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Compressor Coolers (increase in gas flow)???

Compressor Coolers (increase in gas flow)???

Compressor Coolers (increase in gas flow)???

(OP)
kirkwarren (Petroleum) Nov 3, 2009
Hello all, new to the gas world (I am a civil recently picked up by gas company for larger infrastructure/project management work) and have been asked the following question:  How much additional flow can we get through our compressors if we install coolers/chillers.  Seems simple for a gas modeling program but we do not have one.  My question is, is it a simple matter of using any of the flow equations pertinent to transmission flows and if so, does it make sense to use our existing situation i.e. gas discharges at approx 100 and takes 4 or so miles to get back to ambient and compare that to the new scenario which is the gas being discharged immediately at ambient?  I am sure I have shown my naivete but would appreciate any help with this.  
 
 
 

RE: Compressor Coolers (increase in gas flow)???

below is a sample of a system where a 10 mile 6" std wall line is buried in 60F soil.  You can run 20 MMSCFD and get an 870 psai output, you can chill the gas to 60F and get 1MMSCFD more or you can add 70 psia to the inlet and get 5MMSCFD more.

Use excel and the panhandle equation and mimic these caese and then you can go from there. as you can see, the gas coming out in the first case never gets to ground temp.

RE: Compressor Coolers (increase in gas flow)???

Or download Ariel Performance and get a program that handles all of the subtleties an nuances very well.

David

RE: Compressor Coolers (increase in gas flow)???

Ariel?  I see this only as a pipeline flow problem, assuming you have the machinery needed to get the pipeline inlet pressure and temperature you want, but in any case ...

Don't get carried away with using the standard flow equations to analyze thermal cases for long pipelines where you estimate or ignore overall heat transfer coefficients.

Dcasto, I think you will only get those flowrates until the soil warms up.  Have you not modeled the soil as a constant temperature medium for the whole length of the line.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

The question was

Quote:

How much additional flow can we get through our compressors if we install coolers/chillers
which I took to mean "how much flow can we get through our compressors".  And it is in the Compressor Forum.  I don't get where it becomes a pipeline question.
I've done a similar analysis on a 3-stage recip where I augmented the fin-fan interstage, oil, and turbo coolers with plate and frame heat exchangers and the capacity of the compressors increased dramatically.  Since the primary goal of the exercise was to dump the heat into an evaporation pond I also modeled water cooling before the after cooler.  That didn't affect the compressor capacity much, but it sure helped the pipeline capacity.  All this stuff works together and it is nice when the effects are all in the same direction.

David

RE: Compressor Coolers (increase in gas flow)???

I see this as a compression problem as well.  Aftercooling your gas is not going to increase compressor capacity. As zdas mentions, increased interstage cooling can.  The Ariel program is a good tool to evaluate this.

RE: Compressor Coolers (increase in gas flow)???

dvj, How do you don't see it as a compression problem, when at the same time you say, "Aftercooling your gas is not going to increase compressor capacity."  I agree it won't increase compressor capacity, but it does incease your pipeline capacity, hence IMO he is suggesting a pipeline problem.

My reasoning was he mentions nothing that might be relevant to a compressor.  What changes in the compressor, assuming the cooler is not interstage cooler?  Inlet P1,T1 and outlet P2,T2 are the same.  The gas conditios arn't changed until it gets to the cooler and enters the pipeline.  Besides the fact that, if its a compression problem, why is he talking about "flow equations pertinent to transmission flows" and 4 miles.

Its only a compressor problem after you know how much you can flow in the pipe, then you can spec the compressor and the cooler capacity and temperature.  It would be kind of backwards to build a pipeline to suit your compressor, wouldn't it?

I'd only use an interstage cooler to keep the later stages running under the limits, not to increase pipeline capacity.

    

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

Dave, as you say, it didn't affect the compressor capacity much, but it did the pipeline.  Exactly what I'm talking about.  Despite my successfull request to get a "pipeline" forum, there are still going to be some guys that will still get the wrong forum.  I think this is the first. smile

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

Kirkwarren:

By cooling the gas after the compressor you increase the density of the gas. The specific volume of the gas will be reduced. The gas velocity in the pipe will be lower and the pressure loss will be lower.
For the same delivery pressure at the end of the pipe, a lower back pressure is build at the compressor outlet. At a lower back pressure the compressor flow rate is higher.

Regards
 

RE: Compressor Coolers (increase in gas flow)???

