evaluating installed pzv performance

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 : Instrumentation For additional information on this subject, contactFile Reference: PCI11004 D.W. Buerkel on 874-7339

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Determine Whether An Installed PressureRelief Valve Complies With Requirements

Engineering Encyclopedia Instrumentation


Saudi Aramco DeskTop Standards

Content Page

DETERMINING INSTALLED RELIEF VALVE SYSTEM'S COMPLIANCE WITHDESIGN REQUIREMENTS................................................................................................ 2

Certification .............................................................................................................. 2

Flange Rating ............................................................................................................ 2

ISS............................................................................................................................ 2

Vacuum Relief Device ............................................................................................... 2

Other Relief Devices ...................................................................................... 2

Accessories ............................................................................................................... 2

DETERMINING SELECTED DEVICE’S COMPLIANCE WITH TYPESELECTION REQUIREMENTS ......................................................................................... 2

Conventional Pressure Relief Valves.......................................................................... 2

Balanced Bellows Type Pressure Relief Valves .......................................................... 2

Bellows with Balancing Piston Type Pressure Relief Valves....................................... 2

Back Pressure Limitations on Bellows Type Pressure Relief Valves ........................... 2

Pilot Operated ........................................................................................................... 2

Rupture Discs............................................................................................................ 2

Vacuum Relie............................................................................................................ 2

DETERMINING SELECTED MATERIAL’S COMPLIANCE WITH MATERIALSELECTION REQUIREMENTS ......................................................................................... 2

API Standard 526...................................................................................................... 2

Spring-Loaded Pressure Relief Valves............................................................ 2

Pilot-Operated Pressure Valves...................................................................... 2

SAES-J-600 .............................................................................................................. 2

Bodies ........................................................................................................... 2

Springs .......................................................................................................... 2

Bellows.......................................................................................................... 2

WORK AID 1: 1RESOURCES AND GUIDELINES FOR DETERMININGINSTALLED RELIEF VALVE SYSTEM’S COMPLIANCE WITH DESIGNREQUIREMENTS ............................................................................................................... 2



Saudi Aramco DeskTop Standards

WORK AID 2: RESOURCES AND GUIDELINES FOR DETERMININGSELECTED DEVICES’ COMPLIANCE WITH TYPE SELECTIONREQUIREMENTS ............................................................................................................... 2

WORK AID 3: RESOURCES AND GUIDELINES FOR DETERMININGSELECTED MATERIAL’S COMPLIANCE WITH MATERIAL SELECTIONREQUIREMENTS ............................................................................................................... 2

GLOSSARY......................................................................................................................... 2

BIBLIOGRAPHY ................................................................................................................ 2

ADDENDUM....................................................................................................................... 2

Table of Figures Page

Figure 1. Certification Stamps and Nameplate........................................................... 2

Figure 2. Standard Effective Areas and Letter Designations...................................... 2

Figure 3. Steam, Water, and Oil Pressure Temperature Ratings for Carbon SteelFlanges and Flanged Fittings...................................................................... 2

Figure 4. Typical Effects of Superimposed Back Pressure on the OpeningPressure of Conventional Pressure Relief Valves1...................................... 2

Figure 5. Balanced Bellows Safety Relief Valve........................................................ 2

Figure 6. Effect of Back Pressure on the Set Pressure of Balanced Bellows TypePZVs1....................................................................................................... 2

Figure 7. Bellows Safety Relief Valve with a Balancing Piston.................................. 2

Figure 8. Effect of Back Pressure on the Set Pressure of Balanced Piston PZVs1...... 2

Figure 9. Rupture Disk ............................................................................................. 2



Saudi Aramco DeskTop Standards 1

DETERMINING INSTALLED RELIEF VALVE SYSTEM'S COMPLIANCE WITHDESIGN REQUIREMENTS

SAES-J-600 designates the mandatory requirements that govern the design and installation ofPZVs, except for residential and commercial water heating equipment. Paragraph 4 of SAES-J-600 identifies SADP-J-600 as a source of additional design information. SADP-J-600 alsocontains a Safety Relief Valve System Design Checklist, which can be used to review new andmodified PZV installations. SAES-J-600 identifies the following design criteria, which must beapplied to determine if a given design complies with Saudi Aramco standards.

