Section 3 - Understanding & Analyzing Overpressure Scenarios Training 126.96.36.199.pdf
iPRSM Technical Training Understanding and Analyzing Overpressure Scenarios Version 188.8.131.52
iPRSM ® Copyright © Curtiss-Wright Flow Control Service Corp., 2000-2009.
Module 1: Understanding protected systems for pressure vessels
Lesson 1: Defining and evaluating a protected system
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 2: Analyzing, calculating & evaluating relief contingencies
Lesson 1: Rules for analyzing relief contingencies
About contingency relief analysis
Lesson 2: Evaluating blocked outlet
About blocked outlet Blocked outlet vaporization
Upstream piping and fittings
Lesson 3: Evaluating the effects of control valves
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
38 38 40 40 43
Lesson 6: Evaluating fire
About fire scenarios Applicability of fire cases
Fire case physical properties
Fire boiling liquid with vapor generation
Fire vapor expansion
Fire high boiling point liquid
Fire on liquid full equipment
Lesson 7: Evaluating other scenarios
About other scenarios Liquid thermal expansion
Mechanical equipment failure
Check valve failure
Series fractionation, reflux failure, & loss of quench
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. Module 2: Analyzing, calculating & evaluating relief contingencies
This module describes how to decide which relief scenarios to apply to a protected system, and presents methods for calculating, documenting and evaluating orifice sizing and inlet and outlet piping losses for relief scenarios in iPRSM.
Intended use This document provides guidelines for determining required relief rates for contingency scenarios and for the documentation of those scenarios. These guidelines apply to the evaluation of relief requirements and overpressure protection of pressure vessels designed for ≥15 psig and other equipment, and are not relevant for atmospheric tanks and similar low-pressure equipment.
These guidelines are generic. For any given engineering problem, there are many possible approaches to a solution. Each client or plant may have specific guidelines that will supersede these generic guidelines, and it is understood that client/site-specific guidelines must take precedence over the guidelines presented here. It is the responsibility of the user to review and consider client/site-specific guidelines in addition to those provided in this document. Application of the guidelines in this document is entirely at the discretion of the user. Any liability associated with the application of these guidelines is solely that of the user. Farris Engineering Services makes no claim as to the completeness or accuracy of the material presented in this document.
Deviation from these standard procedures may be appropriate in specific situations. It is essential in any case to include complete notes clearly describing your reasoning, as well as related calculations, in the protected system documentation in iPRSM.
Module 1 Understanding Protected Systems for Pressure Vessels This module describes how to define and evaluate a protected system.
Objectives At the end of this module, 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
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
pressure vessels, heat exchangers, piping, etc.
relief valves, rupture discs, pilot valves, etc.
pumps, upstream equipment operating at a higher pressure, etc.
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.
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.
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
DISCUSSION SKETCH 3
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. 1. Begin by reviewing the P&IDs associated with the system.
Identify what is being protected, what is a potential overpressure source, and how the system interacts.
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, consider the high-pressure side as a possible overpressure during tube rupture. 2. Decide what pieces of equipment are being protected by a given relief device, what equipment failures can possibly cause overpressure, and which overpressure scenarios can result in a relieving event. 3. Collect all relevant equipment data, like specification sheets for pumps, heat exchangers and control valves, U-1s and dimensional drawings for pressure vessels, isometrics of inlet and outlet piping for relief devices. 4. Collect all relevant process-specific data. 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. Apply engineering principles to determine if a given failure can actually cause relief. EXAMPLE
If the high side of a heat exchanger operates at a pressure that is less than the low-side hydrostatic test pressure, the tube rupture scenario may not be applicable. Accurate and complete operating data is essential in determining whether a particular scenario may or may not be applicable.
Lesson 2: Analyzing 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.
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. 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, all calculations and evaluations of the protected system should be done based on the RV Set Pressure. 2. If the RV Set Pressure is greater than the lowest MAWP of the protected system equipment, record the deficiency in the relief device Findings and Deficiencies Notes, Equipment Notes and in the Protected System Notes.
