Gas Compression

[Khansahib]

Ladies and Gentlemen,

40 mmscfd-associated gas at 100 psig is to be compressed and supplied to LPG plant at about 1200 psig. Considering the possibility of LPG plant shut down the gas needs to be directed to gas injection well. This however will requires further compression, 3500 psig, to overcome the wellhead pressure. I am looking into the possibility of multithrow reciprocating compressor with single driver. Is it possible that the final throw of compressor, that boost the gas pressure from 1200 to 3500 psig, is kept on idle (unloaded) until required.

 

[pmover]

for long term operation, a requirement would be to continually flow gas through the compressor (cylinder) for piston/cylinder cooling purposes. simply operating a recip compressor without flow may very well result in failure from heat. this may/may not require gas from from compressor, to cooler (?), through bypass valve, and return to compressor. sending gas to injection well would take place when bypass valve is closed and valves in piping to injection well are open.

other considerations are the torque/hp loads being cycled. the compressor mfg and driver need to be appropriately sized. the compressor mfg needs to address this and needs to be appraised of all possible operating scenarios.

good luck!
-pmover

 

[zdas04]

You could possibly do it by blowing through the last stage. I've seen 3 stage recips running for years with second stage discharge greater than third stage discharge. It doesn't seem to hurt them, and they get lubricated just like the other cylinders. The difference between 1200 and 3500 is less than 3 ratios so you'd only have to blow through one stage.

When the last stage takes the load, hp will go up dramatically (probably a 40% increase in a few seconds), it will take some seriously responsive controls to handle that.
You are talking about something on the order of 8,000 hp into the LNG plant and over 11,000 hp to injection--quite a lot of idle/standby hp.

Might just be the cleverest thing I've heard of today (but I have been all by myself all day).

David

 

[dcasto]

We have a system like this. We just open a valve between the suction and discharge of the 1200 to 3600 psi stage, we we need we close the valve and it pumps.

 

[Khansahib]

Zdas04,
Thanks for your valuable input. I understand your concern about the Idle HP, but the question is in any case I will have Idle HP even if I have a separate machine that is activated only when such requirement arises. The advantage in built-in additional cylinder would be space, maintenance, spares and operating cost as compared to a separate machine. Secondly, I will appreciate to know the bases of you HP figures (8000 or 11000 hp). Finally, your last sentence has duel meaning, how should I interpret it…?

Decsto,
Thanks for the encouragement that you have almost same system. Can you explain a little in depth and what is HP consumption when you turn it on. It will verify Zdas04 figures which he must have some bases.

 

[zdas04]

The information you gave in your original post was 100 psig suction, 1200 and 3500 psig discharge, and 40 MMCF/d. I ran those numbers into a compressor wheel and got 185 hp/MMCF to go to the plant and 245 hp/MMCF to go to injection, adding a bit for hotel loads and some rounding gets to 8,000 and 11,000 hp.

Don't worry about my last sentence, it was my feeble attempt at humor.

David

 

[Khansahib]

zdas04
Based on your calculations only (11000-8000)= 3000 hp will be idle when LPG plant is running and no injection takes place.

As a mater of fact, to keep the question simple, earlier I did not write the distribution of 40 mmscfd. Out of 40 mmscfd around 14 mmscfd at 1200 psig will always be used for gas lift operation and only 26 mmscfd will be available to LPG plant/well injection. In other words only 26 mmscfd will be compressed from 1200 to 3500 psig when LPG plang is not in operation. Considering this compresseion capacity, around 1600-1800 hp will be idle while LPG plant is in operation. I will appreciate your confirmation of my understanding.

 

[zdas04]

So, you are taking a side stream to the LNG plant after the second stage, and always sending 14 MMCF/d to injection? When the plant goes down you plan to just send the extra 26 MMCF/d to the third stage. This sounds like it would work, but I would download a copy of Arei's Performance software (it is free) and see what they say.

David

 

[Khansahib]

David,
That is correct.

Further, I am thinking to have 3 machines, 20 mmscfd each. Two of them (let us say, K1 and K2)will have 4 stage and K3 will have 3 stages. After 3rd stage let us assume the pressure will be 1200 psig. The 4th stage in K1 and K2 is to boost the pressure from 1200 to 3500 psig. This 4th stage in compressor K1 and K2 will be of higher capacity, 26 mmscfd. K1 and K2 will be stand-by to each other unless K3 is down for maintenance.

K1 (with idle 4th stage) and K3 shall be running all the time to supply 40 mmscfd at 1200 psig to lift gas and LPG plant. Incase LPG plant is down K3 additional 6 mmscfd (after lift gas) can be diverted to K1 at the 4th stage to compress it to 3500 psig.

By keeping one machine (K3) without 4th stage can bring the idle HP down to half.

 

[Khansahib]

pmover dna dcasto
.....I am waiting for your input too....

 

[zdas04]

Multiple machines make sense, but why four stages? 100 to 1200 psig is 10.5 ratios at sea level (3.2 ratios/stage for a 2 stage machine) and 100 to 3500 is 30.5 ratios (3.1 ratios/stage for a 3 stage machine). It looks like you should discharge from the second stage to LNG and the third stage to injection.

