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

8/14/2019 Section 3 - Understanding & Analyzing Overpressure Scenarios Training 1.0.5.2.pdf

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iPRSM Technical Training

Understanding and AnalyzingOverpressure Scenarios

Version 1.0.5.2


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2iPRSM Copyright Curtiss-Wright Flow Control Service Corp., 2000-2009.


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Contents

Course overview 5

Intended use 6

Module 1: Understanding protected systemsfor pressure vessels 7

Objectives 7

Lesson 1: Defining and evaluating a protected system 8

Defining protected system boundaries 8

Evaluating a protected system 12

Lesson 2: Analyzing a protected system 13

Pressure variable analysis 13

Using set pressure and MAWP 13

Using overpressure for piping losses 13

Module review 15

Module 2: Analyzing, calculating & evaluating relief contingencies 16

Objectives 16

Lesson 1: Rules for analyzing relief contingencies 17

About contingency relief analysis 17

Upstream pressure 17

Double jeopardy 18

Operator intervention 18

Control response 19

Lesson 2: Evaluating blocked outlet 20

About blocked outlet 20

Blocked outlet vaporization 23

Liquid overfill 24

Pump deadhead 26

Upstream piping and fittings 30


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Lesson 3: Evaluating the effects of control valves 32

Automatic control failure 32

Multiple control valves in combination 35

Abnormal heat input 36

Instrument air failure 37

Lesson 4: Evaluating inadvertent valve operation 38

Inadvertent valve opening 38

Lesson 5: Evaluating heat exchanger tube rupture 40

About heat exchanger tube rupture 40

Lesson 6: Evaluating fire 43

About fire scenarios 43

Applicability of fire cases 43

Fire case physical properties 43

Fire boiling liquid with vapor generation 45

Insulation credit 46

Fire vapor expansion 49

Fire supercritical 50

Fire high boiling point liquid 52

Fire on liquid full equipment 53

Lesson 7: Evaluating other scenarios 58

About other scenarios 58

Liquid thermal expansion 58

Cooling failure 60

Power failure 61

Mechanical equipment failure 62

Chemical reaction 62

Steam out 62

Check valve failure 62

Series fractionation, reflux failure, & loss of quench 63

Other 63

Module review 64


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Course overview

COURSE OUTLINEModule 1: Understanding Protected Systems for Pressure VesselsModule 2: Analyzing, Calculating & Evaluating Relief Contingencies

IN MORE DETAILModule 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 aprotected system, and presents methods for calculating, documentingand evaluating orifice sizing and inlet and outlet piping losses for reliefscenarios in iPRSM.


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Intended use

This document provides guidelines for determining required relief ratesfor contingency scenarios and for the documentation of those scenarios.

These guidelines apply to the evaluation of relief requirements andoverpressure 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, thereare many possible approaches to a solution. Each client or plant mayhave specific guidelines that will supersede these generic guidelines,and it is understood that client/site-specific guidelines must takeprecedence over the guidelines presented here. It is the responsibility ofthe 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 theseguidelines is solely that of the user.

Farris Engineering Services makes no claim as to the completeness oraccuracy of the material presented in this document.

Deviation from these standard procedures may be appropriate inspecific situations. It is essential in any case to include complete notesclearly describing your reasoning, as well as related calculations, in the

protected system documentation in iPRSM.


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Module 1

Understanding Protected Systemsfor 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


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EXAMPLE

DISCUSSION SKETCH 1

! The overpressure source is an upstream vessel.

! The control valve downstream of the upstream vessel is ancillaryequipment, and is the starting point of the downstream protectedsystem.

! The relief valve on the upstream equipment is also ancillaryequipment. The pressure vessel is protected equipment, and therelief 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 influenceon the required relief rate for the protected system. These controlvalves wouldbecome ancillary equipment in the downstreamprotected system.


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! Designating each piece of equipment in a plant as protected,protecting, overpressure, or ancillary allows you to easily manageprocess changes for their potential impacts to an entire processunits relief systems.

!

Any process changes made to the system or any of its pieces ofequipment, like changing the set pressure of the upstream pressurevessel relief valve, will uncheck this system as well as the upstreampressure vessels protected system, generating a message iniPRSMnoting that your change may have impacted both systemsand prompting you to evaluate the change and determine if therelief protection of both systems is still adequate.

DISCUSSION SKETCH 2


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DISCUSSION SKETCH 3


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Evaluating a protected system

Successful evaluation of relief systems is based on a thoroughunderstanding 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 overpressuresource, and how the system interacts.

! Look at upstream equipment and downstream equipment for theeffects of control failures as possible sources of overpressure togain a full understanding of how the process operates.

EXAMPLE

When evaluating the low-pressure side of a heat exchangersrelief protection, consider the high-pressure side as a possibleoverpressure during tube rupture.

2. Decide what pieces of equipment are being protected by a givenrelief device, what equipment failures can possibly causeoverpressure, and which overpressure scenarios can result in arelieving event.

3. Collect all relevant equipment data, like specification sheets forpumps, heat exchangers and control valves, U-1s and dimensionaldrawings for pressure vessels, isometrics of inlet and outlet piping forrelief devices.

4. Collect all relevant process-specific data.

EXAMPLE

What are the high-side operating conditions of an exchanger inwhich you are evaluating the low-side relief protection? What are theupstream operating conditions of a control valve that is supplying adownstream pressure vessel?

5. Apply engineering principles to determine if a given failure canactually cause relief.

EXAMPLE

If the high side of a heat exchanger operates at a pressure that isless than the low-side hydrostatic test pressure, the tube rupturescenario may not be applicable.

Accurate and complete operating data is essential in determiningwhether a particular scenario may or may not be applicable.


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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 informationon any of these topics, pick Help on any iPRSM page.

Pressure variable analysis

The following considerations help you to decide the appropriatepressure to use in determining the relief contingencies that should beapplied to a protected system.

USING SET PRESSURE AND MAWP

1. If the RV Set Pressureis equal to or less than the lowest MAWPof theprotected system equipment, all calculations and evaluations of theprotected system should be done based on the RV Set Pressure.

