Danger
Both onshore and offshore process installations are often provided with pressure relief systems.
Having a spare relief capacity enables continuous production while conducting maintenance on your system on a live plant without a shut down of the process.
These twin or multiple pressure relief systems are often fitted with isolation block valves upstream and downstream of each safety relief valve.
At all times it should be prevented that all relief valves are blocked at the same time.
Always open one of the spare lines first before isolating the operational relief valve to avoid overpressure explosions of the pressure vessels.
Solution
To ensure that at least one relief valve will be open at all times a valve interlock can be installed on each upstream and downstream block valve. Taking the start key from the control room commences the change over of the PSV system following the sequence step by step. The system also ensures that the higher pressure rated upstream block valves are closed prior to the lower pressure rated downstream block valves.
The quantity of the (safety relief) valves may vary and is always depending on the client requirements.
Products
Key cabinet
- option 2 bolt lock
Operation
The upstream and downstream block valves of PSV A and the downstream block valve of PSV B are locked open. (V1, V2 and V4)
The upstream block valve of PSV B is locked closed. (V3)
Note: The downstream block valve of PSV B is normally kept open because of the higher pressure rating of the upstream block valve of PSV B.
Taking the start key B from the control room commences the change over of the PSV.
Insert key B in valve V3 to unlock the closed position of the valve. Open valve V3 and take out key C. Valve V3 is now locked open.
All four block valves are locked open consequently both PSV A and B are open.
To isolate PSV A unlock the open position of valve V2 by inserting key C. Close valve V2 and take out key D. Valve V2 is now locked closed. Insert key D in valve V1 to unlock the open position of the valve. Close valve V1 and take out key E. Valve V1 is now locked closed. PSV A is now isolated. With key E back in the control room a work permit can be obtained to service PSV A for testing or maintenance.
To go back to the original position of the valves the same sequence has to be mirrored.
Depending on which key is stored in the key cabinet you have an overview of each status of the valves.