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Surge in centrifugal compressors cannot, in general, be avoided when a unit trip or a major upset occurs, but the energy of surge should be minimized. Surge is a dramatic collapse of flow within a centrifugal compressor, which results in reverse flows within the machine and attached piping and can cause damage to bearings and other components. During normal and slowly changing operations, surge can be avoided by recycling gas through the surge control valve to maintain a minimum flow. However, when a trip or major upset occurs, flow rate drops and the primary means by which surge energy can be reduced is to lower the head (suction to discharge pressure difference) at which the compressor reaches the surge (minimum stable) flow condition. The head across a compressor during a trip or upset is dependent on the response of the entire system including changing performance of the compressor, transient flows within the piping, control system responses, and capacity and opening rate of surge and other automatic valves, such as vent or blow down valves, and check valves. This paper describes tools and techniques that can and have been used to model transient flows and performance, mechanical and control responses, and time dependent head in compressor systems. The tools used by SwRI include a method of characteristic transient flow analysis routine and finite time step programs that simulate control systems, valve actuators, and the opening (or closing) rate of valves with the resulting flows. The effects of volumes and lengths of station piping, scrubbers, and coolers including temperature effects are accounted for. Computer models also track the performance of centrifugal compressors at different speeds, account for the rotation inertia of compressor trains, and evaluate the thermophysical properties of gas streams. These computer routines have been integrated into a computer process that evaluates the design of entire compressor systems including recycle and other valves
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