Screw and Reciprocating Compressor Control
  • Vote Up0Vote Down Matt B May 2008
    Posts: 2
    I am currently designing a coal seam methane plant that has a large number of screw compressors feeding a large number of reciprocating compressors all located in the one plant. With the reciprocating compressors located close to the screw compressors and a single recip comp capacity (25mmscfd) being significantly higher than a single screw (8.5mmscfd), I have a concern that the screw compressors will not be able to respond quickly enough to shutdown of the recip compressors.

    Our compressor vendor is currently supplying a screw compressor package with a 25% capacity recycle valve and not 100% - they have stated that this is typical practice. The screw compressor responds to high discharge pressure by using it's slide vane to unload the screw, then reduce speed and then open recycle. The slide vane takes some time to respond and unload and I am concerned that it will not be quick enough to avoid tripping the screws on high discharge pressure. Immediately opening a 100% recycle valve and unloading the screw as quickly as possible makes more sense to me and I am wondering if we are wise to get the vendor to upsize the valve for this purpose?

    Anyone have any experience with operating screws and recips in series in close proximity?
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  • Vote Up0Vote Down mdraqib43 May 2010
    Posts: 1
    Excellent article.Its really a good article. It gives me lots of pleasure and interest. It’s a most important post. So i want to know some other details about this article. Thanks
  • Vote Up0Vote Down Todd May 2010
    Posts: 3
    This is a fairly normal situation in refrigeration plants.  Usually the booster compressors are recipricating and the high stage are screws.  Slide valves are usually adjustable because they need to be set to opperate at a speed that is condusive to the system stability.  For example, if a screw loads too fast, it can draw down the intermediate pressure too low, causing the recip to unload.  ( vollumetric efficiency - lower discharge, lower suction ), then the screw unloads too fast and causes the recip to load again or shut off on high discharge.  The other reason slide valves need to be adjusted is too prevent over amping.  If the needle valves on the slide valve control are opened up all the way, a screw could load to 100% faster than anything in the system can keep up with.  Depending on your screw manufacturer, the slide valves will be controlled by different methods, but they all use a piston that is controlled by differential pressure usually between suction and oil pressure.  The best sollution is to control the system, not just one compressor.  Look at all the system variables.  Try to evaluate how many of them can be eliminated.  Is your load constant?  If not, can you stabilize
    it with a regulator?  Are you bringing in an intermediate load?  Is it constant?  Recycle valves are usefull, but a well designed system would not rely on them too heavily.  A high quality control system that has inputs from all the compressors pressures and slide valven position along with the recips unloader status and any input from the up and downstream Processes.  I would also want this controller to have total control over the regulators that are bypassing or holding pressure on or before your compressors.  If you employ a good PID strategy and use fast acting control valves, you should be able to prevent any shut downs.  In fact, you should absolutely include a means to digitally record and easily graph every single I/O point.  This all sounds like a lot, but I have gone into countless plants that could not meet their specifications and most of the time it is related to control, not equipment.  Good luck!     
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