Principles Of Pressure Relief Valve Operation

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.

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Principles Of Pressure Relief Valve Operation

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Content Page

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

Mechanical Operating Features Of PZVs .................................................................. 2

Effect of PZV Operation on System Pressure ............................................................ 5

SPRING-LOADED PRESSURE RELIEF VALVES ............................................................ 7

Components of Spring-Loaded PZVs ........................................................................ 7

Body.............................................................................................................. 8

Bonnets ......................................................................................................... 8

Caps And Test Gags .....................................................................................12

Stem Mounted Assemblies ...........................................................................14

Spring Adjustment Screws............................................................................15

Lifting Levers...............................................................................................16

Disk-Seat Assemblies...............................................................................................19

Lift ...............................................................................................................19

Seating .........................................................................................................19

Blowdown....................................................................................................22

Action ..........................................................................................................25

Bellows Type Spring-loaded PZVs...........................................................................25

Bellows ........................................................................................................26

Balancing Piston...........................................................................................26

Principles of Operation.............................................................................................28

PILOT-OPERATED PRESSURE RELIEF VALVE............................................................29

Principles of Operation.............................................................................................29

Main Valve Design ..................................................................................................31

Pop Action Non-flowing Pilot Valve ............................................................32

Pop Action Flowing Pilot Valve ...................................................................33

Modulating Action Flowing Pilot Valve .......................................................35

Modulating Action Non-flowing Pilot Valve ................................................36

Low-Pressure Pilot Operated Relief Valve ...............................................................38

Main Valve Design.......................................................................................39



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Pilot Valve Design........................................................................................39

GLOSSARY........................................................................................................................40

Table of Figures Page

Figure 1. Cutaway of a Typical PZV Assembly........................................................ 3

Figure 2. General Forces Acting on a PZV ............................................................... 4

Figure 3. Graph of System Pressure vs Time for PZV Operating Cycle .................... 6

Figure 4. Major Spring-Loaded PZV Assemblies ..................................................... 7

Figure 5. Open Bonnet PZV..................................................................................... 9

Figure 6. Closed Bonnet PZV..................................................................................10

Figure 7. Vented Bonnet PZV .................................................................................11

Figure 8. Wetted Bonnet PZV .................................................................................12

Figure 9. Caps and Test Gags..................................................................................13

Figure 10. Stem Assembly Parts..............................................................................15

Figure 11. Spring Adjustment Screw.......................................................................16

Figure 12. Typical Lifting Lever Assembly .............................................................17

Figure 13. Summary of Common Spring-Loaded PZV Features ..............................18

Figure 14. Typical Seating Assembly With Soft Seats .............................................20

Figure 15. Huddling Chamber - Effect of Adjustment Rings....................................23

Figure 16. Typical Blowdown Adjustments.............................................................24

Figure 17. Typical Bellows Seal PZV......................................................................27

Figure 26. Pilot-Operated Pressure Relief Valve Operation Principles .....................30

Figure 27. Main Valve Design.................................................................................31

Figure 28. Pop Action Non-flowing Pilot Valve ......................................................32

Figure 29. Pop Action Flowing Pilot Valve .............................................................34

Figure 30. Modulating Action Flowing Pilot Valve .................................................36

Figure 31. Modulating Action Non-flowing Pilot Valve ..........................................37

Figure 32. Low-Pressure Pilot Operated Relief Valve..............................................38



Saudi Aramco DeskTop Standards 1

INTRODUCTION

Pressure Relief Valves (PZVs) are classified within a general valve group called regulators orregulating valves. PZVs are operated by process forces. Valve operation actually involves twotypes of forces; mechanical and hydraulic. Hydraulic forces are used to lift or open the valve andmechanical and/or hydraulic forces are used to close the valve. When the valve is closed,mechanical forces along the valve stem act to prevent valve operation and maintain a tight seatseal. The valve is opened by process generated hydraulic forces that lift the valve disk off of itsseat, breaking the seal. Once the valve is opened or popped, process fluid passes through thevalve.

PZVs are automatic valves that have the following operating characteristics:

• Predictable and repetitive

• Operate independently

• Adjustable operating range

Predictable and repetitive valve operation can be adjusted to any point within an automaticvalve's operating range. The set point on a PZV is defined as the valve set pressure.

PZVs used in refineries fall within two classifications based on power loading:

• Spring-Loaded

• Pilot-Loaded

Selection of a PZV requires an understanding of three valve features:

• Mechanical Operation

• Construction

• Fluid Management

Mechanical operation and construction of a PZV are used to determine if the valve is suitable foruse under certain process conditions and within the particular plant environment. PZVmechanical operation and construction are reviewed in this module.

Fluid management is the rate at which process fluid discharges through a PZV. Fluidmanagement is determined by valve size and process fluid characteristics. Valve sizing and fluidcharacteristics are covered in Module 3.




Mechanical Operating Features Of PZVs

Certain features are found in all PZVs regardless of valve class or manufacturer. Figure 1illustrates the following universal operating features of PZVs:

• Process fluid connections are always located on the bottom, or base, of a PZV.

• Fluid discharge connections are always located on the side of the PZV.

• Process fluid enters the base through an unobstructed orifice tube located within theprocess fluid connection.

• An fixed valve seat is located at the exit annulus of the orifice.

• A moveable valve disk normally covers (seals) the seat to prevent passage ofprocess fluid, but lifts (opens) to allow flow through the valve.

• An adjustable mechanical spring mechanism on a spring-loaded valve, or processfluid on a pilot-operated valve, assisted by a spring, applies a predetermined forceto the top of the disk.

• Part or all of the set pressure forces (closing forces) are transmitted to the disk by aninternal valve stem assembly. Part of the set pressure forces can be supplied fromthe discharge line back pressure acting on the down stream side of the disk.




Stem

ConnectionDischarge

ProcessConnection Orifice

Disk

Seat

FluidCavity

Figure 1. Cutaway of a Typical PZV Assembly

Figure 2 shows why the disk and seat assemblies are the heart of PZV operation. To perform itsfunction, the PZV must:

• Open and close quickly and completely at the designated pressures. (See PoppingAction and Blowdown.)

