Chattering is rapid opening and closing of a pressure relief valve. The rapid viberations may cause cause misalignment, valve seat damage, and mechanical failure of valve internal and associated piping.
Causes for chattering:
- Excessive inlet pressure drop or PSV inlet long line ( This will result in considerable decrease in pressure by the time it reaches the PSV and the PSV shall close and not relieve the excess fluid. Then the fluid will go back to the vessel and then to the PSV and back and front and cause damage).
- Excessive back pressure
- An oversized relief valve
- A relief valve that can handle widely varying florets
- PSV inlet line is smaller than the PSV vale inlet
- Evidence of line pulling with corrosion or process materials
- Set Pressure is equal or very close to the Operating Pressure
- PSV inlet and discharge pressure are very close (Inlet and outlet pressure)
- PSV inlet line pressure drop has to be well below 3 % ( very imp.)
- Multiple PSVs with same pressure setting
-For larger size of safety valves, chatter could occur because they have relatively small outlet area ratio. (the ratio is specified by API526)
Difference between chatter and flutter/simmer
Pressure Relief Valve Chatter is rapid reciprocating motion of the disc where the disc contacts with the seat in cyclic motion. Flutter/Simmer is similar to chatter except the disc does not contact with the seat during cyclic motion.
Why Chatter is destructive ?
Chatter is destructive where it repetitive and rapid reciprocating disc knock on the valve seat may cause
i) damage to the disc & valve seat lead to leakage and passing
ii) damage to bellow lead to gas leak via bonnet vent
iii) damage to the PRV itself and interconnecting piping through vibration
iv) severe noise level
v) Reliving flow rate reduction caused by insufficient valve opening due to chatter
When the relief rate is lower than 25% of PSV rated capacity chattering will be occured because relief gas kinematic force should overcome to PSV spring force and let PSV start to opening and after PSV release if relief flowrate be lower than PSV discharge rate, PSV will be closed again and waiting for increasing of flowrate to minimum rate and after that cycling start again.