introduction to pressure relief systems

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’s employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Intrumentation For additional information on this subject, contactFile Reference: PCI11001 D.W. Buerkel on 874-7339

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Introduction To Pressure Relief Systems

Engineering Encyclopedia Intrumentation


Saudi Aramco DeskTop Standards

Content Page

INTRODUCTION................................................................................................................ 1

FUNCTION AND STRUCTURE OF A PRESSURE RELIEF SYSTEM ANDASSOCIATED CONTINGENCIES ..................................................................................... 2

Function of Pressure Relief Systems ......................................................................... 2

Protection of Plant Equipment and Personnel................................................. 4

Containment of Process Fluid ........................................................................ 5

System Structure and Associated Contingencies........................................................ 6

Relief Valve .................................................................................................. 6

Individual Component Pressure Relief ..........................................................10

Unit Pressure Relief......................................................................................19

Plant Pressure Relief.....................................................................................19

Multiple Systems to Flares/Burnpits .............................................................21

Relief System Summary...........................................................................................22

SAUDI ARAMCO AND INDUSTRY REQUIREMENTS AND PRACTICES FORA PRESSURE RELIEF SYSTEM.......................................................................................23

Saudi Aramco Standards (Mandatory Practices) and Recommended Practices..........24

SAES-J-600..................................................................................................25

SAES-A-005 ................................................................................................26

Inspection Department Procedures................................................................26

34-SAMSS-611 ............................................................................................26

8020-611-ENG SHTS 1, 2, & 3 ....................................................................26

Industry Requirements and Practices ........................................................................26

ASME ..........................................................................................................27

API...............................................................................................................28

NACE......................................................................................................................29

MR0175 .......................................................................................................29

Summary..................................................................................................................29

BASIC TERMS FOR PRESSURE RELIEF SYSTEMS AND DEVICES............................30

Terms for Pressure Relief Devices ...........................................................................31



Saudi Aramco DeskTop Standards

Terms for Operational Characteristics ......................................................................31

GLOSSARY........................................................................................................................31

Table of Figures Page

Figure 1. A Process System Schematic with Typical Pressure Profiles ..................... 3

Figure 2. Heat Exchanger with PZVs Discharging to Various Relief Systems ........... 7

Figure 3. Typical Flare Installation..........................................................................20

Figure 4. Related Codes, Standards, and Practices ...................................................25

Figure 5. Relationship of Terms Used to Describe Relieving Conditions of PZVs ...31



Saudi Aramco DeskTop Standards 1

INTRODUCTION

Saudi Aramco engineers assigned to Loss Prevention, Project Design, Maintenance andOperations organizations are concerned with the protection of plant equipment and personnelfrom equipment and pipe damage due to excess pressure (overpressure). The instrument used toprotect equipment and pipe is a pressure relief valve (PZV). PZVs that discharge into largerrelief systems provide additional personnel protection from thermal, toxic and other hazards byallowing for the safe disposal of fluids released by PZVs.

There are interactions between PZVs and relief systems that must be understood before a reliefvalve can be properly sized and selected. Therefore, this module reviews some general conceptsrelated to Saudi Aramco relief systems. Codes and standards govern every aspect of safetyrelated systems regardless of the engineering discipline. This module introduces the applicablecodes and standards for PZVs and provides an overview of the relationships between SaudiAramco standards and industrial codes and standards. Key terms are also introduced withreference to the role of PZVs in pressure relief systems.

The focus of this course (PCI 110) is pressure relief valves, not pressure relief systems. Onlythose aspects of pressure relief systems that affect the sizing, selection, and maintenance of aPZV are considered in this course. A single exception occurs in Module PCI 110.03, wheredownstream considerations of relief vents and other closed relief systems are related to theselection of a proper discharge system for an individual PZV and the superimposed backpressure on individual relief valves. (Superimposed back pressure is the static pressure of adischarge system connected to a relief valve.)




FUNCTION AND STRUCTURE OF A PRESSURE RELIEF SYSTEM ANDASSOCIATED CONTINGENCIES

Function of Pressure Relief Systems

Pressure relief systems for process systems and units provide protection against conditions thatcause pressures to increase above the assigned safe limits. PZVs are required to protectequipment and personnel from inadvertent over pressurization of piping and/or equipment andare not the normal pressure control device in a process system. PZVs should only operate torelieve pressure when the normal pressure control devices fail. PZVs are used to prevent acatastrophic event such as fire, unit shutdown, plant shutdown and personnel casualties.

PZVs must be designed to relieve a sufficient amount of process fluid to prevent pressure frombuilding up in the system resulting in over pressurization of piping and equipment. Overpressurization can be defined as pressure that exceeds the "Maximum Allowable WorkingPressure" (MAWP) of the equipment and piping. The equipment and controls that are designedto maintain the normal operating pressure and temperatures may fail and allow the pressure inthe system to exceed the MAWP or the "Maximum Allowable Working Temperature" (MAWT).Both the MAWP and MAWT of a process system are determined in the detailed designprocedure of the process system. Then the MAWP and MAWT is assigned to specific pipingand equipment. Once these numbers are available, it can be determined where in the processsystem PZVs will be required.

PZVs are also used to relieve pressure that may occur due to an increase in the process fluidtemperature. Recall Boyle's law regarding increased pressure due to an increase in temperaturewhile maintaining a fixed volume. If the process fluid is "trapped" by isolation valves orblocked piping, the volume will remain the same and pressure will rise with a rise intemperature. An external source of heat from a fire or other sources can cause a rise of fluidtemperature inside the process equipment causing the internal pressure to rise.

These are fundamental considerations in determining the need for a PZV; specific considerationswill be covered later. Figure 1 shows pressure profiles and a simplified schematic for acompressor driven process system that is protected by one pressure relief valve (PZV) set at 275psig. (Figure 1 is from Appendix B of API-RP-520, "Sizing, Selection, and Installation OfPressure-Relieving Devices in Refineries.") The use of a single PZV to protect severalcomponents in a process system, as shown in Figure 1, is not a typical PZV application.However, Figure 1 will help explain where and why pressure relief is required.




Figure 1. A Process System Schematic with Typical Pressure Profiles




Protection of Plant Equipment and Personnel

To provide protection for plant equipment and personnel, design engineers must analyzeconditions that can cause overpressure of the equipment. To illustrate the requirements forprotection of equipment and personnel from the effects of overpressure, hypothetical cases(scenarios) are presented for the compressor and the furnace shown in Figure 1. These casesconsider the effects of overpressure when protection is not provided to this equipment byexamining both contributing factors and consequences. Using the pressure profiles and theprocess system given in Figure 1, we can relate the MAWP limits of equipment to the relievingcapacity provided by the single PZV.

First, suppose that the following criteria apply to the process and the centrifugal compressorshown in Figure 1:

• The gas/liquid separation involves a flammable hydrocarbon vapor that is heavierthan air (e.g., a molecular weight > 60).

• MAWP for the compressor casing is 400 psig.

With these conditions, suppose the following events occurred:

• The compressor casing cracked as a result of either surge or overpressure due to lossof speed control.

• Flammable hydrocarbon vapors accumulated around the compressor until they wereset afire by a random ignition source.

When we consider the compressor failure, we find that the 400 psig MAWP for the compressoris less than the 406 psig that the profile relates to the 275 psig PZV Set Pressure. The eventbegins with loss of the compressor, but its consequences include the effect of release of aflammable fluid. Depending on the amount of accumulated vapor, ignition could result in asimple fire or catastrophic explosion. In either case, the unit would be down and personnelnearby could be injured.

For the second case, consider the effect of blocked flow on the furnace. If discharge flow fromthe furnace was shut off, the compressor would attempt to makeup the reduced volume from thehigh pressure separator by increasing pressure. At 380 psig, well below the MAWP of 400 psigfor the compressor, furnace tubing would fail, thus causing a fire. The furnace might bedamaged, but the unit would shut down.

The equipment and personnel protection function of relief systems is based on careful analysis ofconditions that cause overpressure. Overpressure due to compressor surge and blocked flow arethe two basis for relief used in the hypothetical examples above (in this case, they are alsocontingencies). Saudi Aramco Engineering Standards (SAES) require a thorough evaluation ofthese contingencies and basis for relief. Basis for relief will be reviewed in Module 3.




Containment of Process Fluid

The requirement to contain process fluid depends on the properties of process fluid. Fluidproperties are a safety concern because of adverse effects such as toxicity, flammability, andcorrosion. Relief systems are classified as either 'closed' or 'atmospheric' depending on thedisposal method for process fluids released from a PZV. Closed relief systems discharge processfluids into 'closed' collection systems. Atmospheric relief systems discharge process fluidsdirectly to the atmosphere.

