Selection and Sizing of Pressure Relief
Valves
Outline/Scope/Introduction
Course Outline
1. Codes and Standards
2. Equation Nomenclature
3. Selection and Sizing
4. Summary
5. Additional Resources
6. Glossary* (see note on page 2)
7. References
Scope
There are numerous types of pressure-relieving devices and systems, both re-closing and
non-re-closing (rupture discs). This course contains technical information limited to pressure relief
valves. The primary purpose of a pressure relief valve is the protection of life and property by
venting fluid from over-pressurized equipment. Information contained in this course applies to the
overpressurization protection of pressure vessels, lines, and systems.
The basic formulae and capacity correction factors contained in this course have been, for the most
part, empirically developed over time within the valve industry. The material presented reflects
current and generally accepted engineering practice. Formulations in this course are consistent with
the requirements of ASME Section VIII, Division 1, Section XIII, and API Recommended Practice
520.
1
,Introduction
The function of a pressure relief valve is to protect pressure vessels, piping systems, and
other equipment from pressures exceeding their design pressure by more than a fixed,
predetermined amount. The permissible amount of accumulation is covered by various codes and is
a function of the type of equipment and the conditions causing the accumulation.
*
NOTE:
For ease of learning, the student is encouraged to print the glossary near
the end of the course and while studying, refer to the definitions of bold
italicized words or phrases when they are first encountered.
It is not the purpose of a pressure relief valve to control or regulate the pressure in the vessel or
system that the valve protects, and it does not take the place of a control or regulating valve.
The aim of safety systems in processing plants is to prevent damage to equipment, avoid injury to
personnel and to eliminate any risks of compromising the welfare of the community at large and the
environment. Proper sizing, selection, manufacture, assembly, test, installation, and maintenance of
a pressure relief valve are critical to obtaining maximum protection.
Types, Design, and Construction
A pressure relief valve must be capable of operating at all times, especially during a period
of power failure; therefore, the sole source of power for the pressure relief valve is the process fluid.
The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a
specified overpressure, and close when the system pressure has returned to a safe level. Pressure
relief valves must be designed with materials compatible with many process fluids from simple air
and water to the most corrosive media. They must also be designed to operate in a consistently
smooth manner on a variety of fluids and fluid phases. These design parameters lead to the wide
array ray of pressure relief valve products available in the market today.
2
, FIGURE 1 – TWO TYPES OF RELIEF VALVES
The standard design safety relief valve is spring-loaded with an adjusting ring for obtaining the
proper blowdown and is available with many optional accessories and design features. Refer to
Figure 1 for cross-sectional views of typical valves. The bellows and balanced bellows design
isolate the process fluid from the bonnet, the spring, the stem, and the stem bushing with a bellows
element. Jacketed valve bodies are available for applications requiring steam or heat transfer
mediums to maintain viscosity or prevent freezing. Pilot-operated valves are available with the set
pressure and blowdown control located in a separate control pilot. This type of valve uses the line
pressure through the control pilot to the piston in the main relief valve and thereby maintains a high
degree of tightness, especially as the set pressure is being approached. Another feature of the pilot-
operated valve is that it will permit a blowdown as low as 2 percent. The disadvantage of this type
of valve is its vulnerability to contamination from foreign matter in the fluid stream.
3
, Codes and Standards
Introduction
Since pressure relief valves are safety devices, there are many Codes and Standards in place
to control their design and application. The purpose of this section of the course is to familiarize the
student with and provide a brief introduction to some of the Codes and Standards which govern the
design and use of pressure relief valves. While this course scope is limited to ASME Section VIII,
Division 1, the other Sections of the Code that have specific pressure relief valve requirements are
listed below. The portions of the Code that are within the scope of this course are indicated in red:
List of Code Sections Pertaining to Pressure Relief Valves
Section I Power Boilers
Section III, Division 1 Nuclear Power Plant Components
Section IV Heating Boilers
Section VI Recommended Rules for the Care and Operation of Heating Boilers
Section VII Recommended Rules for the Care of Power Boilers
Section VIII, Division 1 Pressure Vessels
Section VIII, Division 2 Pressure Vessels - Alternative Rules
Section XIII Rules for Overpressure Protection
B31.3, Chapter II, Part 3 Power Piping - Safety and Relief Valves
B31.3, Chapter II, Part 6 Power Piping - Pressure Relief Piping
ASME specifically states in Section VIII, Division 1, paragraph UG-150(b):
“Other than unfired steam boilers, all pressure vessel within the scope of this
Division, regardless of size or pressure, shall be provided with overpressure
protection devices in accordance with the requirements of UG-150 through
UG-156.”
