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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Comprehensive approach to designing safety valves used in oil and gas plants, featuring case studies throughout the text
Safety Valves in Oil and Gas Plants delivers a comprehensive overview of the various aspects of safety valves in the oil and gas industries, including their design and accessories. To help reinforce learning, case studies included throughout the text and multiple-choice questions and answers are included at the end of each chapter.
Written by an industry veteran with extensive publishing and speaking experience, this book includes information on:
- Design details such as sizing and reaction forces, inspection, maintenance, codes and standards, and packing and preservation
- Daily industrial challenges regarding the resizing of existing pressure safety valves (PSVs) or the proper sizing of new PSVs
- Process safety standards for all new operative installations or existing ones in the chemical industry
- Catalysts that can increase the pressure of fluids, including human error and equipment and component failure
- The role of safety valves in protecting properties, the environment, and human life
This is an excellent reference on the subject for engineers and advanced students in chemical and process engineering, mechanical engineering, piping and valve engineering, and safety and instrument engineering.
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Veröffentlichungsjahr: 2024
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Basic Safety Valves Principles
1.1 Introduction and History
1.2 Maximum Allowable Working Pressure (MAWP)
1.3 The Importance of Safety Valves and Safety Devices
1.4 Overpressure Scenarios and Examples in the Plant
1.5 Variations in Safety/Relief Valves
1.6 Safety Valve Parts
1.7 Safety Valve Dismantling
1.8 Basic Safety Valve Operation
1.9 Safety Valve Essential Pressure Settings
Questions and Answers
Further Reading
2 Design Fundamentals
2.1 Introduction
2.2 Spring Force
2.3 Back Pressure Consideration
2.4 Overpressure and Blowdown
2.5 The Relationship Between the Pressure Values of Safety Valves and Pressure Vessels
2.6 Set Pressure Determination
2.7 Actual Discharge Area Calculation
2.8 Simmer
2.9 Pressure Loss in Inlet Piping
Questions and Answers
Further Reading
3 Safety Valves Features and Accessories
3.1 Introduction
3.2 Full Nozzle vs Seminozzle
3.3 Open vs Closed Bonnet
3.4 Metal Seat vs Soft Seat
3.5 Bellows Seal
3.6 Lifting Lever
3.7 Single vs Dual Trim
3.8 Wires
3.9 Test Gag
3.10 Low-Lift, High-Lift, and Full-Lift Disk
3.11 Balanced Piston
3.12 Cooling/Heating Spacer
3.13 Limit Switch (Position Indicator)
3.14 Pneumatic Actuator
3.15 Stroke Sensor
3.16 Pulsation Damper
3.17 Drain Hole
3.18 Teflon Lining and Coating
3.19 Stellite Coating on Seating Surfaces
Questions and Answers
Further Reading
4 Safety Valves Codes and Standards
4.1 Introduction
4.2 American Society of Mechanical Engineers (ASME)
4.3 American Petroleum Institute (API)
4.4 International Organization for Standardization (ISO)
4.5 British Standards
4.6 Australian Standard
4.7 Canadian Standard
4.8 Other Standards
4.9 National Association of Corrosion Engineers (NACE)
4.10 Pressure Equipment Directive (PED)
4.11 Approval Authorities
Questions and Answers
Further Reading
5 Safety Valves Installation
5.1 Introduction
5.2 Prior to Installation
5.3 Location and Position
5.4 Piping Connection
Questions and Answers
Further Reading
6 Safety Valve Reaction Forces
6.1 Introduction
6.2 Calculations
Questions and Answers
Further Reading
7 Safety Valve Sizing
7.1 Introduction
7.2 Sizing for Gas or Vapor Relief
7.3 Sizing for Steam Relief
7.4 Sizing for Liquid Relief
7.5 Sizing for Two-Phase Liquid/Vapor Relief
7.6 Sizing for Saturated Liquid and Saturated Vapor, Liquid Flashes
7.7 Sizing for Subcooled at the Pressure-Relief Valve Inlet
7.8 Sizing for Fire Case and Hydraulic Expansion
Questions and Answers
Further Reading
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Bellows’ effect on the neutralization of back pressure.
Table 2.2 Back pressure limitations for safety valves.
Table 2.3 Set pressure and maximum accumulated pressure limits for safety va...
Table 2.4 Set pressure determination based on overpressure percentage and MA...
Table 2.5 API 526 standard orifice sizes.
Chapter 4
Table 4.1 API standard orifice sizes.
Table 4.2 Well-known approval bodies in the world.
Chapter 5
Table 5.1 Inlet loss coefficient values.
Table 5.2 Piping friction factor (
f
) values.
Table 5.3 The internal diameter of schedule 40 pipes ranging in size from 1/...
Chapter 6
Table 6.1 Definitions of parameters used for pressure-relief valve load calc...
Table 6.2 Provided data for PSV load calculation in gas service and the conv...
Table 6.3 Values of the gas constant (
C
) based on the ratio of specific heat...
Table 6.4 ASME and API standard orifice sizes.
