API Recommended Practice 520 provides a list of potential causes of overpressure (also referred to as “operating contingencies”) that should always be considered when analyzing an equipment item for its pressure protection requirements. While this list covers the contingencies that should always be considered, careful examination should also be made of the specific process being analyzed to ensure that all potential causes of overpressure are covered.
Of the sixteen contingencies listed in RP 520, several apply very often to equipment types that are installed in nearly all process facilities. Other contingencies on the list may apply only to certain equipment types, or perhaps to several types of equipment, but only in relatively rare circumstances. We will first discuss the contingencies that most often apply to typical equipment types, and then direct our attention to contingencies that are less frequently relevant. The following list of contingency names is taken from API Recommended Practice 520. The evaluation of relative frequency of occurrence is not from Recommended Practice 520, but reflects industry experience.
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Prior to discussing causes of overpressure, it is important to have an understanding of the terms accumulation and overpressure.
It has long been common industry practice to set a pressure relief device at the design pressure of the equipment item(s) it protects. However, it is recognized that most pressure relief devices do not open fully as soon as they reach their set pressure. For example, a conventional spring loaded pressure relief device set at 100 psig may not open fully until the pressure at its inlet has reached 110 psig, or 10% above the set pressure. The amount of pressure above the relief device set pressure is referred to as the accumulation.
Equipment design codes take this pressure relief device behavior into account by making it permissible for the internal pressure to rise above the design pressure of the equipment item during a relieving episode. For example, Section VIII of the ASME Boiler and Pressure Vessel Code states (at paragraph UG-125) that all pressure vessels “shall be protected by a pressure relief device that shall prevent the pressure from rising more than 10% or 3 psig, whichever is greater, above the maximum allowable working pressure except as permitted” in two subsequent sections. The amount of pressure rise permitted by the relevant design code is the “allowable overpressure”. Allowable overpressures vary by code, equipment type, and cause of overpressure. The most common allowable overpressure is 10% of maximum allowable working pressure (MAWP).
The phrase “design pressure” will be used below to refer to the maximum set pressure permissible by the relevant design code for a single relief device on the equipment item in question. For the ASME Boiler and Pressure Vessel Code, this is the MAWP; for the ASME Piping Code, this is the design pressure; for a pump or compressor, this is normally the pressure rating of the casing.