Closed Collection Systems transport discharged process fluids from PZVs to treatmentfacilities. PZV discharges are transported through vent headers to a 'seal drum', which removesresidual liquids and condensable vapors. (Seal drums are usually called 'knockout drums.')Process gas is then transported to a flare stack and burned.

Many liquid PZV discharges are transported to holding tanks or directly into the plant processsewer system. Ultimate disposition of liquid waste is either in the plant sewage treatment facilityor by consumption in an off-site burnpit. Two specialized liquid relief systems used in SaudiAramco facilities are known as Open Funnel Relief and relief to Valve Boxes. (Valve boxes areused for thermal relief discharge release from certain off-site pipe lines.) Open funnel reliefsystems and valve boxes are 'atmospheric pressure relief systems' as well as 'closed reliefsystems' (atmospheric pressure closed system relief). Another form of atmospheric pressureclosed system relief is discharge into process sewer drains. Atmospheric pressure closed systemrelief is important when sizing and selecting a PZV and will be reviewed in detail in Module 3.

The final form of closed system liquid relief transfers toxic and dangerous fluids to holdingtanks, which are vented to a 'closed system relief vent header.' Again, the impact of these'nonatmospheric pressure closed relief systems' will be reviewed in Module 3.

Atmospheric Relief Systems are restricted to gas and/or noncondensing vapor materials at thePZV inlet. Toxic gases and vapors must be released to a closed system. Flammable vapors andsafe vapors must be released through a riser that terminates at least 10 feet higher than anyequipment, manway, or pipeline within 50 feet horizontally. Flammable vapor release mustmeet stringent radiant heat requirements. Radiant heat requirements are not a factor innonflammable safe vapor release.

Process vapors discharged to the atmosphere are dissipated by wind and weather. Fluidsdischarged to vent headers are transported through the header to a knockout drum. Condensateliquid is trapped in the drum while the gas is transported to a flare stack for burning. Drainageand vent systems are an extension of the PZVs discharging into them, and the effects ofconnecting a PZV to a containment system must always be evaluated when sizing and selecting aPZV.




Discharge or release of process fluids (liquid or vapor) in relief systems is for personnel andplant protection. Pressure safety valves (PZVs) protect equipment and piping, while reliefsystems protect personnel, unit operations and plant operations from adverse effects caused bydischarged fluids.

System Structure and Associated Contingencies

A relief system begins at the connection of a PZV on the protected equipment or pipe, and endsat either the plant waste treatment facility or at the point of release to the atmosphere. Figure 2provides an introduction to the structure of relief valve systems. The heat exchanger in thisprocess is one piece of equipment in a larger process unit which, in turn, is part of a plantoperating unit. The following discussion will explain PZV system structure in terms oforganizational levels;

• First, relief devices (such as valves) will be explained in terms of actual installation.

• Then the explanation will look at high organizational levels (such as equipment,units, plants and multiple systems that re relieved by flares and burnpits).

Relief Valve

Individual process components are protected by relief valves. PZVs relieve pressure from acomponent (at MAWP) by discharging enough entrained fluid to maintain pressure within thatcomponent at a safe level.

Figure 2 shows a heat exchanger, coolant pump, and compressor in a representative process.Unlike P&IDs, isolation valves are shown with each PZV in Figure 2. The process in Figure 2has a liquid coolant feed going to the shell of the heat exchanger from a centrifugal pump, whilesour process gas is cooled in the tube side of the exchanger.

Isolation Block Valves are required between the equipment or pipe connection and the PZVinlet connection. Also an outlet isolation valve is required in piping connected to the PZVdischarge connection. Isolation valves are needed for conducting maintenance on a PZV, andmust be either 'Car Sealed Open' (CSO) or 'locked open/locked closed' (LO/LC) with a chain andpadlock or other device to prevent accidental movement when the PZV is in service.

Spare PZVs are provided for major process equipment. Spare PZVs are used when the mainPZV is removed for maintenance. The inlet connection to spare PZVs are 'Car Sealed Closed'(CSC) until the PZV is placed in service. Anytime a PZV is in service, both isolation valvesmust be CSO. The following refer to spare PZVs:

• The darkened CSC maintenance valve under PZV-4 on Figure 2 indicates that it is aspare relief valve.




• A spare PZV allows unit operation while a PZV is removed for testing and inspection(T&I).

Figure 2. Heat Exchanger with PZVs Discharging to Various Relief Systems

PZV-1 is protecting the coolant pump from overpressure. The contingency and basis for relief ofPZV-1 is blocked flow. If the maintenance valve downstream of the pump is accidentally closedwhen the pump is running, then PZV-1 protects the pump case by discharging coolant back tothe pump supply vessel. The following structural features associated with PZV-1 as shown inFigure 2 are typical of Relief Valve Installations:

• Vent valves: The piping between the isolation valve discharge connections and thePZV inlet connections must have a vent valve. When the isolation valve is closed,process fluid is trapped between the isolation valve and relief valve. The pressure inthis area is equal to the process pressure at the time the valve was closed. As a safetyprecaution, this fluid/pressure must be released through the vent valve before thePZV is removed for maintenance.




• Bypass: As is customary with overpressure relief of pumps, PZV-1 is 'bypassing'discharge process fluid back to the centrifugal pump supply vessel (not shown).Bypassing process liquids back to a storage vessel or surge tank both reduces the loadon disposal systems and salvages valuable process fluids.

• Closed vent header: Vent valves are required between the discharge of a PZV andthe inlet of its isolation valve ONLY when the valve is connected to a closed ventheader. PZVs which discharge at atmospheric pressure (atmospheric dischargesystems, open funnel vents, etc.) do not produce back pressure on PZVs, andtherefore do not require a vent valve.

PZV-2, located on the coolant feed line to the shell of the heat exchanger, is a thermal reliefvalve. If the coolant line was blocked closed when filled with coolant, heat from the sun couldcause fluid expansion in the pipe and could damage it. PZV-2 protects against this contingency,which is the basis for relief of PZV-2.

Discharges from PZV-2 are sent through a closed system to a holding tank for later recovery. Ifthe coolant were compatible with the disposal system on the plant process sewer system, PZV-2could be released into the process sewer through a drain connection. In either case the dischargeof PZV-2 is at atmospheric pressure, and there is no back pressure.

Discharge lines from PZVs interface with containment piping in closed relief systems. Thefollowing structural features are associated with PZV discharge connections to closed systemcomponents:

• Open funnel relief vents are just what the name implies. The PZV discharge lineinserts down into a funnel and into a larger diameter pipe connected to the bottom ofthe funnel. Open funnels are connected to liquid hold tanks. Insertion of the PZVdischarge line into the larger funnel line ensures the liquid PZV discharges aretransported directly to the hold tank, while the funnel serves as an atmospheric ventfor the PZV relief system.

• Process sewage drains are similar to funnel vents. The PZV discharge line is sealed inthe process sewer drain above any fluid level. Atmospheric pressure is maintained bya separate sewer vent.

• Valve boxes are closed but ventilated (to the atmosphere) sumps in remote locations.Valve boxes are only used on remote pipelines outside of fenced areas. Valve boxesare used on some special 'sun valve' thermal relief valves. Thermal relief valves arereviewed in PCI 110.03.




• Vapor/gas relief discharge vent headers are closed vent systems designed to transporttoxic and otherwise dangerous materials to remote locations for disposal. Ventheader systems are complex piping networks, located above fire risk zones, and aredesigned to service multiple relief valves and/or other vent headers. These are plantwide networks that start in an operating unit and end at a remote disposal site. Reliefdischarge vent headers are often segregated to facilitate disposal.

PZV-3, on the shell of the heat exchanger, releases vapor discharges to the plant fire reliefdischarge vent header. PZV-3 and PZV-4 are protecting the shell of the heat exchanger fromoverpressure during a fire. Fire will boil liquid coolant in the shell of the heat exchanger,causing heat to be carried away from the metal structure into the plant fire vent relief dischargeheader in vapors discharged by either PZV-3 or PZV-4 (whichever one is on-line).

Sour gas discharge from PZV-5 and PZV-6 are released to the plant sour gas relief dischargevent header. Blocked flow is the basis for relief (contingency) of both PZV-5 and PZV-6:

• PZV-5 is protecting the sour gas compressor case. In this application, the PZVdischarge is to a 'safe area', the sour gas relief discharge vent header.

• PZV-6 is protecting the tube bundle in the heat exchanger against overpressurecaused by the compressor if the tube bundle maintenance valve is closed duringcompressor operation.

Notice that discharge piping for the closed systems as described above, and as shown on Figure2 for PZV-3, PZV-4, PZV-5 and PZV-6, are typically extensive and complex. Special structuralfeatures are associated with PZV discharge piping because of the effects of the following forces:

• Back Pressure: Discharge piping from PZVs must be equal or greater in size than thedischarge connection on its PZV. Discharge piping from a PZV must be carefullysized to prevent 'built-up' back pressure on the PZV. (Built-up back pressure resultsfrom fluid friction in the discharge piping of a relief valve. Built-up back pressuremust not exceed 10% of the set pressure of a conventional PZV. Module 3 will coverbuilt-up back pressure.)