• Maintain a leak-tight seal while under normal system pressures.

Disk movement, and consequently disk-seat seal status, is governed by the opposition of forcesacting on the top and bottom sides of the disk as shown in Figure 2. These forces include:

• Mechanical Forces

• Process Fluid Pressure

• Backpressure




Mechanical Force

Backpressure

(Spring Or Pilot)

Process Fluid Pressure

Figure 2. General Forces Acting on a PZV

Stem assemblies act in conjunction with a power source (spring or pilot) to transmit mechanicalforces to the disk. Forces applied above the disk establish PZV set pressure. This PZV setpressure must be at or below the maximum allowable working pressure (MAWP) of theequipment being protected. Recall from Module 1 that MAWP is determined for the vessel andassociated piping, during the design process.




There can be up to three sources of energy pushing downward on the valve disk stem and there isusually only once source of energy pushing up on the disk trying to open the valve. The upwardhydraulic energy source is from the process fluid. The downward or closing source of energy canbe provided by hydraulic backpressure on the discharge line, the mechanical spring force orhydraulic energy provided by a pilot valve that taps the process fluid energy upstream of thevalve disk. Process fluid pressure is applied to the bottom of the disk and when this forceovercomes the combined downward forces, the disk lifts off its seat. Normally, the combinedmechanical and hydraulic forces above the disk maintain disk-seat seal tightness by forcing thedisk against its seat. During abnormal operating conditions the upward force can overcome thedownward force, opening the pressure relief valve passing process fluid through the valvelowering the process system pressure.

When backpressure acts on the disk from above, as shown in Figure 2, a higher process pressureis required to make the valve open. If the valve design does not consider the effect ofbackpressure, the PZV will not open at the set pressure. One method of adjusting for the effectsof backpressure is to reduce the force applied by the power source (spring or pilot). A secondmethod is the use of a shielding enclosure called a bellows to separate fluids inside the PZV fromthe parts connected to the disk. Bellows prevent backpressure forces from acting on the top ofthe valve disk. Bellows are always required when service conditions include high, variablebackpressure.

Effect of PZV Operation on System Pressure

The graph of system pressure versus time in Figure 3 shows how the operational characteristicsthat were introduced in Module 1 relate to PZV operation:

• When process pressure exceeds valve set pressure, the disk lifts off its seat allowingfluid discharge through the valve.

• Discharge continues as the rate of system pressure increase is reduced and until thevalve is fully open.

• The disk then reseats, thereby sealing the valve closed. Unsafe pressure is reducedin protected equipment as a result of the discharge of process fluid through a PZV.




TIME

PZV Fully Open

Set PressureACCUMULATION

BLOWDOWNReseat

Pressure

SYSTEM PRESSUREINCREASING

Figure 3. Graph of System Pressure vs Time for PZV Operating Cycle

The pressure change shown on the graph between the set pressure and the point at which systempressure begins to decrease as a result of valve operation is called accumulation. Notice that thereseat pressure shown on the graph is lower than the set pressure of the PZV. This lower pressureprevents chattering (repeated opening and closing of the valve). The difference in pressureshown on the graph between the set pressure and reseat pressure, expressed as a percentage of setpressure, is called blowdown.

This module describes the construction and operation of relief valves. This type of valve isdesigned and manufactured as a precision instrument. To explain how these valves operatereliably and accurately, the following design features and mechanisms will be discussed:

• Geometries of valve inlet, nozzle orifice, disk, and discharge.

• Balance of forces acting on valve disk from process fluid at inlet, from continuousand superimposed backpressure at the dis-charge, and from the spring or pilot.

• Spring-operated or pilot-operated mechanisms that maintain a seal at the valve seatand provide the force needed to close the valve after blowdown.

• Protection of valve internals from adverse operating conditions including corrosiveprocess fluids and reaction forces.




SPRING-LOADED PRESSURE RELIEF VALVES

PZVs are classified by their power source as either spring-loaded or pilot-loaded. Valvemanufacturers produce different styles of PZVs to meet the application requirements and serviceconditions of their customers. The styles of spring-loaded PZVs are variations of a basic designthat is referred to as the conventional pressure relief valve. There are different PZVs that areused for liquid, gas or steam service conditions. Each style adds or modifies components toprovide special operating characteristics and capabilities. One of the most importantmodifications is the addition of bellows for service conditions that include corrosive processfluids or backpressure. The following section describes design features that are common tospring-loaded PZVs beginning with variations of the conventional valve and proceeding to morecomplex designs.

Components of Spring-Loaded PZVs

Spring-loaded PZVs incorporate three major assemblies as illustrated in Figure 4:

Cap

Bonnet

Valve Body

Figure 4. Major Spring-Loaded PZV Assemblies




Body

PZV body construction varies by valve class and manufacturer. An orifice-seat assembly isalways included in the valve inlet. Some small PZVs combine the process connection, body,orifice, and seat in one machined part. Most valves include both connections (process anddischarge), and the fluid cavity within their body casting. An orifice-seat element screws into thevalve through the process connection flange.

Some PZVs include the flow cavity and discharge connection in the bonnet (such as wettedbonnets). The valve body combines process connection, and the orifice and seat in one part;which screws into the base of the bonnet.

Bonnets

Bonnets provide structural support, and protection for the adjustable spring and stem mechanismof a PZV. There are three general bonnet classifications:

• Open - Required for steam service to keep the spring cool. The bonnet has largeopenings so that the spring assembly can be cooled by air.

• Closed - equired to protect plant personnel from heat, corrosive or toxic hazardsassociated process fluid discharge.

• Vented - or air and other non-toxic gases, the bonnet can be vented to atmosphere.For toxic and flammable gas applications, or if low backpressure must bemaintained, the bonnet is vented to down stream piping or a flare.

Open Bonnet - Vs, as shown in Figure 5, expose most of the spring assembly parts to theatmosphere. Notice that while an open bonnet permits cooling, it cannot protect the spring fromdirt or corrosive elements in the environment. This type of bonnet is normally used on firedboilers or in boiler house (indoors) applications involving hot water, steam, or air.