Table 6.5 Correction factors for the specific heat.
Table 6.6 Ratio of specific heats “
k
” for different gases.
Chapter 7
Table 7.1 ASME and API standard orifice sizes.
Table 7.2 Values of the gas constant (
C
) based on the ratio of specific heat...
Table 7.3
C
factor and “
k
” ratio of specific heats for different gases.
Table 7.4 The critical pressure and temperature for some common substances....
Table 7.5 Superheat steam correction factor,
K
SH
, according to API 520 stand...
Table 7.6 Two-phase flow scenarios.
Table 7.7 Water and stem properties at 830 ° Rankine – the saturated tempera...
Table 7.8 Choosing critical or subcritical flow for PSV sizing in a subcooli...
Table 7.9 Symbols for sizing safety valves in fire cases.
Table 7.10 Value of cubical expansion coefficient for hydrocarbon liquids at...
Table 7.11 Wetted surfaces that should be taken into account when sizing a s...
Table 7.12 Environmental factor.
Table 7.13 Calculation of wetted area (surface) for different vessels.
List of Illustrations
Chapter 1
Figure 1.1 The old-style safety valve was operated by sliding a weight along...
Figure 1.2 Rupture disk between holders and flanges
Figure 1.3 The failure of the pneumatic supply system to the control valve r...
Figure 1.4 There is an overpressure situation in the second vessel on the ri...
Figure 1.5 An incident involving a fire near a pressure vessel and the opera...
Figure 1.6 A ruptured tube in a heat exchanger causes shell-side overpressur...
Figure 1.7 Increase in pressure in the vessel as a result of an increase in ...
Figure 1.8 Check valve arrangements installed after pumps.
Figure 1.9 Low-lift PRV.
Figure 1.10 Full-lift PRV.
Figure 1.11 Reduced bore PRV.
Figure 1.12 Pilot-operated pressure relief valve.
Figure 1.13 Conventional direct spring-loaded PRV.
Figure 1.14 Balanced direct spring-loaded PRV with a bellows.
Figure 1.15 Safety valve parts.
Figure 1.16 An application of force from the inlet fluid to the disk in the ...
Figure 1.17 The essential pressure settings of a safety/relief valve.
Figure 1.18 A PRV.
Chapter 2
Figure 2.1 Simple line diagram of a safety valve.
Figure 2.2 The effect of back pressure on the safety valve disk.
Figure 2.3 Illustration of fluid force, spring force, and back pressure forc...
Figure 2.4 Safety valves in a closed position are subjected to superimposed ...
Figure 2.5 A safety valve with bellows.
Figure 2.6 As the valve is opened, there is a buildup of back pressure or dy...
Figure 2.7 A simple drawing of a safety valve in a closed position.
Figure 2.8 A simple drawing of a safety valve in an open position.
Figure 2.9 A safety valve with a huddling chamber.
Figure 2.10 A safety valve opening and closing curves.
Figure 2.11 A safety valve function curve.
Figure 2.12 Relationship between the pressure values of safety valves and pr...
Figure 2.13 Illustration of curtain and flow areas.
Figure 2.14 Simmer in a safety valve designed for the gas service.
Figure 2.15 A safety valve with high-pressure loss at the inlet piping.
Figure 2.16 Safety valve function curve.
Chapter 3
Figure 3.1 Safety valve with a full-nozzle design.
Figure 3.2 A seminozzle safety valve.
Figure 3.3 Open bonnet vs closed bonnet.
Figure 3.4 A safety valve and its parts including the valve seat (item numbe...
Figure 3.5 The bellows sealing and bonnet hole on a balanced safety valve ar...
Figure 3.6 Effect of back pressure on bellows (there is a neutralization of ...
Figure 3.7 An example of bellows failure.
Figure 3.8 The detection of leaks in bellows is accomplished using a backup ...
Figure 3.9 Balanced diaphragm design safety valve
Figure 3.10 A safety valve with a lifting lever in a red color.
Figure 3.11 A manual lever is provided for the operation of the safety valve...
Figure 3.12 Safety valve with a packed lever (left) and a plain lever (right...
Figure 3.13 A safety valve with a threaded cap that is connected via wire to...
Figure 3.14 The top of a safety valve is fitted with a test gag.
Figure 3.15 On the right, there is a blocked valve with a test gag, and on t...
Figure 3.16 Installed on top of the valve, this valve disk stop consists of ...
Figure 3.17 The safety valve and its part list, item 11 is a bonnet spacer o...
Figure 3.18 The disk and nozzle contact surfaces are coated with a Stellite ...
Chapter 4
Figure 4.1 Center-to-face dimensions defined in the API standard.
Chapter 5
Figure 5.1 On the body of the safety valve, there is a mark indicating the d...
Figure 5.2 As close as possible to the pressure vessel is the installation o...
Figure 5.3 The distance between the safety valve and a source of flow turbul...
Figure 5.4 An illustration of the inlet piping major requirements connected ...
Figure 5.5 The installation of a safety valve on a pressure vessel containin...
