Sales Gas Compressor.Centrifugal or Reciprocating

[tiputyper]

Hi Experts,
Kindly advise what kind of compressor is more suitable for sales gas considering the below available process data.

Flow = 42 MMSCFD.
Inlet Temp = 75 F.
Inlet Pressure = 1110 Psig
Outlet Pressure = 1290 Psig
Molecular Weight = 21.2
Cp/Cv = 1.586
Z Factor = 0.86

Thanks in advance.
Regards,
Tipu

 

[zdas04]

Big volume, very low ratios, clean consistent dehydrated gas? I would tend towards a centrifugal for this application.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[rotw]

42MMSCFD = 42 x 1000000 Std. f3/day = 1.189 x 1000000 std. m3 / day = 1.189 x 41667 std m3/h
at the suction pressure of 77.5 bara the actual volume flow (very very roughly) becomes (going from std conditions):

1.189 x 41667 x (1/77.5) = 639 actual m3/h

hmmm... ok say 700 m3/h actual at inlet. So what do we have:

Pressure ratio is very low, with a MW a bit on the high side here, probably we are talking about one impeller only, so an overhung design, most likely an integrally geared one. The peripheral speed would be very low (and this not a good indicator...not necessarily a "no go" but let say a bit unusual..).

This 700 m3/h is low flow that does not look very nice for a centrifugal, means you may need to spin the impeller very fast (15000 to 20000 RPM) while going for a small impeller diameter (200 mm and lower) to recover the efficiency in a maybe descent range. To be designed very carefully. Maybe some sort of low head but more efficient stage. ok Doable.
Still the low peripheral speed looks ugly.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[georgeverghese]

No information on the background to this unusual compression duty.
Some other options :
a)If you have an existing centrifugal, how about getting a new rotor for this compressor to suit this new duty?
b)Change sales gas pipeline size or reduce sales gas pipeline end pressure ?

 

[poli60]

by the way...it's not a pump ;-)

http://www.eng-tips.com/threadminder.cfm?pid=1036

 

[zdas04]

rotw,
Why ever did you go through that pedantic exercise. If you are going to go to that much trouble, you really should get your numbers right. Using his compressibility, I get a flow rate at actual conditions of 575 m^3/hr. The MW results in a SG of 0.73 which is high by what standard?

Natural gas compressors are rated at standard conditions, not actual conditions. This isn't a very large job but it still takes over 300 hp to do so it isn't tiny.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[georgeverghese]

If there is no choice but to put this on a separate machine, looks like a recip may be more economical from a capex point of view, from the flow maps I have, given that suction flow is only 340ft3/min. There may be some loss in machine reliability when compared to a centrifugal though.

Power demand for a centrifugal at 61% poly eff/60% isen eff = 447hp, while that for a recip = 470hp(approx), so there is hardly anything to gain on power demand at full load.

The low compression eff of the centrifugal option is due to the low inlet flow (compression eff value taken from the GPSA).

At part load, discharge pressure would be lower than 1290psig (if there is no need to maintain 1290psig), given that the pipeline dp would be less - some thing to investigate with more data from the compressor vendors.

 

[rotw]

Zdas,

I dont agree fully ; I was trying to see if I can discard the centrifugal compressor.

So I wanted an order of magnitude only....and for that I need to do the exercise - I dont know how to determine if these are big volumes or not doing it another way. e.g. 700 m3/h not 7000 m3/h was enough for me;

Second point, centrifugal compressor is determine by actual flow in my world. With that I can have a "feeling" ; with standard volumes I have no feeling at all.

And MW is on high side compared to a rich natural gas sales compressor, it does not matter that much except that the head is even lower and so the peripheral speed.... by pointing out at this, I hope I provided some hint to the poster

No disrespect Zdas, I am surprised that I got this reaction back because I thought it was clear I was doing a very quick and dirty screening stuff

oh...and by the way 575 m3/h makes the situation even worse, for me this is not BIG volumes for centrifugal! I dont know if you realize it but to keep a typically acceptable velocity at inlet of the machine, we are here talking about a 4inch flange or so....again looking at standard volumes would have been misleading. and thanks for doing a more precise calc


"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[rotw]

georgeverghese,
I am inline with you. I would also like to add that the advantadge of integrally geared centrifugal compressor is that you can make the impeller spinning much faster to recover efficiency which would be dramatically low on a between bearing type of machine.In this respect I think GPSA will not give a very realistic view of current trend and capabilities available in the market based on all options available and how far you can push. Actually performance can be quite underestimated.

By assuming a 150 mm impeller diameter, with more or less conventional head capability, the speed can be around 21000 rpm. The efficiency trend for the resulting flow coefficient in the machine will give the following calc (I had to assume some gas composition and tweak to arrive somehow close to the Z factor and Cp/Cv etc.):

WEIGHT FLOW (kg/h) 42700
INLET CONDITIONS
PRESSURE(BAR - A) 77.53
TEMPERATURE (°C) 23
RELATIVE HUMIDITY %
MOLECULAR WEIGHT (%) 21.076
Cp / Cv 1.555
COMPRESSIBILITY (Z1) 0.888
INLET VOLUME(m³ / h) 571.59
DISCHARGE CONDITIONS
PRESSURE (BAR - A) 87.59
TEMPERATURE (°C) 33.56
Cp / Cv 1.554
COMPRESSIBILITY (Z2) 0.896
GAS POWER (kW) 185.75
SHAFT POWER (kW) 187.63
ISENTROPIC HEAD(kJ / kg) 12.968
ISENTROPIC EFFICIENCY (%) 82.804
POLYTROPIC HEAD(kJ / kg) 12.941
POLYTROPIC EFFICIENCY (%) 82.629

Like I said before then it is matter of feasibility and staying with experience envelope - the above is theoretical; in other words check with manufacturers...for sure in the world of centrifugal compressors manufacturers to API, this would definitely be a small machine (not yet in the power range of a car turbocharger of course ). What I mean is this looks more a "special design" kind of machine...

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[georgeverghese]

Rotw,
Okay, 250hp is a good 200hp power savings compared to the recip for this poly eff of 83%; as you say, part load performance would also need to be looked at.