Spring Assembly

Bonnet Housing

Cap

Bonnet

Body

Figure 5. Open Bonnet PZV

Closed Bonnet PZVs, as shown in Figure 6, enclose the spring assembly to protect it fromexternal conditions, and more importantly, to prevent the release of process fluid to theatmosphere. Notice that there is no opening (vent) between the valve body and the bonnet. Thistype of bonnet is used to protect plant personnel from hazards associated with process fluids.

The vent on the side of the housing of closed bonnets is not used on conventional (non-bellows)PZVs; it is always plugged. If the PZV has bellows, the vent hole on the side of the bonnet mustbe connected either to atmosphere or a low-pressure collection system. This additional piping isrequired to ensure that the hazardous fluid will still be contained or directed to a safe collectionsite, if the bellows fail and leak. The bellows type of PZV will be discussed in detail later in thismodule.




Cap

Bonnet

Body

Stem

Spring

Bellows

Nozzel

Disk

Bonnet

VentPlugged

Vent

Stem

Guide

Closed Bonnet With Bellows Closed Bonnet Without BellowsVent Open Vent Plugged

Figure 6. Closed Bonnet PZV

Vented Bonnet PZVs, as shown in Figure 7, do not vent through the side of the housing as youmight expect. Instead the bonnet is vented through the body to the valve's discharge piping.When the valve opens, the discharging process fluid creates a suction pressure across the ventopening in the valve guide. Process fluid does not normally enter the bonnet during discharge.Process fluid can enter vented bonnets under the following conditions:

• When fluid discharge does not reach sustained turbulent velocity as in pulsatingPZV operations or low velocity (laminar flow) discharges, when vent suction is notgenerated.




• When fluid trapped in the valve body migrates into the bonnet area. These fluidscan be residuals remaining after a release, or fluids entrained in a vent headerconnected to the valve discharge.

Some PZVs have an aspiration nozzle attached to the vent opening as shown in Figure 7. Thisstructure induces a vacuum in the bonnet during discharge and eliminates excess bonnetpressure. For consolidated relief valves, the term eductor tube is used.

Nozzle

Stem Cap

Bonnet

Spring

Disk Body

Aspiration

Nozzle

Figure 7. Vented Bonnet PZV

Vented bonnets work best when the process fluid is a gas. If the process fluid is a liquid that isnot corrosive, the separation between the bonnet and the valve body can be eliminated. This typeof valve, known as a wetted bonnet PZV, is actually a vented bonnet. Figure 8 shows a wettedbonnet PZV.




Cap

Bonnet

Body

Spring

Stem

Disk

Figure 8. Wetted Bonnet PZV

Caps And Test Gags

PZVs have caps attached to the top of the bonnet providing a platform for valve accessories andproviding a housing over the spring adjustment screw as shown in Figure 9. A cap to bonnet'tamper seal' is required by code to ensure valve adjustments have not been changed. Caps alsoprotect other devices attached to the stem within the cap and contain a removable plug bolt forinstalling a temporary test gag.

Test gags are used to lock the disk in place when a process unit is being tested at or above itsMAWP. Test gags lock the disk seal closed by preventing movement of the stem.

CAUTION: Test gags must be removed before the PZV will function.




Removable

Plug for Use

of Test Gag

Plain Screwed Cap Open Lever (Single Acting Lever)

Packed Lever Open Lever (Double Acting Lever)

Figure 9. Caps and Test Gags




Stem Mounted Assemblies

Common spring-loaded valve construction features, as shown in Figure 10, include theassemblies mounted along the stem. Stem mounted assemblies provide precise seat and diskalignment. At the bottom of the stem, a typical assembly includes:

• disk holder

• stem to disk connection assembly

• parts that ensure correct joining of disk to seat (stem and seat guiding).

At the top, there is a spring retaining cup, the 'upper spring retainer assembly' (spring washer)which is secured to the top coil of the spring. Upper spring retainers provide a bearing surface forthe valve spring adjustment screw. Upper spring retainers are not fastened to the stem.

The 'lower spring retainer assembly' (spring washer) is located inside the bonnet at the base ofthe spring. Lower spring retainers are washer-like cups welded to the stem. The bottom springcoil is secured within the cup. Lower spring retainers transmit mechanical spring compressionforces to the stem assembly.

Stem guides ensure that process fluid turbulence does not affect stem alignment. Stem guidesconnect to the body below the stem entrance from the bonnet. Regular guides are tubular and fitclosely around the stem. Entrained fluids in valve cavities can migrate into stem guides causingcorrosion and/or build up of harmful deposits. This is an important consideration when selectingeither conventional or bellows type PZVs.

A 'stem retainer' (term used by some manufacturers) is the section of the stem between the lowerspring retainer and the disk that passes through the stem guide. Stem retainers are usually heavierthan the section of stem which passes through the spring to the upper bonnet and cap.




Spring

Cap

Bonnet

BodyAdjustment

Ring

Inlet Nozzle

Stem to Disk

Connection

Stem

Spring Adjustment

Screw

OutletDisk

Disk Holder

Disk Holder

Guide

(Stem Guide)

Figure 10. Stem Assembly Parts

Spring Adjustment Screws

The PZV spring adjustment screw is a hollow bolt that is threaded into the top of the bonnethousing as shown in Figure 11. The hollow center guides the top end of the stem as it passesthrough the upper spring retainer and enters the cap housing.

The base of a spring adjustment screw rides on the upper spring retainer cup at the top of thespring. Spring compression is changed by turning the spring adjustment screw in or out of thebonnet. Positioning the spring adjustment screw regulates PZV set pressure.




A lock nut is included on the spring adjustment screw to prevent movement after setting. Atamper seal is connected between the cap and bonnet to ensure the certified adjustment has notbeen changed.

Figure 11. Spring Adjustment Screw

Lifting Levers

The stem surface above the spring adjustment screw is used for an attachment point for PZVaccessories. Lifting levers, and other stem motion transfer devices, require a bearing fixture onthe stem. Other PZV accessories connect to the valve cap.