2. If the RV Set Pressureis greater than the lowest MAWPof the protectedsystem equipment, record the deficiency in the relief device Findingsand Deficiencies Notes,Equipment Notesand in the Protected SystemNotes.

! Set the system to mitigate when calculations are completed.

! Enter the Pseton the relief valve equipment worksheet equal tothe lowest MAWP,and record the actual set pressure of thedevice in the RV Equipment Worksheet Notesto explain thatcalculations are done based on the limiting MAWP.

! iPRSMwill not calculate scenarios with the Pset> MAWP.Evaluating protection with set pressure at MAWPyields the mostuseful information.

USING OVERPRESSURE FOR PIPING LOSSES

! Piping losses are computed for vapor and steam relief cases at thefull valve capacity, not at the required flow rate.

! As a general rule, for existing installations, the capacity at whichpressure drops should be computed is the scenario-requiredcapacity for liquid relief and the capacity of the device for vaporrelief scenarios calculated at the overpressure used in the scenariocalculation.


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! For new installations, the pressure drops in the inlet and outletpiping should be computed at the valve capacity at a maximum of10% OVPfor all scenarios, including multiple valve applications andfire cases. This is the more conservative approach.

! iPRSMhas a plant-level default that lets you specify which OVPisthe default setting for piping loss calculations. The options are

! 10% forASMESection VIIIand Section III,3% forASMESection I

! scenario OVPfor all

The selection can be overridden on the individual piping pressuredrop calculations by picking the OVP Selectcheckbox.

! It is possible to have iPRSMcompute the pressure drops at themaximum valve capacity instead of the required flow rate for liquidrelief cases by picking the Relief Flow Selectcheckbox in the scenariopiping losses view. This may be useful in cases where the valvecapacity is far less than the required capacity. If you make thischange, record it in the Protected System Notes.


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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 beapplied to a protected system during evaluation?


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Module 2

Analyzing, Calculating and EvaluatingContingency Scenarios

This module describes how to decide which relief scenarios to apply to aprotected system, and presents methods for calculating, documenting

and evaluating orifice sizing and inlet and outlet piping losses for reliefscenarios 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 reliefrates

! use a variety of methods to calculate relief contingencies

! apply calculation methods to evaluate relief systems in iPRSM


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Lesson 1: Rules for analyzing relief contingencies




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 pressureto determine if relief is applicable can vary depending on thesystem.

! Use the upstream relief pressure for analysis of potential sources ofoverpressure for the system in these cases:

! if the upstream equipment and the protected equipment havethe same possible occurrence of overpressure (ex. Fire, Inst AirFailure, 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 beingevaluated are set within 5% of the lowest MAWP, anoverpressure of 16% on the downstream relief device is allowedfor this evaluation even if the equipment in question is indifferent protected systems. If the case is not considered as aresult of this condition, be sure to record it in the Scenario Notes.

! Use the upstream high operating pressure as the upstream sourcepressure for analysis if:

! the upstream source equipment would not likely increase inpressure as a result of overpressure in the downstreamprotected equipment

! Use upstream operating pressure from mass balanceinformation or operating records.

! If plant data on the normal high operating pressure is notavailable, 90% of the upstream set pressure can be used.


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DOUBLE JEOPARDY

! Overpressure that would require more than one independent failureis considered double jeopardy and is not considered viable. Cautionin declaring double jeopardy is advised. The failures consideredmust be truly independent of and unrelated to one another.

! Latent failures are those that can occur without being identified, andshould notbe considered as double jeopardy.

EXAMPLES

Double regulators in series are fairly common. If there is noindication that the first regulator may have failed open, or if it isadjusted in such as way that it is normally full open, it cannot beconsidered to respond on the failure of the second regulator inseries. Full flow through both valves would be appropriate, anddouble jeopardy would not apply.

The inadvertent opening of two independent block valves wouldnormally be considered as double jeopardy. If double block valvesare present in a line, a single failure could be considered if theoperator inadvertently lined up the incorrect line to the system. Thataction would be considered a single failure, and double jeopardywould 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 equipmentwithin a reasonable time, the case is not considered viable. Thetime required for response can vary, but it is normally considered tobe anywhere from 15 minutes to 30 minutes within an operatingunit, and up to two hours for a storage area.

! Operator response time should be determined by persons familiarwith the operations of the facility being reviewed.

! Operator response requires an alarm of the problem. The operatorcannot respond if he is unaware of the problem.


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! The alarm must be independent of the possible cause of failure.

EXAMPLE

If overfill can occur when an outlet level control valve on a vesselfails closed, an alarm using the same level bridle or transmittercannot be considered as independent notification of the rising level.The transmitter or bridle could have become plugged, causing thevalve failure simultaneous with failure to activate the alarm. Useengineering judgment.

CONTROL RESPONSE

! The proper response of a control system or valve cannot becredited 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 toprovide protection.

! If the designed response of a control valve would act to increase therelief requirement, it must be assumed to respond in its designedmanner.

! If the response would serve to reduce or eliminate the overpressureit 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, butcare needs to be used to ensure that the upstream pressure will notalso increase in response.


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Lesson 2: Evaluating blocked outlet




About blocked outlet

! A list of all incoming streams with the maximum expected pressureor its pressure source should be recorded in the Blocked OutletContingency Notes. This is the basis from which several of thecontingencies 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 anupstream PSV set point.

! Each stream should be named something easily identifiable fromthe P&ID or system sketch.

! The streams should be documented through listing in the ScenarioNotes.

EXAMPLE

The incoming streams and do not have sufficient pressure to cause relief .

! Do not state relief can occur but overpressure will not occurbecause the relief valve is set below MAWP. The case should becalculated if upstream pressure exceeds the set pressure by morethan the allowable overpressure, even if by less than 10% aboveminimum MAWP.

! Calculations are not required if the source pressure cannot exceed

the set pressure + the allowable overpressure - by 10% for singlevalve installations or 16% for multiple valve installations.

