Supplied in iPRSM. the outlet pressure drop calculated from the scenario used for the inlet pressure drop can be handled in the same manner as described above from the same worksheet. enter the estimated maximum relieving capacity in the Piping Pressure Drop Worksheet as a User Supplied Flow Rate. a typical maximum temperature is 1100°F. estimate the relief capacity based on the ratio of the required relief area to the installed relief area: Wcap = Wrequired (Ainstalled/Arequired) Using the fire vapor generation worksheet in iPRSM in which the heat input was calculated. the temperature of the liquid in the vessel will continue to rise. In this situation. Make sure that the scenario worksheet has the proper vapor density at the inlet to the relief valve. For carbon steel. use 2 Phase (DI) or an alternative method outside of iPRSM to calculate piping losses. a situation similar to a vapor filled vessel can occur. vessel failure may occur before the pressure relief device set pressure is reached. the inlet piping pressure drop can be evaluated as a vapor at the estimated maximum relief capacity of the relief valve. Use the same method outlined above for supercritical fluids to evaluate the relief requirements. If the boiling point is more than 200°F above the fire design temperature. From the required flow and required area calculated. ! FIRE HIGH BOILING POINT LIQUID ! When a vessel contains a liquid with a boiling point at relieving pressure that is higher than the design temperature of the vessel. If the fluid condenses. If the boiling point is high enough. resulting in only vapor relief. Client-specific and fluiddependent. Equipment that is liquid full with a top mounted relief valve on the vessel may be calculated as a vapor generation scenario. The initial relief will be liquid. The calculations assume that sufficient disengagement between the liquid phase and the vapor phase occurs prior to reaching 21% overpressure. followed by a period of two phase flow and finally vapor. The relief rate is calculated as the saturated liquid at the vapor generation rate on a volumetric basis. Equipment that has the relief valve mounted under the liquid level will result in a large relief requirement due to a vapordriven liquid (VDL). ! ! ! ! Some operators calculate this scenario as a vapor and some operators calculate this scenario as discussed below. This calculation methodology varies from site to site. See the fire boiling liquid with vapor generation section for calculation details. This is because the fluid boils along the vessel walls and must push an equivalent volume of liquid out to the relief valve to maintain relief pressure. ! 53 . Consult site-specific guidelines prior to proceeding with calculations.FIRE ON LIQUID FULL EQUIPMENT ! Often called Vapor Driven Liquid. DISCUSSION SKETCH 1 54 . DISCUSSION SKETCH 2 Determine relief load ! Determine the vapor generation rate in the same manner as for standard fire vapor generation. Evaluate the required relief area using the Fire . ! ! ! 55 .2 or D.3 equations may be used depending on the number of constituents and proximity to thermodynamic critical points. Determine the equivalent liquid that must be displaced using the liquid density at relief conditions. or select the 2 Phase Omega method. Convert the vapor generation rate to the volumetric vapor flow rate using the density of the vapor at relief conditions.2 Phase (DI). The D.Given Flow Rate . PROTECTED SYSTEMS CONTINGENCY SCENARIO VIEW 56 . If the Omega method is used. No additional inputs are required. Enter the required relief rate and pick Evaluate Scenario. evaluate pressure drops using the same process as described for supercritical fluid with a liquid on the inlet. flash and phase at the relief device inlet. Piping pressure drops ! If using 2 Phase (DI) scenario calculations. Enter results in iPRSM as User Supplied.! To evaluate the scenario using 2 Phase (DI) calculations. and upload any spreadsheets used. Attach the stream. select Given Flow Rate . The outlet piping losses will have to be calculated outside of iPRSM. pick the Piping Losses link and the Evaluate Scenario & Piping Losses link in the piping losses view.2 Phase (DI). ! 57 . use the clean heat transfer coefficient to determine the duty of the exchanger. For liquid thermal expansion from heat transfer equipment.Lesson 7: Evaluating other scenarios Supplemental information Some of the information that follows is excerpted for your reference from the iPRSM handbook. About other scenarios These additional approaches are also considered in calculating and analyzing scenarios. Scenarios where the heat source is capable of vaporizing the trapped liquid are covered in other scenarios such as blocked outlet. abnormal heat input or automatic control failure. LIQUID THERMAL EXPANSION ! When a liquid system is blocked-in. any heat input will result in overpressure due to thermal expansion. For context-sensitive information on any of these topics. ! ! 58 . pick Help on any iPRSM page. ! ! Although we calculate the capacity of the valve at 10% overpressure for liquid expansion cases with nominal flow rates. 