Lifting lever assemblies are required on all PZVs in steam, air, or water service over 140°F.Lifting levers are used to ensure that a valve disk is not chemically bonded to its seat. Liftinglever design complexity varies from packing gland isolation of cap atmosphere, to simplehandle/lever combinations as shown in Figure 12. A lever acts on a bearing surface (release nut)attached to the stem to force the disk away from the seat when the lever is raised. The liftingfork, a cam on the lever shaft, controls the distance a disk will lift. Process pressure beneath thedisk must be above 75% of set pressure to prevent stem damage by a lifting lever.




Figure 12. Typical Lifting Lever Assembly




The major components and common features of a spring-loaded pressure relief valve aresummarized in Figure 13.

Body Process Connection - required (unrestricted entrance for process fluids)Orifice - requiredImmovable Valve Seat - requiredFluid Cavity - usuallyDischarge Connection - usually

Bonnet Bonnet functions include support and/or housing for the spring-loading mechanismand an attachment for the tamper seal. Bonnet types include:• Open - Spring assembly exposed to atmosphere

Common in power boiler applicationsEquipped with lifting levers

• Vented - Vented to valve body fluid cavityOptional atmospheric vent onlyProcess fluids can enter bonnet to slightly, partially or completely wetthe bonnet and valve internals.

• Closed- Vented to atmosphere or low pressure ventNot totally isolated from process fluidsProvides maximum spring assembly protection

Caps House Spring Adjustment ScrewProtect stem topSupport PZV motion accessoriesProvide temporary test gag connectionProvide second attachment for tamper seal

StemAssembly

Stems extend from disk to cap and transmit mechanical force to top of disk.• Disk Attachment

Disk guide assembly, disk holder, stem-disk attachmentStem retainer between disk and main stem

• Stem GuideAttaches to body below entrance of stem from bonnetPrevents misalignment of Stem due to fluid turbulence in fluid flow cavity

• Lower Spring RetainerWelded to stem and retains lower spring coilInterfaces spring/stem forces

• Upper Spring RetainerRetains upper spring coilProvides bearing surface for spring adjustment screw

SpringAdjustmentScrew

Regulates spring compression fixing PZV set pressureThreaded into cap/bonnet connectionBase rides on upper spring retainerIncludes a lock nut

Figure 13. Summary of Common Spring-Loaded PZV Features




Disk-Seat Assemblies

The disk-seat assembly used in pressure relief valves is unusual in that it provides a large changein flow rate with a minimum amount of valve stem travel. Although all valves employ a disk-to-seat geometry for flow regulation, other types of process valves use plugs, balls, diaphragms,gates, or rotating disks at their seat to modulate flow through the valve, but none provide such adrastic change in flow rate when the valve stem just begins to move. Regardless of how a PZV isoperated (Pilot-Loaded, Spring-Loaded, Conventional, or Bellows Sealed), a disk-to-seatconstruction is used to regulate process flow through the valve.

Geometry and dimensional aspects of the disk-seat assembly will be discussed in the section onPrinciples of Operation. This section addresses alternative disk-to-seat construction features forthe following PZV operating requirements:

• Lift - High Lift Disks/Low Lift Disks

• Seating - Metal-to-Metal/Soft Seat

• Blowdown - Fixed Blowdown Ring/Adjustable Blowdown Ring

• Action - Popping/Modulating

Lift

The concept of high or low disk lift is now archaic. Disk lift is mentioned here because the termsstill appear in PZV literature. Disk lift is the actual linear travel of the disk away from the closedposition when the valve is relieving. Disk lift, established by valve design, is such that the valveachieves its rated relieving capacity. Discharge flow is code certified for each class of valve byits manufacturer. PZVs are no longer classified as high lift or low lift valves.

Seating

Seating is the formation of a seal between the valve disk and the seat (or zero lift of the disk).Seat construction must be such that tight shut-off can be maintained over long periods. Failure toremain seated and leak-tight has the following adverse effects on valve performance:

• Any leakage, even very slight leakage can cause erosion of the seat-to-seal surfaces.Such leakage also results in chemical attack and corrosion problems to otherinternal valve components.

• Slight leakage results in large loss of process fluid inventory.

• Significant leakage could cause the PZV to open before set pressure is reached.




The required tightness for PZV disk-to-seal is achieved by very precise construction, carefulselection of materials, and proper maintenance. Two types of seating surfaces are used in reliefvalves: metal and soft seats. The type of seat that is selected depends on the valve application. Atypical seating assembly with soft seats, as shown in Figure 14, consists of the following:

• Seat - A machined surface at the opening of the nozzle.

• Disk - An insert with a machined surface that matches the opposing surface on theseal. It is threaded into the disk holder.

• O-Ring Seal - An O-ring selected for specific service conditions to improve seattightness.

• O-Ring Retainer - Disk that holds O-ring seal in correct position on disk surface.

• Retainer Screw - A screw used to attach the O-ring retainer to the disk.

Disk

O-ring RetainerRetainer Screw

O-ring Seal

Seat

Figure 14. Typical Seating Assembly With Soft Seats




The advantages and disadvantages of each type of seat are discussed below.In metal-to-metal seating, both disks and seats are made of hardened metal and their matingsurfaces are polished to a mirror finish. Metal-to-metal seals are seldom bubble-tight (0bubbles/minute at Cold Differential Test Pressure). Another problem associated with metal-to-metal seats is that they can bond to each other when the disk remains in contact with the seat forlong periods of time. Contributing factors to the formation of chemical disk-to-seat bondsinclude:

• Electrolytic corrosion

• Adhesive buildup

Electrolytic corrosion takes place when free electrons cross seal boundaries on metal-to-metalcontact surfaces. Chemical salts accelerate electrolytic corrosion by employing free electrons toform complex substrates. Substrates formed in this way become part of each parent metalsurface. Once these substrates are formed they continue to grow and increase in bondingstrength.