Set the system to mitigate when calculations are completed.
Enter the Pset on the relief valve equipment worksheet equal to the lowest MAWP, 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.
iPRSM will not calculate scenarios with the Pset > MAWP.
Evaluating protection with set pressure at MAWP yields the most useful information. USING OVERPRESSURE FOR PIPING LOSSES
Piping losses are computed for vapor and steam relief cases at the full valve capacity, not at the required flow rate.
As a general rule, for existing installations, 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. 13
For new installations, 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, including multiple valve applications and fire cases. This is the more conservative approach.
iPRSM has a plant-level default that lets you specify which OVP is
the default setting for piping loss calculations. The options are
10% for ASME Section VIII and Section III, 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.
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. This may be useful in cases where the valve capacity is far less than the required capacity. If you make this change, record it in the Protected System Notes.
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?
Module 2 Analyzing, Calculating and Evaluating Contingency Scenarios This module describes how to decide which relief scenarios to apply to a protected system, and presents methods for calculating, documenting and evaluating orifice sizing and inlet and outlet piping losses for relief scenarios in iPRSM.
Objectives At the end of this module, 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
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, 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.
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. Fire, Inst Air Failure, etc.)
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, 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. If the case is not considered as a result of this condition, be sure to record it in the Scenario Notes.
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.
If plant data on the normal high operating pressure is not available, 90% of the upstream set pressure can be used. 17
Overpressure that would require more than one independent failure is considered double jeopardy and is not considered viable. Caution in declaring double jeopardy is advised. The failures considered must be truly independent of and unrelated to one another.
Latent failures are those that can occur without being identified, and should not be considered as double jeopardy. EXAMPLES
Double regulators in series are fairly common. If there is no indication that the first regulator may have failed open, or if it is adjusted in such as way that it is normally full open, it cannot be considered to respond on the failure of the second regulator in series. Full flow through both valves would be appropriate, and double jeopardy would not apply. The inadvertent opening of two independent block valves would normally be considered as double jeopardy. If double block valves are present in a line, a single failure could be considered if the operator inadvertently lined up the incorrect line to the system. That action would be considered a single failure, and double jeopardy would not apply. OPERATOR INTERVENTION
Operator intervention is commonly used as a viable prevention of overpressure for liquid overfill cases.
If the operator can respond to the possible overfill of equipment within a reasonable time, the case is not considered viable. The time required for response can vary, but it is normally considered to be anywhere from 15 minutes to 30 minutes within an operating unit, and up to two hours for a storage area.
Operator response time should be determined by persons familiar with the operations of the facility being reviewed.
Operator response requires an alarm of the problem. The operator cannot respond if he is unaware of the problem.
The alarm must be independent of the possible cause of failure. EXAMPLE
If overfill can occur when an outlet level control valve on a vessel fails closed, an alarm using the same level bridle or transmitter cannot be considered as independent notification of the rising level. The transmitter or bridle could have become plugged, causing the valve failure simultaneous with failure to activate the alarm. Use engineering judgment. CONTROL RESPONSE
The proper response of a control system or valve cannot be credited in eliminating an overpressure scenario from consideration, or with reducing the amount of relief flow required.
Control valve response times are not considered rapid enough to provide protection.
If the designed response of a control valve would act to increase the relief requirement, it must be assumed to respond in its designed manner.
If the response would serve to reduce or eliminate the overpressure it is assumed to maintain its normal position.
Credit can be taken for reduced flow through the control valve in its normal position due to the increase in downstream pressure, but care needs to be used to ensure that the upstream pressure will not also increase in response.
Lesson 2: Evaluating blocked outlet 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.
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. This is the basis from which several of the contingencies are determined to be applicable or not applicable, and is crucial to a comprehensive understanding of the protected system.
The supply pressure may be from operating data or based on an upstream PSV set point.
Each stream should be named something easily identifiable from the P&ID or system sketch.
