Gas Compressor and Gas Cooler 4

[Morph711]

Hi Folks,

First post be gentle!

Trying to reduce Export Gas Cooler Temp down. Would speeding up compressor, but opening up recycle help acheive this? I have to maintain u/s manifold pressure the same.

Cheers
Andy

 

[JJPellin]

Not enough information. What style of compressor? How is the recycle cooled? A diagram would be nice.

Johnny Pellin

 

[Morph711]

Yes Sorry,

Centrifugal compressor - final stage compression from 30bar to 120bar. The Cooler downstream is a plate exchanger sitting at 62 deg C. I want to reduce this to 55 deg c as the trip is 70 deg c.
I thought if the recycle valve is opened more and compressor speeded up might help.

 

[Morph711]

Should add the cooler is Meg/water exchanger. Cannot achieve any improvements on the cooling medium side as the seawater/CM coolers are starting to block up.

 

[Morph711]

We actually tried closing in the recycle and that did drop the temp, however the surge/recycle valve was quite erratic. Too much danger in surging.

 

[LittleInch]

No it wouldn't. Basically you're adding more heat into the gas and including the inefficiency of the compressor, it will increase temperature.

Only if you have very good cooling on the recycle line might it do something, but seems unlikely.

final stage compression looks quite high - 4:1 compression ratio?

What was the design temp / flow / pressure ratio.

Not sure what the seawater has to do with it? Remember we can only see what you show or tell us so this is where any sort of diagram / sketch / technical details helps stop all the questions and allows people to understand your issues.

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

 

[Morph711]

We have a very poor recycle line for cooling. The seawater/ Cooling Med(MEG) exchanger has a blockage with poor flow through the exchanger. Ultimately it will need either a flush or intervention.

We have a Booster compressor u/s of Export compressor which boosters the manifold from 10bar to 30bar.

 

[MortenA]

Listen to what Littleinch says. Increasing recycling will only increase temperature because your adding energy. Get the cooler fixed! You could of course flare a volume equivalent to the recycled volume. If LP system connects to the flare and has additional cooling capacity you might recover the gas this way but your energy consumption will increase.

 

[LittleInch]

It sounds like your running the compressor on the lhs of its curve? So higher inefficiencies , plus a clogged cooler. Anything else not working properly?

Can you increase suction pressure?

Over cool the inlet gas?

Could you use less MEG and more water to improve the cooling capacity?

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

 

[georgeverghese]

Is the hot gas recycle to the contactor running? Could that be making this problem worse?

If this line is not running, then root issue is the blockage on the seawater side of this seawater/MEG-water HX as you say. What type of HX is this? Can you air bump the seawater side of this HX to see if the biological fouling can be dislodged? Is there not a spare online HX in this battery of seawater exchangers here? How is the hypo injection into the seawater feed caisson doing? Can you shock dose the seawater with hypo for a day or 2 and see if the fouling matter will let go of the HX( at the risk of slightly more corrosion on the seawater side of these HXs')?

Agreed, 4:1 compression ratio on this final stage seems unusually high.

 

[Morph711]

Thanks for the replies. All good points. We have decided to back flush the plate exchangers. Not hopeful it will work though. The hypo system has been defunct for over a year with the anode antifouling no help.
Kind of limping along until the next shut down in June. Bringing on a cold subsea well will also help. I've also suggested opening up a another 30" gas stream line d/s of the cooler. Also shutting in one cooler might bump the online one or indeed start another seawater pump for a few moments, if the mv load can handle that.

 

[georgeverghese]

How about installing a temporary hypo injection unit onboard upstream of the seawater coolers? Air bumping (with compressed air) may be more effective than reverse flushing. Agreed, if you could start up the spare seawater pump, the extra velocity together with air bumping may help also.
If you have spare inline plate HXs, then removing one and replacing with a new clean one(which you could purchase quickly off the shelf) is another solution. The work your way through similarly with all the other fouled plate HXs'.
Also check why the trip setting is at 70degC? Can you move it up to say 75degC in the short term till your next planned shutdown? Where is the limitation here?