Relieving contingencies

Individual safety relief valve specifications shall be communicated on API 526 specification sheets or equivalent substitute provided by the end user. When the relieving contingency is vapour and liquid, rates and properties for each at the allowable overpressure shall be provided. 

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 ... 

The first step in the design of protechon against overpressure is to consider ah contingencies which may cause overpressure, and to evaluate them in terms of the pressures generated and/or the rates at which fluids must be relieved. 

It is also important to note that the lowest design pressure of any section of the casing must be specified to be no lower than the pressure which it may be subjected to under the PR valve relieving conditions. This is necessary to recognize pressure drop within the casing. The PR valve should be sized to pass the normal steam flow to the turbine, but credit may be taken for steam flow which is withdrawn from an intermediate turbine stage if it would not be blocked by the same contingency as closure of the exhaust. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

The fluid boiling point at relieving pressure is used for sizing a PR valve orifice for die contingency of fire. 

The coldest and hottest relieving temperature, whether resulting from ambient conditions, operational Mure or fire contingency, is used for the purpose of designing for thermal eiqiansion in the complete system of piping and headers. Credit may be taken for temperature rise or drop in the system, as described above. 

It is recommended that computations of relieving loads avoid cascading of safety factors or multiple contingencies beyond the reasonable flow required to protect the pressure vessel. ... 

In case of failure of a drive affecting a single equipment, the contingency can easily be quantified. For example, the failure of a reflux pump will affect cooling of the overhead vapor, and the column will relieve under a reflux failure situation. 

Control valve contingency is calculated with the upstream pressiue assumed as maximum operating pressure and downstream pressiue as PRV relieving pressure. 

If the relieving temperature exceeds 1100°F, then it can be assumed that the vessel, if carbon steel, will fail before the PRV could relieve. In such a situation, PRV protection may not be very helpful. The use of a rupture disk or an alternative for PRV can be considered. Additional protective measures such as water spray, depressuring, or fireproofing can also be considered. The relief contingency can also be calculated as [3]... 

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