BigInch,
I said I DO see it as a compression question. My reasons were exactly the same as zdas - he came to a compression forum and asked how much more flow he could get through his compressors.  That seems very relevant to compressors to me.  As for why he's talking about transmission flows - I don't know - he did claim he's new to the industry and may be naive.  You may very well be right, but he asked about compressor capacity.  I guess kirkwarren will have to chime in again to clarify.

GjFi,
You're correct.  I'm not sure how appreciable it will be though.  In our fleet of compressors we see very little increase in flow rates over wide swings in discharge pressure.   

RE: Compressor Coolers (increase in gas flow)???

Hi

Is the chiller to be located upstream or downstream the compressor?

I beileve we need to distinguish mass flow and volume flow.
Volume flow is basicaly dependant on pressure and temperature and MW. At constant mass flow :
Pressure increase >> Volume flow decrease
Temperature increase >> Volume flow increase
MW decrease >> Volume flow increase

Therefore, if the shiller is upstream the compressor, cooling down the gas will reduce volume flow from V0 to V1.
In order to get back to V1 you need to increase mass flow and consider the appropriate equation of state e.g BWRS for natural gas.

However pay attention to the compressor if it is centrifugal.
Indeed, if you change the inlet gas temperature, and even you get back to original Volume flow value, consider you will shift/move the compressor map, with an impact on original surge and choke limits to be reconsidered as well.

Regards
Wimple

 

RE: Compressor Coolers (increase in gas flow)???

quoted
Therefore, if the shiller is upstream the compressor, cooling down the gas will reduce volume flow from V0 to V1. In order to get back to V1 you need to increase mass flow and consider the appropriate equation of state e.g BWRS for natural gas.
unquoted

I meant get back from V1 to V0

regards
Wimple

RE: Compressor Coolers (increase in gas flow)???

OK.  Can't wait to find out.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

(OP)
Thanks to all for the insight.  DJV hit the nail on the head regarding my claims of being new to the industry.  The reason my question was initially asked was because I was in fact confused on if it were a compressor problem or a pipeline hydraulics problems.  I assumed that it could be approached from both angles.  The question that was posed to me to solve was explicity "How much can we increase our flow's if we were to add chillers to our compressors?".  Our demand is on the verge of increasing beyond our deliverability this winter and we are entertaining many scenarios, i.e. looking at storage option's to help with peaking, looking at increasing the HP of our compressor stations and lastly looking at adding chillers to our compressors.  Storage options are obviously long term thus it leaves us with looking at the economic viability of adding chillers or adding HP, chillers being the less expensive option.  That said, my interest in asking the question was to see if it actually was a compressor problem and I should approach it that way, or (seeing as I know next to nothing about compressors) could I simply analyze this problem "beginning" at the discharge side of compressor and look only at gas flow in the pipe at various gas temperature's given everything else is static i.e. length of line, ambient temperatures, size of pipe etc. Seems it would be easy enough to ignore everything upstream of this point.  All of the posts have been helpful, and I thank everyone for weighing in.

RE: Compressor Coolers (increase in gas flow)???

IMO,  If you cool the gas after the compressor, you're looking primarily at what you can get down the pipeline at that temperature, as you haven't changed any compressor variable, that will tend to remain the same.

But, its true that you can't separate the pipeline flow from the compressor flow, so you'll have to check if the compressor will do that new pipeline flow without changing the compression variables, or find out how much they will change when the compressor tries to do that new pipeline flow.  

If end of pipeline pressure is held constant, the compressor discharge pressure will go down, since the pipeline differential pressure reduced.  If you raise the end of pipeline pressure by the same amount that the pipeline differential pressure dropped, then the compressor discharge pressure will remain the same, so it also depends on how you intend to control the entire system as well.



 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

Where are you considering cooling the gas - upstream of compressor, downstream of compressor, between stages, or some combination?

RE: Compressor Coolers (increase in gas flow)???

(OP)
downstream.  

RE: Compressor Coolers (increase in gas flow)???

then its settled, it's a pipeline problem?

RE: Compressor Coolers (increase in gas flow)???

If they keep end of line pressure constant will the required compressor discharge pressure really drop much since the intent is to pump more gas down the pipeline?  And even if it drops will there be much increase in the compressor capacity?  That still has to be checked and compared with the increasing compressor HP option.   

RE: Compressor Coolers (increase in gas flow)???

kirkwarren:
Do you have any performance (Flow rate vs. back pressure) for your compressor? That will help in your estimation.

djv:
Based on my experience I guess about 1 % increase in the flow rate for each 20 deg C reduction on the discharge gas temperature.
 