Figure 5.6 The safety valves and inlet piping are connected to three safety ...
Figure 5.7 The connection of a long radius 90° elbow to the safety valve thr...
Figure 5.8 Incorrect inlet piping design by connecting the main piping run t...
Figure 5.9 Some examples of improperly designed and installed safety valve i...
Figure 5.10 A safety valve was installed incorrectly when it was connected t...
Figure 5.11 An illustration of a lateral or branch connection to the safety ...
Figure 5.12 Incorrect installation of a safety valve due to the wrapping of ...
Figure 5.13 The installation of gate valves on vertical and horizontal lines...
Figure 5.14 Inlet piping to safety valves, including isolation valves and bl...
Figure 5.15 A three-way ball valve is located on the inlet piping to a coupl...
Figure 5.16 Three-way changeover valve—ball types as per API 520 part II.
Figure 5.17 Interlock systems for isolation valves located before and after ...
Figure 5.18 In the outlet line of a safety valve, an upward bend is installe...
Figure 5.19 Angle of 45° at the end of the safety valve outlet tailpipe conn...
Figure 5.20 The installation of a safety valve on a pressure vessel.
Figure 5.21 The safety valve and its outlet piping.
Figure 5.22 Safety valve inlet piping connection.
Figure 5.23 Overpressure protection arrangement for pressure vessel B.
Figure 5.24 Incorrect installation of a safety valve.
Chapter 6
Figure 6.1 A safety valve with dual outlets.
Figure 6.2 Correction factor for high-pressure steam,
K
n
.
Chapter 7
Figure 7.1 Curve for assessing the
C
factor from the specific heat ratio in ...
Figure 7.2 Back pressure correction factor
(
K
b
)
for balanced bellows pressur...
Figure 7.3 Back pressure correction factor for gas and vapors based on API 5...
Figure 7.4 Compressibility factor determination chart.
Figure 7.5 Values of subcritical flow coefficient
(
F
2
)
.
Figure 7.6 Back pressure correction factor for balanced bellows pressure-rel...
Figure 7.7 Viscosity correction factor for balanced bellows pressure-relief ...
Figure 7.8 Capacity correction factor due to overpressure for noncertified p...
Figure 7.9 Critical flow ratio determination based on Omega parameter.
Figure 7.10 Wetted vessels.
Figure 7.11 Different dimensional designations for wetted vessels that are p...
Figure 7.12 Estimation of parameter
F
′
.
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Safety Valves in Oil and Gas Plants
Essential Design Considerations
Karan Sotoodeh
Senior Engineer, Oslo, Norway
Copyright © 2025 by John Wiley & Sons, Inc. All rights reserved, including rights for text and data mining and training of artificial technologies or similar technologies.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.
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Library of Congress Cataloging-in-Publication Data
Names: Sotoodeh, Karan, author.
Title: Safety valves in oil and gas plants : essential design considerations / Karan Sotoodeh.
Description: Hoboken, New Jersey : Wiley, [2025] | Includes bibliographical references and index.
Identifiers: LCCN 2024034340 (print) | LCCN 2024034341 (ebook) | ISBN 9781394294916 (hardback) | ISBN 9781394294930 (adobe pdf) | ISBN 9781394294923 (epub)
Subjects: LCSH: Relief valves – Design and construction. | Petroleum industry and trade – Safety measures. | Gas manufacture and works – Safety measures.
Classification: LCC TS277 .S685 2025 (print) | LCC TS277 (ebook) | DDC 665.7/70289 – dc23/eng/20240822
LC record available at https://lccn.loc.gov/2024034340
LC ebook record available at https://lccn.loc.gov/2024034341
Cover Design: WileyCover Image: © Eremeychuk Leonid/Alamy Stock Photo
Preface
In our daily lives, valves, including safety valves, play an important role. Fluids are moved through pipes in buildings and plants using valves to ensure their safe and reliable flow. As a matter of fact, this occurs in a wide range of industries – even in the manufacture of products or materials that may surprise you. Worldwide, millions of valves are working day and night in a variety of industries. We are going to discuss safety valves in more detail and reveal some fascinating facts about them.
In the oil and gas industry, various service fluids are used, including hydrocarbons that are flammable, toxic, and corrosive. The pressure of fluids in pressurized systems such as pressure vessels and piping can be increased by a variety of factors, including human error, equipment failure, or component failure, such as damage to control valves. In pressurized systems, safety valves are required to handle and release the overpressure fluid. Safety valves reduce excess pressure by releasing fluid from within the plant when a predetermined maximum pressure is reached. A safety valve plays an important role in protecting properties, the environment, and human life. There is a strong focus in the book on design considerations as well as the selection of pressure safety valves in order to prevent overpressure scenarios and to protect lives, assets, and the environment.
