6 replies to this topic

[kasri]

Dear Sir,
I had a basic doubts regarding Centrifugal and Recriprocating compressor.I want to compress the natural gas from 10 barg to 30 barg of about 5 mmscfd flow rate, so I had selected the compressor( Reciprocating / Centrifugal)for the same.
Please clarify the following.
(1) if the inlet pressure drops from the design value how the reciprocating and centrifugal compressor reacts to this.
(2)if the pressure is same and flow rate is reduced how the both compressor reacts.
Apart from the above how do we control the flow rate for both compressor, does the capacity recycle load valve survive for the both compressors apart from VFD.
Thanks ,
Kasri

[pavanayi]

kasri,
Your questions are too fundamental that one can even write a whole book about these (and it has been so.. ).

I would advice you to read the fundamentals about compressors in a proper book, understanding how a gas gains pressure in each type. Once you have understood that concept fully, you will be able to answer these questions yourself.

And while you are at it, you might want to ask a more competent person to do the compressor selection.

This forum might be able to help you much more if you had very specific doubts in Chemical Engineering, and you would like a second opinion on that.

I hope this advice helps.

[breizh]

Kasri ,
This resource should help you and others.

http://books.google....epage&q&f=false

Breizh

Note : Try the search button for more info

[ankur2061]

Kasri,

One of the fundamental means to decide which type of compressor to select for a given application is to critically look at the volumetric flow rate and the pressure ratio (Discharge Pressure / Suction Pressure) for the said application. Sometime back there had been a post on compressor selection and in my response I had attached a compressor selection chart. You are requested to look at the chart to get an overview of the selection process based on volume flow and pressure ratio. Once you have finalized the basic compressor type, you can further refine it to the level of using a VFD / unloading valve / recycle control valve for capacity control in consultation with compressor manufacturers who understand the wide range of applications for the selected compressor type. Here is that link for the compressor selection chart:

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

Hope this helps.

Regards,
Ankur.

[breizh]

Kasri ,
An other resource to add to previous posts:

http://books.google....lations&f=false

Breizh

[kasri]

Dear sirs,
Thanks for your guidance.
Please calrify the following querry along with earlier one.
For centrifugal pump if the flow reduces the developed head will be high for the same rpm,same liquid.
Then how about reciprocating pump.For ex.
A pump( centrifual/reciprocating) is designed for 5 m3/hr , suction pressure: 3 barg , discharge pressure 10 barg.( ie. pressure differential=7 barg).
(1)If the suction flow reduces , the developed head will be increased for centrifugal pump;similary what happens if the suction pressure reduces to 1 barg( for the same flow of 5 m3/hr), is it develops the discharge pressure of 8 barg( 1+ differential pressure 7 barg).
(2)In case of reciprocating pumps,if the suction flow reduces , what will be the outlet pressure is it increase( driver speed same as 5 m3/hr);similarly what happens if the suction pressure reduces to 1 barg( for the same flow of 5 m3/hr & driver speed same as 5 m3/hr), is it develops the discharge pressure of 8 barg( 1+ differential pressure 7 barg).
Please clarify me eventhough I read with theoretical sentence, it is better to understand through numeric values.

Regards,
Kasri

Edited by kasri, 01 May 2011 - 12:41 AM.

[paulhorth]

Kasri,

You have now changed the subject, to ask a question about PUMPS, not compressors, so this should have been in a new thread. Since no-one else has picked up this topic, I will have a go.

Centrifugal and reciprocating pumps work on completely different principles.

For your question (1), the answer is yes. For a centrifugal pump the differential pressure depends on the flow, so at the same flow, the differential will be the same for 1 barg suction as for 3 barg suction. There is a minimum suction pressure limit defined by the NPSH requirement of the pump. If the suction pressure falls so low that there is vapour in the suction, the pump will no longer work.

For your question (2), you need to understand that a recip pump is a FIXED VOLUME FLOW machine. You cannot reduce the flow, except by varying the speed, or the stroke. For fixed speed and stroke, the only way to vary the nett forward flow to the system is to provide a spillback loop, the flow through the pump is always the same. However it is not a fixed differential pressure machine. The pump will deliver its fixed flow at whatever the discharge pressure is in the outlet piping , until the relief valve opens, or the machine breaks or the motor is overloaded. So if you reduce the suction presssure, the discharge pressure stays the same.

Paul

Edited by paulhorth, 06 May 2011 - 03:21 AM.