ABSTRACT
Introduction 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 the 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-standards, API 520 [1,2] and API 521 [3], are widely used. 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 on 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
contingency)
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.