59 . if the RV discharges back into a separate section of piping that is also protected by a relief valve. or calculated as if it were a liquid only. the constant back pressure should be equal to the normal operating pressure of the line. the valve will never reach that level of overpressure. and using the set pressure without any overpressure is normally acceptable.LIQUID THERMAL EXPANSION CALCULATIONS SPREADSHEET ! For liquid thermal from solar heat input. two phase calculations in the outlet piping can be safely ignored and not calculated. For liquid thermal expansion from either solar or tracing heat input. due to the small flow rates. and the variable back pressure should be equal to the set pressure minus the normal operating pressure. and loss of cold feed or quench. For solar heat input on any system. if condensation is lost. There are a number of different modes of cooling failure. unless the specific client allows higher relief pressures. ! COOLING FAILURE ! Loss of process cooling creates an energy imbalance. the excess vapor will generally have to be relieved. automatic control failure. then the spring set should be adjusted so that it will open at the maximum design pressure of the piping under maximum back pressure conditions. The cooling failure scenario is used for the global loss of cooling case for a system. As a general rule the high variable back pressure should be used for the cooling failure case.F on the exposed surface area.! If the valve discharging into the second section of line is a conventional valve.. Do not include surfaces that will not be exposed to direct sunlight. which can produce major relief loads. Other cooling failure cases can be covered under abnormal heat input.3 piping codes allows set pressures up to 20% above design. like the bottom half of piping systems. etc. loss of cold side circulation in a process heat exchanger. ! ! ! 60 . relief may occur due to flashing caused by the entrance of hot feed. If cooling on a feed stream is lost. as appropriate. including cooling water failure. A relieving situation may not result if only sensible cooling is lost. use Heat Transfer Rate: 300 2 o BTU/hr-ft . However. although unexpected flashing could overpressure downstream equipment. ASME B31. ! ! ! 61 .DISCUSSION SKETCH POWER FAILURE ! The power failure scenario is used for the global or partial loss of electricity to a unit. There may be combined effects associated with partial loss of cooling water. Because power failure is considered a global scenario. do not override the high variable back pressure to evaluate this scenario. Evaluate the effect on a system of losing the motor-driven electrical equipment. can cause significant relieving situations. or the loss of a compressor that.! As in other generic cases like control failure. This might cause the specific scenario of lost reflux. Loss of fin-fan cooler and refrigeration systems also needs to be evaluated. MECHANICAL EQUIPMENT FAILURE ! Evaluate the effect of a failure of a single rotating piece of rotating equipment. Data on chemical reactions must be provided by the client and is normally obtained through laboratory testing. Review the vacuum rating of the equipment in the system and warn of potential damage due to excessive external pressure (vacuum) caused by condensing steam during steaming operations for any equipment that might be steamed-out. ! STEAM OUT ! If the equipment in a system has a lower design pressure than the steam-out steam supply pressure. determine the flow of steam into the system using the piping and fitting from the steam header to the system with pressure drop of Psupply: Prelief in the Gas Pipe Flow spreadsheet. ! 62 . Often in chemical reactions two phase flow is applicable and a 2 phase flow calculation method should be used. a lost bottoms pump that may cause a relieving situation in a system due to overfilling. ! ! CHEMICAL REACTION ! ! iPRSM does not predict chemical reactions or the rates of relief. Often this is evaluated under another scenario. the engineer must analyze the probable consequences of each assumed utility outage to identify those cases where over-pressure may result in the process system under consideration. especially for a multistage. which should be covered under lost quench or reflux. REFLUX FAILURE AND LOSS OF QUENCH ! Series fractionation To be discussed during distillation column training ! Reflux failure To be discussed during distillation column training. even if properly inspected and maintained. leakage across the check valve seat is assumed during reverse flow conditions. Flow can be calculated using the Cv Failure Scenario Hazard Worksheet in iPRSM or in the Gas Pipe Flow or Liquid Pipe Flow spreadsheets. Caution is recommended when taking any credit for the reduction of reverse flow using check valve/s. if properly inspected and maintained. ! Loss of quench To be discussed during distillation column training OTHER This guideline describes methods to evaluate a range of possible overpressure scenarios. ! ! ! SERIES FRACTIONATION. leakage across the valve is assumed to equal the flow through a single orifice with a diameter equal to one-tenth of the largest check valves nominal flow diameter. It may not address all possibilities. 63 . the valve is assumed not to exist. For calculation purposes. for a single check valve.CHECK VALVE FAILURE ! For a single check valve. For a pair of check valves in series. Use site-specific guidance if credit is to be taken. It is the responsibility of the evaluating engineer to fully review the system and verify there are no unidentified relief scenarios before signing off. Module review Have you met the objectives of this module? Can you ! ! define the upstream pressure for a system evaluation? describe rules and limits of using certain credits to reduce relief rates? use a variety of methods to calculate relief contingencies? apply calculation methods to evaluate relief systems in iPRSM? ! ! 64 .