We will begin the first chapter by providing a brief overview of safety valves and their history. As early as the days of mankind, when water was boiled to create steam, it became apparent that a safety device was necessary. Additionally, a comprehensive review of overpressure scenarios (reasons for overpressure) in the plant is presented. This chapter also explains the components of a safety valve, how to dismantle a safety valve, and various terms associated with pressure relief valves (PRVs). Last but not least, we discuss the basic concept of safety valve operation, as well as the safety valve essential pressure settings, including the set pressure, the relieving pressure, and the reseat pressure. A valve’s design plays a significant role in optimizing industrial processes in terms of performance and efficiency. Valves that are well designed ensure accurate fluid flow control, minimize pressure drop, and prevent leakage. The fundamental safety valve designs are provided in the second chapter. The purpose of the third chapter is to provide a comprehensive overview of possible safety valve design options and accessories. The purpose of the fourth chapter is to discuss the codes and standards employed in the oil and gas industry in order to maintain high levels of safety valve quality and reliability. Essential installation considerations for safety valves are discussed in details in the fifth chapter. Finally, safety valves reaction forces and sizing are included in the last two chapters.
8-February-2024
Karan Sotoodeh
Senior Engineer, Oslo, Norway
1Basic Safety Valves Principles
1.1 Introduction and History
In the oil and gas industry, various service fluids are used, including hydrocarbons that are flammable, toxic, and corrosive. The pressure of fluids in pressurized systems such as pressure vessels and piping can be increased by various factors, including human error, equipment failure, or component failure, such as damage to control valves. In pressurized systems, safety valves are required to handle and release the overpressure fluid. Safety valves reduce excess pressure by releasing fluid from within the plant when a predetermined maximum pressure is reached.
In the earliest days of mankind, when water was boiled to create steam, the necessity of a safety device became apparent. The safety valve was thought to have been invented by the Frenchman Papin, who applied it in 1682 for a steam digester. In 1679, a French physicist Denis Papin invented a high-pressure cooker known as the steam digester or bone digester (also known as Papin’s digester). It is a device for extracting fats from bones in a high-pressure steam environment. A lever and a movable weight were used to keep the safety valve closed; by sliding the weight along the lever as illustrated in Figure 1.1, Papin was able to maintain the valve’s position and regulate the steam pressure. It has now been discovered that Papin was only the inventor of the improvements just mentioned and that the German Glauber had already been utilizing safety valves some fifty years earlier. The valve opened once the pressure from the steam pressure acting on the valve exceeded the pressure from the weight acting on the lever arm. For designs that require a greater relief pressure setting, an extended lever arm and/or heavier weights are required. Although this simple system worked, additional space was required. There was also the disadvantage of unintended opening of the valve when the device was subjected to the bouncing movements of the weight. In the early 19th century, boiler explosions on ships and locomotives often resulted from defective safety devices, which led to the development of the first safety relief valves for industrial use. Between 1905 and 1911, there were 1700 boiler explosions in the United States, resulting in 1300 deaths.
Figure 1.1 The old-style safety valve was operated by sliding a weight along the lever
(Courtesy: Shutterstock).
1.2 Maximum Allowable Working Pressure (MAWP)
There is a requirement to install safety valves on piping and equipment (e.g. pressure vessels) where it is expected that their working pressure will exceed the MAWP. An MAWP is a designation established by the American Society of Mechanical Engineers (ASME) for pressure-relief components in vessels. At specific operating temperatures, it determines the maximum amount of pressure that can be applied to the weakest part of the vessel. In addition to the establishment of safety protocols, industrial facilities use the MAWP to prevent explosions by ensuring the system does not exceed a safe operating pressure. The maximum operating pressure (MAWP) of a vessel refers to the maximum level of pressure the vessel may be exposed to, whereas the design pressure refers to the maximum level of pressure it should be exposed to during normal operation. It is generally accepted that the design pressure of a pressurized system is lower than or equal to the maximum acceptable working pressure of the system’s vessel. The MAWP is calculated based on the physical limitations of the weakest part of the vessel, whereas the design pressure of a system is determined by the type of pressure system used. The MAWP also differs from the design pressure since the former characteristic changes over the course of the vessel’s lifetime. A vessel’s MAWP is gradually lowered as a result of wear, use, and corrosion of carbon steel elements.
It was explained that MAWP focuses on the weakest part of the pressure vessel. Using this example, we can identify the weakest part of a vertical pressure vessel. It is assumed that the weakest part of this vessel is located on its body, rather than on the top or bottom dishes. The thickness of the vertical pressure vessel is not constant. Typically, the lower parts of a pressure vessel are thicker than the upper parts. This is due to the fact that the lower parts of the pressure vessel are in direct contact with a larger column of fluid. In spite of this, the upper part of the vessel may have the lowest thickness due to the absence of head pressure. Accordingly, the top of the vessel is the weakest part with the lowest thickness that must be considered when determining MAWP.
The maximum allowable accumulated pressure (MAAP) is specified for vessels protected by pressure relief devices. It is the maximum allowable pressure during discharge from the relieving device. When all relief devices are fully closed, accumulation pressure refers to the maximum pressure that can build within a vessel. In most cases, this pressure is expressed as a percentage of the MAWP. The governing case for opening the relief valve protecting the vessel determines this percentage. A vessel protected against fire has an MAAP of 121% of the MAWP. In other words, the pressure inside the vessel is allowed to rise to 121% of its MAWP before the relief valve opens in order to release the pressure.