• Certification

• Flange rating

• Specific design requirements (Instrument Specification Sheet)

• Vacuum relief devices

• Other pressure relief devices

Each of these criteria play a part in establishing acceptable designs for Saudi Aramco applications,and they are described on the following pages.

Certification

PZV certification is a contractual agreement between the National Board of Boiler and PressureVessel Inspectors (National Board) and the PZV manufacturers and repair facilities. Certificationauthorizes the use of an official “stamp” to certify that the PZV meets all requirements of theASME pressure vessel codes and the National Board inspection procedures. Figure 1 shows fourcertification stamps and a PZV nameplate that bears one of the certification stamps.




Figure 1. Certification Stamps and Nameplate




SAES-J-600 also states that the design shall be in compliance with API Std 526 (Addendum 1),which shows the effective orifice areas of PZVs based on letter designations as shown in Figure 2.

DesignationEffective Orifice Area

(square inches)

D 0.110

E 0.196

F 0.307

G 0.503

H 0.785

J 1.287

K 1.838

L 2.853

M 3.60

N 4.34

P 6.38

Q 11.05

R 16.0

T 26.0

Figure 2. Standard Effective Areas and Letter Designations




Flange Rating

Flange ratings are pressure designations (ANSI B16.5) that are the recommended ratings at thetemperatures shown in the table in Figure 3. The table shows recommended ratings for varioustemperatures.

Steam and water pressure rating (primary) psig 150 300 400 600 900 1500

Service Temperature, °FMax steam, water, and oil pressures, psig

(non-shock)100 230 500 670 1000 1500 2500150 220 480 640 960 1440 2400200 210 465 620 930 1395 2325250 200 450 600 900 1350 2250300 190 435 580 870 1305 2175350 180 420 560 840 1260 2100400 170 405 540 810 1215 2025450 160 390 520 780 1170 1950500 150a 375 500 750 1125 1875550 140 360 480 720 1080 1800600 130 345 460 690 1035 1725650 120 330 440 660 990 1650700 110 315 420 630 945 1575750 100 300a 400a 600a 900a 1500a

Maximum steam and water pressure, psig800 85 250 335 500 750 1250850 70 300 270 400 600 1000

Maximum oil pressures, psig800 92 275 370 550 830 1380850 82 245 330 490 740 1230900 70 210 280 420 630 1050950 55 165 220 330 495 825

1000 40 120 160 240 360 600

Figure 3. Steam, Water, and Oil Pressure TemperatureRatings for Carbon Steel Flanges and Flanged Fittings

Paragraphs 4.1.2 and 8.11 of SAES-J-600 state that the minimum flange rating for inlet valvesshall be ANSI B16.5 Class 300 for all PZVs smaller than 16 inch nominal inlet size. The oneexception is for pilot operated PZVs for atmospheric and low pressure tanks. These PZVs shouldbe ANSI B16.5 Class 150 Flat Faced flanges.




ISS

Paragraph 4.1.3 of SAES-J-600 states that specific design requirements and options should belisted on the ISS.

Vacuum Relief Device

Paragraph 4.1.4 of SAES-J-600 states that relief devices for vacuum services should comply withthe requirements for API Standards 620 and 2000 (Addenda 2 and 3, respectively).

Other Relief Devices

Paragraph 4.1.5 of SAES-J-600 states that other pressure relief devices, such as thermal reliefvalves, pilot operated relief valves and rupture discs should comply with the appropriate ASMErequirements that are consistent with the application.

Accessories

Paragraph 4.2 of SAES-J-600 concerns accessories such as lifting levers and test gags. Therequirements for lifting levers can be found in ASME Code Sections I and VIII.

