Torsional Analysis of a 20-Cylinder Hypercompressor Train

Abstract

The capacity of plants for the production of low-density polyethylene is increasing worldwide and is continuing to challenge designers to provide the larger compression machinery required. The expected level of reliability and availability is very high, and the cylinders and frames of these reciprocating compressors are specially designed to withstand this heavy-duty service. All design aspects have to be properly considered, including the torsional analysis that represents a very critical element. This article considers a train with 2 compressors, each configured with 10 cylinders, with an electric motor drive between them. One machine is directly coupled to the motor while the other has a flexible coupling. This represents the largest reciprocating compressor train in the world, with a total motor power of 33 MW.

Practical torsional analysis and coupling selection for multi-machine complex process rotating equipment trains

“…It requires a direct integration of the motion equations, and the solution is time-dependent. The differential equations of motion are similar to those of the steady state (equation (2)), but they have different exciting torques [4].…”

“…Very high frequency additional modes have no effect on reliability (no resonance is foreseen and also they are high frequency and low amplitude exciting harmonics) [4]. Basically, the model is represented by a number of rotating masses connected to each other by means of mass-less shaft intervals with appropriate torsional stiffness.…”

“…Figure 3 shows a torsional model for a complex rotating machine train. The goal is to locate natural frequencies away (usually by 10 per cent) from potential excitation frequencies that might come from machines [4][5][6]. In addition, it is recommended that natural frequencies are placed 10 per cent outside of the electrical net frequency and 5 per cent outside of its second multiple [4].…”

“…Because of low damping levels [3,4], the undamped natural frequency can represent the natural frequency of a real train. In case the torsional natural frequencies do not meet mentioned margins (10 per cent away from potential excitation frequencies), individual train component should be modified.…”