Overpressure protection is the most fundamental safety requirement for plant design. A large number of situations are possible for a particular pressure relief valve (PRV), and careful consideration is required to quantify all scenarios. The design of a particular pressure protection system depends upon the contingency calculated for a particular situation, and often more than one contingency situation controls the design. The design aspects of a pressure relief system are dictated by the mechanical design code; however, two API-recommended practices, API RP 520 [1,2] and API RP 521 , are widely used. Although these API codes are only recommendatory, the impact of violation requires careful consideration. Contingencies for the design of a pressure protection system can be divided into two major groups:
• Impact on plant design • Impact on individual design
There are contingencies that need not necessarily be considered for the design of a particular PRV but will have an impact in the design of the plant relieving system (flare or cold vent). Relieving contingencies under all possible scenarios need to be established for the following situations:
• Overall plant power failure • Failure of a particular motor control center (MCC) • Cooling water failure • Instrument air failure • Steam failure • Fire (sometimes fire in a particular fire circle controls the total
There are a large number of contingencies that influence the design of a particular PRV, but may have no impact on the design of the overall plant. The controlling contingencies need to be established and reported for the following situations:
• Check valve failure • Blocked discharge • Control valve failure • Thermal expansion of liquid • Heat exchanger tube rupture • Reflux failure and overhead system • Loss of reboiler heat • Venting of storage tank • Failure of individual motor • Accidental closure of valve
Most design houses develop a typical table, normally called contingency quantification, which tabulates all possible contingencies for a particular PRV. Although tabulation of all contingencies under the situation “impact on plant design” is essential, only the single controlling contingency (if applicable) under “impact on individual design” requires tabulation. Some operating houses have the practice of tabulating all contingencies under impact on individual design, which are preferably avoided. These tabulations do not serve any fruitful design purpose but sometimes confuse the designer.