Reciprocating Compressor Capacity Control

It's reciprocating.

Ask the compressor manufacturer's technical staff, or read the operator's manual' or

ask your facilities engineer.

Is the compressor used in a once-through manner; i.e., without receiver?
Is it feasible to install a smaller motor pulley temporarily?
Is the compressor equipped with VFD?

I doubt it has a VFD , if he did his question should have been about one, but this is a poorly worded question (whats new) how about a picture of the unloaders? hydraulic, electric? a model number?

Those too are good questions. I'm guessing that ")%" is "0%", so that the compressor is all or nothing.

Make a loop around the compressor and feed some of the stream back to the side of the inlet of the gas. You can then regulate the flow by opening the bypass valve.

You would probably have to measure the gas flow and steer the bypass valve accordingly.

...either that or switch the compressor on for only 50% of the time.

Many Process Engineers don't live in the real world.

From your description the compressor design is a two (2) step unit at: 0% - 100%.

Yes: The compressor capacity would/will be decreased by 50%.

Yes; If the operating criteria is 0%, 50%, 100% then the compressor must be a three (3) step compressor.

If the compressor you are dealing with has the proper valving and cylinder design it can be converted to a 3 step control.

If not, it is possible to restrict the inlet air supply to the compressor with dampers and/or valving to control the output however, you will need to monitor the compressor operation for temperature rises in the innercooler and aftercooler.

Keep in mind that reducing a recriprocating compressor's speed by using a VFD, will decrease the system pressure which in most cases will not support the process operation requirements.

One more critical issue I forgot to mention when considering adapting/installing a VFD on a recriprocating compressor is that the compressor oil pump in many cases will not provide enough oil pressure unless it is rotating at rated speed.

and all this time I thought oil pressure in the system had to do with the difference between the oil pump output pressure and the suction pressure.

1.You have questioned ,start up capacity 50? or 100? since this is a 2 step capacity control ( you have not mentioned properly 0% and 100% or? ) it has to be 50% and 100%. Normally compressor will start with 50% and once the oil pressure reaches it will step up to 100% like star delta starters.

2.As per your process Engineers request you have to start and maintain at 50%, yes you can do. You have lock at 50% capacity instead of allowing to go to 100%.

3.Compressor capacity? When you lock at 50% the compressor capacity will be 50% not 100%. (means if this is a 100ton system when you operate at 50% it will become 50ton system)

Since you've mentioned, 'make-up gas compressor', I presume this could be 'hydrogen compressor with three stages', right? Mostly these are balanced, opposed, double acting types.

It is possible to incorporate suitable un-loaders for all the suction valves. The unloading sequence could be:

All suction valves in un-loaded position (to start with as not to load the motor initially) = 0% capacity.

All suction valves at crank-ends in loaded position with all suction valves at head-ends in un-loaded position = 50% capacity.

All suction valves in loaded position = 100% capacity.

It is difficult to see what your process engineer is trying to do. Does 50% mean full load for half the time, or half load for all the time.

Assuming your compressor has suitable load control then it will off-load at the system pressure - and stay off-load until the pressure drops.

If your process engr has managed to get his end of the system running at 50% of normal demand then your compressor will automatically respond which means it also run at 50%. You don't have to do anything with it.

If your process engineer wants you to limit your compressor to running at 50% of the time, then you have to stop the compressor. But when, and for how long ?

Compressor motors do not like frequent stop-starts (they overheat) - usually they must be limited to 4-per hour, and when there is low demand, the compressor runs but does not pump - it off-loads - (during which time the motor power factor is poor).

If you need to maintain pressure to meet the 50% demand, and to avoid too many starts per hour, you therefore need a 'run-on timer' in the control. This will stop the compressor if running on no-load for 15 minutes when there is no demand. In fact a run-on timer is a good idea anyway.