Seal Gas Pressure Higher than Compressor Suction System

[DoraeS]

Hi,
I would appreciate if someone could help me with the following question:

I have a compressor with suction system designed for 20 barg. The seal gas supply pressure is at 22 barg, which has taken into account of normal suction pressure and worst case settle out pressure (around 19 barg). My question is that during a compressor shutdown and when it remains pressurized, will the seal gas overpressurise the compressor suction system since it has higher supply pressure than suction design pressure? or is this common and how to ensure overpressurisation does not occur?

Thank you for your contribution.

 

[LittleInch]

A drawing would help, but yes, in theory the seal gas can slowly pressurise the compressor, but unless your seal is pouring gas out or your inlet system vey small you shouldn't have an issue.

An inlet design pressure of 20 bar seems low.

Isolating the seal gas supply whn the machine isn't operating seems like a good idea to me.

Got a drawing of the seal assembly?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Is there a PSV upstream of this compressor casing set at 20barg, which cannot be normally isolated from the compressor casing? That PSV should easily handle this seal gas leakage into the compressor casing. Is there a firecase PSV on the suction scrubber set at 20barg?

 

[DoraeS]

Hi LittleInch and georgeverghese,
Thanks for your help.
The compressor suction system contains a suction scrubber, without any isolation from the compressor casing. And yes the suction scrubber has a fire PSV. The suction system design pressure of 20 barg is based on worst case settle out pressure + 10%, as per client standard.
Is this a normal scenario where for prolong pressurized shutdown of a compressor, the seal gas will gradually increase the system settle out pressure? Then we rely on the suction PSV to handle this?
Thanks.

 

[LittleInch]

I'm not really a compressor man and you might get better answers if you posted in the gas compression forum in the mechanical engineers section.

But also have a look at and post the seal parameters and gas flow, volumes etc so you can work out how long it takes to pressure up your suction space.

Also why is this being kept pressurised, but shutdown. My experience is that compressors usually get blown down when they are tripped or after a limited period, in part because of this issue.

It is isn't good practice to use a PSV for something you can foresee happening. PSVs have a margin on them below set point so if it's set at 20 barg could easily start chattering at 18-19 barg.

SO first figure out why this is being kept "live", how much gas is going past the seal and how long are you keeping it in this condition.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

In your case, there is no choice but to rely on this suction scrubber PSV. Normally, with bigger margins between settleout and suction side mechanical design pressure, the seal supply design pressure would be less than compressor suction design pressure (and so would the PIC controlling seal supply pressure). In your case, if you want to avoid this PSV from lifting every now and then for too long in such a scenario, you can consider an automatic total blowdown after say a given time. But check with Operations first if they would agree to blowdown after this xx minutes in pressurised conditions, since they would be reluctant to go through the entire purge step in the startup sequence after total blowdown.