• Reaction Forces: Discharge piping from a PZV must also be carefully designed towithstand reaction forces generated by discharging fluids at high pressures. PZVreaction forces are reviewed in Module 4. Consider PZV-5 which protects the sourgas compressor case. The discharge piping between PZV-5 and the sour gas ventrelief header must be sized and supported to withstand any reaction forces generatedwhen PZV-5 'pops'. Reaction forces in piping are caused by high velocity dischargeat high pressure. Reaction forces are discussed in detail in Module 4.




Individual Component Pressure Relief

Figure 1 provides additional examples of individual components. The following observationsprovide an overview of representative contingencies that are involved in providing overpressureprotection for these components.

Compressor Component - A centrifugal compressor is the first individual component displayedin Figure 1. PZVs are required on all centrifugal compressors if the maximum develop head isgreater than the case or piping MAWP. PZVs are also required on interstages when includedwith the compressor. Main PZV set pressures can never be higher than the surge pressure of thecompressor (110% of the pressure rating of the case), but main PZV set pressures can be setlower to protect downstream process equipment or piping. Interstage PZVs can be set higherthan surge pressure to avoid driving the compressor into surge. Saudi Aramco EngineeringStandards require the PZV to discharge to a safe area or flare, not to the compressor suction.

For a centrifugal compressor, a surge contingency is always required. This PZV must be sized todischarge the maximum amount of process fluid generated during surge. However, when thePZV is set at a lower set pressure to protect downstream equipment, the PZV must be sized forthe maximum flow rate the compressor can generate. Protection of downstream equipment bythe PZV is seldom required since the compressor is sized for optimum process performance justas all other components in the process unit. MAWPs of downstream equipment will usuallyexceed the MAWPs of the prime movers feeding them after allowances for intermediate pipingand equipment. (Generally, for example, a typical heat exchanger tube bundle will produce apermanent pressure drop of 10 psig in line pressure through the exchanger.)

Since compressors and other prime movers are usually located at ground level for ease ofmaintenance and structural considerations, they are often located in fire relief areas (fire zones).However, fire contingencies are never required for prime-movers or pipe lines. The volume ofentrained liquid is so small that a managed discharge is useless. Thermal relief valves or PZVsprotecting the components will discharge; however, pipe, prime-movers, and gas or vaporcontainers located in the fire relief zone will usually be destroyed by a fire.

Blocked flow is another contingency which must always be considered. The following are blockflow events:

• Blocking the feed to the compressor would drive it into a stall condition, but thecompressor is protected from a surge condition by a required PZV.

• Blocked compressor discharge flow would not cause overpressure directly. Thecompressor would run to 100% output, which is 10% below its MAWP. When speedcontrol is lost, the compressor would 'wind-out' to a point on its performance curvewhere the impellers begin to slip (spin without increasing flow). If the pressuregenerated at this point is higher than the MAWP, then the PZV must be sized tohandle the greater of either surge flow or high speed flow (runaway speedcontingency).




Customarily, the manufacturer of a compressor provides the PZVs and all other necessarycomponents piped and mounted on a compressor 'skid'. Manufacturers have various ways ofpreventing runaway speed control and are experienced in providing the correct PZV for theirequipment. However, Saudi Aramco design engineers are responsible for preparing thecompressor specifications and for checking the delivered unit and the discharge piping from thecompressor PZVs even if the compressor is skid mounted. Toxic or dangerous gases and/orvapors must discharge to a closed system vent header. In some cases (i.e., Hydrogen Sulfide[H2S]), the closed system must be a segregated vent header.

If the discharge fluid from the compressor PZV/PZVs is safe, it can be discharged to theatmosphere through a riser 10 feet above any equipment or manway within 50 feet horizontally.If it is safe and flammable, rigorous radiation calculations must be used to establish the elevationfor release.

Feed/Product Exchanger (1) Component - The next individual component in the process unitdepicted in Figure 1 is a feed/product exchanger (1). (The first exchanger after the compressordischarge.)

If this heat exchanger is air cooled, then fire, loss of fan, or the louver would be considered asbasis for relief. Blocked Liquid Product flow, however, is a reasonable contingency that couldcause overpressure. The following are contingencies to consider:

• If the feed control valve (FCV) were closed to the feed/product exchanger (1), theexchanger tube bundle pressure would not increase. There would not be a basis forgenerating overpressure since fluid would expand downstream and no pressure buildup would occur.

• If both control valves (FCV & LCV) are closed, then thermal relief must beconsidered as a contingency. Whether the fluid is liquid or gas, calculations arerequired because thermal relief in blocked pipe or tubing containing liquid is a SaudiAramco required contingency.

• Remember, thermal relief is a special case of blocked flow where a liquid trapped in acontainer expands as the result of external heat. If hydraulic expansion exceeds theMAWP of the container, then thermal relief must be considered as a contingency. Inthe absence of a fire contingency, thermal relief would also be the basis for relief.

NOTE: If the process fluid is a liquid and saturated with gas (from thecompressor), it can still expand enough due to external heat sources to exceed theMAWP of the heat exchanger tubing.

• Blocked flow due to a closed outlet valve (LCV) is a valid contingency if themaximum operating pressure in the product feed line into the process unit is higherthan the MAWP than any equipment in the system.




• If the MAWP of the compressor exceeds the MAWP of tubing in feed/productexchanger this becomes a valid contingency.

• If the potential mass flow available through the FCV is greater than the mass flowpotential through LCV and the Gas Bleedoff (Purge Gas) valve, a contingency forrelief consideration exists.

If this heat exchanger is air cooled, then fire, loss of fan, or the louver would be considered asbasis for relief (contingencies). Blocked flow, however, is a reasonable contingency that couldcause overpressure for the following events:

• If the feed maintenance valve were closed to the feed/product exchanger (1), theexchanger tube bundle pressure would not increase. There would not be a basis forgenerating overpressure since fluid could still travel (or expand) downstream.

• If both maintenance valves are closed and the entrained fluid is a gas, it is unlikelypressure limits will be exceeded due to thermal expansion. If the entrained fluid is aliquid, then thermal relief must be considered as a contingency.

• Thermal relief is a special case of blocked flow where a liquid trapped in a containerexpands as the result of external heat. If hydraulic expansion exceeds the MAWP ofthe container, then thermal relief must be considered as a contingency. In the absenceof a fire contingency, thermal relief would also be the basis for relief.

Even if the feed fluid is a liquid, it is unlikely that a liquid saturated with gas (fromthe compressor) can expand enough due to atmospheric heat to exceed the MAWP ofthe heat exchanger tubing. However, calculations are required; thermal relief inblocked pipe or tubing containing liquid is a Saudi Aramco required contingency.

• Blocked flow due to a closed outlet maintenance valve is another valid contingency ifthe MAWP of the compressor exceeds the MAWP of tubing in feed/productexchanger (1). If two MAWP values are closed, the pressure drop of interconnectingpiping is deducted from the compressor MAWP. If the resulting value is still higherthan the MAWP of feed/product exchanger (1) tubing, a blocked flow contingencymust be considered.

Another case would occur if the maximum operating pressure in the product feed line into theprocess unit were to go higher than the MAWP of the feed/product exchanger (1) tubing. Ifproduct feed pressure can exceed the MAWP of tubing in feed/product exchanger (1), PZVprotection is required.




Note that back pressure from the product feed to the unit was not considered as a contingency forthe PZV on the compressor. Common engineering piping practice would require a check valve indischarge pipe from the compressor and a check valve in the product feed piping. Industrialcheck valves fail in the closed position, thereby blocking flow. A check valve that failed in theopen position and simultaneous overpressure in the product feed piping would be a very remotecontingency or 'double jeopardy'. (Double jeopardy is defined as a combination of two remotecontingencies, neither of which singly constitute a risk appropriate for consideration of a PZV.)

If a PZV is required for protection of the process tubing in the feed/product exchanger (1), theneeded PZV must discharge in the same manner as the PZV on the above compressor. Thefollowing paths for discharge depend on fluid properties:

• Toxic or dangerous gas or vapors will discharge to a unit vent header for transportand disposition by a plant vent header, with header system segregation required forsome process gases or vapors.

• Safe vapors are discharged through a riser to the atmosphere at least 10 feet aboveany equipment or manway within 50 feet horizontally.

• Toxic and/or flammable liquid discharges will be released through a funnel vent ordrain into a hold tank for later plant disposition. (Hold tank selection depends onentrained toxic vapors in the discharged liquid. Open funnel vents are open to theatmosphere while other hold tanks are vented to unit or to plant vent headers.)

• Safe nonflammable fluids are discharged through drains into the plant process sewagesystem.

