Operation of reciprocating gas compressors driven by reciprocating gas engines has several inherent problem areas. The gas flows in a surging pattern in the suction and discharge headers which may result in excessive fuel use and harmful vibration. Normal operation requires loading changes to achieve the desired pressures and flow rates as system requirements change. These loading changes are typically, changes of compressor cylinder clearance or deactivation of compressor cylinders. The program developed by Panhandle uses the cylinder loading changes to achieve the smoothest possible gas flow. This results in reduced pulsation, vibration, and engine adjustment as operating conditions change. A Cooper Bessemer Z-330 (10,000 horsepower) and a Cooper Bessemer GMWS (3,400 horsepower) have been selected to demonstrate the effects of loading changes but the closer the match between instantaneous power developed and power required, the less energy the flywheel is required to absorb and release through rotational velocity changes, and the smoother the flow, the less the pulsation and vibration in the attached piping. The model calculates power cylinder torques by adding the harmonic torque components from each power cylinder at the appropriate phasing or crank interval. These are from empirical test data for I.C. engines, as published by Porter in the March, 1943 Journal of Applied Mechanics. The amplitudes are then adjusted up or down to create the desired horsepower at the operating speed of the unit in question. Most engines have even firing intervals and it is generally assumed that no improvement can be made in the input power curve. The line pressures, gas characteristics, cylinder dimensions and crank angles, and unloader type, size, and location are input for the compressor. The internal cylinder pressures throughout compression, discharge, re-expansion, and intake are calculated using ideal gas equations for each end of each cylinder. Composite torques and flows in the suction and discharge piping are then developed by adding the values for each end of each cylinder at the appropriate phasing or crank intervals. The compressor cylinder timing for the two examples are shown in Diagrams 1 and 2.
As previously described, the composite compressor torque curve for one cycle is developed for the unit with one unloader activated. A Fourier Analysis is done to determine the amplitudes of any specified harmonic components of the torque curve. As for pulsation, the model is used to sequence through all available unloaders to minimize the torque variation at any specified frequency. This can be used to minimize torsional vibration and the associated stress when operating near a crankshaft critical speed. This results in less frequent need to balance power cylinders with compressor load changes, as the load applied more nearly matches the power developed on an instantaneous basis than with a randomly or statically selected unloading sequence. This results in smoother operation and reduced fuel consumption. Graphs IX through XII show the instantaneous torque (horsepower) curves for the two sample units. The best and worst cases at typical conditions are given for both units.