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Reciprocating compressor volume pocket unloading

Renoyd (Mechanical) (OP)
6 Dec 08 1:40
Hello,

When evaluating the capacity of a reciprocating compressor with multiple volume pocket unloading, at the same discharge pressure, do we consider the flow in the unloading conditions (which may be higher at higher suction pressures and unloading keeping the HP requirements lower), or just consider the flow at fully loaded conditions (which is lower due to HP limitation)?

What I am concerned is the heat generated due to unloading. When you put a compressor into service, you don't want it to be running for long with some or all of the pockets open. Right?
Montemayor (Chemical)
6 Dec 08 13:17

Wong.

You have a misunderstanding about the function and action of unloader pockets on reciprocating compressors.  Unloader poackets are probably the optimum way to unload a recip.  They merely decrease the volumetric efficiency of the cylinder affected and reduce the horsepower requirement proportionately to the reduced compressor capacity.

The unloader pockets should be designed in accordance with the expected suction pressure increase (which increases the gas density and, consequently, the horsepower requirement.  This suction pressure increase should have been included in the calculation for the unloader sizing and the installed horsepower requirements.  There is no heat generated due to unloader pocket unloading.  You can run the compressor with the pockets fully open.  All that is happening is you have no gas being positively compressed and flowing through the machine.  It is called 100% unloading.  The volumetric efficiency is effectively 0.0%.
 
Renoyd (Mechanical) (OP)
6 Dec 08 15:24
Montemayor,

Thanks for your response!

If under the same suction pressure, no doubt that the capacity is higher when fully loaded and the HP required is higher.

Now I wanna increase the suction pressure and hence the capacity. However the HP is limited. From the compressor performance curves, If I keep some pockets open, in the increased suction pressure condition, the capacity is increased and the HP required is within limit (otherwise too high if fully loaded).

Do you mean after unloading the capacity still not increased even the suction pressure is increased with the same HP? How to explain the performance curves then?
zdas04 (Mechanical)
6 Dec 08 19:58
Renoyd,
You need to read Montemayor's post carefully, it has the answer to you second question.

To slightly rephrase, the end result of increasing clearance is to reduce the efficiency of the cylinder.  This manifests itself by either reduced throughput or reduced compression ratios that are done across a cylinder or both.  Clearance is used often on multi-stage machines to balance the flow rate and compression ratios from one stage to the next.

The thermodynamics are really complex, but the bottom line is that by increasing clearance you decrease the efficiency of the machine so you can do less work.

David
Renoyd (Mechanical) (OP)
7 Dec 08 0:31
David,

Yes, I understand that unloading is a way of reducing the volumetric efficiency and hence the capacity. But if you increase the suction pressure to a certain level, and then unload to certain steps, the throughput may still be higher than that before the suction pressure is increased. This is exactly what I was talking about. Do you consider the increased throughput (due to suction pressure increase and unloading for HP limitation) as compressor capacity? Or just consider the fully loaded condition (which is lower due to HP limitation)?

For example: the drive maximum HP=1200. When fully loaded, at suction pressure Ps=300 Psig, capacity is 18 MMscfd (HP=1200). If increasing Ps to 400 Psig, capacity becomes 24 MMscfd, required HP=1400, but if unloading to certain step, capacity becomes 20 MMscfd, and required HP=1200. My question is: do you consider the capacity of the compressor as 18 MMscfd (fully loaded) or 20 MMscfd (partially loaded but at higher suction pressure)?
 
zdas04 (Mechanical)
7 Dec 08 11:43
"Do you consider ..." it for what purposes?  Are you selling compressors?  

You install enough hp to do the job required.  You configure the compressor to do the job required within available hp.  So if the job you are trying to do is 300 --> 900 psig and you want to load to the full 1200 hp then the  machine has a capacity of 18 MMCF/d.  If the job is really 400 --> 1100 psig then the fully loaded (with extra clearance) machine has a capacity of 20 MMCF/d.

Are we just talking semantics here or are you trying to show a capability number on a drawing or PowerPoint?  If that is the case then use the as-configured capacity (i.e., the 20 MMCF/d)

David
Renoyd (Mechanical) (OP)
7 Dec 08 18:15
I should have explained the background of this question.

I am currently working on a project trying to increase the throughput of a gas plant. The compressor capacity becomes an issue. IF considering fully loaded (no pockets open), then the capacity is not enough considering the HP limitation. But if increasing the suction pressure and unloading to certain step, the throughput will be increased within HP limitation. This is what this question was about.

  
nickelkid (Mechanical)
8 Dec 08 10:28
Renoyd,

I believe you will have capacity rating for each suction pressure. I.E. at Ps = W, capacity = X, at Ps = Y, capacity = Z, etc...

These tabulations are excellent documentation and needed throughout the life of the plant. A lot of head scratching can be avoided when they are available and understandable.

 
gurmeet2003 (Mechanical)
14 Mar 09 14:02
Renoyd,

When you talk about capacity you are referring to mass through put. Is my understanding correct? If this is the case, then the compressor hp will be limited by the driver (for an existing unit). Now let us try to  think about the problem thermodynamically. You have mentioned that you can increase the suction pressure. How will you do this? If you do this and the discharge pressure does not change, then for the same hp one should be able to push through a bigger mass flow thermodynamically.

When the suction pressure is increased, the pressure ratio across the compressor will also reduce and mass through put will increase. But for a multistage compressor the pressure ratio is distributed acorss all stages and is dependent on the relative values of cylinder diameters. When you change the pressure ratio, you can use the volume pockets to rebalance the pressure ratios. I think that what you want to do is quite doable. But as pointed out by RCHandy, you will have to work with the manufacturer's performance programe to verify that you can get the new operating points you want.

Gurmeet
dcasto (Chemical)
15 Mar 09 18:06
i love this discussion.  It is the same as the one on the capacity of a TEG, Amine, or a gas plant.  What is the capacity of a TEG unit?  I'll see them listed as a 25 MMSCFD unit, right, what does that mean, can it do 25MMSCFD at 300 psig?.  Or a 100 GPM amine unit, so, can I put 100 MMSCFD through the 24" contactor.

Your simple data suggest a 300 to 900 psig at 18 MMSCFD and it will do 25 MMSCFD at 400 to 900. To me it's a 1200 HP single stage unit, nothing more or less.

As for more throughput, there is always a trade off.  You'll get more volume through the plant at a lower recovery efficiency.

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