Adhesives are produced when hydrocarbons in the process fluid leak through the valve seal.Hydrocarbon molecules that enter into a disk-to-seat seal become trapped by mechanical forcesas they try to escape into the valve discharge area. Lighter hydrocarbon fractions separate fromheavier molecules and 'bubble' out of the seal. In time, the accumulation of heavy hydrocarbonsforms tars which bond the seat to the disk.

Soft seats have an intervening O-ring installed between a metal disk and a metal seat. If a softseat fails, the metal-to-metal seat serves as backup. Soft seat O-rings are fabricated from typicalelastomers used in process valves. Buna-N, Viton and Teflon formulations are the most commonO-ring materials. Elastomers have fluid surfaces which shape to the polished metal surfaces andform bubble-tight seals.

Advantages of soft seats include:

• Repeatable tight sealing characteristics.

• Low cost seat replacement.

• Longer periods between required maintenance.

• Seldom form chemical bonds with disks.

A disadvantage of soft seats is that they have limited temperature and pressure ranges.Manufacturers catalogs give the temperature and pressure limits of each 0-ring material.Typically, they range from -150°F to 500°F and 15 psig to 1440 psig. Manufacturers do notrecommend the use of soft seats for steam service.




Soft seats are also subject to electrochemical processes that can result in disk-to-seal bonding.Elastomers, unlike metals, do not have sufficient free electrons to form complex substrates.Some elastomers will host salts and allow a salt bridge to develop across metal surfaces incontact with the elastomer. Teflon formulations, however, will not support salt formations.Adhesives can bond soft seat elastomers to the metal surface of the disk, but since bubble-tightseals do not release hydrocarbon vapors, adhesives are not deposited unless the soft seal has beendeformed or degraded.

Discussions of seat bonding illustrate an important concept regarding PZVs. A PZV is amechanical device which remains static. Contact parts within a PZV do not undergo the'cleansing' afforded by movement of sliding parts. This property makes it is cost effective tochoose extra quality when specifying a PZV.

Blowdown

Blowdown is a term that identifies the difference in pressure between the opening and closingpressure of a PZV. The term blowdown is often used incorrectly to say that a PZV has popped,operated or lifted. The most accurate definition of blowdown is found in API RP-520 I paragraph1.2.3.2.6:

"Blowdown is the difference between the set pressure and the closing pressure of a pressure-relief valve, expressed as percent of set pressure or in pressure units."

Recall that this term was introduced by looking at how system pressure changes during the timebetween opening and closing of a PZV (See Figure 3). Recall also that PZVs are designed to givereliable and precise operation and that such operational characteristics are achieved by usingdesign simplicity. An example of design simplicity is that PZVs generally do not requireadjustments. For a specific system pressure and a correctly sized PZV, both opening and closingof the PZV depends on its spring and the system pressure. For stable applications, a PZV can beinstalled and expected to perform without any adjustments. When system conditions requirevalve blowdown adjustments to obtain better performance, there are PZVs that have adjustmentrings attached over the nozzle. PZVs used as safety relief valves in steam service also haveadjustment rings attached over the stem guide.

Figure 15 shows the effect of adjustment rings on the flowpath away from the seating surfaces.Streamlined pressure waves are formed in this area during disk lift as a PZV opens. This areabetween the upper surface of the nozzle ring and the bottom surface of the disk holder thatextends to the guide ring is called the huddling chamber. It is also referred to as a secondaryorifice or a secondary control chamber. This adjustable mechanical configuration is designed toamplify the lifting forces beneath the disk. Immediately after disk-seat separation, huddlingchamber fluid forces begin to accelerate disk lift. A 'popping' sound is produced in the huddlingchamber during disk lift.




Disk Holder

Nozzle RingNozzle

Disk with

Proboscis

Huddling Chamber

Figure 15. Huddling Chamber - Effect of Adjustment Rings

Huddling chamber forces are necessary for brisk valve operation to prevent exceeding theoverpressure valve specifications. Overpressure is the pressure tolerance between the set pressureand the pressure at full flow discharge. A PZV must reach 100% flow rate at or before exceedingallowable overpressure. Huddling chamber influence on fluid flow direction ensuresoverpressure compliance.

These same forces also prevent the valve from reseating until the pressure under the disk fallsbelow the initial opening pressure or set pressure. Huddling chamber configuration directs theforces derived from fluid inertia to oppose the mechanical stem forces. The disk cannot reseatuntil the initial mechanical stem force overcomes both of the following forces:

• Process pressure force

• Fluid inertia force

The fluid inertia force is not applied until fluid flow begins. Thus, once the disk comes off theseat, the upward force is increased by the force developed by the fluid's inertia. The disk cannotreseat until the process pressure plus the force of the fluid inertia is less than the original setpressure. In summary, the forces that cause accelerated upward disk lift also cause delayed diskclosure, and lower valve reseating pressure.




Nozzle rings are commonly called blowdown rings because they not only direct streamlinehuddling chamber fluid flow, as shown in Figure 15, but also to amplify disk opening (at lift) byusing fluid momentum to accelerate the lifting action. This same fluid inertia will opposemechanical stem forces to delay the PZV reseating until the pressure has fallen below the setpressure.

Blowdown rings are either fixed or adjustable depending on valve class and manufacturer. PZVshaving adjustable blowdown rings provide lock screw ring retainers with tamper seals.Blowdown adjustments are a certified setting on a PZV, and therefore require tamper seals.Blowdown ring performance is determined by reseating pressure versus the set pressure. Aminimum blowdown ring adjustment would produce 0% blowdown. Blowdown above 5% isunacceptable. Figure 16 shows typical blowdown adjustments. In the valve on the left, theblowdown ring is set all the way up so that its upper surface touches the disk holder. Thisposition would increase the blowdown. In the valve on the right, the blowdown ring has beenmoved down and will decrease the blowdown.