4
Valves
Outline/Scope/Introduction
Course Outline
1. Codes and Standards
2. Equation Nomenclature
3. Selection and Sizing
4. Summary
5. Additional Resources
6. Glossary* (see note on page 2)
7. References
Scope
There are numerous types of pressure-relieving devices and systems, both re-closing and
non-re-closing (rupture discs). This course contains technical information limited to pressure relief
valves. The primary purpose of a pressure relief valve is the protection of life and property by
venting fluid from over-pressurized equipment. Information contained in this course applies to the
overpressurization protection of pressure vessels, lines, and systems.
The basic formulae and capacity correction factors contained in this course have been, for the most
part, empirically developed over time within the valve industry. The material presented reflects
current and generally accepted engineering practice. Formulations in this course are consistent with
the requirements of ASME Section VIII, Division 1, Section XIII, and API Recommended Practice
520.
1
,Introduction
The function of a pressure relief valve is to protect pressure vessels, piping systems, and
other equipment from pressures exceeding their design pressure by more than a fixed,
predetermined amount. The permissible amount of accumulation is covered by various codes and is
a function of the type of equipment and the conditions causing the accumulation.
*
NOTE:
For ease of learning, the student is encouraged to print the glossary near
the end of the course and while studying, refer to the definitions of bold
italicized words or phrases when they are first encountered.
It is not the purpose of a pressure relief valve to control or regulate the pressure in the vessel or
system that the valve protects, and it does not take the place of a control or regulating valve.
The aim of safety systems in processing plants is to prevent damage to equipment, avoid injury to
personnel and to eliminate any risks of compromising the welfare of the community at large and the
environment. Proper sizing, selection, manufacture, assembly, test, installation, and maintenance of
a pressure relief valve are critical to obtaining maximum protection.
Types, Design, and Construction
A pressure relief valve must be capable of operating at all times, especially during a period
of power failure; therefore, the sole source of power for the pressure relief valve is the process fluid.
The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a
specified overpressure, and close when the system pressure has returned to a safe level. Pressure
relief valves must be designed with materials compatible with many process fluids from simple air
and water to the most corrosive media. They must also be designed to operate in a consistently
smooth manner on a variety of fluids and fluid phases. These design parameters lead to the wide
array ray of pressure relief valve products available in the market today.
2
, FIGURE 1 – TWO TYPES OF RELIEF VALVES
The standard design safety relief valve is spring-loaded with an adjusting ring for obtaining the
proper blowdown and is available with many optional accessories and design features. Refer to
Figure 1 for cross-sectional views of typical valves. The bellows and balanced bellows design
isolate the process fluid from the bonnet, the spring, the stem, and the stem bushing with a bellows
element. Jacketed valve bodies are available for applications requiring steam or heat transfer
mediums to maintain viscosity or prevent freezing. Pilot-operated valves are available with the set
pressure and blowdown control located in a separate control pilot. This type of valve uses the line
pressure through the control pilot to the piston in the main relief valve and thereby maintains a high
degree of tightness, especially as the set pressure is being approached. Another feature of the pilot-
operated valve is that it will permit a blowdown as low as 2 percent. The disadvantage of this type
of valve is its vulnerability to contamination from foreign matter in the fluid stream.
3
, Codes and Standards
Introduction
Since pressure relief valves are safety devices, there are many Codes and Standards in place
to control their design and application. The purpose of this section of the course is to familiarize the
student with and provide a brief introduction to some of the Codes and Standards which govern the
design and use of pressure relief valves. While this course scope is limited to ASME Section VIII,
Division 1, the other Sections of the Code that have specific pressure relief valve requirements are
listed below. The portions of the Code that are within the scope of this course are indicated in red:
List of Code Sections Pertaining to Pressure Relief Valves
Section I Power Boilers
Section III, Division 1 Nuclear Power Plant Components
Section IV Heating Boilers
Section VI Recommended Rules for the Care and Operation of Heating Boilers
Section VII Recommended Rules for the Care of Power Boilers
Section VIII, Division 1 Pressure Vessels
Section VIII, Division 2 Pressure Vessels - Alternative Rules
Section XIII Rules for Overpressure Protection
B31.3, Chapter II, Part 3 Power Piping - Safety and Relief Valves
B31.3, Chapter II, Part 6 Power Piping - Pressure Relief Piping
ASME specifically states in Section VIII, Division 1, paragraph UG-150(b):
“Other than unfired steam boilers, all pressure vessel within the scope of this
Division, regardless of size or pressure, shall be provided with overpressure
protection devices in accordance with the requirements of UG-150 through
UG-156.”
4