The streams should be documented through listing in the Scenario Notes. EXAMPLE
The incoming streams and do not have sufficient pressure to cause relief .
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, even if by less than 10% above minimum MAWP.
Calculations are not required if the source pressure cannot exceed the set pressure + the allowable overpressure - by 10% for single valve installations or 16% for multiple valve installations.
This scenario assumes that all outlets that can be blocked are blocked.
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.
The relief device should pass all fluid entering the system at relief conditions and any change in volume generated by energy entering the system.
Generally, the sum of all entering streams should be considered when calculating relief rate. For the streams to be combined, they should also be entering the system simultaneously during normal operation.
For equipment with multiple operational modes - batch processes, dryers with regeneration cycles, operations with varying piping lineups - additional blocked outlet contingency scenarios may be needed to address all possible relief cases.
Review the operation of any control valve on inlet streams. 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. The full flow rather than the normal flow would be appropriate for the relief flow rate.
All pumps, upstream equipment, 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.
All upstream relief valves that are used to demonstrate the inability to cause relief should be linked to the protected system as ancillary equipment.
Record all assumptions in the Scenario Notes.
BLOCKED OUTLET VAPORIZATION
Change in volume due to liquid thermal expansion should be considered under the thermal expansion contingency. Change in volume due to vaporization of a trapped liquid is normally considered under blocked outlet.
The design duty of heat exchangers using a clean coefficient of heat transfer should be used. The duty may be adjusted for the relief Log Mean Temperature Differential (LMTD) vs. the design LMTD.
Create a spreadsheet showing the adjustment to the LMTD. 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
If the vapor generated at relief pressure exceeds the thermodynamic critical point, use the MERR 1 spreadsheet to determine the relief rate, and enter the case as a given flow rate 2 phase using the appropriate direct integration or Omega method calculations. Enter the determined flow rate in iPRSM using the hazard type Given Flow and an appropriate flow type - Vapor 2 Phase or 2 Phase (DI). 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.
Review the source vessel to ensure that sufficient volume in the source equipment exists to overfill the protected equipment, or that the control valve flow is not greater than the pump capacity, as appropriate.
Include calculations for all except the very obvious cases.
iPRSM calculates the vessel fill time on the vessel equipment
worksheet. The fill time is based on the high (alarm) liquid level on the equipment design criteria. VESSEL EQUIPMENT DESIGN CRITERIA
If the protected system contains a predefined amount of residence time above the high liquid level alarm, operator response may be credited.
Blocked outlet in these cases is typically not considered for a liquid overfill case which meets the operator response time requirements. This time frame will vary based on the facility and site-specific guidance.
In order to credit operator response, a fully independent alarm is needed. You cannot expect response without a method of notifying the operator of the problem.
Verify site or plant-specific guidelines time limits and document in Scenario Notes why the case is not applicable. VESSEL EQUIPMENT CALCULATION REQUIREMENTS
Evaluate pumps that feed the system to determine if the deadhead pressure plus the effects of any hydrostatic head exceed relief.
Evaluate the pump deadhead at the installed impeller size. If the installed impeller is unknown, use the maximum possible impeller size. For design of new systems, use the maximum impeller if practical. Use of installed or maximum impeller size is project or site specific.
From the pump curve or pump specification sheet, determine the deadhead pressure, and input the required data into iPRSM on the pump equipment worksheet, including the source pressure above the liquid in the upstream of the pump.
Obtain the height of the suction equipment from the field inspection or design data for that equipment. Include high liquid level and assume vessel at high operating pressure.
Do not include the hydrostatic head in the suction pressure as these are added by iPRSM. 26
If the operating pressure of the suction is unknown, assume 90% of the set pressure of the suction equipment’s relief device. EQUIPMENT VIEW - PUMP EQUIPMENT PARAMETERS PANEL
PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW
In the blocked outlet scenario view, select the Hazard Type: Pump Pressure and the Flow Type: Liquid.