Disk

Disk

Holder

Blowdown

Ring

Blowdown Ring Adjustment

For Set Pressure Test

Blowdown Ring Adjustment

For Service Operation-Vapors

Figure 16. Typical Blowdown Adjustments




Action

Pressure relief valves fall within two operational classes: Pop Action and Modulating. Pop actionPZVs drive to full release on initiation. Modulating PZVs release in proportion to the level ofoverpressure driving the disk. All PZVs reach full lift (100% lift) before reaching allowableoverpressure. Pop action PZVs release more fluid during a blowdown period and generally takelonger to reseat than modulating PZVs. Modulating PZVs exercise 'on demand' releases. If theprocess pressure continues to rise the valve discharge rate increases. If the process pressure startsto fall during release the PZV starts to reseat.

Process considerations will dictate the choice of characteristic. Use modulating PZVs in surgingprocesses to allow time for the process to recover before implementing full release.Compressors, distillation towers, pump discharges and the like are candidates for modulatingPZVs. Thermal relief, liquid and heavy fluid services are also candidates for modulating PZVs.

Run-away processes are always candidates for pop action PZVs. If the emergency is self-sustaining (i.e., it cannot be controlled by external control systems), use pop action PZVs. Alsouse pop action PZVs for gas and vapor systems which must respond quickly to high volumereleases. In addition to equipment protection, the PZV might stabilize the system until normalcontrol can be regained.

It is important to realize that PZV selection characteristics are not always governed on the basisof size. Valve sizing will establish the amount of fluid a PZV must release. Characteristicselection is based on how quickly full release should be implemented. Full release will always beimplemented within allowable overpressure limits.

Bellows Type Spring-loaded PZVs

Spring-loaded PZVs are either Conventional or Bellow types. Conventional PZVs are used inclean services where little or no backpressure is present. This means most conventional PZVswill discharge to atmosphere or low-pressure collection systems. Conventional PZVs used in gasor vapor service will range in size from 2-inches to 10-inches and have set pressures rangingfrom 30 psig to 300 psig. Conventional PZVs do not perform satisfactorily when connected todischarge piping that causes backpressure to develop as the process fluid flows through the valveand its discharge piping. This type of backpressure is called built-up backpressure. It will bediscussed in the following section on Principles of Operation.




A bellows type PZV is a conventional PZV with bellows protection around its disk and stemwithin the fluid cavity. Bellows PZVs also have sealed bonnets. Bonnets on all bellows PZVsmust be vented to atmosphere or piped to funnel vents on low-pressure collection systems. Extraprotection can be provided by incorporating a balancing piston inside of the bellows. Should thebellows leak, the balancing piston will neutralize backpressure forces. Bellows must be used iftotal backpressure exceeds 10% of set pressure. Balancing bellows must be specified with allSpring-Loaded PZVs used on corrosive, dirty, and flammable processes. Use balanced bellowsPZVs when discharging into a major plant vent header.

Bellows

The bellows used in PZVs is a device which seals all stem-disk related parts between theattachment of the stem guide to the valve body and a second attachment on the disk holder.Every bellows has a distinctive 'accordion-like' shape. This shape allows flexure, and isolatesturbulent fluid forces and/or backpressure forces acting within the flow cavity from changingstem-to-disk driving forces. Bellows also isolate fluids in the valve flow cavity from contact withparts contained inside the bellows.

Balancing Piston

An auxiliary balancing piston is a common accessory in bellows seal PZVs. A balancing pistonis a machined surface extending outward from the stem retainer within the guide. See Figure 16.It provides a lifting surface inside a bellows. Lifting surface area provided on a balancing pistonequals or exceeds the average sectional area across the bellows and/or the surface area above thedisk retainer. If a bellows leaks, fluid forces acting downward onto a disk retainer are balancedby equal lifting forces acting upward against the bottom of a piston.

Note that atmospheric pressure within a PZV bonnet extends into a bellows via stemretainer/guide separation discussed earlier. Bellows are used to seal the valve fluid flow cavityfrom the bonnet, stem, and disk, but include bonnet atmosphere inside the bellows. Balancingpistons also offset the effects of bonnet pressure acting on spring forces.




VentSpring

Cap

Bonnet

BodyAdjusting

Ring

Nozzle

Balancing

Piston

Sealing

Bellows

Figure 17. Typical Bellows Seal PZV

Although there is a pressure balance between the bonnet and the bellows area, the pressure pathalong the stem within its guide acts as a restricting orifice. Pressure balance occurs over timeacross the restriction, it is not instantaneous. Balancing pistons compensate instantaneously toany change within a bellows cavity.

The term 'balanced bellows' means that a balancing piston is included within a bellows. Theexpression 'bellows seal' is used when balancing pistons are not used. Bellows seal valvesprovide protection against backpressure, but they do not provide the internal pressure forcebalance protection afforded by balanced bellows.




Protection from fluid corrosion and/or contamination is always lost when a bellows leaks orruptures. Therefore, while bellows type PZVs provide significant operational protection, theyshould not have longer T&I periods than conventional PZVs.

Principles of Operation




PILOT-OPERATED PRESSURE RELIEF VALVE

A Pilot-Operated Pressure Relief Valve consists of two major components: the main valve andthe pilot valve. Opening and closing actions of a main valve are controlled by a pilot valve.

From the characteristics of valve action, there are two types of pilot valves in existence today:the pop action pilot valve and the modulating action pilot valve. The pop action pilot valve popsfully open and actuates the main valve to full lift without overpressure. The modulating actionpilot valve opens slowly in proportion to the system pressure in the first couple percents ofoverpressure. The main valve also opens slowly, but reaches full lift at about 4 to 5% ofoverpressure.

From venting characteristics, the pilot valve can be either flowing or non-flowing. A flowingpilot valve allows system medium to continuously flow out of the pilot valve when the mainvalve is relieving. A non-flowing pilot valve, on the other hand, stops venting of the systemmedium when the pilot valve opens and the main valve is relieving.

In all, there are four types of pilot valves in use in industry today:

• Pop action flowing pilot

• Pop action non-flowing pilot

• Modulating action flowing pilot

• Modulating action non-flowing pilot

Principles of Operation

Figure 26 shows a pilot-operated pressure relief valve in a closed position and in a relievingposition. As the system is being pressurized, the pressure pick-up transmits the pressure fromthe inlet of the main valve through the pilot valve and into the dome of the main valve. Thepressure area of the piston in the dome of the main valve is greater than the inlet seat area.Greater pressure force acting upon the top of the piston in the dome pushes the piston downwardfirmly against the seat on the nozzle in the main valve and the valve is closed. Moreover, thegreater the system pressure the greater the force holding the piston on the main valve seat.