Furnace Component - Figure 1 depicts a furnace which is vaporizing the process fluid (aheterogeneous gas, considered as a single process fluid) in the illustrated process.

A process furnace is another type of heat exchanger containing a tubing arrangement (tubebundle) for transporting process fluid through the heat exchange area (furnace burner, or hot gasarea). Therefore, relief valves are required on furnace outlets and inlets in order to maintain flowthrough the furnace tube bundle and keep the tube bundle temperature below its MAWT.

Since the fluid in a furnace is a gas, the following conditions occur:

• If the inlet valve to the furnace is closed during furnace operation, the inlet PZVwould bypass the inlet block valve and discharge into the tube bundle.

• If both maintenance valves are closed during furnace operation the discharge PZVwould discharge bypass fluid from the inlet PZV into a closed vent systemappropriate for the process fluid.




Therefore, unless the process tubing in the furnace is rated for the MAWT of the furnace, PZVsare required by Saudi Aramco standards for protection of furnace tubing. Consequently, thebasis for relief is blocked flow. Process fluid flow would be disrupted for personnel protectionduring a fire in the operating unit outside of the furnace.

Liquids entrained in a hot but inoperative furnace can be protected by thermal relief PZVs.However, it is unlikely that discharge flow due to thermal relief would exceed the blocked flowcontingencies outlined above. It is still important to consider that residual radiation from furnacewall liners (the ceramic brick lining the walls of furnaces) continues heating tube bundles longafter burner shutdown, and pressure in the tube bundles will increase when heated process fluidsare entrained. (Normal operating procedures for furnaces require process fluid flow duringstartup and shutdown to protect tubing against thermal shock. After shutdown, flow ismaintained until the furnace cools. However, safety and emergency analysis requiresconsideration of all unsafe conditions [contingencies] beyond standard operations.) Thermalrelief is a required contingency for PZV sizing and selection anytime liquids can be entrained ina container, whether the container is pipe, tubing, or vessels.

Another source of blocked flow to the furnace in Figure 1 is catalytic plugging in the reactor.Even with maintenance valves, CSO on the furnace PZVs would still be required unless the tubebundle MAWT is rated for the maximum burner temperature. Note that if the furnace operatedon hot gas rather than being a direct fired unit, it is likely that the tube bundle would have aMAWT rating higher than the MAWT of the gas. In this case, a different basis for relief mightbe considered; or no relief valves would be required. In any event, the catalytic pluggingcontingency is accounted for in the blocked flow case discussed above.

Fluids discharged from PZVs on the furnace are released in the appropriate manner. Toxic anddangerous gases are discharged into a segregated or common process unit vent header fordistribution to the plant vent header system and disposal. Safe gases are released to theatmosphere through a riser 10 feet above any equipment, piping, or manway within 50 feethorizontally. Safe flammable gases are released through a riser at an elevation based on radiationcalculations of the burning gas.

Liquid releases from the inlet PZV are discharged into the furnace tube bundle to ensureadequate cooling of the tubes. After vaporization, the gas is discharged through the outlet PZV inthe appropriate manner, which is described in the previous paragraph.

Reactor Component - A reactor is a specialized pressure vessel. The obvious function of thereactor in the hypothetical process depicted in Figure 1 is to rearrange the molecular structure ofprocess gas from the furnace to favor extraction of a liquid product yield by the HP separator.Since no secondary process feeds are depicted, it can be assumed that the reactor 'floats' (has noexternal controls) on the process between two process units, a furnace and a feed/productexchanger, under temperature control.




If the reactor is a catalytic reactor, considerations regarding the catalyst will be included incontingency determinations for the reactor. The following are pressure related contingencies:

• Excess pressure and temperature in a catalytic reactor can cause catalyst packing andcan also poison a catalyst. Therefore, blocked outlet flow from the reactor is a validcontingency.

• If the catalytic reaction is exothermic, blocked inlet flow and total flow blockagewould prevent process gas from removing excess heat from the catalyst. In this case,blocked inlet flow and total reactor blockage are both valid contingencies.

Fire is not a contingency since the reactor has no wetted surface. Other contingencies, like arunaway reaction, are not likely since no external heating or cooling systems are depicted inFigure 1. However, overpressure caused by high feed pressure is a valid contingency. Thesystem pressure drop between product feed/compressor discharge into feed/product exchanger(1) and the reactor is 40 psig. If the MAWP of the reactor is less than 360 psig, a reactor PZVlocated above the catalyst bed must be sized to discharge 100% of product feed.

If excess pressure can poison the catalyst or cause catalyst packing, a maximum operatingpressure (MOP) is established for the catalyst bed. If catalyst MOP is lower than the reactorMAWP, the set pressure of the PZV would be set to the MOP of the catalyst rather than thereactor MAWP. However, the PZV would be sized to discharge 100% of product feed not thelower product feed rate at the catalyst MOP. The PZV will protect both vessel and catalyst.

Note that the lowest pressure in the reactor, its discharge pressure of about 290 psig, is higherthan the set pressure on the HP separator. This observation rules out a contingency that backpressure from the HP separator could damage the catalyst bed in the reactor.

Gases discharged from the reactor PZV would be released in the manner outlined above for thefurnace and other components in the process unit.

Feed/Product Exchanger (2) is a shell and tube type heat exchanger. It is depicted in Figure 1as using coolant on the shell side of the exchange to cool product gas from the reactor.

Shell and tube heat exchangers have two separate, pressure-rated containers (sides). The heatexchanger shell surrounds a sealed tube bundle and has a singular MAWP. The tube bundlelikewise has a singular MAWP. The MAWP of one container is lower than the MAWP of theother, and the container (side) with the lower MAWP is classified as the low pressure (LP) sideof the heat exchanger. Usually, for cost saving reasons, the shell is the LP side.

A broken tube contingency should be considered when the shell is the LP side. However, ifpressure resulting from a broken tube cannot exceed the MAWP of the shell, then a broken tubecontingency is not the basis for relief. The shell side of a heat exchanger can have a lowerMAWP than the tube side and can still be rated higher than either the MOP of the process fluidor the MOP of the heat transfer fluid.




A contingency resulting from water or light hydrocarbon mixing with hot oil as the result of atube break must be considered. If process fluids used in a heat exchanger react violently whenmixed, usually PZVs cannot react quickly enough to protect the exchanger from the resultingoverpressure. However, design engineers should test the contingency on a case by case basis tolearn if a valid basis for relief is possible.

When the heat exchanger is located in a fire relief area, a fire contingency must be explored.When the heat exchanger contains liquid, there are wetted surfaces to transfer heat away fromthe structure; and the heat exchanger can be protected by a PZV. Because the tube bundle isusually insulated from external heat by liquid in the shell, fire is not usually the basis for relief ofthe tube bundle.

Fire is a contingency for the shell of the feed/product exchanger because this exchanger containsliquid coolant. The tube side of the exchanger contains hot process gas and cannot be protectedfrom fire by a PZV. The shell side PZV will protect the tube bundle by continued cooling untilthe supply of coolant is exhausted. When fire is a valid contingency, it is usually the basis forrelief. However, plant design practices often locate heat exchangers above fire relief zones inopen bays more than 25 feet above any fire bearing surface. Since the process fluid is a gas in theoperating unit under discussion, and since the feed to the HP separator is located above a liquidlevel, both feed/product exchanger (2) and product condenser would be located in a higher bayabove the fire relief zone. Therefore, a fire contingency would not be a basis for relief.

Blocked flow in the tube bundle cannot cause overpressure since the gas is cooled in theexchanger. (Vacuum relief might be required, but this is unlikely. Most shell and tube heatexchangers are rated for full vacuum [FV].) Blocked flow of coolant in the shell is a contingencythat is probably the basis for relief in this example. Trapped liquid exerts hydraulic forces on theshell of the exchanger due to thermal expansion caused by hot gas in the tube bundle.

Therefore, the most likely contingencies are:

• Process overpressure at MOP. Since a 360 psig MAWP is not excessive for a tubebundle or heat exchanger shell, this is not a likely contingency.

• A ruptured tube which may be beyond protection by a PZV.

• Blocked flow in the shell.

Discharge of released gas from the tube bundle PZV on feed/product exchanger (2) would be toeither an appropriate relief vent header or to the atmosphere. Liquid discharge from the shellPZV is required to release to a closed system. It might be reasonable to discharge the shell PZVto the coolant discharge line in blocked flow or thermal relief cases. Superimposed back pressureconsiderations on the PZV and/or blocked coolant discharge piping would probably make'bypass' PZV release impracticable. Many heat transfer fluids can be discharged in the plantsewage systems, or the fluids can be sent to atmospheric pressure segregated holding tanks.




In the event that fire was a basis for relief, the PZV discharge would be a saturated vapor whichmight form solids when cooled. Special isolated PZV discharge treatment would be required inthis case. This is another good reason to locate feed/product exchanger (2) outside of the firerelief zone.

