Comparision between 2 reciprocating compressor

[Sabergg1981]

Hi all.
I want to know about advantages and disadvantages of these 2 configurations form your approach.
I want to compress 780 m3/h from suction condition of t=65 & p=5bara to discharge pressure of 126bara. Molecular weight is about 32 and i have sour gas to compress.
I want to use LP,MP,HP configuration to overcome increasing discharge temperature. But as a parallel i can use a single reciprocating compressor with 6 throw to meet my condition with a 1 electro motor.
What do you suggest? And why? Consider intercooler's prices.
Thanks

 

[zdas04]

You were doing find till the last line. Now it reads like homework.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Sabergg1981]

Why homework? It's an industrial problem. Selection one of these configuration deeply depends on economical approach,where i have not enough data about prices. A reciprocating compressor with 1 electro motor( also 1 in spare) or 3 compressor in series( LP,MP,HP) ( also with 1 spare in every stage) with 6 electro motor. Which one? It seems 2 configuration are equal. I want to know which one do you select?

 

[zdas04]

I've only seen the construct "What do you suggest? And why? Consider intercooler's prices." in text books. Never in real life. If you'd left that out, I would have answered your question.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Sabergg1981]

How do i explain my problem dear zdas? Drop the last line and answer. I know your approach is very useful. Thank you

 

[LittleInch]

Can you sketch the two options please so we are clear what the choices are. I'm confused by the phrase "6 throw", but maybe that's just me. ...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Sabergg1981]

1) a single recip compressor with 6 throw
2) 3 recip compressor in series( LP,MP,HP)

 

[petrochemical87]

your query is not clear
1)Is this six throw recip a single stage compressor? a compression ratio of 126/5 in a single stage is not possible?
2) The LP/MP/HP setup implies a three stage compressor with intercoolers?


please clarify

 

[Sabergg1981]

@petrochemical
1)it depends on your discharge temperature
2)yes

 

[zdas04]

OK. You have 26 compression ratios. The normal way to do this is with a 3 stage compressor, but 32 MW gas is definitely not a plant product unless you're compressing Germanium. If this is a hydrocarbon gas mixture (and your "sour" comment seems to confirm that) then I have to think in terms of variable composition and a lot of condensible hydrocarbons.

I would be reluctant to do this in a single 3 stage machine (regardless of the number of throws). I would probably make the first machine a single stage recip (the gas is just too difficult to ever get a compatible screw oil) and do the rest with a 2-stage recip. The single stage has some reasonable ability to handle variability, and getting the gas up to around 20 bara fixes a lot of low pressure operations problems for the 2-stage skid. Doing 6 ratios in two stages is a cake walk.

As to how many frames you need. I would twin both frames. Design two single stage recips in parallel each able to move 90% of your required flow and discharging into a common header. Then put two 2-stage machines (each able to do 70% of the design conditions) in parallel sucking on that header. By oversizing the first stage you are accounting for all of the unpleasantness of your gas and low pressures, which allows the 2-stage machines to have a chance to work. The compressor guys will tell you that you are wasting available power by doing that, but what are you selling, KPI's or gas?

Putting this into three frames is a waste of money since every skid have a fixed cost component that can be significant (a two stage cooler costs about 120% of the cost of a single stage cooler or 60% of two single stage coolers). Doing it in a single frame forces the aggressive use of suction control to keep all the stages balanced. Two frames is a compromise.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Sabergg1981]

@ zdas04
I cant truely understand why we should have change our mind to a parallel 90% recip in first compressor and to 70% 2-stage recip after header? Why dont we use 100% recipcompressor

 

[zdas04]

I'm just telling you how I would handle a very difficult gas. Do what you want, but remember that compressors need maintenance. Compressors go down, sometimes with problems that require considerable time to get parts. For a compressor station whose outage would cause an entire field to be shut in, I'm going to install some redundancy. You could also put one 100% single stage in front of one 100% two stage and hope that the capacity of the field never changes and that nothing ever breaks, but that is a lot of "hope".

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Sabergg1981]

@zdas
I dont want to account hope in design and i also find out behind of every comment of you, a lot of experience is hidden. I ask you because this problem is related to the process guys and i want to know my problem very well from process approach. I understand because of variable behaviour of gas field you dont use a (1+1) 100% recip compressor. I must explain that these compressor are used in a off-gas unit of gas refinery, so we dont face of variable behaviour of gas field. From now on, i want to know you insist on 90% (1+1) recip in first stage and a 2 stage of 70% (1+1) recip?
Then i want to know about unpleasantness of gas for oversizing the compressor. Would i ask you to explain more?

 

[zdas04]

For field gas that heavy, the composition changes from second to second. Often considerably. One component will change phase faster than another component at a given pressure and temperature, but when the first component gets close to being all gas, the next component will use that energy and accelerate its phase change. I've gotten 5 samples within an hour on a rich gas field and some of the components would change 30% from sample to sample. That is really hard on compressor performance. That is what I mean by unpleasantness.

I've never worked inside a plant, so I don't know how consistent the off-gas is in real life. If the off-gases have a reasonably consistent composition then I would back off on my safety factors. Probably to no lower than each compressor sized for 60% of full stream because running at a higher load than that in steady state has a long history of high rate of failures.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"