! This scenario assumes that all outlets that can be blocked areblocked.


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! Any inlets with streams that cannot provide sufficient pressure tocause relief are also assumed closed. The vessel is assumed tocontain 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 enteringthe system.

! Generally, the sum of all entering streams should be consideredwhen calculating relief rate. For the streams to be combined, theyshould also be entering the system simultaneously during normaloperation.

! For equipment with multiple operational modes - batch processes,dryers with regeneration cycles, operations with varying piping line-ups - 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 controlvalve set to open on flow may respond to reduced flow rate causedby increased down stream pressure by going to its full openposition. The full flow rather than the normal flow would beappropriate for the relief flow rate.

! All pumps, upstream equipment, and headers should be linked tothe protected system as overpressure sources even if they aredetermined to not be able to cause overpressure or relief.

! All upstream relief valves that are used to demonstrate the inabilityto cause relief should be linked to the protected system as ancillaryequipment.

! Record all assumptions in the Scenario Notes.


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DISCUSSION SKETCH


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BLOCKED OUTLET VAPORIZATION

! Change in volume due to liquid thermal expansion should beconsidered under the thermal expansion contingency. Change involume due to vaporization of a trapped liquid is normallyconsidered under blocked outlet.

! The design duty of heat exchangers using a clean coefficient of heattransfer should be used. The duty may be adjusted for the relief LogMean 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 ofthe calculations used to determine the clean heat transfercoefficient. Attach the spreadsheet calculations in the ProtectedSystem Documentsin iPRSM.

EXAMPLE: SPREADSHEET CALCULATIONS


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! If the vapor generated at relief pressure exceeds the thermodynamiccritical point, use the MERR 1spreadsheet to determine the reliefrate, and enter the case as a given flow rate 2 phase using theappropriate direct integration or Omega method calculations. Enterthe determined flow rate in iPRSMusing the hazard type Given Flow

and an appropriate flow type - Vapor 2 Phaseor 2 Phase (DI).

EXAMPLE: MERR 1 SPREADSHEET

! Set property flash linked to scenario at the relief pressure and relieftemperature calculated in the MERR 1spreadsheet to use theappropriate properties in the iPRSM scenario calculations.

LIQUID OVERFILL

! Review the source vessel to ensure that sufficient volume in thesource equipment exists to overfill the protected equipment, or thatthe control valve flow is not greater than the pump capacity, asappropriate.

! Include calculations for all except the very obvious cases.


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! iPRSMcalculates the vessel fill time on the vessel equipmentworksheet. The fill time is based on the high (alarm) liquid level onthe equipment design criteria.

VESSEL EQUIPMENT DESIGN CRITERIA

! If the protected system contains a predefined amount of residencetime above the high liquid level alarm, operator response may becredited.

! Blocked outlet in these cases is typically not considered for a liquidoverfill case which meets the operator response time requirements.This time frame will vary based on the facility and site-specificguidance.

! In order to credit operator response, a fully independent alarm isneeded. You cannot expect response without a method of notifyingthe operator of the problem.


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! Verify site or plant-specific guidelines time limits and document inScenario Noteswhy the case is not applicable.

VESSEL EQUIPMENT CALCULATION REQUIREMENTS

PUMP DEADHEAD

! Evaluate pumps that feed the system to determine if the deadheadpressure plus the effects of any hydrostatic head exceed relief.

! Evaluate the pump deadhead at the installed impeller size. If theinstalled impeller is unknown, use the maximum possible impellersize. For design of new systems, use the maximum impeller ifpractical. Use of installed or maximum impeller size is project or sitespecific.

! From the pump curve or pump specification sheet, determine thedeadhead pressure, and input the required data into iPRSMon thepump 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 inspectionor design data for that equipment. Include high liquid level andassume vessel at high operating pressure.

! Do not include the hydrostatic head in the suction pressure as theseare added by iPRSM.


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! If the operating pressure of the suction is unknown, assume 90% ofthe set pressure of the suction equipments relief device.

EQUIPMENT VIEW - PUMP EQUIPMENT PARAMETERS PANEL


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PROTECTED SYSTEM CONTINGENCY SCENARIO VIEW

! In the blocked outlet scenario view, select the Hazard Type:PumpPressureand the Flow Type:Liquid.

! Select the overpressure pumpfrom the Overpressure Pumpdropdownmenu. To be available for selection, this pump must be linked to theprotected 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 Curveand Pump Head At Reliefcalculated by iPRSM,determine relief rate for Pump with Installed Impeller. Note that thisvalue is not displayed if you are using input mode.

! Record any cases that cannot cause relief in the Protected SystemNotes.


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SCENARIO VIEW PUMP PRESSURE WORKSHEET FULL VIEW NOT INPUT MODE


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! This discussion assumes all liquid relief to the point of discharge. Ifthe relieving material flashes either across the relief valve or in theoutlet piping, refer to the two-phase calculation methodologies fordetermining the area calculation and piping losses.

UPSTREAM PIPING AND FITTINGS

! The normal flow rate might be used to evaluate the system with flowfrom an upstream system at a higher pressure under pressurecontrol. The correct response of any control valve that would act toreduce the flow to the system cannot be credited.

EXAMPLE

If there is an upstream pressure control valve that would normallyclose 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, aflow control valve on the inlet line that would see a reduction in flowas the downstream pressure increases would be calculated at thefull flow through the wide open control valve.

! Flow reduction due to resistance from the piping is normallyignored, but can be considered if a more thorough evaluation isrequired.

! If you want to determine the maximum flow through a section ofpiping a spreadsheet should be used. The Gas Pipe Flowor LiquidPipe Flow

spreadsheets can be used to calculate the maximum flowthrough a section of piping. Be sure to document any assumptionsused such as length of piping.


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! In a first-pass analysis, field isometric inspection data is notnormally requested.

GAS PIPE FLOW SPREADSHEET


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Lesson 3: Evaluating the effects of control valves




Automatic control failure

! For each automatic control failure case, give the scenario a uniqueextension name to help identify which control valve has failed.