1.3 The Importance of Safety Valves and Safety Devices
A safety valve is primarily designed to protect life, property, and the environment. The purpose of a safety valve is to open and relieve excess pressure from vessels or equipment and to prevent further fluid release once normal conditions have been restored. Safety valves are a part of safety systems, which include safety devices such as safety valves, as well as associated piping systems and process equipment used to handle fluids released under excessive pressure. It is important to understand that safety valves are safety devices and are often the last line of defense. The safety valve must be capable of operating at all times and under all conditions so it should be completely reliable. A safety valve should not be misunderstood as a process valve or pressure regulator. Overpressure protection should be its sole purpose.
It is unavoidable to use pressure relief devices in a number of industrial processes. There are safety valves installed on boilers and reactors, pipes and pipelines, pumps and compressors as well as cooling and heating circuits. In safety systems, safety valves are not the only component. Generally, rupture disks as non-reclosing or one-time used safety devices consisting a thin metallic diaphragm are used to protect pressure vessels, equipment, and systems from over pressurization. They are also called pressure safety disks, burst disks, or bursting disks. When the inlet pressure reaches the rupture disk set pressure, the disk bursts. Due to the fact that it is a one-time-use device, it will need to be replaced after it bursts. Rupture disks are either installed directly between flanges or inserted into a corresponding rupture disk holder, which is then mounted between flanges (see Figure 1.2). Typically, rupture disks are designed to relieve pressure at 1.5 times the vessel’s MAWP. In addition to being simple and requiring no moving parts, rupture disks are also inexpensive and leak-tight. In spite of this, it is not possible to test them, as they degrade with age and corrosion, and in order to reinstall them, the process area must be shut down. There is the possibility of using a rupture disk in place of a safety valve or of using a rupture disk in conjunction (parallel or series) with a safety valve. If the fluid relief must occur rapidly or if the fluid could disturb the proper functioning of the valve, a rupture disk can be preferred over a safety valve as the primary relief method. For instance, some fluids can freeze during pressure drops and block the safety valve or connected inlet and outlet lines. In comparison with the safety valve, the rupture disk has a higher response pressure. In combination with the safety valve, a rupture disk could be installed upstream of the valve in order to prevent valve plugging, corrosion, leakage, and wear caused by pressure fluctuations and frequent opening and closing of the valve. Therefore, the lifecycle of safety valves can be extended, and the need for maintenance on the safety valves can be reduced. Whenever the pressure rises significantly and rapidly, the rupture disk serves as a reliable backup system for relieving the pressure if the safety valve cannot respond quickly or fails to open. It should be noted that the reasons mentioned above that rupture disks can be installed in place of safety valves are also true when rupture disks are installed in parallel with safety valves.
Figure 1.2 Rupture disk between holders and flanges
(Credit: Faiz/Adobe Stock).
1.4 Overpressure Scenarios and Examples in the Plant
There is a detailed discussion of the potential for overpressure in American Petroleum Institute (API) 521 and International Organization for Standardization (ISO) 23251. It is important to note that the pressure vessels, heat exchangers, operating equipment, and piping are designed to contain the system pressure. In designing the system, the following factors are taken into account: (a) the normal operating pressure at operating temperatures, (b) the effects of any combination of process upsets that may occur, (c) the differential between the operating and set pressures of the pressure-relieving device, and (d) the effect of earthquakes and winds as supplemental loadings.
1.4.1 Closed Outlet of a Vessel
In the event that a manual block valve on the outlet of a pressure vessel is accidentally closed while the equipment is operational, the vessel may be exposed to a pressure greater than the MAWP. In the event of an outlet-block valve being closed, there may be an overpressure, which requires the use of a pressure-relief device most likely. Every valve has the potential to be operated by mistake. Case Study 1.1 illustrates a pressure vessel with a closed outlet due to a closed control valve in the outlet line. It is important to note that closed outlets are not solely the result of the outlet valve being operated by mistake. Various factors can contribute to blocked outlets, including failure of the control valve, mistaken operation of the valve, and failure of the instrument air or power supply. There are, however, other ways of closing outlets in a system in addition to block and control valves. Blocked outlets can be caused by frozen lines, plugged filters on the outlet line, and stuck or misinstalled check valves.
1.4.2 Unintentional Valve Opening
The possibility of an accidental opening of any valve from a source of higher pressure (e.g. the valve located on the inlet line to equipment handling high-pressure steam or process fluids) should be considered. Pressure-relieving capacity may be required for this condition. A process flow diagram in 1.2 illustrates how an open control valve can result in an overpressure situation in a pressure vessel.
