The primary purpose of a Pressure Relief Device is the protection of life and property by releasing excess fluid in vessels, line or system in abnormal or emergency situations and thus maintaining allowable limits of pressure at all times.



  1. In a direct spring loaded safety valve the closing force or spring force is applied by a helical spring which is compressed by an adjusting screw. The spring force is transferred via the spindle onto the disc. The disc seals against the nozzle as long as the spring force is larger than the force created by the pressure at the inlet of the valve. The figure shows the enlarged nozzle and disc area of a safety valve with the forces acting on the disc.


  1. In an upset situation a safety valve will open at a predetermined set pressure. The spring force Fs is acting in closing direction and Fp, the force created by the pressure at the inlet of the safety valve, is acting in opening direction. At set pressure the forces Fs and Fp are balanced. There is no resulting force to keep the disc down on the seat. The safety valve will visibly or audibly start to leak (initial audible discharge).


  1. 1 Inspection frequency can be fixed from 1 year to 4 years based on the performance of PSV during pre-pop test as received condition.
  2. 2The First inspection frequency is normally 2 years.
  3. 3Frequency remains same if the same set point during pre-pop test.
  4. 4It can be increased to 1 year more the set point is found to be within +5% to -10% during pre-pop test.
  5. 5Frequency can be decreased 1 year if it exceeds the above limit.

Pressure Safety Valve Calibration - Follow up Tool