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Reciprocating compressor control method

Reciprocating compressor control method

Dear Sir/Madam

The reciprocating compressor with fixed speed motor and compressor capacity is constant.

My query :
1.Reciprocating compressor can have both capacity control and by pass control?. Actually, there is a flow transmitter on the compressor drum discharge line(instead of the pressure sensors) to
send signal back to spill back valve on the bypass line.

2. If variable speed driver used in the reciprocating compressor,
which control is commonly used? capacity control or spill back control?

3. There is two discharge line from the compressor drum. One line is control 70% total capacity, and the other is control to 30% total capacity. And, there is flow transmitter on these two line to send signal to there FIC. So I wonder could I pull these two FIC signal back to the spill back valve to control the bypass flow.

Thank you.


RE: Reciprocating compressor control method

There is no such thing as "by-pass control".  You don't control what you call the "by-pass".

There is Capacity Control on a reciprocating compressor, and it can be one (or a combination) of:

1. Variable compressor speed;
2. Clearance pockets on the compressor cylinders; ("plug" valves are one form)
3. Valve unloaders;
4. A recirculation around the compressor; (this is not a "by-pass" because it doesn't by-pass anything.  The term "by-pass" is a misnomer given in ignorance).

The compression efficiency (and consequently the energy consumption efficiency) is highest in #1 and decreases in the numerical order given.  The worst energy consumer (& most inefficient) is the recycle method (call "by-pass" by you).

Obviously, if you install speed variation control you have the best capacity control and –therefore – require no other method (as long as the compressor can operate withing the capacity range you are required to control.  You are mixing up your terms (and consequently, your logic) by equating the terms "capacity control" and "spill back control".  Your wording doesn't make sense.  What you call a "spill back" valve is nothing more than the recycle valve used in recycling the compressed gas back to the suction side of the compressor.  You waste the amount of energy and time used to compress the gas you recycle.  It is that simple.

I cannot address your query #3 because it doesn't make sense without a detailed sketch of the piping and the controls – in the form of a P&ID.  Furnish a sketch or P&ID and we can address your concern/question.

RE: Reciprocating compressor control method

Engineers at Ariel used to describe Montemayor's #1 as "If you never compressed it, the effeciency on the part you skipped by slowing the machine down is 100%".  That statement always made a lot of sense to me.


RE: Reciprocating compressor control method

Hey knapee,

In one of my plants we just installed a very automated multistage recip compressor that has unloaders, pockets, and a spillback (what you are probably calling a bypass). Previously the operators were able to set a capacity control parameter at values of 25%, 50%, 75%, and 100% on the DCS, which would would open or close the unloaders or pockets based on a preconfigured scheme. The spill back would then assume whatever value was needed to control the pressure. Seeing that the operators would allow the spillback to assume large energy wasting values by leaving the capacity setting at 75% or 100% even at low loads, our control engineer automated the capacity controls to keep the spillback to a minimum. Automating this control scheme has probably saved between 0.5 to 1 MW of electricity.

On the otherhand variable speed reciprocating compressors are not that common in my experience, but obviously are the most energy saving control system of all.

The last question needs a picture to understand.

best wishes always,

RE: Reciprocating compressor control method

Dear Montemayor, sshep,  

Thank you all your kindly reply. Please see my attached drawing if
you have time.

There are three options for the FT location

option 1) FT is located on the discharge of the drum
option 2) FT is located after the two discharge line intersection
          point but before the PSV inlet line.
option 3) FT is located after the PSV inlet line.

Could you please tell me which option is best for the FT to send signal to FIC and back to spill back control valve?

Thank you so much.

Best Regards,

RE: Reciprocating compressor control method

Well, the diagram isn't labelled very well but it looks like the line exiting the left side is the bypass line.  Given that this is a reciprocating compressor, I'm not sure what you're trying to accomplish?  

Normally a bypass valve is controlled by a combination of suction pressure with a discharge pressure over-ride.  I'm not sure that flow control is what you want in this application.  However, based on what I'm seeing in the picture, I would probably locate the flow element on the top of the drum, but it will almost certainly conflict with the signals being sent the other two(2) valves from the columns.  

What you could do is set the bypass on suction pressure to maintain a design flowrate and then pick one of the columns as the master and the other as the float.  This needs to be looked at by a controls guy - or talk to your compressor vendor about control schemes.

RE: Reciprocating compressor control method

I would like to clarify why this discharge line is control through flow instead of pressure cause there is a specific reason that
the feed flow to column have to control in a fixed ratio comparing
to other feed to the column.

And, there is two spill back control valves,
one is between the compressor pulsation dampers, and the other is
between the feed tank and discharge drum. But, I don't know what is advantage for this design and how is the control logic inside the compressor function, ex. is this shutdown function generated from motor and send alarm(shutdown) signal to DCS.  


RE: Reciprocating compressor control method


First and foremost, your poor sketches leave a lot of information undefined:

1) The flow arrows are not identified as to where they are going;
2) The equipment is not all labeled and identified;
3) There is no identification of where the suction stream is coming from and at what controlled conditions;
4) There are lines shown that are unidentified;
5) The conditions of each stream are not given – flow rate, pressure, temperature, etc.
6) Is this an existing installation?  Or is it a proposed design?

