PSV(pressure safety valve) outlet isolation valve & expander/reducer location

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1. According to API RP 520 PART2, isolation valve shall be full bore to minimize built up back pressure. And the flow area of the isolation valve shall be same or bigger than PSV outlet flow area. 

There is no requirement that PSV outlet isolation valve size shall be same or bigger than PSV outlet size.

Usually PSV discharge isolation valve is located downstream of expander in order to reduce the back pressure but, it is not mandatory by API.

<API RP 520 PART2>

2. According to API 521, the sizing criteria for the PSV discharge piping is based on total back pressure. If there is no problem on total back pressure which is lower than PSV's MABP(Maximum Allowable Back Pressure),  there is no need for isolation valve is located downstream of expander in the PSV discharge line. 
In case of conventional PSV application,  MABP is 10% of set pressure.

API 521
 3. In case Fluid is gas phase, it is required to avoid sonic velocity or critical velocity. if fluid velocity reaches sonic velocity due to expansion in the PSV discahrge, of course, the isolation valve shall be located after expander. However, for liquid phase, it is not applicable. 

API 521 7.3.1.3.4

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