Coolant failure to the exchanger would upset the condenser downstream and possibly the HPseparator. It should not be a contingency in feed/product exchanger (2).

Product Condenser Component - If cooling water failure to a condenser cannot causeoverpressure in related columns or gas/liquid (HP separator) separators, as would be the case inthe process depicted in Figure 1, then condenser vapor flow alone would not constitute a validcontingency. HP separator pressure in the process unit in Figure 1 is regulated by compressorsuction, not the downstream condensation and a reboiler as typified by a distillation column.

Figure 1 does not show a liquid discharge pump on the condenser. Therefore, either gravity feedor process pressure is used to transfer condensate to the HP separator. Hot process gas from thereactor is driven up to feed/product exchanger (2) and is cooled and transferred by processpressure to the condenser located at the same level. Heavier condensate then drains into the HPseparator. If this is the case, then fire is not a likely contingency for establishing a basis for reliefin the condenser. Like feed/product exchanger (2), the component would be located above the 25foot elevation of a fire relief zone in the process unit.

If the condenser was located in a fire zone, then fire would be a basis for relief (contingency),since coolant is passing through the shell of the condenser. Blocked flow would be a more likelybasis for relief. Entrained liquid in the shell would be heated by gas from feed/product exchanger(2) and would cause hydraulic force overpressure.

Since the gas in the condenser is cooled, a ruptured tube would only cause overpressure in thecondenser if the shell side was the LP side and the MAWP of the shell was lower than the MOPof the product gas. This is an unlikely design.

Loss of coolant could be a contingency for relief of the tube bundle if the MAWP of the bundlewas lower than the MOP of the product gas. The same would hold true for blocked dischargeflow from the condenser. Both cases are unlikely. A flooded condenser would cause a pressureincrease in (i.e., block the flow from) feed/product exchanger (2), but would not causeoverpressure in the condenser. Since the condensate is insulated from solar radiation by coolingfluid in the shell, thermal relief of the tube bundle would not be a contingency.

The most likely contingencies for a basis of relief from the condenser are: blocked flow, thermalrelief on the shell, and none on the tube bundle.

Coolant PZV relief from the shell is a liquid and would be discharged to a closed system. Watercoolant would be discharged through a drain connection into the plant process sewer. Othercoolant would be discharged to a holding tank.




HP Separator Component - The HP separator is a pressure vessel, and pressure reliefprotection is regulated by the same codes and standards as the reactor. However, the base of theHP separator would be located at ground level, in a fire relief area, and the HP separator has awetted surface. Therefore, a fire contingency is the likely basis for relief of the HP separator.

A wetted surface within a pressure vessel protects structural metal in contact with liquid fromdamage caused by heat from a fire. Liquid in contact with hot metal transfers heat away from themetal and into the liquid. Heat from the liquid is in turn transferred to vapor when the liquidboils. When the pressure from boiling liquid reaches the MAWP of the pressure vessel, the PZVon the vessel discharges. Heat contained in the discharged vapor or gas is carried away from thepressure vessel in the PZV discharge release either to a vent header or the atmosphere.

Pressure in the HP separator is also regulated by the compressor. Compressor suction draws gasout of the HP separator while residual gas from the condenser discharge creates pressure in theHP separator. Energy is added to the process fluid entering the process unit by the compressor.(It is assumed that the reactor could be endothermic, but this is not likely. The catalytic reactorcauses energy contained in the process fluid to be released to or absorbed from the systemthrough chemical changes in the fluid. The assumption in the process shown in Figure 1 is thatenergy is released into the process unit by the reactor.)

Energy is removed from the process unit by feed/product exchangers (1) and (2) and by thecondenser. The net energy contained in feed process fluid entering the process unit equals the netenergy leaving the process unit in the process liquid stream outlet on the HP separator.

Total system energy is manifested by three factors: temperature, pressure, and volume. Thesystem volume is fixed by process equipment and its interconnected piping. Process unit volumeis fixed when the equipment and interconnecting pipe is manufactured. If the process unitvolume is altered, the equipment or pipe is deemed unsafe beyond the limits set by codes andSaudi Aramco Engineering Standards. However, direct measurement of volume changes in largeequipment and pipe is expensive and complex. Therefore, volume is fixed, or held constant, andtemperature and pressure in process equipment and pipe are used to pipe determine their safeoperating limits. These limits are the MAWPs and MAWTs that were described earlier; and,since volume is deemed constant, changes in pressure and temperature represent energy changes.

When unwanted energy is added to or removed from a process unit or a component within theprocess unit, this energy must be balanced by opposing energy transfer. Hence, pumps, heatexchangers, catalytic reactors, condensers, compressors, and other process equipmentcomponents are used to manage energy transfers as process fluids are tailored by the process.PZVs are used on these process components and their interconnecting pipe to remove energy (inthe form of pressure) that has exceeded a level that has been specified as safe. Whentemperature is altered to control energy levels, a heat exchanger is used.




The measurement function of unwanted energy that is to be removed by a PZV is the MAWP ofthe component that is protected by the relief valve. The quantity of energy that a PZV mustremove is the volume of process fluid which contains that energy. The time available to removeexcess energy is equal to the rate at which excess energy is added to the system. In the case offire heating the HP separator, the rate at which excess energy is added is the BTU transfer rateacross the vessel wall into the entrained liquid at the wetted surface. Therefore, the rate at whichenergy is expelled in a flowing volume of vapor or gas through the PZV must equal the rate atwhich heat energy is transferred into the vessel from a fire. Discharge piping and disposalsystems connected to the PZV must not reduce the required volumetric flow rate out of a PZV.

Compensation for any causal effect on flow rate through the relief valve must be made by thedesign engineer when sizing and selecting the PZV on the HP separator. This compensation isrequired whether the PZV discharges into a riser for atmospheric relief, or into a closed system,unit or plant, vent relief header. In turn, any spontaneous energy release out of a PZV intodischarge piping (reactive force) must be accounted for in the support design of the dischargesystem.

Unit Pressure Relief

PZV relief discharges from process units can take any acceptable configuration. Relief forequipment in a potential fire risk zone might be discharged into a local vent header fordisposition in the plant vent header system. Equipment extending above the fire risk zone (suchas tall distillation columns) might release atmospheric discharges safely beyond lower levelequipment.

Equipment containing toxic or dangerous materials will discharge as vapors into segregated localheaders connected to a suitable plant system. Liquid discharges of toxic materials are collectedin hold tanks, while nontoxic liquids discharge into the plant process sewage system.

Finally, harmless vapors such as steam are vented to the atmosphere even within a fire risk zone.Again, as with PZV-1 in Figure 2, we consider a relief system that consists of individual PZVsprotecting valuable process components. Types of fluids (such as toxic, flammable, vapor, orliquid) within a process component (such as process equipment, pipe, or pump) and the locationof a component within a process unit (such as a fire risk zone or the height above platforms)dictate final fluid disposition in an emergency.

Plant Pressure Relief

Plant pressure relief is accomplished by a network of vent headers that are routed above fire riskzones. Vent headers are routed more than 25 feet above all fire bearing surfaces. A fire bearingsurface is any surface that will hold burning liquid materials. (Solid metal decking is a firebearing surface while a deck made of metal grating is not a fire bearing surface.) Plant ventheaders collect pressure relief discharges from unit vent headers and/or individual PZVs andtransport these discharges to a disposal site (Figure 3) at the end of the plant vent header.




Figure 3. Typical Flare Installation




A fire in one operating unit could be discharging vapors through the operating unit vent headerto one section of a plant vent header while a runaway reactor could be discharging vapors toanother section of the same header. Plant vent headers and their disposal systems must bedesigned to handle simultaneous emergencies along the header.

Below ground are equally sophisticated and complex relief systems. There are plant-widenetworks of process sewers which terminate in a treatment plant. Process sewers are designed fordisposal of safe process fluids.

Toxic materials are collected in individual holding tanks: i.e., tanks that are serviced by openfunnel relief PZV discharge lines or sealed holding tanks that are vented to a closed relief ventheader. Toxic materials holding tanks comprise a plant-wide disposal problem. However, theyare usually considered part of process unit relief. The ultimate disposal of materials collected inholding tanks is a plant level function. Actual collection is at the process unit level.

Multiple Systems to Flares/Burnpits

Process fluids discharged from PZVs must be managed and treated in order to protect theenvironment, the plant, process units, and personnel in the vicinity from any harm caused by therelease. To accomplish this goal, PZV discharges are regulated and require specific reliefengineering design based on the nature of the released fluid and other conditions. Designrequirements for PZV discharge relief systems range from Valve Boxes to complex systems ofsegregated vent headers and disposal systems, as depicted in Figure 2. PZV discharge systems,vent headers, and disposal systems will be discussed in a separate course. PCI 110 focuses on theheart of all overpressure protection, the PZV.

