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]

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.

 

[zdas04]

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

David

 

[BigInch]

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/

 

[zdas04]

The question was

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

 

[ARenko]

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.

 

[BigInch]

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/

 

[BigInch]

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.

**********************
"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/

 

[GjFi]

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

 

[ARenko]

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.

 

[wimple]

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

 

[wimple]

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

 

[BigInch]

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/

 

[kirkwarren]

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.

 

[BigInch]

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/

 

[ARenko]

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

 

[kirkwarren]

downstream.

 

[dcasto]

then its settled, it's a pipeline problem?

 

[ARenko]

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.

 

[GjFi]

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.