For vessels that are subject to the requirements of ASME Power Boiler Code Section I, PZVsmust be specified with test gags to permit testing in place. After the test is complete, the testgags should be removed.




DETERMINING SELECTED DEVICE’S COMPLIANCE WITH TYPE SELECTIONREQUIREMENTS

The type of relief valve that is selected depends on the effect of back pressure and corrosion onthe valve’s ability to reliably provide the required relieving capacity. This section discusses theselection requirements for the following safety relief valves.

• Conventional

• Bellows type

• Bellows type with balancing piston

• Pilot operated

• Rupture Discs

• Vacuum Relief

Conventional Pressure Relief Valves

Conventional PZVs are used when the discharge is either through a short tail pipe that vents tothe atmosphere or through a low-pressure manifold system that carries the discharged fluid of oneor more valves to a remote location for disposal. Normally, the spring force is the differentialbetween the set pressure and the atmospheric pressure. The set pressure, therefore, will beincreased by superimposed back pressure unless the spring force is adjusted accordingly. Built-up back pressure may also affect the PZV's performance. As a result, when one or more valvesdischarge into a common manifold, the impact of downstream pressure should be determined byreferring to the PZV manufacturer's catalog.

The interaction of forces within the valve and the effects of back pressure are shown in Figure 4.




Sp

rin

g FS

PB

PBPB

PV

Spring BonnetVented to Atmosphere

Backpressure DecreasesSet Pressure

VentedSpringBonnet

Disk

Sp

rin

g FS

PB

PBPB

PV

Spring BonnetVented to Valve Atmosphere

Backpressure IncreasesSet Pressure

SpringBonnet

Vent

DiskGuide

Disk

PB

PV AN = FS - PB (AD - AN) PV AN = FS + PB AN

AD > AN

Where:

AD = disk area

AN = nozzle seat area

FS = spring force

PV = vessel pressure in psig

PB = superimposed back pressure in psig

Figure 4. Typical Effects of Superimposed Back Pressureon the Opening Pressure of Conventional Pressure Relief Valves1




Conventional PZVs have a disk area, AD, that is greater then the nozzle seat area, AN. If thespring bonnet is vented to the atmosphere (Figure 4A), the back pressure acts with the vesselpressure, PV, to overcome the spring force, FS. In effect, this situation makes the openingpressure less than it would be if the valve discharged to the atmosphere.

If the spring bonnet is vented to the valve discharge (Figure 4B), not vented to the atmosphere,the back pressure acts with the spring force to increase the opening pressure. Any variation in thesuperimposed back pressure, PB, will directly affect the opening pressure; therefore, PB shouldevaluated in the system design.

Saudi Aramco has established criteria regarding the combined effect of superimposed backpressure and built-up back pressure on the performance characteristics of valves. Paragraph 5.1of SAES-J-600 states that conventional PZVs shall be used when the following conditions aremet:

5.1.1. The sum of the maximum variable superimposed back pressure plusthe built-up back pressure is less than 10 percent of the set pressure.

5.1.2. Fouling and corrosive conditions are not expected.

The criteria in Paragraph 5.1 of SAES-J-600 can help to identify possible errors in the selection ofPZVs. One example of an application that would violate this standard is the use of a conventionalPZV in wet sour service, such as a production trap.

Balanced Bellows Type Pressure Relief Valves

A bellows type is a conventional PZV with bellows protection around its disk and stem within thefluid cavity (see Figure 5). The bellows is a device which seals all stem-disk related parts betweenthe attachment of the stem guide to the valve body and a second attachment on the disk holder.The bellows' accordion-like shape allows flexure, and it isolates turbulent fluid forces and/orbackpressure forces that act within the flow cavity. Bellows also isolate fluids in the valve flowcavity from contact with the parts that are contained inside the bellows.

