Booster, Primary and Hyper compressors are the pieces of machinery used to reach the extremely high pressures needed for the reaction in Low-Density Polyethylene plants. Wide operating ranges are necessary due to the different products and conditions required by the process to obtain both homo and co-polymers required by the market. A suitable capacity control system has to be studied during the design phase to optimize the operation particularly for Booster and Primary compressors, that are supplying the required flow to the Hyper, then compressed to the reactor. In the latter the traditional capacity control cannot be used because of the very high pressures involved and for start-up and various preparation activities the by-pass along the reactor is the only flow regulation possibility. On the contrary, in Booster and Primary there is a wider choice of CCS, from "clearance pockets" to "variable spee1d drive", from step to step-less. This paper gives a brief overview of these systems, explains their pros and cons and examines the impact they have on valve behaviour explaining the CCS-valve interactions. In particular it focuses on the new generation of electromechanical step-less control systems.
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