Compressor Coolers (increase in gas flow)???

[kirkwarren]

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.

 

[dcasto]

djv, GjFi,

I posted models above.

 

[GjFi]

dcasto:

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

Regards

 

[ARenko]

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.

 

[dcasto]

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.

 

[GjFi]

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

 

[BigInch]

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/

 

[pmover]

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

 

[BigInch]

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/