Bellows PZVs also have sealed bonnets. Bonnets on all bellows PZVs must be vented toatmosphere or piped to funnel vents on low-pressure collection systems. Bellows must be used iftotal backpressure exceeds 10% of set pressure. Balancing bellows must be specified with allspring-loaded PZVs for corrosive, dirty, and flammable processes. Balanced bellows PZVsshould be used when discharging into a major plant vent header. Unbalanced bellows valves maybe specified when only corrosion protection is needed.




Cap

Bonnet

Body

Stem

Adjusting

Screw

Spring

Bellows

Nozzle

Blowdown•

Ring

Disk

Bonnet

Vent

Vent

Figure 5. Balanced Bellows Safety Relief Valve

The design of balanced bellows type pressure relief valves reduces the effect of back pressure onthe set pressure (see Figure 6). The effective bellows area, AB, is the same as the nozzle seatarea. The arrangement of the bellows in the valve prevents the back pressure from acting on thetop side of the disk within AB. The disk area, AD, that extends beyond the bellows and theopposing nozzle seat area cancel the effect of the back pressure on the valve disk so that there areno unbalanced forces under any downstream pressure variations.

The design also minimizes the effect of built-up back pressure on performance characteristics suchas opening and closing pressure, lift, and relieving capacity.




AB = ANWhere:

AB = effective bellows areaAD = disk areaAN = nozzle areaPV = vessel pressure gaugePB = superimposed back pressure in psigFS = spring force

Figure 6. Effect of Back Pressure on the Set Pressure of Balanced Bellows Type PZVs1

Paragraph 5.2 of SAES-J-600 states that bellows type PZVs shall be used when the followingconditions are met:

5.2.1. The sum of the variable superimposed back pressure plus the built-upback pressure, per paragraph 5.1.1, exceeds 10 percent of the setpressure.

5.2.2 Fouling and corrosive conditions are expected and protection cannotbe afforded by using alternative materials or devices.

The criteria in Paragraph 5.2 of SAES-J-600 can help to identify possible errors in the selection ofPZVs. One example of an application that would violate this standard would be a steam reliefvalve that is vented to the atmosphere.




Bellows with Balancing Piston Type Pressure Relief Valves

Extra protection can be provided by incorporating a balancing piston inside of the bellows (Figure7). If the bellows leak, the balancing piston will neutralize backpressure forces.

Figure 7. Bellows Safety Relief Valve with a Balancing Piston

As shown in Figure 8, the top face of the piston is vented so that the back pressure on theopposing faces of the valve disk are canceled. The top face of the piston has area, AP, which isthe same as the nozzle seat area, AN.




Disk

Balanced Disk And

Vented Piston Type

Spring

Bonnet Vent

P P B

P B

P v

F s

Piston Vent

Vent

B P B P

Pis

ton

AP = AN

Where:

AB = effective bellows area

AD = disk area

AP = piston area (top)

AN = nozzle area

PV = vessel pressure gauge

PB = superimposed back pressure in psig

FS = spring force

Figure 8. Effect of Back Pressure on the Set Pressure of Balanced Piston PZVs1

Paragraph 5.3 of SAES-J-600 states that bellows type PZVs shall be equipped with auxiliarybalancing pistons when the valves are used in fouling or corrosive service.




Back Pressure Limitations on Bellows Type Pressure Relief Valves

Paragraph 5.4 of SAES-J-600 states that the total back pressure on bellows type PZVs shall belimited to the smaller of the following two determinations:

5.4.1 Relieving Capacity Restriction - Total backpressure shall not reducedifferential pressure across the PZV to a value that would limit PZVrelieving capacity to less than design capacity. For gas services, thiscapacity corresponds to a total backpressure of approximately 50% ofset pressure.

5.4.2 Bellows Mechanical Strength - Total backpressure shall not exceed thatwhich the mechanical strength of the bellows can support. Vendor’srecom-mendation shall be followed.