Select the overpressure pump from the Overpressure Pump dropdown menu. To be available for selection, this pump must be linked to the protected system as OVPSource or protected equipment.
Enter the height of the relief device in the pump pressure scenario Worksheet. This can be estimated from the isometric.
Using the Pump Curve and Pump Head At Relief calculated by iPRSM, determine relief rate for Pump with Installed Impeller. Note that this value is not displayed if you are using input mode.
Record any cases that cannot cause relief in the Protected System Notes.
SCENARIO VIEW – PUMP PRESSURE WORKSHEET – FULL VIEW – NOT INPUT MODE
This discussion assumes all liquid relief to the point of discharge. If the relieving material flashes either across the relief valve or in the outlet piping, refer to the two-phase calculation methodologies for determining the area calculation and piping losses.
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. The correct response of any control valve that would act to reduce the flow to the system cannot be credited. EXAMPLE
If there is an upstream pressure control valve that would normally close to reduce the flow rate, assume it stays in its normal position. 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 - that is, 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.
Flow reduction due to resistance from the piping is normally ignored, 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. The Gas Pipe Flow or Liquid Pipe Flow spreadsheets can be used to calculate the maximum flow through a section of piping. Be sure to document any assumptions used such as length of piping.
In a first-pass analysis, field isometric inspection data is not normally requested. GAS PIPE FLOW SPREADSHEET
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. For context-sensitive information on any of these topics, pick Help on any iPRSM page.
Automatic control failure
For each automatic control failure case, give the scenario a unique extension name to help identify which control valve has failed.
Each control valve that requires calculations should have its own scenario.
Control valves that don’t result in relieving cases may be listed together in the general automatic control failure scenario. For this scenario, clear the Apply Scenario checkbox. Be sure to include appropriate Scenario Notes to document cases that don’t apply.
When calculating flow through a control valve, use the Installed Cv for the control valve at 100% open. 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, not as the Cv at maximum flow.
If the manufacturer’s data is not available, the values in Table 1, provided represent a conservative assumption for globe style valves, and may be used if the specific valve data cannot be located. Do not use these values until after you have attempted to obtain specific data on the valve. Be sure to include appropriate comments in the Scenario Notes if assumptions are used.
When using a Cv from Table 1, record in the Equipment Data Notes that specific valve Cv data was not available, and that, conservatively, the Cv for an inch full port globe valve is used. Use the Cv for the port diameter if known and the pipe size if unknown.
TABLE 1: FULL PORT GLOBE VALUE CV DATA
This table lists the calculated Cv (for liquid) and Cg (for gas) values for globe valves. For other types of valves such as ball and gate, use the equations in Crane Technical Paper 410. Full Port Globe Valve Cv Per Crane
Full Port Globe
=29.9 = f (L/D) d^2/sqrt(K)
Steam below 1000 psig
Friction factor L/D
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
Select the control valve for evaluation by picking its checkbox in the Selected Control Valves list. To be available for selection, the control valve must be linked to the protected system as ancillary equipment.
iPRSM has equations to calculate the flow rates for automatic
control failure for liquids that do not flash and vapors. The equations assume that the entire pressure drop is taken across the control valve. The required relief rate for a failed control valve is typically the flow rate through the failed open valve less the normal flow rate. A control valve failure simultaneous with a blocked outlet is normally considered a double jeopardy.
Use W adjust in iPRSM to increase or reduce the control valve flow as needed to account for normal flows.
is normally used to subtract the normal flow rate through the control valve being considered, but can also be used to increase the flow rate for relief when needed. Value must be negative if subtraction is desired.
To be conservative, use the Low Normal flow rate for the credit.
Record in the Scenario Notes that the required relief rate is the difference, as well as the source of data for the normal flow rate used, like spec sheet for equipment or control valve, PFD, etc.
