Selection And Sizing Of Pressure Relief Valves

PDHonline Course M112 (3 PDH)

Selection and Sizing of Pressure Relief Valves

Instructor: Randall W. Whitesides, PE

2012

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SELECTION AND SIZING OF PRESSURE RELIEF VALVES Randall W. Whitesides, P.E.GENERAL/SCOPE/INTRODUCTION

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 that a fixed predetermined amount. The permissible amount of overpressure is covered by various codes and is a function of the type of equipment and the conditions causing the overpressure.

Note:For ease of learning, the student is encouraged to print the glossary and refer to the definitions of words or phrases as they first appear while studying the course material.

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 safetysystems 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 reliefvalve 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 bedesigned 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 of pressure relief valve products available in the market today.

FIGURE 1 - TWO TYPES OR RELIEF VALVES

The standard designsafety 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 areavailable 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 asthe set pressure is being approached. Another feature of the pilot-operated valve is that it will permit a blowdown as low as 2 %. The disadvantage of this type of valve is its vulnerability to contamination from foreign matter in the fluid stream.
CODES AND STANDARDS

Introduction Since pressure relief valves are safety devices, there are many Codes and Standards in place to control their designand 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...