A PC-compatible computer program that identifies and quantifies malfunctions in reciprocating compressor performance has been developed and is undergoing initial field tests.
Southwest Research Institute, San Antonio, is developing the compressor diagnostics software in a program jointly sponsored by the Gas Research Institute (GRI) and the Southern Gas Association's Pipeline and Compressor Research Council, according to GRI.
Field tests using version 2.0 of the software are taking place at Amoco Production Co.'s Painter, Wyo., plant; Tennessee Gas Pipeline's Channel Industries Gas' Station 809; Transcontinental Gas' Stockbridge, Ga., compressor station; Pacific Gas & Electric's Hinkley compressor station; and Marathon Oil Co.'s Yates field.
The final version will be commercially available next year.
PROBLEM
GRI says that reciprocating compressors in U.S. gas-transmission pipelines total at least 10 million hp installed. And the costs of operating them average $60 for fuel and $30 for maintenance for every 1 MMcf of gas transported.
For a large compressor, the yearly cost of valve leaks alone, if uncorrected, can run to hundreds of thousands of dollars.
Because of these large operating costs, compressors must operate at peak efficiency, which also helps minimize emissions such as nitrogen oxides (NO,).
Maintaining peak compressor efficiency, says GRI, requires an accurate, easy-to-use method to identify compressor malfunctions and to determine when operational corrections or maintenance are required.
GRI SOLUTION
The PC-compatible computer program, Compressor Diagnostics Software, identifies and quantifies malfunctions in reciprocating compressor performance.
By pinpointing the source of the problem-valve leak, piston ring leak, or piston top-dead-center errors, for example-the operator can determine what preventive or corrective action may be necessary.
The software can also be used to control and continuously monitor compressor operation, says GRI. And it has application for compressors used in gas storage, processing, and production facilities.
By monitoring the mechanical "health" of a reciprocating compressor, says GRI, the software indicates when maintenance action is necessary. Timely maintenance that leads to efficient operation saves natural gas fuel, maximizes pipeline throughput, reduces transmission costs, and avoids unnecessary shutdowns.
CONCEPT
The software is based on a set of mathematical equations (algorithms) that analyze the variation of pressure with the position of the compressor piston. The software accounts for thermodynamic conditions (heat transfer between the gas and the cylinder walls, for example) and valve dynamics.
With inputs of important compressor cylinder dimensions (bore, stroke, rod diameter, so forth) and information on gas quality, the model predicts what the variation of pressure and temperature should be through a full cycle of piston motion.
The software obtains its information from a data-acquisition system measuring cylinder operating conditions (cylinder pressure, suction temperature, so forth) at regular intervals of the crankshaft rotation, says GRI.
With this information, the software compares predicted pressure variation with measured values and selects potential causes for any significant discrepancies, particularly piston top-dead-center errors, leakage of suction and discharge valves, and piston ring leakage.
The suspected causes are incorporated in the model, and the predicted and measured values are compared again.
The procedure is repeated until the predicted and measured pressures agree for both expansion and compression cycles of the cylinder. The quantitative information generated by these simulations permits calculation of the magnitude of valve leaks, wasted pumping effort, performance, and throughput.
PROSPECTUS
The initial software has been refined to improve temperature-rise prediction and to identify sources of valve problems. The user interface and data base management have been improved based on the initial testing.
Further refinements will include analysis of multi-cylinder, multistage reciprocating compressors, indication of performance trends, and diagnosis of piston top-dead-center errors.
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