I like to know about the thermal expansion PSV function. We got single thermal PSV in ethane reflux pump suction line as well as butane reflux pump suction line, whenever these PSV are taken for maintenance, we are isolating the particular pump and depressurizing. In addition we are de-engerizing the pump motor power supply too. We got two set of argument regarding the isolation procedure for this thermal PSV related equipment.
With above operation it has been treated the thermal PSV like normal fire PSV or the PSV on discharge of positive displacement pump which requires pump isolation. Note: Pump being isolated by block valve only not by spading and there is chance of getting pressurization due to block valve passing or vaporization holdup liquid.
In other way, Thermal PSV given to protect the pump during isolated the condition, which may subsequently buildup hydraulic pressure due to ambient conditions. So pump has to be kept on unblocked conditions to avoid any hydraulic pressure buildup which may get equalize with system pressure. This PSV must be on service when the pump kept isolated. So during PSV maintenance, pump must be kept on line.

I hope you will give us correct procedure to follow for thermal PSV.

Regards
Babu Prasad

Babu,

The way you describe indicated that the system lacking of POSITIVE ISOLATION. Please check the Isolation philosophy. I don't think this kind of issue has not been discussed and documented for a plant handling Ethane & Butane.

Valve leakage is one of the credible scenario. A positive action such as spading, double block and bleed, diverting pipe, etc are required.