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Reciprocating compressor capacity

Reciprocating compressor capacity


I just graduate and I am new in a real engineering work. I dont have experience and I need your help :)

we want to use existing reciprocating compressor, the calculated reciprocating compressor capacity at suction condition (55 MMscfd) is higher than the natural gas supplied to compressor (30 mmscfd). actually i just want to know what's the effect if i dont control the capacity of the reciprocating compressor ? the first one is excess power required in the prime mover, how about the other effects (discharge pressure and flow rate)? and why ?

thanks in advance :)

RE: Reciprocating compressor capacity

You are not providing enough information. How do you intend to run this compressor at the lower capacity? You can reduce the operating speed. You can partially unload the compressor using suction valve unloaders or pocket unloaders. You can use a controlled spill-back to return the excess flow to the suction. You can pinch on an inlet control valve to reduce the suction gas density. Some of these options could be wasteful, foolish or even dangerous. The least likely result would be high load on the driver. But, we don't know, because you did not provide enough information.

Johnny Pellin

RE: Reciprocating compressor capacity

Thanks JJPellin for your respond :)
actually i want to know why we need to control the capacity of the compressor ? what will happen if we dont reduce the operating speed ? it will give higher power than needed, there any other effect ?

RE: Reciprocating compressor capacity


You may just be graduated, not sure what discipline, but you need to take a step back and look at your question and system.

A pump or compressor or other thing that changes something (pressure, temperature, composition) is normally only one part of an overall system and without understanding the impact of other things on the bit you're looking at you won't get anywhere.

Compressors are sized at standard onditions to make life easy for everyone, bunt in reality operate at different conditions, both inlet and outlet.

Your reciprocating compressor is a simple thing - just pistons going back and forward. If you don't feed it with enough gas, it will still keep going, but just lower the pressure at the inlet. if you don't want the pressure at the inlet lowered then you need to recyle some of the gas it is pumping back to the inlet.

recip compressors at a fixed inlet pressure move the same amount of fluid up to the point where their compression ratio is exceeded. The amount of power depends both on flow and differential pressure. A centrifugal or axial compressor works differently.

So in short, if you do nothing, your compressor will pull down the inlet conditions until it gets into equilibrium with the inlet and outlet conditions.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Reciprocating compressor capacity

LittleInch stated the case well. This is a positive displacement machine. It will displace the same volume whether you want it to or not. If you do not change something to reduce the capacity, it might draw down the suction pressure, as LittleInch suggests. But, this assumes that this is possible. If it was taking suction from a vast reservoir or a much larger pipeline, or atmospheric air, for example, the suction pressure would remain constant and the compressor would continue to move its full capacity. You have to do one of the things I listed or have a system that will provide the same affect.

If the suction pressure is drawn down, then the differential pressure across the compressor will increase (assuming constant discharge conditions). This can result in overload of the piston rods and catastrophic failure. The machine would be destroyed and a massive release of gas could result in a vapor cloud explosion and fire with the potential to kill anyone in the area. Overloading the motor is the least of your concerns.

Johnny Pellin

RE: Reciprocating compressor capacity


First off, remember that although the unit SCFD seems to convey a volume flow, this unit doesn't actually define a volume flow, but rather a certain amount of molecules per unit time. Keep this in mind through the following explanation.

First of all, a common misconception with positive displacement compressors is that they are a "fixed volume" machine. For reciprocating machines, in terms of the stroke volume, this is very true. However, in terms of the volumetric flow of gas through the machine, this is not the case. Based on differential pressures across each compression stage, both flow in ACFM and SCFM can vary. If your compressor is "rated" for higher flows in SCFD at the given suction pressure than the amount of gas that can be physically provided to the machine as measured in SCFD, then the following will likely happen:
A) Suction pressure will drop
B) Discharge pressure will increase
C) Flow in SCFD will balance to the rate at which gas can be provided to the machine (law of conservation of mass dictates that this must happen). This flow may increase from what you claim based on the system resistance as a whole.

You should therefore check with your equipment manufacturer as to what the anticipated suction/discharge pressure conditions could be given the SCFD flow through your machine. Additionally, given the increased differential pressure, you need to perform an analysis to determine what the change in loading of your piston rods will be. You may very well exceed the integrity limitations of the mechanical components. I would also review the pressure-retaining capabilities of your cylinders, piston rings, compressor rod packing, valves, and upstream and downstream equipment to ensure you do not exceed these limits (both on the low end for suction piping and high end for discharge piping).

As for power requirements for your driver, without additional data, I cannot tell you as to whether or not your power requirements will increase. You need to evaluate a PV diagram for the compression process in order to determine whether or not your power requirements will be greater or less. Reciprocating compressors are a bit tricky in this regard.

Hope this helps.

RE: Reciprocating compressor capacity

There is nothing wrong with the answers above, but they seem to assume that you know more about your job than you actually do. The issues have absolutely nothing to do with wasting power on the driver, you've already budgeted the power and small deviations are rarely an issue.

Let's say you have a single compression chamber that is turning at 1200 rpm, moving 55 MMSCF/day of natural gas (SG=0.6, k=1.28) at 100 psia and 60F suction and 300 psia discharge. This chamber has a physical volume of 5.1 ft^3. At 100 psia suction you are putting 0.23 lbm of natural gas into that volume.

Now you lower your flow rate to 30 MMSCF/day. You are providing less gas than is required to fill the cylinder. The cylinder will be filled, so since PV=mRT, at constant volume and temperature, the pressure has to come down to compensate for the lower mass available. This is manifested by the pressure within the cylinder dropping to around 55 psia.

In the assumed conditions, 100 psia to 300 psia is 3 compression ratios. Now I still need 300 psia to open the cylinder outlet valve so with 55 psia minimum in the cylinder, your compression ratios have gone from 3 to 5.5. 5.5 compression ratios can break things and generate more heat than 3 ratios. I would expect your discharge temp to go from about 211F which is pretty easy to manage to 295F which is on the boarder of a problem.

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

RE: Reciprocating compressor capacity

My mistake. Discharge pressure will likely decrease unless you have a pressure controlling device, but overall differential pressure across your compressor will increase.

RE: Reciprocating compressor capacity

Discharge pressure is controlled by everything but the individual compressors. It is generally the last thing to change dramatically with a volume change. If the station is discharging into the nationwide grid, then that volume change is too small to be noticed. If it has a lengthy line to tie into the grid, then friction in that line might go down some, or (more likely) the grid pressure will hold a check shut, so with reduced friction losses I would expect the compressor discharge pressure (absent a backpressure valve designed to keep the machine at a minimum discharge pressure) go down a touch, but it is not a foregone thing.

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

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