5 replies to this topic

[Matteo Giorgio Marrano]

Goodmorning,
i have a question concerning a fuel gas compressor for a Power Plant.
This is my first time in compressors so, waiting for a reply to a request of offer submitted to a manufacturer some days ago, I would like to understand more in order to be able to comprehend what the manufacturer will say to me.

The fuel gas will be compressed in order to enter a turbogas.

1) First, these are my input data:
-fluid: natural gas (principally methane)
-flowrate: 8750 kg/hr
-specific gravity: 0.6
-mol. Weight: 17.527
-Inlet pressure: 30 barg
-Inlet temperature: to be advised in order to obtain 70°C on outlet
-Outlet Pressure: 48 barg
-Outlet Temperature: 70°C

2) Using a spreadsheet taken from this forum some months ago (sorry I dind’t remember the author, see attached snapshot of calculation), I obtained the required power=approx 205 kW. The suction temperature in order to obtain 70°C on the outlet should be 24°C approx.

Is it right my calculation?I don’t have experience on numbers concerning compressors, so I don’t know if this number is right or not.

3)In the hypotesys that the data are correct, what would be the type of compressor to be used in this case?reciprocating or centrifugal?

Thanks you so much

Matteo

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[ankur2061]

Matteo,

For a centrifugal compressor, polytropic process, for your conditions I get the following results with my spreadsheet:

Inlet temperature: 27.8°C
Gas Power Requirement: 210 kW
Mechanical losses: 3%
Shaft power requirement: 216 kW

According to the standards of a globally reputed company for flow rates above 300 Am³/h (175 CFM), centrifugal compressors are recommended. Your flow rate turns out to be 377 m³/h at inlet conditions of pressure & temperature, so a centrifugal compressor should do the job well.

Regards,
Ankur.

[Matteo Giorgio Marrano]

Matteo,
For a centrifugal compressor, polytropic process, for your conditions I get the following results with my spreadsheet:

Inlet temperature: 27.8°C
Gas Power Requirement: 210 kW
Mechanical losses: 3%
Shaft power requirement: 216 kW

According to the standards of a globally reputed company for flow rates above 300 Am³/h (175 CFM), centrifugal compressors are recommended. Your flow rate turns out to be 377 m³/h at inlet conditions of pressure & temperature, so a centrifugal compressor should do the job well.
Regards,
Ankur.

Dear Ankur, thank you for your prompt reply

Thanks, my results seems to be similar to yours.
Unfortunately I cannot say who is the author of my spreadsheet, maybe someone of this board (in fact I downleaded from it), but I think that my spreadsheet could be used in first approssimation. Do you think the spreadsheet I am using is right?

Let me understand and clarify if m calculation are ok on a basis of G=8750 kg/hr:
Actual cubic meters: 390 m3/hr (approx)
Normal cubic meters: 11000 Nm3/hr (approx)
Actual cubic feet per minute: approx 240 ACFM
MMSCFD: 20 MMSCFD
Considering eventual approximation (I have to re-control it), are my conversion “quite” right?
I understand that a centrifugal compressor will be ok and this what I would like to expect from the manufacturer.

Let me ask you another three question, if you can reply to them it would be great:
1) In my case the inlet temperature of gas will probably changes due to the fluctuation of gas B.L. characteristics. This thing will affect the outlet temperature of compressor or there is some device that, in any case, will give me an outlet temperature of 70°C? What happens if the inlet temperature is not 25°C but, in example, 15°C or 35°C?
2) Do the centrifugal compressor need a constant inlet feederate at a constant pressure or not?In my case do I have to guarantee at inlet of compressor the flowrate of 8750 kg/hr and 30 barg pressure or I can give to it other flowrates and pressures?

3) Before the gas enters the compressor I have a reducing station that will reduce the pressure to 30 barg. There is a specified reason why I have to reduce the pressure to a constant value in this kind of plants?(this question can be attached to question 2). Maybe, according to point 2, I have to feed the compressor with a constant pressure?

Sorry for my ignorance in this matter…

Thank you so much ankur

Matteo

[Zauberberg]

Your unit conversions are right, I got the same (~11,000 Sm3/hr) suction flow. Now you have expanded your question further, and the answer cannot be so simple unless you give us the range of desired operating conditions (cases) for this particular machine.

1) In case of lower suction temperature, discharge temperature will drop as well (for the same compression ratio). What is the concern for having discharge temperature different (lower, higher?) than 70C?

2) If you opt for centrifugal machine (fixed speed), it will operate on its flow/head curve. There is a minimum suction flow for each centrifugal compressor - called Surge point - and if you envisage this operating condition sometimes in the future, you should install antisurge system or consider another type of compressors (e.g. rotary or recip), or evaluate feasibility of having variable speed driver. How many design cases (flow/pressure) you have?

3) No one can answer this question except you


Best regards

Edited by Zauberberg, 24 November 2009 - 06:44 AM.

[Matteo Giorgio Marrano]

Your unit conversions are right, I got the same (~11,000 Sm3/hr) suction flow. Now you have expanded your question further, and the answer cannot be so simple unless you give us the range of desired operating conditions (cases) for this particular machine.

thank you Zauberberg for you reply.
I'll try to explain more, sorry but all my questions come in like a "stream of consciousness" like in Joyce:)
The contribute of you and the entire forum is very very important for me in order to improve my culture on every new thing that i found during my work.

1) In case of lower suction temperature, discharge temperature will drop as well (for the same compression ratio). What is the concern for having discharge temperature different (lower, higher?) than 70C?

The battery limit condition (inlet of gas turbine) is 48 barg and 70°C, so i think this is a fixed point.
Maybe the turbine is designed to work at these conditions and is not allowable to change the output of compressor.

2) If you opt for centrifugal machine (fixed speed), it will operate on its flow/head curve. There is a minimum suction flow for each centrifugal compressor - called Surge point - and if you envisage this operating condition sometimes in the future, you should install antisurge system or consider another type of compressors (e.g. rotary or recip), or evaluate feasibility of having variable speed driver. How many design cases (flow/pressure) you have?

I have one design case, sincerely it is very poor of data. Anyway i can assume in first approximation that the flowrate will remain +or- the same. Regarding the pressure i understand that if the compressor is given from 30 to 48 abrg, i cannot let the inlet pressure to be different from 30 barg. Is it right?

3) No one can answer this question except you

i cannot answer:) The only thing is that i suppose che the compresso need a specified and constant pressure in order to work.
The fact that the value is 30 barg, maybe can be explained considering that the fuel gas coming from battery limit has a pressure variable from 30 to 66 barg, so 30 barg is the minimum pressure that we can reach.

Thanks
Matteo

[Zauberberg]

If the client hasn't supplied you with full set of data, you have to work with what you've got - and that is essentially one set of operating conditions. So you have an easy task.

If the inlet pressure drops below 30barg, a fixed speed centrifugal machine - if designed for a single pressure case - would not be able to deliver full flow to the turbine. Or, if you want to play it safe, overdesign the compressor and use suction throttling or consider variable speed driver. That's a safe option. Or, you can always revert back to the client and ask for additional clarifications.