Reliability of a leaking balanced bellows PSV

[rower740]

I am wondering if anybody can offer any thoughts on the following issue:

A balanced bellows PSV in olefin service has shown leakage from the bonnet vent hole and also from the body joint. The bonnet has been tightened down and the body joint leak stopped and the leak from the tell tale reduced to a drip. My question is that there is an obvious failure with the integrity of the valve but how reliable is it that it will still relieve at its set pressure? Also given that there is an external leak path, is it likely that any lifting of the valve would result in a larger leak occurring?

Any views any one has would be good to hear.

Thanks.

 

[don1980]

When the valve is in the closed position, the cracked bellows creates an open path between the discharge system and the atmosphere. The leakage rate will obviously depend on the size of the crack and the pressure in the discharge system.

The effect this has on the set pressure depends on the pressure inside the bellows, due to leakage from the discharge system. That pressure is probably low, but you'll have to be the judge of that. The initial set pressure was based on no pressure (atm pressure) inside the bellows. An elevated bellows pressure, due to this leak, will effective increase the set pressure. But again, that's effect is probably pretty small, assuming there's no a high pressure in the discharge system.

When/if this PSV opens you'll get leakage from the relief stream through the bonnet vent hole.

 

[rower740]

Thanks for your reply.

The system pressure is around 18barg on the inlet side and the discharge of the PSV relieves at 22.5barg into a 9 barg system. It is in place downstream of a feed preheater to a distillation column with the discharge of the PSV venting into the column which itself is protected by two PSV's set for 12barg.

My concern is that if the PSV lifts then there could be a loss of hydrocarbon to the local atmosphere and if the valve did not reseat properly then the leak would continue until the unit is shutdown.

The other concern that it won't lift until a higher pressure is still a concern. As you say the effect on valve performance would be small if the discharge side was a low pressure but at 9barg there could be some issues then.

Thanks for your input.