Low Density Polyethylene (LDPE) production involves reactors operating at up to 3,000 bar pressure or even higher. The final stage of gas compression is done with a plunger-type machine called a “Hyper Compressor”. A “packing stack” for pressure sealing consists of multiple bronze packing rings and oil lubrication injection points; which are positioned in place by numerous packing cups. The distribution of pressure drop in a new cylinder and the changes in sealing behavior over its life are critical to design, but have long been prohibitively difficult to verify experimentally.
Four cylinders were extensively instrumented; two in each of two successive test campaigns over four years’ time. Strain gages were located at the outside diameter of each packing cup to infer internal dynamic pressure across the packing stack. Included also in the high-speed data acquisition were strain gages on numerous additional components, thermocouples on packing cup faces, a pressure transducer in the low-pressure lubrication line, and existing eddy-current proximity probes on the plunger rods. Continuous monitoring was performed from the initial installation to final removal of each cylinder. Analysis of this high speed data along with long-term trends led to several conclusions regarding the design and operation of the Hyper cylinder. Testing also revealed various unusual events that can be further studied to better understand some of the unique behaviors that take place in the highly complex cylinder design of Hyper compressors.