MULTIPLE CONTROL VALVES IN COMBINATION
Remember that you cannot take credit for the correct response of a control valve, so even though one of the two valves is not considered to fail, it is not double jeopardy to assume that one does not respond. Series
Depending on their set pressures, often one of the two control valves in series is full open.
There are two possible methods for determining the maximum flow through two control valves in series.
The easiest method to use to determine the flow is to combine the Cv’s of the two valves using the following formula. Cv combined = 1/ SQRT (1/CV1^2 + 1/CV2^2 + 1/CV3^2 ….)
The Cg is computed based on the combined Cv calculated above, and C1 = 34 for a globe valve, etc. There is some error in this method of calculating Cg.
A more rigorous approach is to iterate the flows through the two valves until they balance. This is not normally required as a first-pass calculation.
For two or more control valves in parallel the Kv or Cv can be calculated as: C = Cv1 + Cv2 + …
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.
The heat input is limited for these cases by either a limit in the heat transfer capacity of the equipment, a supply limitation in the fuel or heating medium, or a combination of both.
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.
Distillation reboiler: If the normal bottoms composition bubble point temperature exceeds the hot side supply temperature, the column feed composition should be used.
For screening calculations, assume there is an infinite supply of the heating medium so the hot side temperature is the supply temperature, or condensing temperature at supply pressure for steam systems.
If the relieving capacity of the system is adequate based on the screening calculations, the more rigorous and time consuming calculations need not be performed.
The spreadsheet referenced for blocked outlet vaporization can be used to predict the relief flow rate.
INSTRUMENT AIR FAILURE
For the global instrument air failure scenario, evaluate the effect of all of the control valves in a system going to their failure position.
Often this results in no relieving case. Nonetheless, evaluate each of your automatic control failure scenarios to see if any cause relief situations with the valve in the design failure position.
Also look at other overpressure sources such as compressors that might cause a relief if they were to fail. EXAMPLE
It is likely that plant-wide instrument air failure might shut down the process gas compressor in an ethylene unit.
Instrument air failure is a global failure in which the high variable back pressure should be used.
Lesson 4: Evaluating inadvertent valve operation 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.
Inadvertent valve opening
Inadvertent valve operation is the scenario that reviews the possible overpressure created by the opening or closing of any valve.
Review any feed streams that can cause overpressure - these should all be included in the Blocked Outlet Scenario Notes - and determine if they have a single valve that can be opened or closed that would result in overpressure.
Calculate the required relief as the maximum flow through a valve and piping segment.
If there are two valves in a line but one is typically left open, then that line would be subject to the single failure criteria. A single failure can also be an operator making an incorrect line up involving two block valves, so the presence of multiple block valves does not rule out the case. Consult plant-specific guidelines and operating instructions.
Include control valve bypasses as susceptible to inadvertent valve opening.
The maximum calculated flow through the bypass at relief pressure is the relief flow. This assumes that control valve bypasses are not used to supplement the control valve capacity.
Assume that the control valve stays in its normal position and the normal forward flow continues.
Some facilities have procedures that make the consideration of the control valve bypasses not applicable for relief sizing. Consult the specific plant or client guidance. 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.
Calculate the flow for the inadvertent valve opening scenario using the gas flow or liquid pipe flow spreadsheets as appropriate, and enter the flow as given flow rate into iPRSM. 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, include the obvious fittings from the P&ID to determine the K’s and equivalent lengths.
For control valve bypasses shown as globe valves, assume it is a full port valve.
If the relief valve is inadequate based on these simplifying assumptions, a more detailed review of the piping configuration from the pressure source to the protected system may be needed.
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. Clearly record all assumptions. DISCUSSION SKETCH
Lesson 5: Evaluating heat exchanger tube rupture 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.
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.
If low side test pressure is greater than or equal to the high side MAWP, no relieving case is required to be evaluated. Use caution when declaring that a tube rupture scenario is not applicable. Be sure to review the test pressure of other equipment in the system to ensure that, should a tube rupture occur, that it is not exceeded.
