Abstract

This paper discusses a computer program that develops operating curves for single and multi-stage reciprocating compressors. The operating curves, which show horsepower requirements as a function of pressure, are used as an aid in operating compressors.

The series-parallel flow of the gas in a staged unit sets up interrelationships between the stages and also between the cylinders of each stage. At constant horsepower, small suction pressure changes cause large changes in the discharge pressure. The interrelationships and the suction pressure sensitivity cause convergence problems in modeling staged compressors. The convergence problem becomes more pronounced as the number of stages increases.

The first part of the paper discusses a technique that solves the convergence problems in modeling staged units. The technique is discussed along with the results of a computer program written to test the modeling techniques. program written to test the modeling techniques. The second part of the paper discusses how this test model is built into a production computer program that develops and plots operating curves. This program automatically produces the curves using the compressor's data (bore, stroke, clearance, etc.)

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Northern Natural Gas Co. uses operating curves as an aid in operating reciprocating compressors. These curves show horsepower and volume as a function of suction and discharge pressure for a given cylinder clearance; a pressure for a given cylinder clearance; a separate plot is used each clearance configuration. Figs. 1 and 2 show typical curves for single and multistage compressors. The operating curves are generated by a compressor model.

There are convergence problems in modeling staged compressors. Fig. 3 is a schematic of a typical staged unit. The series-parallel flow of the gas sets up interrelationships between the stages and also between the cylinders of each stage. The flow distribution, for example, to a stage's cylinders may change with pressure because of different clearances. As noted in Fig. 2, small suction pressure changes at constant horsepower cause large changes in the discharge pressure. The interrelationships and pressure sensitivity cause convergence problems pressure sensitivity cause convergence problems in modeling staged compressors.

A modeling technique was developed and tested that solves the convergence problem. An ideal compressor handling an ideal gas is assumed in describing this model. The actual test model used modified equations that compensate for real compressor and gas behavior, such as compressibility, valve leakage, etc.

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