3 replies to this topic

[camad]

I would like to clear out my understanding of gas compressor. If it is moving at constant RPM, is it mean that it can only allow same volumetric flow rate all the time? So the power will remain constant too, (P=VI) then voltage and current will varies. My investigation is to find why there is changes of pressure and flow rate entering the compressor. Could anyone provide me some good explanation please. Thanks. I would like to know why pressure inlet of gas compressor decrease  and I am wondering is it related to volumetric flow rate (in MMSCFD) as the flow rate increase?

Attached Files

•  Pressure drop data.docx   19.14KB   13 downloads
•  problem 1.pdf   106.36KB   17 downloads

Edited by camad, 04 August 2015 - 06:40 AM.

[Art Montemayor]

Camad:

 

If you know the type, brand or make of your gas compressor, then identify it and let us know.  I believe, from your sketch, that it is a centrifugal type; but then, your use of non-standard instrument symbols is also another amateur mistake in communicating engineering data.

 

However, if it is a dynamic (centrifugal) type - as opposed from a positive displacement (reciprocating) type, the flow rate the machine displaces depends on the “Head” or resistance that opposes the flow.  This is clearly explained and described in all compressor textbooks and in our Forums.  This topic has been discussed in our Forums for many years now and the related threads can be read by using our Search Engine.

 

Are you a student?  Your query is so elementary and basic that I wonder if you have had the normal, required academic courses to get a degree in engineering.  Of course, a gas compressor will continue to compress gas while it is rotating - even a centrifugal device.

 

The reason why your pressures (you fail to identify which specific pressures you mean) and your flow rates are varying is probably due to the manner in which your instrument controls are functioning.  Your sketch, as I stated, does not use normal, standard instrument symbology and there are what appears to be pressure signals being sent to a throttle valve in the suction side of a centrifugal (?) compressor, upstream of the suction scrubber.  What a “signal selector” really is and really does is anyone’s guess.  Your diagram does not show any surge controller - if indeed, it is a centrifugal type.

I highly recommend you resort to studying the instructions found for all industrial compressors (both centrifugal and reciprocating) which are routinely supplied with all manufactured compressors.  These instructions cover all operational, maintenance, and safety issues specifically related to the machine and should clearly explain the reasons and the methodology of the instrumentation furnished on the machine.  Additionally, you should read and study as many compressor textbooks as you may have available to you.  Amazon books is a good source on the internet.

[HABIB092]

As in the attached files  all  process informations are not complete in P&ID. I will assume some conditions and try to give you the answer.

 

motor is fixed speed.

compressor is centrifugal with ASCV  surge control valve

 

P= Q*T(P2/P1)

 

power equation qualitatively

 

P= power  KW or hp

Q= gas flow rate     mmscfd

T= absolute  inlet temp

P2= abs discharge pressure 

P1= abs  suction pressure

P2/P1 = compression ration

 

as a fixed speed motor  ,as rpm are fixed ,  

If suction pressure increases, the working inlet volume will remain same   but  in standard condition volume will increase. and power consumption will increase.

 

as on 13 date   suction pressure is high   ,density of gas in high , inlet volume is high , compressor is taking more load  , out let flow is less  and  some percentage ASCV is open 25%.

 

as on 14 date  suction pressure is less, density of gas is less, inlet volume less, compressor is taking less load , outlet flow is high  and ASCV is less open 20%.

 

and  so on

keep power equation in mind  

[shan]

Please see the attached typical centrifugal compressor Capacity vs. Suction Pressure curve at the constant RPM and specified discharge pressure.

Attached Files

•  Compressor Capacity vs Suction.pdf   102.28KB   35 downloads