Figure 3 shows a typical disposal system at the termination of a plant vent header system. Notethe open funnel vents beneath the 'XCV' devices, an the oil/slop recovery pump and line inFigure 3. Figure 3 displays a complex unit operation which might exert back pressure oninterconnected plant vent headers. Back pressure in headers (superimposed back pressure) is animportant consideration in PZV sizing and selection. Module PCI 110.03 provides a detailedreview of basis for relief and 'single contingency' with regard to PZVs.

One single contingency that is frequently a basis for relief is fire. Process equipment itemslocated in a fire risk zone during a fire are subjected to intense heat generated by the fire.Process materials within vessels, heat exchangers, and other containers with large wettedsurfaces boil, creating internal pressures which exceed the MAWP of the container.

PZVs protecting process fluid containers during a fire release vapors in order to maintaincontainer pressures at their MAWPs. Process fluid vapors must then be transported away fromthe fire risk zone either by atmospheric vent risers or by unit vent headers and/or plant ventheader systems.




Segregated headers are often necessary to prevent mixing of toxic or dangerous vapors. Anexcellent example of process fluids that require segregation are the sour gases Hydrogen Sulfide(H2S) and Ammonia (NH3). When mixed in the vapor state, these gasses form solid AmmoniumSulfide and can block headers.

High concentration levels of H2S require both segregated vent headers and special provisions fordisposal. Segregated vent systems will normally have dedicated disposal systems. A disposalsystem can be as simple as a dedicated flare high above an operating unit designed to burn H2Sdischarges from three PZVs connected to the operating unit vent header. Or, a disposal systemcan be a complex operating unit which separates gases from vapors, neutralizes liquid residuals,pumps the treated residuals to a plant process sewer, and burns the separated gas in a flare stack.

Hydrocarbon and oil bearing disposal systems collect liquid residual in a seal drum at the end ofone or more plant vent headers. Gases from the plant vent headers are burned in a local flarestack. Liquid residuals are transported (either pumped through a pipeline or shipped by truck) toa remote site and burned in a burnpit.

Relief System Summary

A PZV provides protection to the relief system in which it is installed. If the relief system(atmospheric or closed system) exerts more back pressure on the PZV than was calculated whenthe valve was sized or that was anticipated when the valve was selected, less process fluid willbe discharged than is required to maintain MAWP conditions in the protected equipment, pipe,or vessel. All components in a relief system (from the individual PZV to the plant disposalsystem) function together in an emergency.

A rigorous system of interlocking procedures and responsibilities insures that a PZV will releasea certified volume of process fluid at a specified temperature and pressure before the pressurewithin the protected equipment or container accumulates to a specified overpressure. Protectionreliability requires proper engineering judgment and adherence to Saudi Aramco EngineeringStandards as well as PZV reliability for true safety. Applicable Saudi Aramco EngineeringStandards and recommended practices will be reviewed in the following section, together withrelevant industry recommended practices and standards.




SAUDI ARAMCO AND INDUSTRY REQUIREMENTS AND PRACTICES FOR APRESSURE RELIEF SYSTEM

The formal documentation items that govern or otherwise apply to the design of industrialfacilities and to the fabrication and construction of these facilities are referred to as codes,standards, specifications and practices.

Engineering codes include the minimum level safe practices incorporated in a body of standards.In industry, standards consist of mandatory practices or of other formal requirements that mustbe followed or met. For example, ASME Section I is an American code incorporating standardsrelating to power boilers, direct fired pressure vessels, and related equipment used in thegeneration of steam for the power industry. Standards for designing, manufacturing, andapplying safety valves, and relief valves (PZVs) for use on power boilers under varioussubsections of ASME Section I are legal requirements in the USA.

Different bodies of standards can draw on each other without the need to repeatedly resolvehighly focused technical details. For example, API Standard 526 'Flanged Steel Safety-ReliefValves', simply adopts ANSI standards B16.5 and B16.34 information regarding flange andvalve connection for use on PZVs.

Guidance for the application of codes and standards to engineering problems is often provided inrecommended practices. API recommended practices outline acceptable engineeringmethodology used by the North American petroleum industry. Recommended practices draw onand support codes, standards, and acceptable engineering procedures and outline acceptablemethods for solutions to engineering problems. In one sense, recommended practices areadvanced engineering tutorials. They start with definitions of terms, references to relevant codesand standards, and applicable engineering equations. Next they demonstrate how to combine thisinformation to solve related engineering design problems. For example, API RP-520 providesgeneral methodology for the sizing and selection of PZVs. Other specific methodology for sizingand selecting PZVs is provided by individual manufacturers.

Standards for a specific industrial corporation adopt the content and methodology of industrywide standards and practices to the specific requirements of the organization preparing thestandard. Each company has a working methodology that provides it with a competitive edge inits industrial field. The standards of a company ensure compliance with the company'smethodology. Companies are free to use industry wide standards and practices as they see fit orto ignore them altogether. However, much time and expense devoted to standards preparationcan be saved by copying useful industry wide standards when they meet corporate objectives.Also, most large companies participate in the preparation of industry wide standards throughtheir representatives in the industrial standards and/or professional engineering societies.




Saudi Aramco Standards (Mandatory Practices) and Recommended Practices

In the titles of formal Saudi Aramco documentation items, the terms standards, specifications,practices and instructions carry the definitions listed below:

• Standards, in the title Saudi Aramco Documentation Standards (SAESs), refers tothe minimum mandatory requirements for the design, construction, maintenance, andrepair of equipment and facilities for Saudi Aramco.

• Specifications, in the title Saudi Aramco Materials System Specifications (SAMSSs),refer to the minimum mandatory technical requirements (as opposed to commercialrequirements) for materials supplied to Saudi Aramco by vendors and manufacturers.

• Procedures, in the title Saudi Aramco Engineering Procedures (SAEP), refers to setsof instructions (steps or directives) for administrative record-keeping andauthorization procedures.

Figure 4 lists standards, practices, specifications, and instructions that are related to pressurerelief valves and that will be referenced in PCI 110. Each of the Saudi Aramco documentationitems listed in the right column references one or more of the industry wide standards orpractices in the left column. Each of Saudi Aramco items also references other related SaudiAramco items. In the event of a conflict between the Saudi Aramco items and the referencedindustry items, the Saudi Aramco standards, specifications and practices take precedence.




Industry Codes, Standards, andPractices

Saudi Aramco Standards, Specifications, andPractices

ASME Section I (Codes)ASME Section VIII (Codes)ASME/ANSI PTC- 25.3-1988 (Code)

Pressure Relief Valves:API STD-2000-1992 (Standard)API STD-527-1984 (Standard)API STD-526-1984 (Standard)

API RP-520, Pt.I-1993 (Practice)API RP-520, Pt.II-1994 (Practice)API RP-521-1997 (Practice)

Other Related Standards & Practices:NACE MR0175-94 (Standard)

Instruments - Pressure Relief:SAES-J-600, Pressure Relief Devices34-SAMSS-611, Safety Relief Valves34-SAMSS-612,SAES-J-002, Acceptable Instruments

Inspection & Maintenance:SAEP-318, PZV AuthorizationSAEP-1131, Form 3099ASAEP-319, PZV T&I and QA RegulationsSAEP-1132, PZV Test StandSAEP-1133, Form 3750 T&I ReportSAEP-1134, RV Tech. Certification

Other PZV Related Safety:SAES-A-005, Safety Instruction SheetSAES-B-067, Safety ID & Color Coding

Related Piping & Equipment:SAES-L-043, Thermal Relief in PipingSAES-B-057, Refrigerated Storage Vessels32-SAMSS-021, Water-Tube BoilersSADP Safety and Facility Design,05, CentrifugalPumpsSADP Safety and Facility Design,06, PositiveDisplacement Pumps31-SAMSS-001, Centrifugal Compressors31-SAMSS-005, Centrifugal Refrig. UnitsSAES-K-501, Steam Turbines

Figure 4. Related Codes, Standards, and Practices

SAES-J-600

SAES-J-600, 'Pressure Relief Devices', is the foremost standard for sizing, selecting andspecifying individual PZVs and other relief devices that are beyond the scope of PCI 110. Afterconceptual design of a process system is complete, and essential process information exists(P&IDs, PFDs, Engineering data forms for equipment and vessels, etc.), SAES-J-600 is the firstresource used for selecting a PZV. It includes all the rules for, and is essential to, PZV sizing,PZV selection, and installation of PZVs on Saudi Aramco facilities.




SAES-A-005

SAES-A-005, 'Safety Instruction Sheet', is an informational standard relative to PCI 110. SafetyInstruction Sheets (SISs) are administrative forms for vessels and equipment. They arecompleted by the specifying engineers that are responsible for designing the vessel or equipment.SISs must include the location and set pressure of each PZV provided on the equipment. Whensizing and specifying PZVs, instrumentation engineers will only use an SIS for referenceregarding MAWP and MAWT. Engineering data forms for the equipment would also providethis information.