A reciprocating compressor is an inherent metering device.  It displaces a fairly accurate amount of gas on each and every stroke.  Therefore, the use of flow control on the discharge of a reciprocating compressor is rarely – if ever – done.  If you require an accurate generation of high pressure gas at two, specific flow rates through use of a single reciprocating compressor, then please so state it at the very outset.  We are being forced to "guess" or assume what you are trying to do.  Why don't you simply just tell us ALL of your scope of work and basic data?  It would be so much simpler.

Another reason that flow control is not usually used on reciprocating compressors is that the flow is pulsating and therefore is difficult to measure and transmit in order to generate a constant and steady control signal to a control device.  If, in fact, you are trying to control two simultaneous, parallel flow rates from a reciprocating compressor, then you are going to ensure that the compressor is always producing an excess of product gas in order to ensure that both flow rate demands are satisfied and the excess compressed gas is recycled back to the suction of the compressor as an inherent inefficiency resulting in lost work and power.

Your sketches are very bad and difficult to interpret.  It is obvious to me – as it should be to all who see them – that you have spent a minimum of effort in producing a communications tool with these diagrams.  What you call "option 3" cannot be understood since you show no "PSV" in either of the two diagrams.  I guess that by "option 2" you mean that you have two parallel, flow-controlled product gas lines (going somewhere?) that are controlled by independent control valves.  These two control valves produce a back pressure that is limited by a recycle valve dumping excess gas back to the suction of the compressor.  This assumes that the suction can take the recycle.  Since we don't know the nature and control of the suction to the compressor, we have to assume that the recycle gas can be handled by the suction of the machine and that the recycle will not cause an accumulation at the suction conditions.

If the above assumptions are true, then I would do what you show in the sketch – BUT bear in mind that there are limitations.  You cannot expect to vary the flow rates beyond a certain range of flows because there may be limitations as to how much gas you can safely recycle and to what extent you can raise the compressor's discharge pressure due to the throttling of the flow control valves on the discharge.

You do not show a discharge aftercooler on the sketch of the compressor, so I have to assume that isn't one.  If that is the case, you should not recycle hot, discharge gas back into the suction of the compressor because this will compound the discharge temperature of the compressor and this may be a serious jeopardy.


RE: Reciprocating compressor control method

Dear Montemayor

Thank you very much for your comment.
I apologize for my last drawing and attached another one this time.

This reciprocating compressor has two spill back valves.

One is located between the inlet/outlet of the compressor through
the PIC control, and the other one between the inlet Hcl tank line and compressor discharge line through the FIC control. And, this FIC control is for minimum flow control.

Hope this may clarify your querry.

RE: Reciprocating compressor control method

There's really only 4 ways to control the flow through a reciprocating compressor:

1.) Modify the suction pressure to allow more or less gas to enter the compressor.

2.) Modify the speed of the compressor - essentially it's proportional to flow.

3.) Modify the displacement of the machine via clearance pockets, disabling HE and CE valves, etc.

4.) Auto-bypassing discharge back to suction.

You can not control the flow through the reciprocating compressor by pinching the discharge on the compressor.  If you try and auto-bypass back based on discharge flow or pressure, the suction pressure will rise and the throughput will increase even higher.  You will likely require a suction pressure control valve for capacity control but trying to control both discharge flows as you have drawn is not likely to work.  The sketch attached to today's email is even more confusing than ever.  Can you vary the speed of the compressor?

RE: Reciprocating compressor control method

ear Sirs:

Please bear with my poor English again.

1. This compressor is fixed speed, and it use pressure control/flow control on the discharge side of the compressor.
The flow control is only to maintain the minimum flow rate from discharge drum to suction in case there is no flow back to compressor.

2. There is split range pressure control located on the HCl drum that is the suction drum to the compressor.
Hcl is stored in this drum and tobe control pressure at 8 kg/cm2 by either adding or take off heat. During normal, Hcl be vaporized to
keep the pressure. So, there will no kind of problem such as, suction pressure suddenly rising,and throughput higher.

Another question is that this Hcl compressor is used N2 to purge the compressor while start up, but in license's data sheet, they
required veneer to advised inter gas such as Hydrogen to be used as inert gas seal. But it's kind of weird.

Thank you very much for your comment.

RE: Reciprocating compressor control method

Dear Mr.Montemayor

Sorry to bother you this trivial again.

I wonder why the vender remove the inlet psv and add the two psv on the discharge line. And, it seems to me that pressure relief valves on reciprocating compressor discharges shall be located at least
10 pipe diameters downstream of the pulsation dampener and have a set pressure at least twenty percent above the average operating pressure, but not sure apply to this diaphragm compressor.

Look forward your help, and thank you very much.


RE: Reciprocating compressor control method


I have no idea as to what your vendor is doing with respect to the PSV location modifications.  I have no knowledge what the scope of work or scope of supply is on the part of your vendor, so I can't even guess.

If you have a lump sum, turn-key arrangement with your vendor, then it should be his responsibility (or right) to make whatever modifications in order to meet with his deliverables to you – within the constraints of your purchase agreement.

You should have the right (normally) to approve whatever he proposes, so you can make whatever comments, recommendations, corrections you feel are appropriate.


RE: Reciprocating compressor control method

reciprocating compressor d/c is pressure controlled to meeet the requirement

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