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Chapter 12
Pressure-Relief Valve Requirements

Excerpt

The ASME Boiler and Pressure Vessel Code was developed between 1911 and 1914 as a set of safety rules to address the serious problem of boiler explosions in the United States, which at that time were almost a daily occurrence. Average steam pressure in boilers in those days had reached about 300 psi (2 MPa), and the explosions took a heavy toll of lives and property. The principal design basis for boilers and pressure vessels is the safe containment of design pressure. Protection against overpressure was therefore a very important aspect of pressure vessel design. Accordingly, that first edition of what is now Rules for Construction of Power Boilers, Section I of the Code, included rules for the overpressure protection of boilers, based on the best industry practice at the time. The principles of today's Code rules for overpressure protection are little changed from those of the first Code.

That first ASME Code of 1914 has now grown into 12 book sections, covering a wide variety of subjects. Six of these book sections deal specifically with the design of pressure vessels and include requirements for overpressure protection. The various pressure-relief devices used for overpressure protection may be categorized as pressure-relief valves (commonly called safety valves), and nonreclosing devices, such as rupture disks.

The function of a pressure-relief device is to open at a specified pressure and pass a sufficient amount of fluid to prevent pressure in the vessel or system being protected from exceeding an allowable overpressure above the design pressure. In some book sections such as Sections I and VIII, the design pressure is referred to as the maximum allowable working pressure (MAWP). Each book section provides rules for determining the required relieving capacity of pressure-relief devices, for establishing allowable overpressure, for determining the maximum set pressure of the pressure-relief devices, and for ensuring that pressure-relief devices will function as required should an overpressure condition occur. Although fundamentally similar, the rules vary somewhat in detail and complexity, reflecting the nature of the application and the somewhat different design philosophies of the committees governing the various book sections. It happens that the Code offers only limited explanation of its rules on overpressure protection, and aside from certain functional requirements, very few rules to guide the designers of pressure-relieving devices. The objective of this chapter is to summarize the Section I rules and to provide some explanation and discussion of them and how they are applied. Since similar rules apply to all the non-nuclear book sections, a brief explanation of those other rules is included, to point out some differences. Thus, the discussion which follows covers all the non-nuclear book sections, with the major emphasis on the rules in Section I.

  • Introduction
  • Overpressure Protection
  • Selection of Pressure-Relieving Devices
  • Design of Pressure-Relief Valves
  • Establishing and Certifying Relieving Capacities
  • Qualification of Pressure-relief Valve Manufacturers
  • Installation Guidelines
  • Pressure-Relief Valves in Service

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