Inspection Department Procedures

Inspection SAEP series establish administrative record-keeping rules regarding authorization anduse of PZVs, testing and inspection (T&I) of PZVs, test standards for PZV testing, andcertification of PZV technicians. The bulk of the instructions relating to PZVs in the InspectionSAEP series cover administrative authority and responsibility, and provide information oncompleting forms 3099A and 3075. These procedures are essential to maintenance personnelworking with PZVs.

34-SAMSS-611

Saudi Aramco Material System Specification, 34-SAMSS-611, 'Safety Relief Valves FlangedConventional And Balanced Types', outlines Saudi Aramco requirements regarding PZVs foruse by PZV vendors and design engineers. This SAMSS is included as part of all requests forquotations and purchase orders that are sent to vendors. It references API RP-526 as a source ofadditional requirements. 34-SAMSS-611 must be reviewed for each PZV specified by a designengineer to verify that the manufacturer's information meets all Saudi Aramco requirements.Vendors which cannot comply with requirements of 34-SAMSS-611 should not be on thebidders list for a particular PZV.

8020-611-ENG SHTS 1, 2, & 3

Saudi Aramco form 8020-611-ENG, consisting of sheets 1, 2, and 3, is the engineering datasheet for PZVs. It contains all data essential for the purchase of a PZV, including sizinginformation. Module PCI 110.03 will examine form 8020-611-ENG in great detail. It remainspart of the record of every active PZV on Saudi Aramco facilities.

Industry Requirements and Practices

Industry wide standards that are authorized by Saudi Aramco are referenced, and therebyadopted, in SAESs, SAMSSs and SADPs. Major industry wide standards which have a directbearing on pressure relief systems and that are authorized by Saudi Aramco are listed in the leftcolumn of Figure 4 and are reviewed below.




ASME

Several industry wide standards prepared by the American Society of Mechanical Engineers(ASME) are referenced in SAESs and SAMSSs and thus become mandatory in Saudi Aramco.Only requirements included in the referenced sections constitute Saudi Aramco requirements.Additionally, Saudi Aramco SAESs and SAMSSs sometimes place limits on the application ofreferenced standards. Applicable limitations on referenced standards regarding sizing, selection,and maintenance of pressure relief valves will be listed throughout PCI 110.

ASME standards that cover PZVs and that are referenced by Saudi Aramco are summarizedbelow.

Section I - 'Power Boilers' of ASME, 'Boiler and Pressure Vessel Code' (ASME Section I)outlines all requirements for the design, construction, and certification of power boilers, directfired pressure vessels, and compressed air systems. Under ASME Section I, pressure reliefdevices required for equipment is the responsibility of the certified manufacturer of theequipment. Therefore, PZV requirements in ASME Section I are not reviewed in detail in PCI110.

Section VIII - 'Pressure Vessels' of ASME, 'Boiler and Pressure Vessel Code' (ASME SectionVIII) outlines all requirements for the design, construction, and certification of unfired pressurevessels. Under authority of ASME Section VIII, the end user (not the equipment manufacturer)is responsible for pressure relief devices used for protection of pressure vessels. For this reason,user company design engineers are responsible for sizing, specifying, selection, procurement,and installation of PZVs on pressure vessels. ASME Section VIII is the primary basis for APIstandards and practices discussed in PCI 110.

ASME Section VIII contains two parts relating to PZVs: Division 1, 'General Requirements',and Division 2, 'Mandatory Requirements'. The latter contains Appendix 10, 'MandatoryCapacity Conversion For Safety Valves', and Appendix 22, 'Mandatory Acceptance Of TestingLaboratories And Authorized Observers For Capacity Certification Of Pressure Relief Valves',which relate directly to Saudi Aramco requirements for regulating PZVs.

Certification Tests - ASME/ANSI PTC 25.3-1988, 'Safety and Relief Valves - PerformanceTest Codes', Section I, 'Power Boilers' (ASME Section I) outlines all requirements for thedesign, construction, testing, and certification of PZVs. This is the parent code of ASMESections I and VIII relating to design, construction, testing, and certification of PZVs covered bythe sections.

Capacity calculations are included in every ASME code related to relief devices. They are alsoincluded in API codes, standards, and practices, and in certified PZV manufacture's literature.Module PCI 110.03 relates these various references to Saudi Aramco requirements.




API

The American Petroleum Institute (API) is an industrial standards organization devoted to thetechnical requirements of the petroleum and petrochemical industries. API publications related toPZV and referenced in SAESs and SAMSSs are included in PCI 110. These publications arelisted in the Industry Standards column of Figure 4 and are outlined below. Specific details fromthese publications that are related to topics discussed in PCI 110 are explained and reviewed inapplicable Modules.

API RP-520 'Sizing, Selection, and Installation Of Pressure-Relieving Devices in Refineries' is atwo part outline of recommended practices regarding PZVs used in refineries.

• Part I - Sizing and Selection is a comprehensive tutorial for sizing, selecting, andspecifying PZVs in general. This publication must be understood by design engineerswho specify and select PZVs for Saudi Aramco. Saudi Aramco requirements relatedto PZVs cannot be applied by design engineers without a through knowledge of themethodology incorporated in API RP-520

• Part II - Installation contains the methodology for installing PZVs in a relief system.It does not address design features of relief systems, only the installation methods forPZVs attached to a system.

API RP-521, 'Guide for Pressure-Relieving and Depressuring Systems' is a comprehensivetutorial for the design of pressure relieving systems. Many of these design considerations havebeen addressed above and will be reviewed in detail in Module PCI 110.04.

API STD-526, 'Flanged Steel Safety-Relief Valves' outlines standard requirements designatedby the API for PZVs. Understanding of this standard is essential to selection of PZVs in accordwith Saudi Aramco requirements, particularly to the completion of Saudi Aramco specificationform 8020-611 ENG. Module PCI 110.03 will discuss API STD-526 in detail.

API STD-527, 'Seat Tightness of Pressure Relief Valves' outlines standard requirements andmethodology for testing leakage through a PZV. This API standard is referenced by SaudiAramco standards related to testing and Saudi Aramco certification of PZVs. API STD-527 willbe reviewed in detail in Module 5.

API STD-2000, 'Venting Atmospheric and Low-Pressure Storage Tanks - Nonrefrigerated andRefrigerated' outlines standard requirements for relief devices beyond the scope of PCI 110(Low Pressure Storage Vessels). However, Appendix C -'Types And Operating CharacteristicsOf Vents' contains useful information about Pilot-Operated PZVs which are part of PCI 110 anddiscussed in Module PCI 110.02.

Also, SAES-B-057 defines a class of storage vessel which bridges a gap between Low PressureStorage Vessels (those having a MAWP below 15 psig) and Pressure Vessels (those having aMAWP equal to or above 15 psig).




NACE

The National Association of Corrosion Engineers (NACE) International is an industrialstandards organization devoted to the standardization of technology related to corrosion ofmetals used in industrial devices, containers, and/or equipment. NACE Group Committee T-1 isdevoted to the study of issues confronting the Petroleum Production industry. NACE standardsare referenced throughout SAESs and SAMSSs, but only part the of MR0175-94 outlined belowrelates to PZVs.

MR0175

Section 9.1 of NACE Standard MR0175-94, 'Standard Material Requirements - Sulfide StressCracking Resistant Metallic Materials for Oil field Equipment' (NACE MR0175-94) outlinesrequirements for 'Valves and Chokes'. Information from these and related paragraphs has beenextracted by some manufacturers of PZVs who now manufacture PZVs having constructionmaterials certified for use in sour gas service. SAESs and SAMSSs related to PZVs (refer toFigure 4) require conformance to NACE MR0175-94 for all PZVs in contact with sour gas. Thedetails in NACE MR0175-94 are not essential to the design engineer. However, the standardmust be referenced in Saudi Aramco PZV specification form 8020-611 ENG, when the PZV is incontact with sour gas, and only vendors certifying conformance to NACE MR0175-94 areacceptable for procurement under these specifications.

Summary

The relationships among standards, specifications and practices is a complex issue in anycompany. It is important to review Saudi Aramco requirements and practices first in order toascertain the applicability of related industry wide codes, standards, and practices. It is equallyimportant to check the issue and revision dates of SAESs, SAMSSs and SADPs to resolveconfusion related to language and referenced sections. PCI 110 will reference standards,specifications, and practices in relationship to course material only.




BASIC TERMS FOR PRESSURE RELIEF SYSTEMS AND DEVICES

Figure 5 is a graphic representation of important terms used to describe various relievingconditions of PZVs used in accordance with ASME codes and API practices.