Truthfully it is too early in the morning to follow your K1 stuff. I don't know why you'd size the site for 60 MMCF/d, I'd probably look at three machines (all configured the same) capeable of 15 MMCF/d each and take a side stream after the second stage of all 3.

David

 

[Khansahib]

David,
The stages are just assumptions it can vary. Regarding 3 machines, I want to keep one machine stand-by instead of running all together.

 

[dcasto]

Sorry gentlepersons, I was out of town.

I've attached a run from a well know supplier, no not soliciting, just an example of what you can do.

At on of my facilities we take field gas from 40 psig to 850 psig to a LPG plant. It takes 3 stages, so 3 throws on the compressor perform that service. The residue is returned at 250 psig and must be compressed to 900 psig. The 4th cylinder does this service and we call it the 4th stage. We sized the the 4th stage cylinders large enough that all the gas could be compressed, so we have excess capacity in the 4th stage. On two of the units we installed a valve that opens the 4th stage discharge to the suction. This unloads the engine by 25%. If we need the cylinder to move more gas, we just SLOWLY close the valve and the engine governor opens up and we move the gas. If we do not need it, we slowly opn the valve.

 

[zdas04]

I've seen that tried before, never very successfully. I saw some comprehensive data a while back to pointed very strongly towards machines running consistently below 80% of full load lasted longer than anyones rotate-an-idle-machine scheme (most of which run the compressor skids at over 95% of rated hp).

I would do it with three machines, sized for 15-16 MMCF/d going into 3500 psig, each running at about 75% of rated hp when the LNG plant is up, increasing over to 95% when it is down. You would want to play around in Ariel's program to see if there is a reasonable way to move the entire amount with two machines for short periods of maintenance, but there is often a reasonable way to do this.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Khansahib]

dcasto,
One thing I do not understand, I will appreciate your clarification. The fourth stage discharge is opened to suction that means the rich and lean gas is mixed? Need more explaination to understand. Meanwhil, the performance sheet you attached will be very helpful when I run my own program provided I can download Ariel software on my Laptop. Thanx

David,
Your proposed configuration of 3 machines will be advantagious only when no breakdown takes place. In practice we need to give operation/maintenance a flexibility of down time incase of breakdown. Further, with 3 machines in operation there may be high excess idle HP when LPG plant is in operation as last stage on all three compressors will not be operational. decasto Ariel performance run (which he attached here) indicates that a 3 stage compressor will be runing at 75% load if last stage is not activated. In my case, when LPG plant is in operation, a two stage compressor will be running at 95% capacity and in parallel a 3 stage compressor will be running at 75% capacity. Whereas, in case of LPG plant is down, the 2-stage compressor will be running at the same 95% while the 3-stage will be loaded to close to 100% due to lining up of 3rd stage.

Gentlemen, I hope none of you are bored or feel challenged. I need very clear concept befor I make presentation of my concept, that is why I am going in depth and engaging your exeperties....Thanks

 

[pmover]

Khansahib,

i am following this thread . . .

the information you are receiving from David and dcasto is adequate and very useful.

sorry, i do not believe in performing design work/calcs for others that are being paid to accomplish. only you know the design criteria and system requirements in full and yes, the users of this forum will gladly assist you upon request.

it appears that your conceptual design is feasible and practical. again, if using a single driver for all stages, with/without multiple units, then the load applied to the driver when transferring to injection should be done slowly (slowly open/close bypass valve (?)). rapid changes to the engine load may lead to shutdown (i.e. overspeed trips, engine overload, etc.).

good luck!
-pmover

 

[dcasto]

The gas won't mix because you will have a check valve farther down stream from where the suction and discharge connects are made.

As pmover says, we try to give some practicle experience when others are trying to do something different. I believe the submitters will pont out their opinions and areas where they had trouble and keep posters from going to far away.

I believe if you do a P&ID of what you want, you'll see quickly where the controls will be required to sence where the process is and so that you can make the system work automatically. The one I spoke of isn't automated and our operators manually adjust, there are controls that cut back production or recirculate gas until the manual valves are adjusted.

 

[dcasto]

For my own clarification, is this an Liquified Petroleum Gas (LPG) RECOVERY Plant or a Liquified Natural Gas (LNG) Plant?

The reason for the question, I can see no reason in going to 1200 psig into the plants as you'll be above the criticle point. I've only gone that high one time because I need to reject ethane prior to a LNG plant.

 

[Khansahib]

pmover and dcasto,

I am sorry if anyone of you feel that I am getting paid while making you work, if you have that thinking than I will appreciate if you do not respond to this thread.

People in this forum are very well aware that this forum is to share knowledge and experiences and not a consultancy service therefore one should refrain from such comments.

dcasto, to you question, it is an LPG plant and battery limit requirement is 965 psig. I did not go deep in probing why such a high pressure is required but may be designer thinking is to run the turbine for refrigeration.

Gentlemen thanks for your valuable comments and input. I am sure that a lot of people in the forum will take advantage from these discussions, particularly the young generation.