Reciprocating Compressor Control

I think recycling the nitrogen pressure is waste of energy and N2 ,Instead a reciprocating compressor have a facility of load and unload, call the vendor to fixed load unload valves over the cylinder and that should solve ur problem further connect the discharge header pressure switch to solenoid valve so that at hi pressure the load unload valve will unload the compressor and at low pressure it will load the compressor . and alsoadding pulsating damper at discharge also will reduce the fluctuation .it also possible to run the compressor on partial load by selecting some unload valve on and some unload valves off using two solenoid .

Adil

Perhaps Steve S. will have some input on this thread as well, he has a lot of experience with this. In my experience, it is very typical to have a recycle valve coming off of the discharge of the compressor and sending gas back to the front end of the machine. They are used to trim capacity control and also to start the machines up on no load, especially when there are no unloaders present.

Even with unloaders as the previous poster suggested, I would still install a recycle valve that is sized for a generous portion of the compressor's rated discharge flow, if not 100% of the compressor's discharge flow. The recycle valve while costing power will tremendously smooth out your pressure spikes, right now the compressors are interlocking to protect either themselves from excessive rod load or your downstream piping/vessels/PSVs from over-rating.

Setting up the control is fairly easy, in my experience you have a PID controller for the suction pressure and a PID controller for the discharge pressure and whichever output is greater, that is sent to the recycle valve as output. The suction pressure controller is reverse acting, that is as the suction pressure drops, the valve opens to try and raise the pressure. This will protect you from pressure fluctuations on the front end of the compressor as well as the discharge.

You need to be careful with wind up and some other minor problems but it is a pretty established method of controlling the compressor. Others may be able to explain this better. The valve response will be based on the tuning of the control valve which you will have to set once the valve is in place. I would usually make the valve fail-open, but others may have a different opinion.

The previous poster is correct, there can be substantial power savings to installing unloaders on the compressor, be it over the suction valves, on volume pockets on the head end of cylinders, etc. Usually however, these are discrete devices in that they open or close and create specific load steps on the compressor. A good example would be if you put unloaders only over the head end suction valves of each cylinder such that when they were turned off, the compressor would only be compressing gas on the crank end and would be about 50% loaded. Depending on the number of throws and stages you have, you can get a number of different loads. One machine I operated had 37 load steps between 43% and 100%, and some others had 12 or 13 load steps. Sometimes they only have 3. I have heard of limitless unloading, but have also heard that it can be a pain to manage so I won't suggest that.

If you go the unloader route, you need to employ or consult a reciprocating compressor engineer (I am not one of those). By changing the load on the compressor in this way, it is easy to get unbalanced forces on the machine, poor rod reversals, etc. As I stated, I would also still put in a recycle valve as it seems you have some rapid disturbances.