"PURPOSE The purpose or intent of this procedure is to establish a method for conducting tests to determine performance characteristics of a given design of pressurerelieving device which is required to operate under variable superimposed back-pressure conditions with compressible fluids. 1 It is not intended to be used in testing individual production units at the time of manufacture. The following items should be taken into consideration: a. The variation of opening pressure from set pressure as a result of variations in back pressure. (This item is included because of a divergence between opening pressure and set pressure as superimposed back pressure increases. As this back pressure increases above atmospheric@ a valve may tend to lift gradually rather than to ""pop."") b. The change in reseating pressure as a result of variations in back pressure. c. Capacity under variable back pressure. d. Mechanical operating characteristics."
API RP 525-1960 history
1960API RP 525-1960 TESTING PROCEDURE FOR PRESSURE-RELIEVING DEVICES DISCHARGING AGAINST VARIABLABLE BACK PRESSURE (First Edition)