Interstage pressure problems

[paulpaul]

Hi to all, I am new to this forum!

I briefly present myself: I am an european reciprocating compressor designer: our compressors are specifically designed for CNG service, but also fit good for a wide range of applications (nitrogen, hydrogen, biogas, and other). Rated power ranges, depending from the model, from some kW to about 500 kW.
I have a question for you!

I am facing a never-experienced problem with two identical natural gas compressors installed in the same plant. Specs of each one of them:
Rated power: 50 kW about
Suction press: 4 barg
Discharge press: 250 barg
3 cylinders on "W" arrangement, 4 stages
Speed: 1500 rpm
Driver: AC motor, directly coupled by means of an elastic joint
Regulation: ON/OFF
Unloading at start: valve lifters on 1st and 2nd stage, bypass on 3rd and 4th stage.

After some days of normal operation, I experienced a start problem on 2nd stage pressure, regardless it was a hot or a cold start and regardless of discharge pressure.
The 2nd stage pressure, wich normal value is 55/60 barg, contiunues to rise to 75 at start (reached in a few seconds after start), causing compressor to stop in alarm and in some cases opening safety valve.
We have changed 3rd and 4th stage suction and discharge valves many times: for a few days the compressor run properly, then the same start problems comes out again.
We cheched valves proper sizing with the manufacturer, we changed valve disc material and springs, we replaced cylinder liners and piston rings on 3rd and 4th stage, but after some time of proper running the problems comes out again and again, sometimes after a few hours, sometimes after a few days.
Normally, at the 5th or 6th start attempt the 2nd stage pressure rise but stops rising at lower values, about 70 barg: after few seconds, pressure drops to regular values (55/60 barg), and if the compressor doesn't stop anymore, continues to run properly.

The problems comes out only at startup (and not always), regardless of discharge pressure and regardless of machine and external temperatures and other conditions.
For us it's a true rebus, I am thinking about something in the gas (I have an inlet particulate filter of 5 micron filtration degree) but the valve manufacturer (a primary european manufacturer) is sceptical about this. I never found water in the gas. I thought also about pulsation phenomenon at startup.
I will be very grateful if you can help me. If you need some more informations, don't hesitate to ask!

 

[zdas04]

I'm assuming that the first stage discharge pressure is about normal during this event.

I think you are on the right track looking at the 3rd stage valves, have you looked at the valve lifters too? The only thing I can think of that would make a particular cylinder spike discharge pressure with a reasonably constant suction is that the gas has no place to go. Since this is only a start up phenomena then I think you can rule out blockage in an interstage cooler. It keeps coming back to a third-stage suction valve problem (either installation or design) or a problem with the valve lifter.

Sorry not to be more help.

David

 

[monaco8774]

Is this compressor taking its gas from a subsea reservoir, on start up from a production well sometimes the production comes in cold initially and this means you klnock out more liquids upstream and have a lighter mol weight for the gas untill main production comes on, also sometimes an alternative source gas is used for start up.

If this is your case a drop in Mol weight would have to be quite significant say 19 to 17 assuming interstage temperatures stay the same.

Maybe a long shot but if this is close to your case you could look at upstream conditions for the times you have had teh interstage problem

 

[rzrbk]

I would also check for leakage in the spillback line. If i read correctly, it recycles 4th stage discharge back to the 2nd stage discharge.

 

[paulpaul]

First of all thank you all for your interest.

We checked some times ago interstage cooler and it seemed ok. One of the first things we checked was also the correct closing of 3rd and 4th stage bypass at startup...

@ monaco8774: compressor does not take gas from a subsea reservoir, but from a pipeline. Howewer, since the problem begins to appear always at the first startup in the morning (and then after some time appears also more frequently at successive startup) I thought about something wrong in the gas composition.

In the last two days I replaced in one machine 3rd and 4th stage cylnder liners and piston, slightly increasing their bore. Now 2nd stage pressure is about 42/45 barg and 3rd stage pressure is about 76/80 barg (about 20 barg decrease from previous condition, almost exactly as planned by my calculations).
Now we'll see if it valves work in better condition with those lower pressures.
I'll keep you informed! Thanks to all