Pressure relief valve definition

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

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. 

On the smaller, non-API type systems, the relief valve is also the pressure control valve. This definitely must be an external valve. While a compromise for the smaller system, the requirements are also not so severe. 

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. 

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. 

Obviously, definition 2 would apply to a safety audit. The word audit implies a standard. The purpose of the audit is to find out if the standard is being met. This could be a standard for the number and location of fire extinguishers, whether operators have met certain training requirements, whether the relief valves on pressure vessels have been tested in the time period required, or virtually anything else that management deems essential to safety. Audits can involve any or aU of the following physical inspections, interviews, and review of documents and records. 

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. 

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