1 reply to this topic

[BGH]

Good day to all,

I have a question about the fail-safe-mode of a recycle valve of a reciprocating compressor that is in commissioning.

We have a 3 stage reciprocating unit, going from 90 to 1300 psi, there is an automatic recycle valve from the 3rd to the 1st stage, we are workig with natural gas. The problem we are facing is that when the unit stops (normal or emergency stop) the recycle valves fully open (safe mode FO), during this the first stage goes up to 720 psi (above the setting of the 1st stage psv) so basically the PSV open every time the unit stops.

Every where i have looked on internet i found that this recycle valve should work like this, fail-open, but no one explain clearly why.

So my questions are:

1. Why a recycle valve in a reciprocating unit should be Fail-open? it is for protecting the unit? or it depends on the process?

2. If the recycle valve is FO, how to avoid the overpressure in the 1st stage during normal stop or emergency stop? should we have installed a restriction orifice?

Attached you can find the PID of our installation.

Any suggestion will be appreciate.

Attached Files

•  Comp 2.pdf   244.93KB   171 downloads
•  Comp 1.pdf   332.98KB   177 downloads
•  comp 3.pdf   345.68KB   121 downloads

[Chris Haslego]

Bumping this topic back up at the request of BGH.

[thorium90]

Hi, for these compressors, you need to unload them and open the discharge flare before stopping. I believe the compressor will be able to run with recycle fully open and the suction and discharge fully closed. I shall detail how it should be done for both cases to prevent popping of first stage PSV.
For normal stop:
First place recycle valve on manual and open slowly. This will dump gas into the first stage, therefore close the suction slowly at the same time to maintain first stage pressure. At the same time, slowly close the discharge valve also. The reason is that the compressor has lesser load when the suction is being close and the recycle is being opened and so closing the discharge valve keeps the pressure stable.
Continuing doing this until the suction and discharge is fully closed. The compressor is now running on its own gas. The recycle valve may or may not be fully open (depends on sizing of valve and line but it most likely will not be otherwise its undersized). At this time start to completely unload the compressor by opening the flare valve at the discharge line and at the same time opening the recycle valve fully. The purpose of this is so that once the compressor stops, the gas at the discharge (higher pressure) will not all go the suction but will go to the flare (atm pressure).

In the event of an emergency stop, its actually almost the same steps. Only this time, first open the discharge flare fully immediately to prevent the gas from getting dumped into the suction and popping the PSV. After the flare is open, close the suction and discharge valves.
I didnt really scrutinize your P&ID to see if it can be done, but I think you get the idea how.

Also, recycle valves are fail open because if it fails closed it can overpressure. For example, if the plant IA fails but the electricity is still ok, the compressor is therefore still running but without IA, the recycle valve fail close will cause the discharge pressure to fly to the sky along with the rest of the compressor. Therefore they are fail open. Better to overpressure the low pressure suction than to overpressure the already high pressure discharge.

Edited by thorium90, 22 January 2013 - 10:44 PM.

[Bobby Strain]

My philosophy is to always fail a valve closed unless there is good reason not to do so.

Bobby

[Neelakantan]

two things to note:

when the system is designed, somebody didnot see the settling pressure or calculated the setlling volume wrongly?
what is the control/sd requirement.

personally, when the compressor trips recycle valve opening is a catch-all easy option. only thing is that when the compressor restarts, there has to be a vent-off before "ready to start condition", unless the system is designed otherwise.

with recyle valve FO, then sombody has to calculate the settling pressure of the system, so that the cylinders and suction pots are designed accordingly. normally, we also include BDV which will open when the unit has a process or emergency shutdown.

so get back to the designer or design basis and see what is the setlling pressure originally intended.

regards
neelakantan