! Each control valve that requires calculations should have its ownscenario.

! Control valves that dont result in relieving cases may be listedtogether in the general automatic control failure scenario. For thisscenario, clear the Apply Scenariocheckbox. Be sure to includeappropriate Scenario Notesto document cases that dont apply.

! When calculating flow through a control valve, use the Installed Cvfor the control valve at 100% open. Wide open Cv from themanufacturers data should used and may be obtained from thecontrol valve spec sheet. This will be shown as the valve Cv, not asthe Cvat maximum flow.

! If the manufacturers data is not available, the values in Table 1,provided represent a conservative assumption for globe stylevalves, and may be used if the specific valve data cannot belocated. Do not use these values until after you have attempted toobtain specific data on the valve. Be sure to include appropriatecomments in the Scenario Notesif assumptions are used.

! When using a Cvfrom Table 1, record in the Equipment Data Notesthat specific valve Cvdata 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 ifunknown.


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TABLE 1: FULL PORT GLOBE VALUE CVDATA

This table lists the calculated Cv(for liquid) and Cg(for gas) values forglobe valves. For other types of valves such as ball and gate, use theequations in Crane Technical Paper 410.

Full PortGlobe ValveCv Crane

CraneA-27 K Cv C1 Cg Cs

PerCrane A-26

FullPortGlobe = f (L/D)

=29.9d^2/sqrt(K) Assumed Fisher

Steambelow1000 psig

Nom.Size Sch. ID

Frictionfactor L/D Crane A-31

GlobeValve Catalog Cg/20

1/2" 80 0.546 0.027 340 9.18 2.94 34 100 5.0

3/4" 80 0.742 0.025 340 8.5 5.65 34 192 9.6

1" 80 0.957 0.023 340 7.82 9.79 34 333 16.6

1.25" 80 1.278 0.022 340 7.48 17.86 34 607 30.4

1.5" 80 1.5 0.021 340 7.14 25.18 34 856 42.8

2" 80 1.939 0.019 340 6.46 44.23 34 1504 75.2

2.5" 40 2.469 0.018 340 6.12 73.68 34 2505 125.3

3" 40 3.068 0.018 340 6.12 113.76 34 3868 193.4

3.5" 40 3.548 0.0175 340 5.95 154.30 34 5246 262.3

4" 40 4.026 0.017 340 5.78 201.58 34 6854 342.7

5" 40 5.047 0.016 340 5.44 326.54 34 11102 555.1

6" 40 6.065 0.015 340 5.1 487.02 34 16559 827.9

8" 40 7.981 0.014 340 4.76 872.94 34 29680 1484.0

10" 40 10.01 0.014 340 4.76 1373.21 34 46689 2334.512" 40 11.938 0.013 340 4.42 2026.86 34 68913 3445.7

14" 40 13.124 0.013 340 4.42 2449.58 34 83286 4164.3

16" 40 15 0.013 340 4.42 3199.95 34 108798 5439.9

18" 40 16.876 0.012 340 4.08 4215.80 34 143337 7166.9

20" 40 18.812 0.012 340 4.08 5238.55 34 178111 8905.5

22" STD 21.25 0.012 340 4.08 6684.35 34 227268 11363.4

24" 40 22.624 0.012 340 4.08 7576.70 34 257608 12880.4

26" STD 25.25 0.012 340 4.08 9437.65 34 320880 16044.0

28" STD 27.25 0.012 340 4.08 10991.93 34 373726 18686.3

30" STD 29.25 0.012 340 4.08 12664.64 34 430598 21529.932" 40 30.624 0.012 340 4.08 13882.41 34 472002 23600.1

34" 40 32.624 0.012 340 4.08 15754.90 34 535666 26783.3

36" 40 34.5 0.012 340 4.08 17618.92 34 599043 29952.2


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! When entering scenario data in iPRSM:

! use a unique name for the scenario to help identify whichcontrol valve has failed

! Hazard Type:Control Valve Failure

! Flow Type:Vaporor Liquid

! Select the control valve for evaluation by picking its checkbox inthe Selected Control Valveslist. To be available for selection, thecontrol valve must be linked to the protected system as ancillaryequipment.

! iPRSMhas equations to calculate the flow rates for automaticcontrol failure for liquids that do not flash and vapors. The equationsassume that the entire pressure drop is taken across the controlvalve. The required relief rate for a failed control valve is typicallythe flow rate through the failed open valve less the normal flow rate.

A control valve failure simultaneous with a blocked outlet is normallyconsidered a double jeopardy.

! Use W adjustin iPRSMto increase or reduce the control valveflow as needed to account for normal flows.


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! W adjustis normally used to subtract the normal flow ratethrough the control valve being considered, but can also beused to increase the flow rate for relief when needed. Valuemust be negative if subtraction is desired.

! To be conservative, use the Low Normalflow rate for the credit.

! Record in the Scenario Notesthat the required relief rate is thedifference, as well as the source of data for the normal flow rateused, 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 acontrol valve, so even though one of the two valves is not considered tofail, it is not double jeopardy to assume that one does not respond.

Series

! Depending on their set pressures, often one of the two controlvalves in series is full open.

! There are two possible methods for determining the maximumflow through two control valves in series.

! The easiest method to use to determine the flow is to combinethe Cvs of the two valves using the following formula.

Cv combined = 1/ SQRT (1/CV1^2 + 1/CV2^2 + 1/CV3^2 .)

The Cgis computed based on the combined Cvcalculatedabove, and C1 = 34for 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 twovalves until they balance. This is not normally required as afirst-pass calculation.

Parallel

For two or more control valves in parallel the Kvor Cv can be calculatedas:

C= Cv1 + Cv2 +


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ABNORMAL HEAT INPUT

! Abnormal heat input is a special case of control failure involving theflow 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 orheating medium, or a combination of both.

! The heat transfer of the equipment may be adjusted based on thetemperature difference of the hot side supply vs. the cold side fluidbubble point temperature at relief pressure.