Figure 26. Pilot-Operated Pressure Relief Valve Operation Principles

When the system is pressurized, the system medium is also acting on the pilot valve disk. Assystem pressure rises to the set point, pressure force overcomes the spring force in the pilot valveand the pilot valve opens. Once the pilot valve opens, system medium in the dome is evacuatedthrough the pilot valve vent. The downward pressure force on the dome side of piston becomessmaller than the upward pressure force on the inlet side of the piston allowing the main valve toopen thus relieving the system overpressure.

As system pressure drops below the valve reseat pressure, the spring force in the pilot valveovercomes the system pressure force on the disk and the pilot valve closes. Once the pilot valvecloses, the pilot valve redirects the system medium back into the main valve dome. Whenpressure force on top of the piston becomes greater then the upward pressure force on the bottomof the piston, the piston is pushed down against the nozzle to close the main valve and returns thesystem to the normal operating mode.




Main Valve Design

A typical main valve consists of a body, a cover, a guide, a piston, a nozzle and a preload spring.Figure 27 illustrates a typical main valve. The majority of main valves use elastomer O-ringseals in the cover, guide, piston, nozzle and seat. Therefore, the temperature limit of the pilotoperated relief valve is determined by the temperature limit of the elastomer material.

Spring

Cover

Body

Piston

Nozzle

Guide

Sensor For Pressure Pickup

Nozzle Seal

Guide Seal

Piston Seal

Figure 27. Main Valve Design

Pressure pick up can be either at the inlet neck of the body as shown in Figure 20 or at a remotelocation in the system to be protected. System medium, and thus system pressure, enters thepressure pick up and channels through the pilot valve to the top side (or dome side) of the piston.Piston pressure area on the top side is much greater than the seat area on the bottom. Hence, thehigher pressure force on the top pushes the piston downward against the nozzle seat to seal offthe system medium. Opening of the pilot valve reduces the pressure in the dome and, hencereduces the downward force on the piston. Once upward force on the bottom of the pistonbecomes higher than the downward force on top of the piston, the main valve opens. Closing ofthe pilot valve reintroduces system pressure into the dome and the higher downward pressureforce pushes the piston down to close the main valve.

The preload spring ensures that the valve is in the closed position when the system is beingpressurized. In addition, the spring also improves the stability of the valve when it is subjectedto backpressure.




Pop Action Non-flowing Pilot Valve

The most commonly used pilot valve is the pop action non-flowing type as illustrated in Figure28. Port A is connected to the pressure pick-up by either a piece of tube or a piece of pipe.Pressure pick-up can be located either at the main valve inlet neck or at remote locations on thepipe or vessel. Port B is connected to the dome of the main valve by a piece of tubing.

Figure 28. Pop Action Non-flowing Pilot Valve




System medium is picked up by the pressure pick-up and channels to Port A. From Port A,system medium flows through the strainer and along the inside surfaces of the adjuster and seatinsert to Port B. From Port B, system medium travels to the dome of the main valve through apiece of tubing. System medium in the dome generates a force to push the piston down to closethe main valve. System medium also flows through the clearance between the upper seat andspacer rod to the pilot disk. The disk seals off the system medium by a predetermined springforce. The magnitude of the spring force which can be adjusted by turning the spring adjustingscrew. This adjustment determines the set pressure of the valve.

need flow schematic diagram 1. 2. 3. -according to Aramco review

When system pressure reaches the relief valve set pressure, the pilot valve pops open and thesystem medium in the dome flows through Port B, through the pilot seat and finally vents outthrough Port C. Reduction of the dome pressure causes the main valve to pop open. When thepilot valve opens, the lower disk moves up with the spacer rod and seats against the seat insert.This stops the system medium from flowing beyond the seat insert, i.e., once the pilot valve popsopen, flow through the pilot valve is stopped. Therefore, the pilot valve is called a pop actionnon-flowing PZV.

Once the system pressure drops down to the relief valve reseat pressure, the pilot valve closesand pushes the spacer rod down. The spacer rod, in turn pushes the lower disk down to let thesystem medium flow from Port A to Port B and then, to the dome to close the main valve.

Valve blowdown is determined by the location of the blowdown adjuster. Blowdown isincreased by moving the blowdown adjuster down. Conversely, moving the blowdown adjusterup will shorten the blowdown of the valve.

Pop Action Flowing Pilot Valve

A schematic diagram of a pop action flowing pilot valve is shown in Figure 29. When thesystem pressure is below the set pressure of the valve, the pilot disk is pushed down against theseat by the pilot spring. Pilot supply line channels system pressure to the pilot valve body andthe main valve dome. Once the system pressure reaches the set pressure, the pilot disk popsopen and allows the dome pressure to drop causing the main valve piston to open. While thepilot valve is open and relieving, system medium is continuously flowing through the pilotsupply line and out through the pilot exhaust.




Pilot Supply Line

Optional Pilot Filter

Piston

Seat

Internal Pressure Pickup

Outlet

Inlet

Pilot Seat

Dome

Main Valve

Pilot Exhaust

External Blowdown Adjustment

Pilot Spring

Pilot Disk

Figure 29. Pop Action Flowing Pilot Valve

Once the system pressure drops down to the reseat pressure, the pilot valve closes. Closing ofthe pilot valve causes the dome pressure to increase and eventually closes the main valve.Blowdown of the valve can be adjusted by either opening or closing the variable orificeconnected to the pilot supply line.

By removing the spacer rod, seat insert and lower disk and adding a variable orifice to Port B ofFigure 28, the pop action non-flowing pilot valve is converted to a pop action flowing pilotvalve.