RE: Compressor Coolers (increase in gas flow)???

djv, GjFi,

I posted models above.

RE: Compressor Coolers (increase in gas flow)???

dcasto:

What is the slope of the compressor chart (flow rate vs. back pressure) on your model?

Regards

RE: Compressor Coolers (increase in gas flow)???

dcasto,
I saw those and they support what I was getting at - we're not necessarily going to lower compressor discharge pressure (as has been suggested) because the intent is to increase flow rate through the pipeline.  

Really what I was getting at was what BigInch said - they still need to check that the compressors will provide the flow.  

 

RE: Compressor Coolers (increase in gas flow)???

GjFi,

How can I get slope of a system with no points to begin with.  I just assumed at the same discharge pressure, what would be the new capacity of a line with cooler inlet gas.  

RE: Compressor Coolers (increase in gas flow)???

dcastro,

I think the compressor provides the gas flow. The pipe causes restrictions to the flow.  By cooling the gas after the compressor, the head losses in pipe will be reduced. For the same pressure at the end of the pipe, the back pressure at the compressor will be reduced. At lower back pressure the compressor will provide more flow. The slope of the performance chart (flow rate vs. back pressure), I think, is needed to estimate the amount of the flow gained by cooling the gas.  
 
Regards
 

RE: Compressor Coolers (increase in gas flow)???

Dcasto has chosen to control the compressor at constant discharge pressure, in which case the PL end pressure will need to be raised to match PL dP and maintain consistant boundary conditons and continuity of mass flow.  If dcasto wants to control it that way, that's its his choice.  Its a reasonable assumption for the data given.  You could do the problem your way, but you'd also need the compressor curves to do it.  They wern't given.  

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Compressor Coolers (increase in gas flow)???

all,

i'm well-acquainted with this matter . . . and value the responses provided.

only two compressor stations, stn 1 at mp2.3 and stn 2 at mp 25.1.  2-12-inch pipelines.  artic environment . . .

compressor performance data is attached with normal and lower inlet temps at station #2, assuming cooling at stn 1 & assuming same efficiencies and orifice flow dP.  the mBWR EoS and AGA #3 were used for analysis.

on a larger-scale, lowering inlet temps may be more economical.  on this smaller-scale, likely not?  i believe the best option is to decrease system resistance rather than to change the process fluid conditions.

hope this helps . . .



feel free to further comment.
-pmover

RE: Compressor Coolers (increase in gas flow)???

I think that there may be some flow increase, some from the gas chiller, but mostly during additional cooling provided by pipeline contact with the adjacent cooler soil, until the ground is heated and the system stabilizes at a new temperature profile.  Over this short distance, that final temperature profile might be relatively high and I would suspect that the initial increase in flow might eventually reduce by 1/2 or more when finally stabilized.  

I think the contrary may be true.  The effects on a short line may be relatively beneficial compared to a long line.  In a long pipeline the gas will progressivley near colder soil temperature, so in effect a chiller only steepens the temperature drop of the initial segments of pipeline and when that local temperature near the chiller is averaged over the entire still colder length of pipeline.  In an artic environment, the temperature would tend to lower that much quicker to soil temperatures, chiller or none.  The average operating temperature might not have changed appreciably at all over a long length.  For a short line the resulting average pipeline operating temperature might decrease significantly with a chiller and flow increase accordingly.  However its also true that gas temperatures might never get close to the average soil temperature a few feet away from the pipeline.  By the time you get to the end of the short line, gas temperature might still be many degrees higher than the average soil temperature, so the percentage increase in flow would not be nearly the same percentage that you would see in a long line, but it might still represent a significant improvement over the previous performance.  I don't think you can draw too much into what the improvement might be without modeling the heat transfer rate between gas and soil.  At low flowrates, there maybe a no advantage, as the soil would cool that gas all by itself.  At medium flows, the chiller might kick in some advantage, but at high flowrates, perhaps relatively little of the gas would be cooled by either the chiller or the soil.  

I would say, the benefit of adding a chiller will be less than what you might initially expect, unless you had high flowrates in a relatively small line diameter over a short length.  That's where a chiller is most likely to help the performance, but whether it can pay for itself, might still be in doubt.  That depends on the value of the increased sales gas flow.  

Offhand I would think that higher pressure operation would be a more efficient method of increasing flow than playing around with cooling, but if the pipe is in the ground, there may be no other option.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

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