For existing installations, you can compare the low side test pressure against the maximum operating pressure on the highpressure side, or 90% of the set pressure of the pressure relief device, or 90% of the set pressure of the low set valve for multiple valve applications on the high-pressure side. Remember to compare all of the low pressure equipment in the system, not just the exchanger.
iPRSM can be used to calculate the flow through the broken tube.
Unless site specific guidance applies, use an Orifice Coefficient: 0.60 and 2 full tube areas.
Be sure to link the high pressure side of the exchanger to the protected system as an overpressure source. SCENARIO VIEW - 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.
Verify that there are no control valves or other possible restrictions that would limit the low pressure side flow, 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.
If two-phase or flashing flow through the tube, flow calculations will have to be performed outside of iPRSM. Several models exist that can be used to perform this calculation.
The Guidelines for Pressure Relief and Effluent Handling Systems (CCPS, 1998) contains software tools that are widely used to document this calculation.
Optionally, for flashing liquid, you can calculate the tube rupture rate as a liquid, which yields a conservative result as a first pass evaluation. Then convert the liquid tube rupture rate into a vapor rate for the relief rate.
If the liquid flashes at relieving pressure, review the system to determine if there is adequate disengagement for the liquid. This can be done by inspection, 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 disengagement seems practical, the relief can be evaluated as the flashed vapor portion of the tube rupture flow. Calculate the surge time to verify. Include surge time calculation as documentation if credit is taken.
The fill time should be determined based on the liquid inflow from the rupture minus the normal liquid outflow of the low pressure side.
Credit for operator response should not be taken unless there is a level alarm to initiate operator response.
Lesson 6: Evaluating fire 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.
About fire scenarios Calculation and evaluation of fire scenarios takes the following approaches into account. APPLICABILITY OF FIRE CASES
Unless specifically directed otherwise, assume that all equipment is in a potential fire zone.
Check your specific plant or client guidelines to determine if fire calculations apply to cooling water (CW) side of heat exchangers. Verify that there are no control devices on the cooling water return line that might become closed. Record in Scenario Notes that fire calculations do not apply to CW systems; plant procedures require cooling water to be drained when not in service and will relieve to the cooling tower when in service.
Thermal expansion relief remains a viable contingency on the cooling water side of heat exchangers.
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. Verify the plant specific guidelines.
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.
If the relief valve is mounted under the liquid level, select pressure/bubble point with the pressure set at relief pressure.
For multicomponent streams, select the latent heat flash type in the stream flash view. 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.
The point of the maximum relief rate is predicted based on the effective latent heat and volume of the relief fluid.
Input the pressure for the flash, 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.
Input the maximum percentage of the stream to be vaporized for the analysis. The system will determine the point at which the maximum vapor relief requirement is reached as a factor of the effective latent heat, molecular weight, temperature, compressibility, and specific heat ratio of the vapor generated.
To see the details in the calculations, pick the View Graph link. Additional details on the analysis are available from the Export S/S link on the LH flash maximization plot view.
The vapor phase reported from this flash is the relief fluid. DISCUSSION SKETCH
FIRE BOILING LIQUID WITH VAPOR GENERATION Determine heat load
iPRSM will calculate the wetted area for the fire case on the
equipment worksheet. All data required for wetted area calculation must be entered on the equipment worksheet. The wetted area will be based on either the Normal Liquid or the High Liquid level as selected, and elevation data entered in the vessel data worksheet.
Liquid heights are evaluated from the bottom of the vessel. Be sure to add the height of the bottom head if the liquid level is known from the tangent line.
Unless otherwise directed, use the high operating liquid level for the fire case.
If the normal liquid level is to be used, adjust by setting the He Select dropdown menu.
Record in the Equipment Notes what liquid level is used and other pertinent data elements. The liquid level may be available from the P&ID, equipment drawings, specifications, or operating data.
As a worst case assumption, if data is not available, use the liquid level corresponding to 100% of the field level transmitter. Record what was assumed in the Scenario Notes.