1.4.3 Check Valve Leakage or Failure
In a check valve, fluid flows in one direction and is automatically prevented from flowing backward when the fluid reverses direction. It is common to use a check valve arrangement when sending fluid from a lower-pressure system to a higher-pressure system in order to prevent reverse flow. The use of a single check valve is not always an effective method of preventing overpressure due to reverse flow from a high-pressure source. Therefore, in some cases, the present author proposes to install two check valves in series to increase the safety and reliability of the pressure equipment and piping system in the event of check valve failure. If there is only one check valve installed and it fails to function, high-pressure fluid will flow to the piping, pump, and pressure vessel that are installed upstream of the check valve. This can lead to serious overpressure scenarios in the facilities located upstream of the check valve. A single check valve may be adequate if the backflow high-pressure fluid applies pressure to the upstream pressure vessel which is less than the vessel’s test pressure. In most cases, however, the focus should be on preventing reverse flow. Additionally, reverse flow through equipment such as pumps and compressors can result in the destruction of mechanical equipment in addition to overpressure in the upstream system. An additional means of backflow prevention (double check valves in series) should be provided if this hazard is of concern. See 1.5 for more information.
1.4.4 Utility Failure
It is possible for overpressure in related equipment to result from the failure of utilities such as electricity, instrument air, steam, fuel, and cooling water. It is important to consider the possibility of utility failure when analyzing the possibility of overpressure in equipment. For example, the failure of electricity (for the fans of air coolers or circulation pumps for cooling water) or cooling water may lead to the failure of heat exchangers to perform as expected. Consequently, the temperature of the process can rise, resulting in overpressure in the equipment that is protected. In process vessels, overpressure can result from the failure of electrical or mechanical equipment that provides cooling or condensation. In the event of a loss of instrument air, all air-operated valves are driven into their specified fail position resulting in an overpressure scenario in the vessel. The steam can be used to drive turbines, which, in turn, drive pumps, compressors, blowers, and fans. The partially available utility can be considered to evaluate possible overpressure in protected equipment in case one source of utility fails, but another independent source is still available simultaneously to carry the complete or part of the extra load caused by partial utility failure. Therefore, independent utility alternatives can lower the risk of possible overpressure, and they should be considered during the design process.
1.4.5 Heat Exchanger Tube Failure
The tubes in shell-and-tube heat exchangers are susceptible to failure due to various factors, including thermal shock, vibration, and corrosion. Whatever the cause, the high-pressure stream may overpressure equipment on the low-pressure side of the exchanger. A determination should be made as to whether the low-pressure system is capable of absorbing this release. It is necessary to determine the possible pressure rise in order to determine whether additional pressure relief is required if the flow from the ruptured tube discharges into the lower-pressure stream or shell side. Case Study 1.4 provides further information regarding heat exchanger tube failure.
1.4.6 Transient Pressure Surges
Water Hammer
A liquid-filled system should be carefully evaluated for the possibility of hydraulic shock waves, known as water hammer. As a result of very rapid closure of valves, such as check valves, water hammer can occur in liquid services. A water hammer is a type of overpressure that cannot be controlled by typical pressure-relief valves, as their response times are often too slow. In milliseconds, changing peak pressures can raise the normal operating pressure by a significant amount. In the absence of proper safeguards, these pressure waves may damage pressure vessels and piping. In long pipelines, water hammer is often prevented by limiting the speed at which valves can be closed. A pulsation dampener could be considered in cases where water hammer occurs.
Steam Hammer
There is a possibility that a peak-pressure surge, called steam hammer, may occur in piping that contains compressible fluids. Rapid valve closure is generally the cause of the most common occurrence. As this oscillating pressure surge occurs in milliseconds, the normal operating pressure may rise by many times, causing vibration and violent movement of piping and possibly resulting in equipment rupture. Due to their slow response time, pressure-relief valves cannot be effectively used as protective devices. Steam hammer can be prevented by avoiding the use of quick-closing valves.
Fire
Equipment exposed to plant fire has a risk of overpressure in case of fire, resulting from fluid expansion or vaporization as per 1.3.
1.4.7 Process Change/Chemical Reaction
A significant change in operating process parameters such as the temperature increase may make equipment susceptible to damage. When reacting systems shift equilibrium, excess gas or excess heat can result, causing overpressure. For the design of relief valves, these possibilities must be carefully examined.
1.4.8 Summary and Case Studies
The purpose of this paragraph is to summarize some of the main causes of overpressure scenarios in pressurized systems. High pressure can damage piping and equipment for various reasons, including failure of the cooling system, which causes liquid or gas to be heated and pressurized. Excessive fluids are flowing through the piping and equipment as a result of control system failure, disconnection of electricity or compressed air, fire in the plant, failure of the heat exchanger tube, chemical reaction, blocked discharge, or ambient temperature increase. There is a possibility that each of the events listed above may occur individually and separately from one another. It is also possible for them to occur simultaneously.