Modulating Action Flowing Pilot Valve

The modulating action flowing pilot valve design is more complicated than the pop action pilotvalve described earlier. System medium is channeled to the main valve dome through the pilotsupply line and variable orifice as shown in Figure 30. The pilot disk is pushed down against theseat by predetermined spring force. When total system pressure force on the bottom side of thepilot piston and sense diaphragm is greater than the spring force, the pilot disk opens and allowsthe system medium to evacuate from the dome to the pilot vent through the pilot seat. The pilotdisk opening force is primarily developed from the pressure under the sense diaphragm and,therefore, increases and decreases slowly providing the pilot disk, as well as the main disk, witha modulating action. While the main valve and the pilot valve are open and relieving, the systemmedium is continuously flowing out through the pilot vent and, hence it is a flowing pilot valve.




Optional Pilot Filter

Piston

Seat

Internal Pressure Pickup

Outlet

Inlet

Dome

Main Valve

Variable Orifice

Pilot Supply Line

Seat

Pilot Stem

Sense Diaphragm

Sense Chamber

Pilot Exhaust (Tubed to Main Valve Outlet)

Pilot Piston

Figure 30. Modulating Action Flowing Pilot Valve

Modulating Action Non-flowing Pilot Valve

Among the four types of pilot valves in use today, the modulating action non-flowing pilot valveis the most complicated. The system medium flows to the main valve dome through the pilotsupply line, the center passageway of the feedback piston and the inlet seat as shown in Figure31. The pilot outlet seat is closed by the spring inside the feedback piston. When systempressure is below the set pressure, the main pilot spring pushes the feedback piston down to openthe inlet seat and close the outlet seat. Opening the inlet seat and closing the outlet seat directsthe system medium to the dome to close the main valve.




Figure 31. Modulating Action Non-flowing Pilot Valve

When system pressure increases, the upward pressure force under the sense diaphragm increasesaccordingly and carries the feedback piston upward. When the system pressure reaches the setpressure, the feedback piston moves up sufficiently to close the inlet seat and open the outletseat. Closing the inlet seat and opening the outlet seat causes the dome pressure to drop andopens the main valve. Simultaneously, the inlet seat is closed which stops the system mediumfrom continuously flowing out of the pilot valve when main valve is relieving.

Once pilot spring is adjusted, opening and closing of the pilot valve depends on the pressureforce underneath the sense diaphragm. The pressure force on the sense diaphragm rises and fallsslowly which provides both the pilot valve and the main valve with a modulating action.




Low-Pressure Pilot Operated Relief Valve

The pilot operated relief valves described in earlier sections are generally used for services withoperating pressures of 15 psig and above. For low pressure applications, such as storage tankprotection, set pressures are mostly below 15 psig and can be as low as a few inches of watercolumn. A typical low pressure pilot operated relief valve is shown in Figure 32.

Figure 32. Low-Pressure Pilot Operated Relief Valve




Main Valve Design

Under low pressure system, the pressure force acting on the main disk is relatively small andtherefore, a diaphragm type main disk is used. The piston type main disk used in a high pressurevalve cannot be used in low pressure valve because the piston is too heavy and the friction forcebetween the piston and the guide is too high for the available pressure force to open and to closethe main disk.

Aside from the fact that the dome of a low pressure valve is formed by a diaphragm and cover,the basic design and the operational principles of the low pressure valve are identical to the highpressure valve.

Pilot Valve Design

A typical low pressure pilot valve is a modulating action flowing type (refer to Figure 32).System pressure is channeled to the pilot valve from the pressure pickup on the main valve inletneck. When the system pressure is below the set pressure of the valve, the pilot disk is pusheddown against pilot seat by the spring to prevent system medium from escaping. System pressurefills the pilot valve and the dome of the main valve.

When system pressure reaches the set pressure of the valve, the sum of the upward pressureforces on the pilot disk and on the sense diaphragm becomes greater then the spring force andcauses the pilot valve to open. Opening of the pilot valve reduces the dome pressure through thepilot exhaust and causes the main valve to open. While the pilot valve remains open, pressure inthe boost cavity and the sense cavity is also being evacuated through a fixed orifice and avariable orifice. In the mean time, system medium is continuously flowing out from the pressurepickup through the orifices and to the pilot exhaust.

As soon as system pressure drops down to the reseat pressure, spring force overcomes totalupward pressure forces on the pilot disk and the boost diaphragm and closes the pilot valve.Closing of the pilot valve redirects system pressure to the dome to close the main valve.Blowdown can be lengthened or shortened by adjusting the variable orifice.




GLOSSARY

adhesive bond A bond produced by hydrocarbon migration through a valve sealarea.

balanced bellows A balancing piston included within a bellows.

balancing piston A machined surface extending outward from the steam retainerwithin the guide which provides a lifting surface inside thebellows.

bellows seal An accordion-shaped device which attaches to the valve fluidcavity.

blowdown A generic term which means a PZV has discharged. Thedifference between the set pressure and the closing pressure of apressure-relief valve, expressed as percent of set pressure.

blowdown rings Devices which disrupt streamline huddling chamber fluid flowwhen a disk starts to close.

bubble-tight seal A seal that allows 0 bubbles per minute at CDTP.

blowdown period The amount of time for a valve to open, release and close.

chemical bond A bond formed by electrolytic forces generated when freeelectrons cross seal boundaries on metal to metal contactsurfaces.

huddling chamber The area between the disk and the seat when the PZV opens.

modulating action valve A valve which opens slowly in proportion to the system pressurein the first couple percents of overpressure.

pilot-operated valve Consists of a main valve and a pilot valve. Opening and closingactions of the main valve are controlled by the pilot valve.

pop action valve A valve which pops fully open.

seat bonding When a disk and seat become chemically bonded.

simmer Exhaust valve leakage that occurs just before the processpressure reaches the pressure set point.




soft seats An O-ring installed between a metal disk and a metal seat.

stem guide Used to ensure process fluid turbulence does not affect steamalignment.

stem retainer The section of stem between the lower spring retainer and disk.

tamper seal Used to ensure that valve adjustments have not been changed.

Documents

Principles Of Pressure Relief Valve Operation