All fire calculations on equipment within the unit boundaries are subject to a maximum fire height of 25’. 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. More stringent default fire heights can be entered as a plant-level parameter and option.
The plant level defaults should be set to the fire height used at the facility you are working on. These can be overridden for any specific fire evaluation if needed.
For the liquid level in a distillation column, enter the normal and high liquid levels as done for other types of equipment. Based on the distillation tower entries, iPRSM calculates the wetted surface area for the liquid holdup in the tower.
For a sphere, the fire level includes all wetted area up to the maximum vessel diameter. This may exceed the normal fire height limit. iPRSM will make these computations for you.
In many instances, the area of a vessel head that is enclosed by a skirt may be excluded from fire calculations. IPRSM wetted area calculations allow you to include or exclude the area of the bottom head. EQUIPMENT VIEW – WORKSHEET PARAMETERS
On the equipment worksheet enter the appropriate heat absorption rate. The options are listed as: for good drainage
API 521 Adequate
API 521 Inadequate
for poor drainage or NPFA equipment propane, propylene, LPG storage
The environmental factor on the equipment worksheet in iPRSM is used to define the insulation credit.
When insulation credit is taken for any fire case, record in the Protected System Notes that insulation credit is needed to ensure
adequate relief protection in the event of fire. Insulation on protected equipment must be maintained to ensure adequate protection.
If insulation credit is not needed, calculations can be performed with F: 1.0, however this will increase the flare header loading for the global fire case.
Typical environment factors to be used per API:
for full insulation with SS banding and jacketing .05 for jacketed vessels 1.0 for no insulation or non-fire proof 1.0 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, iPRSM will calculate the environmental factor based on the values provided. .03
EQUIPMENT VIEW - ENVIRONMENT FACTOR SELECTOR
If the wetted area is determined outside of iPRSM, 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.
The vapor rate from the protected vessel and the W area fire vapor are additive.
During a fire, heat will enter a heat exchanger through all surfaces exposed to the fire. Heat may also be transferred internally across the tubes.
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, then the entire heat exchanger external surface area should be used.
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. Note that the comparison is with the temperature at set and not at full relief.
In the fire scenario, select Hazard Type: Fire Vapor Generation and Flow Type: Vapor.
Fire should be calculated at 21% overpressure.
Select the equipment that is participating in the scenario. You may have multiple pieces of equipment involved in the fire. To add their wetted area to the calculation, you must attach the involved equipment to the system as either protected or overpressure sources.
All equipment involved in a given scenario is evaluated based on the single latent heat and fluid properties attached to that scenario.
Use multiple fire scenarios for varying physical properties if needed. The flows and streams will need to be added outside of iPRSM and re-entered in a different scenario.
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.
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.
An assumed wall temperature (carbon steel vessel) for a fire case is 1100 deg F based on API 521.
Select the vapor expansion hazard type in iPRSM to calculate the relief temperature and relief flow rate.
If the relief temperature is predicted to exceed the wall temperature, set the scenario to not apply, and record in the Scenario Notes and Protected System Notes that relief will not occur in a fire case, vessel wall temperature will exceed allowable temperature prior to reaching the relief pressure, other methods of cooling the vessel in the event of fire should be evaluated. These may include systems such as water curtains, fire monitors, etc.
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.
Obtain the physical properties needed by flashing the fluid at relief pressure and the relief temperature predicted by iPRSM. Note that this value will not be shown if you are using input mode.
The higher the operating pressure and the lower the operating temperature, the more conservative the results will be.
To calculate the relief requirements, select the Hazard Type: Fire Gas Expansion in the fire scenario worksheet.
Pick Evaluate Scenario to have iPRSM calculate the relief temperature for the gas expansion.
Evaluate a flash of the relief stream at the relief pressure and the fire relief temperature. Select the fire relief flash and phase in the scenario and reevaluate to have iPRSM calculate the required relief rate and orifice area.
If Z>0.8, calculate iPRSM as vapor. If Z