2 replies to this topic

Dear Friends,
We have a 3-stage 4 cylinders reciprocating compressor (1 1 2 3 cylinders arrangment) with interstage coolers that recovers hydrocarbons (mainly Methane,Ethane and Ethylene) operated by electric motor. We have an issue that the recycle valve is always open to maintain suction pressure. I know that this is not an efficient way of running recips, but every time I try to close the recycle valve , suction pressure drops. I also tried to reduce capacity control on compressor to 50% (since compressor can only be loaded to 50% or 100%) but suction pressure increased and discharge temperatures on 1st and 2nd stages discharge increased . The operating process conditions are as follows:-

Suction Pressure = 0.1 barg
Discharge Pressure=18.5 barg

Suction Temperature= 42 deg. C
Discharge Temperature=127 deg. C

What is the possible cause of not being able to maintain suction pressure?

Regards

[benoyjohn]

YoungProf,

You did not mention the actual gas load available for the machine.

Please indicate that value as a % of the machine capacity. From your description, it is very likely that the actual gas load is between 50% to 100% of the machine capacity. In such a case the only way to run your machine is to run it on 100% load and recyle the balance capacity.

regards,
Benoy

[Art Montemayor]

Young:

What you seem to describe is a Vapor Recovery Unit (VRU) reciprocating compressor. If that is the case, then your gas source is very unpredictable with regards to flow rate. This is the case in practically all VRU units. The designer is forced to design the compression unit for an anticipated maximum capacity and turndown (control) the capacity handled as the available gas flow is reduced or is not available.

I could go into a description of all the capacity control options you have for a reciprocating compressor, but since I have done this many times on these Forums in the past, I will refer you to prior posts that you can access through our SEARCH feature. I recommend you do this because you seem to be unfamiliar with how to control the capacity of a reciprocating compressor. You state your compressor "can only be loaded to 50% or 100%" – but you fail to say by what method. I'm going to take a guess that what you are using to control the capacity of your recip is a recycle control valve (for verneer adjustments) and volume pockets for the macro adjustment (50 and 100%). In other words, any flow rate between 0 – 50% or 50 – 100 % is handled by the recycle control valve.

I am not going to go into a compressor control tutorial exercise at this point because we don't have the time nor the space to run such an exercise. I expect that you are (or will become) knowledgeable in reciprocating capacity control and will understand what I say since you must have access to the operating manuals or instruction for your compressor. If I am correct in my guess of the type of compressor control you have been given, then you must rely on the operation of the recycle control valve to maintain a positive suction pressure on the first stage. This is critical when handling hydrocarbon gases because if you don't, you will establish a potential vacuum in the suction of the compressor, draw in atmospheric air into the compressor and create a potential explosive atmosphere with the hydrocarbon gases. IT IS VERY IMPORTANT TO MAINTAIN A POSITIVE SUCTION PRESSURE IN THE FIRST STAGE. NEVER CLOSE THE RECYCLE VALVE.

If you close the recycle valve, it is common sense that a positive displacement machine (such as a reciprocating compressor) operating at constant speed will draw a vacuum when the supply of gas is reduced below its rated capacity.

Also, when you manually reduce the compressor capacity to 50% it is obvious that the gas flow rate trying to get into the compressor is more that 50% of the rated capacity and, therefore, the pressure in the suction increase. You simply have reduced the capacity too much. That's why the recycle control valve is required.

You have not given us any control diagram or any P&ID, so I can't comment any more since we don't have the details on your installation. But be aware that you must fully understand what you are doing when you mess with the capacity controls.