Hyper-compressors are subjected to very high operating pressure, maximum and fluctuating, strongly impacting design and life of all cylinder components. Components subjected to fatigue (both high cycle fatigue and low cycle fatigue) can experience a failure in a very short time that may cause large production losses. Packing cups proved to be one of the most critical cylinder components and the fatigue behavior of the packing cups subjected to alternating loads, due to the suction and discharge conditions, is their key design factor. A thorough study has been performed in order to increase the safety factors in all critical regions of the packing cups analyzing a new manufacturing cycle that allows optimizing the compressive pre-stresses on the lube oil parts due to the autofrettage procedure. The approach included a dedicated sensitivity FEM analysis of the autofrettage pressure to determine the correct residual stresses at the lube oil parts in order to keep them under compressive stress in operating conditions and Fracture Mechanics Calculations based on BSI7910 aimed at achieving a design able to survive with a threshold defect larger than the minimum detectable defect.

The life of valves is extremely important for the availability of hyper-compressors in LDPE plants. The valves operate due to the action of the differential pressure across a moving element, typically with a poppet shape, and close due to the action of dedicated counteracting springs housed into the same valves. A proper design of the spring is a balance between ensuring the “on time” opening/closure of the valve, a low impact velocity of the poppet and low fluttering. Particular attention shall be paid to the dynamic behavior avoiding to excite the spring and poppet-spring system close to themselves resonance frequencies. Different factors such unexpected transient conditions or vortex shedding phenomena could shorten the life of valves springs and valves bodies, since the valve spring is forced to operate out of the design conditions. A compressor application presenting evidence of spring surge phenomena (resonance of spring itself) is presented. Frequencies exciting spring natural modes frequencies can be introduced in the process gas by vortex shedding generated along the gas outflow through the valve during the transient conditions or in regime conditions. Since the vortex shedding frequency predictability is affected by some bias inaccuracy level coming from process variability and model approximation, then a robust design approach is to employ progressive spring that is insensitive to surge phenomena. This paper gives an overview of progressive and non-linear springs and the benefits they offer compared to linear springs and how progressive springs has to be designed for hyper-compressor valves application. Several spring designs are analyzed with a simplified prediction model, the results (load deflection, stiffness, natural frequencies, fatigue lives) are then compared with finite element analysis and bench tests results using a specifically developed test bench allowing to measure spring natural frequencies and to conduct accelerated fatigue testing.