Pressure-relief terms/definitions

The terminology in pressure relief devices is very specific and therefore the terms used in this field are explained in order for the reader to better understand the literature on the subject. Some definitions are as they are given in the API. 

Pressure relief device is the general term for a device designed to prevent pressure or vacuum from exceeding a predetermined value in a pressure vessel by the transfer of a fluid during emergency or abnormal pressure conditions. There are, however, different definitions for specific devices, their testing and their operating characteristics. 

Reclosing pressure relief devices have a variety of names, although there used to be a clear definition based on the US market and API. However, when the Pressure Equipment Directive (PED) came into effect in 1997, this somewhat added to the confusion, as PED uses the overall term safety valve for every pressure-relieving device subject to the PED code. Originally the following were the definitions for the different terms per API and are still in use today. 

API 520 Definition A nominal or computed area used with an effective discharge coefficient to calculate the minimum required reheving capacity for a pressure relief valve per the preliminary sizing equations contained in API Standard 526. API Standard 526 provides effective discharge area for a range of sizes in terms of letter designations D through T. 

The term set pressure is used to denote the pressure at which the relief device is open fully. This is not the definition used by some — for example, British Standards. For a safety valve the set pressure is often 10% above the set pressure as specified by the manufacturer or measured in the workshop, since this higher pressure is needed to open the valve fully. For a bursting disc, the set pressure is the nominal burst pressure plus any tolerance (or alternatively it is the maximum specified bursting pressure). The term overpressure is used here to mean the difference between the (redefined) set pressure and the maximum pressure reached during the venting process. 

Accumulation The rise of pressure above the MAWP of the pro-tec ted system, usually expressed as a percentage of the gauge MAWP. Note The MAWP and accumulation terms are not included in the ANSI definitions since they relate to the protec ted system instead of the relief device. 

A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a certified quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been Restored. See 5.2.2. In this Workbook, "safety valve" is used to mean any type of safety or relief valve (following the definition in BS 6759). The term is sometimes used to denote relief valves designed for gas/ vapour service only. 

Definitions of terms used in connection with overpressure relief systems. (Reproduced courtesy of the American Petroleum Institute from API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I - Sizing and Selection,... 

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