! Distillation reboiler: If the normal bottoms composition bubble pointtemperature exceeds the hot side supply temperature, the columnfeed composition should be used.

! For screening calculations, assume there is an infinite supply of theheating medium so the hot side temperature is the supplytemperature, or condensing temperature at supply pressure forsteam systems.

! If the relieving capacity of the system is adequate based on thescreening calculations, the more rigorous and time consumingcalculations need not be performed.

! The spreadsheet referenced for blocked outlet vaporization can beused to predict the relief flow rate.


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DISCUSSION SKETCH

INSTRUMENT AIR FAILURE

! For the global instrument air failure scenario, evaluate the effect ofall of the control valves in a system going to their failure position.

! Often this results in no relieving case. Nonetheless, evaluate eachof your automatic control failure scenarios to see if any cause reliefsituations with the valve in the design failure position.

! Also look at other overpressure sources such as compressors thatmight cause a relief if they were to fail.

EXAMPLE

It is likely that plant-wide instrument air failure might shut down theprocess gas compressor in an ethylene unit.

! Instrument air failure is a global failure in which the high variableback pressure should be used.


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Lesson 4: Evaluating inadvertent valve operation




Inadvertent valve opening

! Inadvertent valve operation is the scenario that reviews the possibleoverpressure created by the opening or closing of any valve.

! Review any feed streams that can cause overpressure - theseshould all be included in the Blocked Outlet Scenario Notes- and

determine if they have a single valve that can be opened or closedthat would result in overpressure.

! Calculate the required relief as the maximum flow through a valveand piping segment.

! If there are two valves in a line but one is typically left open, thenthat line would be subject to the single failure criteria. A singlefailure can also be an operator making an incorrect line up involvingtwo block valves, so the presence of multiple block valves does notrule out the case. Consult plant-specific guidelines and operatinginstructions.

! Include control valve bypasses as susceptible to inadvertent valveopening.

! The maximum calculated flow through the bypass at relief pressureis the relief flow. This assumes that control valve bypasses are notused to supplement the control valve capacity.

! Assume that the control valve stays in its normal position and thenormal forward flow continues.

! Some facilities have procedures that make the consideration of thecontrol valve bypasses not applicable for relief sizing. Consult thespecific plant or client guidance.

Note that valves that are part of a CSC/CSOinspection program aretypically assumed to be in their given position and are not subject toinadvertent valve operation.


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! Calculate the flow for the inadvertent valve opening scenario usingthe gas flowor liquid pipe flow spreadsheetsas appropriate, and enterthe flow as given flow rate into iPRSM. You can also determine theflow with the control valve failure calculations and enter the Cvforthe type of block valve being reviewed.

! For the first-pass calculations, include the obvious fittings from theP&IDto determine the Ksand equivalent lengths.

! For control valve bypasses shown as globe valves, assume it is afull port valve.

! If the relief valve is inadequate based on these simplifyingassumptions, a more detailed review of the piping configurationfrom the pressure source to the protected system may be needed.

!

Often an ISOis not needed and a fitting count and estimate of thelength of pipe is sufficient to calculate the flow resulting from theinadvertent valve opening using the gas flowor liquid pipe flowspreadsheets. Clearly record all assumptions.

DISCUSSION SKETCH


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Lesson 5: Evaluating heat exchanger tube rupture




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 pressureside MAWP.

!

If low side test pressure is greater than or equal to the high sideMAWP, no relieving case is required to be evaluated. Usecaution when declaring that a tube rupture scenario is notapplicable. Be sure to review the test pressure of otherequipment in the system to ensure that, should a tube ruptureoccur, that it is not exceeded.

! For existing installations, you can compare the low side testpressure against the maximum operating pressure on the high-pressure side, or 90% of the set pressure of the pressure reliefdevice, or 90% of the set pressure of the low set valve formultiple valve applications on the high-pressure side.Remember to compare all of the low pressure equipment in thesystem, not just the exchanger.

! iPRSMcan be used to calculate the flow through the broken tube.Unless site specific guidance applies, use an Orifice Coefficient:0.60and 2 full tube areas.


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! Be sure to link the high pressure side of the exchanger to theprotected system as an overpressure source.

SCENARIO VIEW - TUBE RUPTURE SCENARIO WORKSHEET PARAMETERS

! The flow required for relief is the difference between the flowthrough the two tubes and the normal volumetric flow on the lowpressure side.

! Verify that there are no control valves or other possible restrictionsthat would limit the low pressure side flow, then enter the volumetricflow on the low pressure side as the CapCredit liqor CapCredit vapon

the Tube Rupture Worksheet.

! Be sure to record in the Scenario Notesthat you are taking credit forthe normal volumetric forward flow rate on the low pressure side aswell as where you got the flow rate information for the LPside.


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! If two-phase or flashing flow through the tube, flow calculations willhave to be performed outside of iPRSM. Several models exist thatcan be used to perform this calculation.

! The Guidelines for Pressure Relief and Effluent Handling

Systems (CCPS, 1998) contains software tools that are widelyused to document this calculation.

! Optionally, for flashing liquid, you can calculate the tube rupturerate as a liquid, which yields a conservative result as a firstpass evaluation. Then convert the liquid tube rupture rate into avapor rate for the relief rate.

! If the liquid flashes at relieving pressure, review the system todetermine if there is adequate disengagement for the liquid. Thiscan be done by inspection, for instance disengagement may be

possible in the shell side of a kettle type exchanger but not in thetube side of any exchanger.

! If disengagement seems practical, the relief can be evaluated as theflashed vapor portion of the tube rupture flow. Calculate the surgetime to verify. Include surge time calculation as documentation ifcredit is taken.

! The fill time should be determined based on the liquid inflow fromthe rupture minus the normal liquid outflow of the low pressure side.

! Credit for operator response should not be taken unless there is alevel alarm to initiate operator response.


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Lesson 6: Evaluating fire




About fire scenarios

Calculation and evaluation of fire scenarios takes the followingapproaches into account.