The terms introduced and used earlier in this module will help you understand Figure 5. One ofthese key terms is "MAWP", the Maximum Allowable Working Pressure. Notice that Figure 5shows the relationship between:

• the requirements of pressure vessels, and

• the characteristics of relief devices.

The "Percent of MAWP" gradations in the center column are used to relate these requirements tothe characteristics of PZVs. The left side of Figure 5 shows the internal vessel pressures that arecontrolled by a single PZV or by multiple PZVs at the various settings that are listed on the rightside of Figure 5. The "relieving conditions" and terms used on Figure 5 will be discussed belowin the section "Terms for Operational Characteristics."

Definitions for the terms used in Figure 5 are available in Section 1, the Introduction to API RP520, Part 1. You should review the precise definitions listed in API RP 520 at this time, butshould recognize and expect that a deeper understanding will come from the experience ofanalyzing and calculating relief valve requirements and then actually sizing and selecting PZVs.The exercises in Module PCI 110.03 and Module PCI 110.04 of this course will provide youwith an opportunity to gain this valuable experience.

Figure 5 contains only a few of the terms essential to sizing PZVs. Other terms are essential tounderstanding the operation of PZVs, selecting PZVs, specifying PZVs, and installing PZVs inaccordance with Saudi Aramco requirements. In the references you use in this course, you willfind differences in the definitions for these terms, especially terms used for parts of PZVs.Manufacturers of PZVs name valve parts according to their own preferences. To maintainneutrality among vendors, the names of valve parts described in Module PCI 110.02 are notspecific to any relief valve manufacturing company.




Figure 5. Relationship of Terms Used to DescribeRelieving Conditions of PZVs




Terms for Pressure Relief Devices

"Pressure relief devices" as defined in API RP 520 include valves and rupture disks that: (1)actuate by inlet static pressure, and (2) open to prevent pressure in excess of a specified value.Unlike rupture disks, pressure relief valves (PZVs) are capable of closing automatically.Different types of PZVs are identified on the basis of application requirements such as backpressure and whether the fluid is compressible or incompressible.

The single term 'relief valve' combines the three valve types that are identified in API RP 520 as:'Safety Valve', 'Relief Valve', and 'Safety Relief Valve'. The valve types are defined in API RP520, section 1.2.1, page 1. API RP 520 defines these valves as types of spring-loaded pressurerelief valves that have different applications.

Module PCI 110.02 of this course, PCI 110, explains both 'spring-loaded' and 'pilot operated'relief valves. By defining three relief valves as types of "automatic pressure relieving devices",Saudi Aramco has incorporated 'pilot operated' relief valves into all three industry wideclassifications of relief valves as well as the Saudi Aramco designated PZV. However, API RP520, section 1.2.1 provides definitions of relief valve classified by valve features other than theoperator providing sealing force within the PZV (a spring or pilot: spring-loaded or pilotoperated). These terms will be understood by the end of Module PCI 110.02, which describes thestructure and function of PZVs.

Terms for Operational Characteristics

The purpose of presenting Figure 5 at this time is to review the terms that were introduced earlierin this module and to apply them again to establish an overview of what happens when a reliefvalve performs its function. Since the components and operating principles of PZVs will not becovered until Module PCI 110.02, this discussion is limited to introductory remarks only. Aswas mentioned above, much of what is shown on Figure 5 will be more fully understood aftereach of the modules in this course has been completed.

To describe "Pressure Vessel Requirements," the following terms introduced earlier can bestated in relation to Figure 5:

• Maximum Allowable Working Pressure (MAWP)

The maximum allowable gauge pressure that is permitted for an equipment item at adesignated temperature. It is based on calculations that identify the weakest point inthe structure (metal at bends, or flanges) and adjusts for corrosion.




• Design Pressure

The gauge pressure for the most severe combination of pressure and temperature thatcan be expected during operation. Design pressure is always less than or equal to theMAWP. Design pressure is used to specify thickness and other requirements for thevessel.

• Maximum Operating Pressure

The maximum pressure expected during operation.

With definitions for these pressure vessel requirements, the following term can be introduced:

• Set Pressure

The pressure at the inlet of a PZV that will open the valve. Since the PZV has acritical safety function, precise adjustments are made to set this pressure. SAES-J-600requires this setting to be at least 10% or 15 psig above the Maximum OperatingPressure.

When the pressure in a vessel that is protected by only one PZV is greater than the set pressure,the valve will open, release fluid, and then close again. The following terms are used todescribe this event:

• Simmer

A pressure which causes the seating surfaces of a PZV to leak slightly just before thevalve "pops" (lifts) open when set pressure is reached. The term simmer refers to arelatively small release and to the sound that can be heard as the gas escapes. Figure5 shows that a typical simmer occurs at about 98% of the set pressure.

• Closing Pressure

The decreasing inlet pressure at which the PZV closes after it has lifted (popped).Figure 5 shows a closing pressure of 93% of MAWP.

• Blowdown

The difference between the set pressure and the closing pressure for a PZV. This isalso shown on Figure 5.




For a pressure vessel that is protected by more than one PZV, the pressure inside the vesselchanges during the time that one or more PZVs are open. How long a PZV stays open dependson the condition causing pressure to increase in the vessel, the size of the PZV, and what isconnected to the discharge side of the PZV. The following terms are used to describe theseeffects:

• Overpressure

The pressure increase over the set pressure of the first PZV (expressed as percent ofset pressure). In Figure 5, the Maximum Relieving Pressure for one PZV is 110% ofMAWP. The Maximum Allowable Set Pressure for a single PZV is 100% ofMAWP. Overpressure in this case is 110%.

• Accumulation

The pressure increase over the MAWP of the vessel that occurs while a PZV isdischarging (expressed as psig or percent of MAWP). In Figure 5, three MaximumAllowable Accumulations are shown for operating and fire contingencies at 110%,116% and 121% of the Maximum Allowable Accumulation for design pressure.

The effects of various relieving conditions will be considered in greater detail later in the course.'Blowdown' is discussed in Module PCI 110.02. 'Simmer' and 'Leak Test Pressure' are discussedin Module PCI 110.04. Information needed to size PZVs will be taken up in detail in ModulePCI 110.03.




GLOSSARY

API An abbreviation for American Petroleum Institute.

ASME An abbreviation for American Society of MechanicalEngineers.

basis for relief A single cause of overpressure for process equipment thatrequires the largest relieving capacity. This "worst case"relieving scenario is determined by calculating the reliefcapacity for all possible causes (single contingencies) toidentify the largest one. It is used to determine the size of thePZV that will provide the protection that is required.

built-up back pressure Pressure at the discharge side of a PZV that is caused by fluidfriction from flow in the discharge piping after the valve opens.

car sealed closed (CSC) An administrative control that ensures that a valve remains inthe closed position until a clearance is issued to open it. Wax isplaced on a wire clip to identify valves not operated withoutthe required authorization (Work Permit/Clearance).

closed collection system A system made up of devices, sealed vessels, and piping thatcollect hazardous fluids from a process system into theenvironment. Closed collection systems are used to carry thehazardous fluids to safe disposal devices such as hold tanks andflare systems.

containment system A system consisting of structures, devices, vessels, and pipingthat prevent the release of hazardous fluids from a processsystem into the environment.

contingency An abnormal event that can cause overpressure (an emergency)in a particular component of a process system.

double jeopardy Two independent contingencies, neither of which constitutes asingle risk appropriate for consideration of a PZV.

emergency An interruption from normal operations in which personnel orequipment are endangered.

fire risk zone Usually the area of a process within 5000 square feet and up toa height of 25 feet above any and all fire bearing surfaceswithin an area.




LO/LC A Locked Open/Locked Closed valve is one that has beenpadlocked and chained in either the open or closed position,thus ensuring that the position is not changed without therequired authorization (Work Permit/ Clearance).

MAWP The Maximum Allowable Working Pressure for one or morePZVs that are protecting one or more vessels and theirinterconnecting piping.

NACE An abbreviation for the National Association of CorrosionEngineers.

operating pressure The pressure that a vessel or system of vessels is normallysubjected to while in service.

overpressure Generally, an emergency condition where pressure may exceedMAWP. For pressure vessel relieving require-ments, theincrease in pressure in a vessel over the set pressure of the firstPZV that occurs while that PZV is discharging.

PZV An abbreviation for any type of relief valve.

set pressure The pressure at the inlet of a PZV that causes the valve to open.

single contingency A single abnormal event causing an emergency.

single risk The equipment affected by a single contingency.

superimposed backpressure

Static pressure at the discharge side of a PZV that affects itsoperation. The pressure against which a PZV must begin toopen. This back pressure is caused by pressure existing in thedischarge piping system or other valves discharging into it.

wetted surface The area within a container covered by process fluid that issubjected to heat from a fire.

worst case basis Another term for relief basis. The relief rate that is used to sizeand select a PZV.

introduction to pressure relief systems

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