Pilot Operated

The principle of operation of pilot operated PZVs was described in detail in Module 2. Basically,there are four types of pilot operated valves in use today:

• Pop action flowing pilot

• Pop action non-flowing pilot

• Modulating action flowing pilot

• Modulating action non-flowing pilot

A flowing pilot valve allows system medium to continuously flow out of the pilot valve when themain valve is relieving. A non-flowing pilot valve stops venting of the system medium when thepilot valve opens and the main valve is relieving. Paragraph 5.5 of SAES-J-600 states that allpilot operated PZVs shall have non-flowing pilots, and they shall be furnished with testconnections. Paragraph 5.5 also states that:

• The pilot pressure connection shall be external to the PZV and it shall beattached to the tank or vessel. Integral pilot connections shall not be used.

• Pilot operated valves are limited to sweet, non-corrosive, clean gas services.

• Other requirements may be imposed as each proposed case may warrant.




• Connecting flanges for all pilot operated valves for atmospheric and lowpressure tanks shall be ANSI B16.5 Class 150 Flat Faced flanges. (Paragraph8.11)

The application of pilot operated valves for any service that does not meet the criteria aboverequires written approval from the Manager, Process & Control Systems Department, Dhahran.The requirements that must be met for such approval are listed in Paragraph 5.5 of SAES-J-600(Addendum 2).

Rupture Discs

A rupture disk is a pre-bulged solid metal disk that is designed to burst when it is overpressuredon the concave side (Figure 9). A rupture disk may have a flat seat or an angular seat. A rupturedisk may have a satisfactory service life when it operates at 70 percent or less of its rated burstpressure and when limited pressure cycling and temperature variations are present.




Figure 9. Rupture Disk

Paragraph 5.6 of SAES-J-600 states that rupture discs may be considered in special cases as asubstitute for PZVs or to isolate PZVs from materials that can cause damage or interfere withPZV operation.

In general, the use of rupture discs is an advanced topic in PZV sizing and selection so it is notreviewed in this course.

Vacuum Relie

Paragraph 5.7 of SAES-J-600 states that for applications of vacuum relief devices refer to APIStd 620 and 2000.




DETERMINING SELECTED MATERIAL’S COMPLIANCE WITH MATERIALSELECTION REQUIREMENTS

Material requirements mainly concern corrosion and fouling problems. Paragraph 6.1 of SAES-J-600 states that the materials for Saudi Aramco PZV applications shall be selected in accordancewith API Standard 526 (Addendum 1). The material requirements in API Standard 526 and thematerial restrictions and exceptions in SAES-J-600 are described below.

API Standard 526

Materials for spring-loaded and pilot-operated pressure relief valves should be selected inaccordance with the required temperature ranges shown in Tables 2 through 29 of API Standard526.

Spring-Loaded Pressure Relief Valves

The body and bonnet materials for spring-loaded relief valves are listed in Tables 2 through 15 ofAPI Standard 526. The Standard states that these materials shall be equivalent to or exceed thefollowing types and grades:

Carbon steel ASME SA 216, Grade WCBChromium molybdenum steel ASME SA 217, Grade WC6Austenetic stainless steel ASME SA 351, Grade CF8M

The body and bonnet may be of different materials, but they must meet the minimum pressure andtemperature requirements in the standard. Materials for the internal parts of the valve should bein accordance with the manufacturer's standards for the temperature and service that are indicatedon the ISS.

Pilot-Operated Pressure Valves

The body material for pilot-operated relief valves are listed in Tables 16 through 29 of APIStandard 526. These materials shall be equivalent to or exceed the following types and grades:

Carbon steel ASME SA 216, Grade WCBAustenetic stainless steel ASME SA 351, Grade CF8M

Materials for the pilot and internal parts of the valve should be in accordance with themanufacturer's standards for the temperature and service that are indicated on the ISS.




SAES-J-600

Exceptions to API Standard 526 are contained in Paragraphs 6.2 through 6.5 of SAES-J-600.These material exceptions are described below.

