API RP 521, Section

API RP 521 Section 3.15 contains extensive guidance relative to relief requirements for external fire (as discussed earlier in this book). 

Any situation not covered by the rest described herein, but covered by the 16 overpressure scenarios in API RP 521 Section 3.1. 

For some relief scenarios, correlations have been established for the relieving load. For the external fire case, API RP 521 (Section 3.15.2) gives... 

The release of hydrocarbons or other flammable vapors to the atmosphere should comply witii API RP 521, Section 4.3. 

API RP 521, Section 3.18.5, indicates that, where the low-pressure side is in the vapor phase, full credit can be taken for the vapor handling capacity of the outlet and inlet lines, provided that the inlet lines do not contain check valves or other equipment that could prevent back flow. The same approach would apply in cases in which the low-pressure side is liquid full, provided that the released material also remains in the liquid state. However, when the low-pressure side contains liquid and vapor is released or generated through a rupture tube, the effective relieving capacity with which the piping system can be credited should be based solely on an equivalent vapor flow. 

While this is the general recommendation, compliance with this is not always possible. As seen earlier in this book, higher oudet pressure drops may result in reduced valve capacity and instability. In that case, other types of SRVs as discussed in Chapter 9 need to be taken into consideration. API RP 521 also states the outlet pressure drop for individual relief valves should be based on the actual rated valve capacity consistent with the inlet piping pressure drop as discussed in Section 6.1. 

Since there are many potential causes of failure, it would be nice to have a checklist to make the analysis organized and somewhat standard. As a guidance, a pretty good checklist is given by the Guide for Pressure-Relieving and Depressuring Systems, better known as API Recommended Practice 521 (API RP 521) table 1 in Section 3 (Table 13.1). 

Design codes and standards such as API RP 521 and the DIERS Project Manual (Fisher et al., 1992) should be consulted for other correlations and recommended methods for calculating relief loads. The DIERS Project Manual also discusses calculation of relief loads for underpressure scenarios (Section 13.17.6). 

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