With referance to: Pressure Level Relationships chart

PRV Testing – Blowdown; 

Blowdown is the % difference between Closing Pressure (Reseating Pressure) and Set Pressure. In general, Blowdown is set to 7% for ASME Section VIII and 4-6% for ASME Section I. If the blowdown is too much, PRV Will close slowly and cause a lot of Process Fluid loss. If the Blowdown is too low, PRV will cause a lot of Simmer, which causes Disc and Seat ‘s Valve can damage.

Refer to: What is PRV pop test? (Popping test)

PRV – Requirement of Overpressure and Blowdown per ASME Section I; 

For Pressure Relief Valve (PRV) is designed in accordance with ASME Section I Power Boiler has to be Overpressure or % of the pressure that exceeds the Set Pressure of the PRV open no more than 6% and will be Accumulation or % of pressure overload. From the MAWP of the system not more than 6% as well

In addition, for Blowdown (% difference between Reseating Pressure and Set Pressure), there must be at least 2% Minimum Blowdown of Set Pressure or 15 kPa in the case of Set Pressure below 750 kPa to prevent chattering between Seat and Disc of the PRV and the maximum blowdown must not exceed the values ​​in the table divided by the set pressure of the PRV .