Case Study 1.1
According to Figure 1.3, there is a control valve located on the outlet line of a pressure vessel, which is used to adjust the level of fluid within the vessel. Pneumatic air is used to operate (open and close) the control valve. Assume that the pneumatic air supply system has failed, resulting in the control valve remaining closed. Consequently, the fluid cannot be discharged from the outlet line of the pressure vessel, and its accumulation inside the vessel increases the vessel’s pressure above MAWP. To release the excess pressure, the pressure relief valve (PRV) on the top of the vessel must be opened. Moreover, another overpressure scenario or pressure build-up in the vessel which leads to the operation of PRV is blockage of the vessel discharge, which is also the case here as the control valve remains in the closed position and blocks vessel discharge.
Figure 1.3 The failure of the pneumatic supply system to the control valve results in a scenario of overpressure within the vessel and the operation of the PRV.
Case Study 1.2
Two vertical pressure vessels are shown in Figure 1.4. A liquid is exiting the first vessel on the left and entering the second vessel after passing through the control valve. Assume that the second pressure vessel is full of liquid and is under pressure. When this occurs, the control valve must be closed in order to prevent liquid from flowing to the second vessel on the right. The pressure in the second pressure vessel will rise above the MAWP if the control valve is left open, so the pressure safety valve (PSV) must release the overpressure liquid.
Figure 1.4 There is an overpressure situation in the second vessel on the right as a result of the upstream control valve remaining open.
Case Study 1.3
Figure 1.5 illustrates a fire case near a pressure vessel that resulted in an overpressure scenario and catastrophic rupture of the vessel. A PRV will discharge the excess pressure in such a situation.
Case Study 1.4
As shown in Figure 1.6, a tube rupture may occur as a result of corrosion, fatigue, or brittle fraction in a heat exchanger. In the event of a rupture of the tube, the fluid inside the tube enters the shell side, which increases the shell side pressure, requiring the use of a PSV to release the excess pressure.
Figure 1.5 An incident involving a fire near a pressure vessel and the operation of the PRV.
Figure 1.6 A ruptured tube in a heat exchanger causes shell-side overpressure and causes the PSV to operate.
Case Study 1.5
As shown in Figure 1.7, an increase in ambient temperature can result in an increase in temperature within the vessel. The vessel contains gas whose pressure increases with increasing temperature. Due to the pressure increase exceeding MAWP, the PRV at the top of the vessel releases the excess pressure.
Figure 1.7 Increase in pressure in the vessel as a result of an increase in ambient temperature.
Case Study 1.6
According to Figure 1.8, a pressure vessel with tag number V-01 is used to separate liquids from gases. Liquid from the bottom of the vessel is pumped forward. The system is equipped with two pumps. The top pump is the main pump and the bottom pump is a bypass used in the event that the upper pump requires maintenance. Immediately after the pump there is a check valve with a red circle around it that prevents backflow from downstream to upstream. If the check valve fails, fluid enters the pump from the discharge side to the suction side. As a result of this reverse flow, the pump may be damaged and its performance may be compromised. The high-pressure fluid can enter the V-01 pressure vessel and increase the vessel’s pressure above MAWP. As a consequence, a PSV needs to be installed on V-01 in order to release the overpressure in this case as well as in other potential overpressure scenarios.
Figure 1.8 Check valve arrangements installed after pumps.
1.5 Variations in Safety/Relief Valves
In general, “safety valves” and “safety relief valves” refer to various pressure relief devices designed to prevent excessive accumulation of internal fluid pressure. A wide range of safety relief valves are available for various applications and performance requirements. The use of safety valves is governed by various national standards, which require different designs to meet those standards. Furthermore, the terms “safety” and “relief” are frequently used interchangeably, but this is not appropriate. Safety valves are used to control the flow of compressible fluids, such as steam and other gases. As a result of this compressibility, overpressure must be relieved quickly. The gas or steam may be discharged into the atmosphere or directed back into the piping system. Fluids that are not compressible, such as water and oil, require relief valves. The immediate discharge of full flow is not required since a very small flow significantly reduces the risk of overpressure.
The terms associated with safety and safety relief valves are defined in most national standards. The terminology used in the United States and Europe differs significantly. There are several important differences between Europe and the United States, including the fact that a valve referred to as a “safety valve” in Europe is referred to as a “safety relief valve” or a “pressure relief valve” in the United States. The ASME PTC25 standard addressing pressure relief devices defines the following generic terms:
Pressure relief valve (PRV):
As a pressure relief device, it opens to relieve excess pressure and then closes and prevents the flow of fluid after normal conditions are restored. Its opening is generally characterized by a rapid opening action or by an opening that is proportional to the increase in pressure above the opening pressure. The device may be used for either compressible or incompressible fluids, depending on the design, adjustment, or application. As a general term, this term includes safety valves, relief valves, and safety relief valves as well. There are several types of PRVs listed in ASME PT25, which are explained as follows:
Low-lift PRV:
PRVs in which the actual discharge area is the curtain area determined by the position of the disk (see
Figure 1.9
). During its relief cycle, it often remains relatively unstable since it will not necessarily lift fully open to its capacity and will act more or less proportionately to the pressure increase. Noncompressible fluids are typically handled by low-lift PRVs.