APPLICABILITY OF FIRE CASES

! Unless specifically directed otherwise, assume that all equipment isin a potential fire zone.

! Check your specific plant or client guidelines to determine if firecalculations apply to cooling water (CW) side of heat exchangers.Verify that there are no control devices on the cooling water returnline that might become closed. Record in Scenario Notesthat firecalculations do not apply to CWsystems; plant procedures requirecooling water to be drained when not in service and will relieve tothe 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 anydirect fired equipment.

! Fire is not considered applicable to steam systems unless theytypically have a condensate level. Verify the plant specificguidelines.

FIRE CASE PHYSICAL PROPERTIES

! If you are working with a pure component:

! obtain physical properties by selecting the relief pressure anddew point to obtain the vapor properties.

! If the relief valve is mounted under the liquid level, selectpressure/bubble point with the pressure set at relief pressure.


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! For multicomponent streams, select the latent heat flash type in thestream flash view. This is used to determine the vapor compositionand effective latent heat for a multicomponent fluid stream through aseries of successive flashes and calculations.

!

The point of the maximum relief rate is predicted based on theeffective latent heat and volume of the relief fluid.

! Input the pressure for the flash, and select whether the latentheat is to be corrected for the specific heat of the liquid byselecting no-fire from the dropdown menu.

! Input the maximum percentage of the stream to be vaporizedfor the analysis. The system will determine the point at whichthe maximum vapor relief requirement is reached as a factor ofthe 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 Graphlink.Additional details on the analysis are available from the ExportS/Slink on the LHflash maximization plot view.

! The vapor phase reported from this flash is the relief fluid.

DISCUSSION SKETCH


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FIRE BOILING LIQUID WITH VAPOR GENERATION

Determine heat load

! iPRSMwill calculate the wetted area for the fire case on theequipment worksheet. All data required for wetted area calculation

must be entered on the equipment worksheet. The wetted area willbe based on either the Normal Liquidor the High Liquidlevel asselected, and elevation data entered in the vessel data worksheet.

! Liquid heights are evaluated from the bottom of the vessel. Be sureto add the height of the bottom head if the liquid level is known fromthe tangent line.

! Unless otherwise directed, use the high operating liquid level for thefire case.

! If the normal liquid level is to be used, adjust by setting the HeSelectdropdown menu.

! Record in the Equipment Noteswhat liquid level is used and otherpertinent data elements. The liquid level may be available from theP&ID,equipment drawings, specifications, or operating data.

! As a worst case assumption, if data is not available, use the liquidlevel corresponding to 100% of the field level transmitter. Recordwhat 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 aresubject to a 30 fire height. Verify any plant specific guidelines incase your plant has adopted the more stringent 30 NFPAlimitinstead of the 25APIstandard. More stringent default fire heightscan be entered as a plant-level parameter and option.

! The plant level defaults should be set to the fire height used at thefacility you are working on. These can be overridden for any specificfire evaluation if needed.

!

For the liquid level in a distillation column, enter the normal and highliquid levels as done for other types of equipment. Based on thedistillation tower entries, iPRSMcalculates the wetted surface areafor the liquid holdup in the tower.

! For a sphere, the fire level includes all wetted area up to themaximum vessel diameter. This may exceed the normal fire heightlimit. iPRSMwill make these computations for you.


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! In many instances, the area of a vessel head that is enclosed by askirt may be excluded from fire calculations. IPRSMwetted areacalculations allow you to include or exclude the area of the bottomhead.

EQUIPMENT VIEW WORKSHEET PARAMETERS

! On the equipment worksheet enter the appropriate heat absorptionrate. The options are listed as:

! API 521 Adequatefor good drainage

! API 521 Inadequatefor poor drainage or NPFAequipment -propane, propylene, LPGstorage

INSULATION CREDIT

! The environmental factor on the equipment worksheet in iPRSMisused to define the insulation credit.

! When insulation credit is taken for any fire case, record in theProtected System Notesthat insulation credit is needed to ensure


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adequate relief protection in the event of fire. Insulation on protectedequipment must be maintained to ensure adequate protection.

! If insulation credit is not needed, calculations can be performed withF:1.0, however this will increase the flare header loading for the

global fire case.

! Typical environment factors to be used perAPI:

! .03for full insulation with SSbanding and jacketing

! .05for jacketed vessels

! 1.0for no insulation or non-fire proof

! 1.0for cold insulation (normally non-fire proof)

! calculated value if insulation configuration is known and vesselhas SSjacketing and banding

! If the Environment Factor Selectoris set to Fire Proof Insulation,

iPRSMwill calculate the environmental factor based on thevalues provided.

EQUIPMENT VIEW - ENVIRONMENT FACTOR SELECTOR

! If the wetted area is determined outside of iPRSM, the wetted areacan be entered in iPRSMas User-Supplied Wetted Areaor the vaporrate can be entered as W Area Fire Vaporif it is to be added to therate calculated by iPRSMfor a vessel on the fire scenarioworksheet.

! The vapor rate from the protected vessel and the Warea fire vaporare additive.

! During a fire, heat will enter a heat exchanger through all surfacesexposed to the fire. Heat may also be transferred internally acrossthe tubes.

! If the boiling point of the fluid on the side opposite to the side youare working on has a bubble point temperature at its relief pressurethat is higher than the bubble point temperature at the set pressureof the fluid on the protected side, then the entire heat exchangerexternal surface area should be used.


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! If the bubble point temperature at relief pressure of the oppositeside is less than the bubble point temperature at set pressure of theside you are working on, only the exposed surface area of theprotected side need be used. Note that the comparison is with thetemperature at set and not at full relief.

Evaluate relief

! In the fire scenario, select Hazard Type:Fire Vapor Generationand FlowType:Vapor.

! Fire should be calculated at 21% overpressure.

! Select the equipment that is participating in the scenario. You mayhave multiple pieces of equipment involved in the fire. To add theirwetted area to the calculation, you must attach the involvedequipment to the system as either protected or overpressure

sources.

