7 replies to this topic

[Kriss]

Hi,

 

I am a Rotating equipment engineer and new to this forum.I have following question's in Centrifugal compressor. Appreciate if any one of the meber can advise me for the blow quries.

 

1. Normally maximum number of stages in centrifugal compressor are betwwen 8 ~ 10, not exceeding 10. But there is no restriction in API 617 and other codes. Can any one explain why the number stages not exceeding to 10. What will happen if it exceeds more than 10 stages ?

 

2. I have seen that for low molecular weight gases, we do not use centrifugal compressor because head required is high. Is there any molecular weight range has been defined for centrifugal and reciprocationg compressor ?

 

3. Is there any tip speed limitation while selecting a compressor ?

 

4. If any one is having centrifugal compressor spread sheet for power calculation, please share with me.

 

Thanks,

Krishna

[PingPong]

1) The more impellers in a casing, the longer the unsupported shaft and the thicker that shaft has to be. Has to do with bending, vibration, first and second critical speed(s), .....

 

2) Selection between centrifugal or recip or other type compressor is based on economics, not on a specific mol wt.

 

3) Tip speed is limited by the tensile strength of the selected impeller material. Presence of corrosive or acidic or basic components in the gas can lower the maximum allowable tip speed further.

 

4) I am sure that if you google you will find several online calculators, and spreadsheets, for calculating compressor power.

Edited by PingPong, 16 June 2014 - 04:35 PM.

[Kriss]

Dear Pingpong,

 

Thanks for your response.

 

1) Noted. I need a small  clarification on first and second critical speed. What I understand from your explanationa that we will not get required margin betwwen critical speed and operating speed as reuired by API. Please confirm if my understanding correct.

 

2) Not clear to me.For less Mol. wt gases like hydrogen, we go for Reciprocating compressors. Could please clarify what economics to be considered while selecting between Recip and centrifugal compressor.

 

3) Noted.

 

4) ok.

[ankur2061]

krishnapadhy,

 

Compressor selection is based on the combintation of volumetric flow rate and the pressure ratio. Low flow rates and large pressure ratios favor selection of reciprocating compressors and vice versa for centrifugal compressors. Refer the compressor selection chart provided as an attachment to the link below:

 

http://www.cheresour...3854#entry43854

 

If you are interested in a comprehensive calculation tool related to centrifugal compressors which will not only allow you to calculate the power but also help in selecting the number of stages refer to the link below:

 

http://www.cheresour...wer-calculator/

 

Regards,

Ankur

[Kriss]

Ankur,

 

Thanks for the response and the selection chart.

 

Please advise if we can also use centrifugal compressor for low molecular weight gases like hydrogen.

[arvind]

Ankur,

 

Thanks for the response and the selection chart.

 

Please advise if we can also use centrifugal compressor for low molecular weight gases like hydrogen.

 

Hi Kriss,

 

Yes.

 

We can use centrifugal compressor for hydrogen too, provided flow rate and pressure ratio criteria must be fall under centrifugal compressor operating range as mentioned in earlier post by Mr. Ankur.

 

Regards,

Arvind

[Kriss]

Thanks Arvind, Thnaks Ankur, Its clear to me now.

 

krishna

[Babu Prasad]

We used Centrifugal compressor to recycle hydrogen in the Hydrocracker plant which is used to develop DP of 8 bar but it delivers large quantity flows to reactors. Where as reciprocating compressor used to make up fresh hydrogen to this recycle gas compressor loop with DP of 170 bar but less flow. As Mr.Ankur said , Compressor selection is based on the combination of volumetric flow rate and the pressure ratio. Low flow rates and large pressure ratios favor selection of reciprocating compressors and vice versa for centrifugal compressors