Figure 1.9 Low-lift PRV.
Figure 1.10 Full-lift PRV.
Full-lift PRV:
This type of PRV has a bore area that serves as the discharge area. In a full-lift PRV, the disk lifts sufficiently so that the curtain area determined by the position of the disk no longer affects the discharge area. The bore area determines the discharge area, and therefore the valve’s capacity. Compressible fluids such as steam can be handled by full-lift PRVs.
Figure 1.10
illustrates a full-lift PRV in which the disk is fully lifted and the liquid in the discharge line is enclosed in the bore of the discharge pipe, covering the entire internal area of the pipe.
Reduced bore PRV:
This type of PRV is characterized by a flow path below the seat that is smaller than the flow path at the inlet to the valve, which creates a venturi effect (see
Figure 1.11
).
Figure 1.11 Reduced bore PRV.
Full bore PRV:
The bore area of a PRV is equal to the flow area at the inlet, and there are no obstructions in the bore area between the inlet and flow areas.
Direct spring-loaded PRV:
A PRV in which the disk is held closed by a spring.
Pilot-operated PRV:
Normally, conventional PRVs are held closed by a spring or similar mechanism that presses a disk or piston against the seat, which is forced open when the pressure exceeds the mechanical value of the spring. Pilot-operated pressure relief valves (POPRVs) are PRVs in which the principal relieving device is controlled by an auxiliary PRV that is self-actuated.
Using a POPRV, a small amount of fluid is piped to the rear of the sealing disk in order to keep the valve shut. POPRVs consist of a pilot valve (or control pilot), a main valve, a pilot tube also called a sensing line, a dome, a disk or piston, and a seat. While there are many designs available, the control pilot is essentially a conventional PRV with the special function of controlling the pressure on the main valve dome. Domes are the volumes located above pistons. Typically, a small pilot tube is used to supply pressure to the dome from the upstream side (the protected system). POPRV exhaust is directed into a pipe or open air on the downstream side. As a floating and unbalanced component, the piston moves to open and closed positions in response to fluid pressure.
The pilot-operated valve is a self-contained system that does not require external power or pressure. Using system media and pressure, the pilot valve controls the main valve operation by either opening or closing it automatically. Due to the way that the pressure is routed around to the dome above the piston, when the upstream pressure tries to push the piston open, it is opposed by the fluid pressure above the piston. When the system is operating normally, the system pressure is collected at the main valve inlet and routed to the dome through a line located under the pilot valve. Based on the illustration in Figure 1.12, since the area of the piston on which fluid forces are acting in the dome is greater than the area on the upstream side, the dome side has a greater force than the upstream side. As a result, a net sealing force is produced during normal operation. Upon reaching the set pressure, the pilot valve actuates and releases the fluid into another line or path rather than sending it to the dome. Consequently, the pressure under the piston exceeds the pressure in the dome, causing the main valve to open.
There are a number of advantages associated with POPRVs, including their high capacity, precision control, and wide range of applications. As pilot-operated valves have the ability to handle high flow rates and pressure drops, they are ideal for large-scale industrial applications. These valves provide exceptional control over fluid flow, providing a high degree of accuracy and responsiveness. They are suitable for a wide range of industries and applications, making them an excellent choice. Complexity and higher maintenance requirements and frequency are two disadvantages of POPRV. As a result of the additional components and interconnections required for pilot-operated valves, their complexity and cost can be increased. Due to the potential for wear and tear on the pilot valve and sensing line, pilot-operated valves may require more frequent maintenance.
Figure 1.12 Pilot-operated pressure relief valve.
Conventional direct spring-loaded PRV:
A direct spring-loaded PRV whose operation including opening and closing pressure as well as relieving capacity is directly influenced by changes in the back pressure exerted at the outlet and that is held closed by a spring force. Therefore, conventional direct spring-loaded PRVs are best suited for applications with no back pressure. A conventional direct spring-loaded PRV is shown in
Figure 1.13
, where the discharge of the safety valve is connected to the spring housing or spring area. The back pressure applied through the discharge line affects the operating characteristics of the valve, such as the opening and closing pressure and the relieving capacity.
A back pressure may be caused by the valve outlet being connected to a normally pressurized system or by other PRVs venting into a common header. Back pressure that may occur in the downstream system while the valve is closed is known as superimposed back pressure. There is an impact of superimposed back pressure on the opening of conventional relief valves. In the closed position, this back pressure will provide an additional spring force to the valve disk. Consequently, it may be possible to compensate for a constant back pressure by reducing the spring force. Back pressure can have a significant impact on a valve’s performance by increasing its set pressure and reducing its capacity. As an example, a valve set at 100 pounds per square inch gauge (psig) that is subjected to 10 psig of back pressure will not reach its set point until the system pressure is 110 psig. If there is too much back pressure, the valve may chatter (open and close rapidly), which may cause damage to it. It should be noted that back pressure has no effect on POPRV operation.
Figure 1.13 Conventional direct spring-loaded PRV.
Balanced direct spring-loaded PRV:
