API plan 54 instrumentation

[joji90]

Guys,

I have a glycol sealing unit( plan 54) for agitator of a vessel, below is the seal PID and arrangement proposed by the vendor

however in the old system ( which is being replaced now) had PSLL tripping the agitator at 2 barg which is not included in the new system by the vendor. Shouldn't there be a low pressure trip also incase the oil unit motor fails or stops??

 

[JJPellin]

It depends. What is the process pressure on the primary seal? What is the reverse pressure capability of the primary seal? How flammable, toxic or environmentally destructive is the process fluid? What are the expected consequences of a release of this process fluid? What are the expected consequences of a sudden trip of the agitator? We have equipment that I would absolutely want to trip on the loss of barrier fluid pressure. We have other equipment that I would absolutely not want to trip because a sudden trip would be much more dangerous or destructive than continuing to run without barrier fluid pressure.

What would happen if this machine trips?
What would happen if this machine continued to run with no barrier fluid pressure?

Which of these carries the most risk to personnel safety, environmental impact, equipment damage, lost production, wasted product, etc.?

Johnny Pellin

 

[joji90]

Johnny, the question is not really regarding the process safety . the consequences of tripping the agitator on the reactor can be assessed by the client process department, however what instrumentation on the seal oil unit ensures continuous sealing fluid to the agitator? lets say all those signals are just alarms and not trips. now my question is shouldn't I include a low pressure switch to get an alarm when there is low flow/pressure in the system say the motor on the seal oil unit fails. the high pressure switch is set at 9 bar cos the PSV pops at 10 bar and if it happens there is no flow to the agitator seal unit .

Regards,
joji

 

[JJPellin]

You didn't ask about a low pressure alarm. You asked about a low pressure trip. Once you add a trip, there is always a possibility of a false trip. If you are asking if this system should include a low pressure switch that could be set up to alarm or to trip. then yes, that is probably a good idea. A pressure transmitter would be even better. Is it a good idea to arm it to trip? I can't answer that with the information provided.

Johnny Pellin

 

[joji90]

Thanks @jjpellin.

Finally the client decided to go with trip only for high temperature of the seal and high pressure of the seal fluid.

 

[georgeverghese]

Dependence on the pump would be much less if you have an accumulator with pressure bladder on the discharge of the pump. Once the bladder has reached max control high operating pressure, the pump stops, and thereafter, the seal runs on rundown from the bladder. The pump restarts when accumulator pressure drops to control low pressure.

Maybe your client feels the backpressure regulator on the return line should be reliable enough to avoid the PSL ?

There is a PSV on pump discharge for some failures, but you need a pressure regulator(set say 8barg or so, since the PSHH is at 9barg) to bleed excess pressure back to the tank in normal operation. You'd destroy the PSV trying in a few days if you used it as a pressure regulator.