! All equipment involved in a given scenario is evaluated based onthe 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 iPRSMandre-entered in a different scenario.


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! Multiple fire scenarios may also be needed in cases where someequipment in the system is diked and separated from other piecesof equipment.


FIRE VAPOR EXPANSION

! Protected systems that contain no liquids and are vapor filled mayrequire fire calculations.

! During a fire the walls of a vapor filled vessel quickly reach elevatedtemperatures. Failure of the walls due to the elevated temperaturescan occur before relief pressure is reached.

! An assumed wall temperature (carbon steel vessel) for a fire case is1100 deg Fbased onAPI 521.


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! Select the vapor expansion hazard type in iPRSMto calculate therelief 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 Notesand

Protected System Notesthat relief will not occur in a fire case, vesselwall temperature will exceed allowable temperature prior to reachingthe relief pressure, other methods of cooling the vessel in the eventof fire should be evaluated. These may include systems such aswater curtains, fire monitors, etc.

! iPRSMwill calculate the relief temperature based on the operatingtemperature from the relief valve equipment data worksheet andhigh 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 thatthis value will not be shown if you are using input mode.

! The higher the operating pressure and the lower the operatingtemperature, the more conservative the results will be.

Evaluate relief

! To calculate the relief requirements, select the Hazard Type:Fire GasExpansionin the fire scenario worksheet.

! Pick Evaluate Scenarioto have iPRSMcalculate the relief temperaturefor the gas expansion.

! Evaluate a flash of the relief stream at the relief pressure and thefire relief temperature. Select the fire relief flash and phase in thescenario and reevaluate to have iPRSMcalculate the required reliefrate and orifice area.

! If Z>0.8, calculate iPRSMas vapor. If Z


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! The relief rate is calculated at the point where the density changeand the physical properties maximize the required relief area.

! Use the MERR 1Model spreadsheet to identify the point at whichthis function is maximized. The MERR 1Model identifies the point at

which the supercritical expansion and resultant factor of reliefparameters is maximized. Input this relief case as a hazard type ofGiven Flow/Two Phaseor Given Flow Rate/2 Phase (DI).

MERR 1 MODEL SPREADSHEET

Determine heat load

! Set up a fire scenario using the fire vapor generation hazard type tohave iPRSMcalculate the heat input during the fire case. Set liquidheight of the protected vessel to 100% full.

! Set this scenario to not apply.

Evaluate relief

! Complete the MERR 1 spreadsheet to determine the relief point andflow rate.

! Enter required information in iPRSMusing Given Flow 2 Phase (DI)or 2Phaseto use API omega calculations for supercritical vaporsentering the relief valve.

Piping pressure drops

! Piping pressure drops are not calculated in iPRSMwhen using the 2phase Omega calculation methods.


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! For the supercritical fluid, the inlet piping pressure drop can beevaluated as a vapor at the estimated maximum relief capacity ofthe relief valve.

!

From the required flow and required area calculated, estimate therelief capacity based on the ratio of the required relief area to theinstalled relief area: Wcap= Wrequired(Ainstalled/Arequired)

! Using the fire vapor generation worksheet in iPRSMin which theheat input was calculated, enter the estimated maximum relievingcapacity in the Piping Pressure Drop Worksheetas a User Supplied FlowRate.

! Make sure that the scenario worksheet has the proper vapor densityat the inlet to the relief valve.

! If the outlet fluid does not condense, the outlet pressure dropcalculated from the scenario used for the inlet pressure dropcan be handled in the same manner as described above fromthe same worksheet.

! If the fluid condenses, use 2 Phase (DI) or an alternative methodoutside of iPRSMto calculate piping losses. Enter to Outlet Lossas User Suppliedin iPRSM.

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, asituation similar to a vapor filled vessel can occur.

! In this situation, the temperature of the liquid in the vessel willcontinue to rise, but it will not boil.

! If the boiling point is high enough, vessel failure may occur beforethe pressure relief device set pressure is reached. For carbon steel,a typical maximum temperature is 1100F.

!

If the boiling point is more than 200F above the fire designtemperature, use MERR 1to determine relieving rates and directintegration or the Omega method to calculate the relief valve orificearea.

! Use the same method outlined above for supercritical fluids toevaluate the relief requirements.


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FIRE ON LIQUID FULL EQUIPMENT

! Often called Vapor Driven Liquid. Client-specific and fluid-dependent.

! Equipment that is liquid full with a top mounted relief valve on thevessel may be calculated as a vapor generation scenario. See thefire boiling liquid with vapor generation section for calculationdetails.

! The calculations assume that sufficient disengagement between theliquid phase and the vapor phase occurs prior to reaching 21%overpressure, resulting in only vapor relief.

! This calculation methodology varies from site to site.

!

Some operators calculate this scenario as a vapor and someoperators calculate this scenario as discussed below. Consultsite-specific guidelines prior to proceeding with calculations.

! Equipment that has the relief valve mounted under the liquidlevel will result in a large relief requirement due to a vapor-driven liquid (VDL). This is because the fluid boils along thevessel walls and must push an equivalent volume of liquid outto the relief valve to maintain relief pressure. The initial relief willbe liquid, followed by a period of two phase flow and finallyvapor. The relief rate is calculated as the saturated liquid at thevapor generation rate on a volumetric basis.


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DISCUSSION SKETCH 1


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DISCUSSION SKETCH 2

Determine relief load

! Determine the vapor generation rate in the same manner as forstandard fire vapor generation.

! Convert the vapor generation rate to the volumetric vapor flow rateusing the density of the vapor at relief conditions.

! Determine the equivalent liquid that must be displaced using theliquid density at relief conditions.

! Evaluate the required relief area using the Fire - Given Flow Rate - 2Phase (DI), or select the 2 Phase Omegamethod. The D.2or D.3equations may be used depending on the number of constituentsand proximity to thermodynamic critical points.