Bodies

The exceptions to API Standard 526 for valve bodies are listed in Paragraph 6.2 of SAES-J-600which is shown in Work Aid 3. The exceptions concern the flowing conditions.

• Valve bodies for hydrocarbon service at temperatures above 0°C (32°F)

• Valve bodies for non-sour water service

• Valve bodies for service at temperatures from 0 to -46°C (+32 to -50°F)

• Valve bodies for service at temperatures below -46°C (-50°F)

Springs

The exceptions to API Standard 526 for valve springs in sour service are listed in Paragraph 6.3of SAES-J-600, which is shown in Work Aid 3. The standard states that springs shall beprotected from direct exposure to H2S by using a balanced bellows or upstream rupture disc. Ifthe spring cannot be protected from direct exposure, it should be made of a sulfide stresscorrosion cracking-resistant material in accordance with NACE Standard MR-01-75.

Bellows

The exceptions to API Standard 526 for bellows in sour service are listed in Paragraph 6.4 ofSAES-J-600, which is shown in Work Aid 3. The standard states that bellows for sour service orfor water with more than 200 ppm chloride shall be fabricated from Inconel Alloy 625 ofHastelloy Alloy C-276.




WORK AID 1: 1RESOURCES AND GUIDELINES FOR DETERMININGINSTALLED RELIEF VALVE SYSTEM’S COMPLIANCE WITHDESIGN REQUIREMENTS

This Work Aid provides the procedures, equations, and tables required to determine if the designcriteria in SAES-J-600, Chapter 4 have been met.




WORK AID 2: RESOURCES AND GUIDELINES FOR DETERMININGSELECTED DEVICES’ COMPLIANCE WITH TYPE SELECTIONREQUIREMENTS

This Work Aid provides procedures, and tables required to determine if the "type selectioncriteria" in SAES-J-600, Chapter 5 have been met.




WORK AID 3: RESOURCES AND GUIDELINES FOR DETERMININGSELECTED MATERIAL’S COMPLIANCE WITH MATERIALSELECTION REQUIREMENTS

Use this Work Aid procedure and the appropriate sections of API STANDARD 526 and APISAES-J-600 (Addenda 1 and 2) to determine whether the materials of a selected PZV complieswith material selection requirements.




GLOSSARY

built-up back pressure The increase in pressure in the discharge pressure in the dischargeheader that develops as a result of the pressure in the dischargesystem. It is the sum of the superimposed built-up backpressures.

critical flow rate The flow rate that corresponds to the limiting velocity

critical flow pressureratio

The absolute pressure ratio of the pressure in the throat of a PZVat sonic velocity (Pcf) to the inlet pressure (P1).

critical flow rate The flow rate that corresponds to the limiting velocity.

limiting velocity The maximum mass flow rate through the PZV nozzle, which isequal to the velocity of sound in the flowing media at thatlocation.

originator An engineer who initiates a RV installation or change thatrequires authorization approvals by providing the design andspecification information. He can be an engineer for anydiscipline (i.e.; Operations Engineering, Maintenance Engineer,Inspection Engineer, Projects Engineer, Plant Engineer, etc.)

relieving pressure The total of the set pressure plus overpressure plus atmosphericpressure. For gases and vapors, the relieving pressure isexpressed in absolute units (psia). For liquids, the relievingpressure is expressed in gauge units (psig).

set pressure The inlet gauge pressure at which the pressure relief valve is set toopen under service conditions.

superimposed backpressure

The static pressure that exists at the outlet of a pressure reliefdevice at the time the device is required to operate. It is the resultof pressure in the discharge system coming from other sources,and it may be constant or variable.




BIBLIOGRAPHY

1. API Recommended Practice 520. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries. Part I. American Petroleum Instititute. SixthEdition. 1993




ADDENDUM

1. API Standard 526

2. SAES-J-600, Pressure Relief Devices




Addendum 1, API Standard 526




Addendum 2, SAES-J-600, Pressure Relief Devices

evaluating installed pzv performance

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