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PROTECTED SYSTEMS CONTINGENCY SCENARIO VIEW


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! To evaluate the scenario using 2 Phase (DI)calculations, selectGiven Flow Rate - 2 Phase (DI).Attach the stream, flash and phase atthe relief device inlet. Enter the required relief rate and pick EvaluateScenario.

Piping pressure drops

! If using 2 Phase (DI) scenario calculations, pick the Piping Losseslinkand the Evaluate Scenario & Piping Losseslink in the piping lossesview. No additional inputs are required.

! If the Omega method is used, evaluate pressure drops using thesame process as described for supercritical fluid with a liquid on theinlet. The outlet piping losses will have to be calculated outside ofiPRSM.

Enter results in iPRSMas User Supplied, and upload any

spreadsheets used.


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Lesson 7: Evaluating other scenarios




About other scenarios

These additional approaches are also considered in calculating andanalyzing scenarios.

LIQUID THERMAL EXPANSION

!

When a liquid system is blocked-in, any heat input will result inoverpressure due to thermal expansion.

! Scenarios where the heat source is capable of vaporizing thetrapped liquid are covered in other scenarios such as blocked outlet,abnormal heat input or automatic control failure.

! For liquid thermal expansion from heat transfer equipment, use theclean heat transfer coefficient to determine the duty of theexchanger.


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LIQUID THERMAL EXPANSION CALCULATIONS SPREADSHEET

! For liquid thermal from solar heat input, two phase calculations inthe outlet piping can be safely ignored and not calculated, orcalculated as if it were a liquid only, due to the small flow rates.

! For liquid thermal expansion from either solar or tracing heat input,if the RVdischarges back into a separate section of piping that isalso protected by a relief valve, the constant back pressure should beequal to the normal operating pressure of the line, and the variable back

pressure should be equal to the set pressure minus the normal

operating pressure.

! Although we calculate the capacity of the valve at 10%overpressure for liquid expansion cases with nominal flow rates, thevalve will never reach that level of overpressure, and using the setpressure without any overpressure is normally acceptable.


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! If the valve discharging into the second section of line is aconventional valve, then the spring set should be adjusted so that itwill open at the maximum design pressure of the piping undermaximum back pressure conditions, unless the specific client allowshigher relief pressures.ASME B31.3piping codes allows set

pressures up to 20% above design.

! For solar heat input on any system, use Heat Transfer Rate:300BTU/hr-ft

2-oFon the exposed surface area. Do not include surfaces

that will not be exposed to direct sunlight, like the bottom half ofpiping systems.

COOLING FAILURE

! Loss of process cooling creates an energy imbalance, which canproduce major relief loads. A relieving situation may not result if only

sensible cooling is lost, although unexpected flashing couldoverpressure downstream equipment. However, if condensation islost, the excess vapor will generally have to be relieved. If coolingon a feed stream is lost, relief may occur due to flashing caused bythe entrance of hot feed.

! There are a number of different modes of cooling failure, includingcooling water failure, loss of cold side circulation in a process heatexchanger, and loss of cold feed or quench.

! The cooling failure scenario is used for the global loss of cooling

case for a system. Other cooling failure cases can be covered underabnormal heat input, automatic control failure, etc., as appropriate.

! As a general rule the high variable back pressure should be usedfor the cooling failure case.


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DISCUSSION SKETCH

POWER FAILURE

! The power failure scenario is used for the global or partial loss ofelectricity to a unit.

!

Evaluate the effect on a system of losing the motor-driven electricalequipment.

! There may be combined effects associated with partial loss ofcooling water.

! Because power failure is considered a global scenario, do notoverride the high variable back pressure to evaluate this scenario.


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! As in other generic cases like control failure, the engineer mustanalyze the probable consequences of each assumed utility outageto identify those cases where over-pressure may result in theprocess system under consideration.

MECHANICAL EQUIPMENT FAILURE

! Evaluate the effect of a failure of a single rotating piece of rotatingequipment. This might cause the specific scenario of lost reflux,which should be covered under lost quench or reflux, a lost bottomspump that may cause a relieving situation in a system due tooverfilling, or the loss of a compressor that, especially for amultistage, can cause significant relieving situations.

! Loss of fin-fan cooler and refrigeration systems also needs to beevaluated.

! Often this is evaluated under another scenario.

CHEMICAL REACTION

! iPRSMdoes not predict chemical reactions or the rates of relief.

! Data on chemical reactions must be provided by the client and isnormally obtained through laboratory testing.

! Often in chemical reactions two phase flow is applicable and a 2

phase flow calculation method should be used.

STEAM OUT

! If the equipment in a system has a lower design pressure than thesteam-out steam supply pressure, determine the flow of steam intothe system using the piping and fitting from the steam header to thesystem with pressure drop of Psupply: Preliefin the Gas Pipe Flowspreadsheet.

! 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 anyequipment that might be steamed-out.


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CHECK VALVE FAILURE

! For a single check valve, even if properly inspected and maintained,leakage across the check valve seat is assumed during reverse flowconditions. For calculation purposes, for a single check valve, the

valve is assumed not to exist.

! For a pair of check valves in series, if properly inspected andmaintained, leakage across the valve is assumed to equal the flowthrough a single orifice with a diameter equal to one-tenth of thelargest check valves nominal flow diameter.

! Caution is recommended when taking any credit for the reduction ofreverse flow using check valve/s. Use site-specific guidance if creditis to be taken.

! Flow can be calculated using the Cv Failure Scenario Hazard Worksheet

in iPRSMor in the Gas Pipe Flowor Liquid Pipe Flowspreadsheets.

SERIES FRACTIONATION, REFLUX FAILURE AND LOSS OF QUENCH

! Series fractionation

To be discussed during distillation column training

! Reflux failure

To be discussed during distillation column training.

! Loss of quench

To be discussed during distillation column training

OTHER

This guideline describes methods to evaluate a range of possibleoverpressure scenarios. It may not address all possibilities. It is theresponsibility of the evaluating engineer to fully review the system andverify there are no unidentified relief scenarios before signing off.


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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?