Load Sharing Compressor

Compressor Load Sharing is used to maintain an equal throughput flow (or capacity) through multiple compressor units which are connected to common suction and discharge lines as seen in Figure 1. The Load Sharing function is normally integrated as a function block into the PLC based unit control system. Stand-alone controllers are also available. A central controller (usually supplied by the customer) is used to provide a common setpoint for each compressor Load Sharing Controller based on suction or discharge pressure or flow for all the units. The compressor throughput flow is a calculated value comprised of the compressor inlet flow minus the recycle flow where the recycle flow is calculated as a function of the recycle valve position. The speed of the compressor driver, the inlet throttle valve position, or the guide vane position is used as a feedback signal to the load sharing controller. The Load Sharing output signal is used to vary the compressor throughput flow by varying the compressor driver speed on variable speed units or an inlet throttle valve or guide vane position on constant speed units.

FE Suction Flow Element HS High Select I/P Current to Pressure Transducer ^Pc Compressor Differential Pressure

PI Proportional Integral PID Proportional Integral Derivative Ps Suction Pressure Feedback Signals in RED

The output of each Load Sharing Controller has a time ramp function which limits either the rate a unit's speed may be changed or the rate the inlet throttle valve or guide vane position may be changed. This is used to limit the rate of control action for stability purposes, and to minimize negative interaction between Load Sharing Controllers, Surge Controllers and other elements in the process. For example; during low flow conditions, if the speed of a unit is being decreased so fast as to cause the flow to decrease below the surge control line (a limit at about 1% of the total flow of the individual compressor to the left of the control line), the rampdown of speed will be temporarily stopped long enough to allow the surge control action to open the recycle valve to increase flow above the limit. This action prevents the speed of the unit from being decreased so quick as to drive the compressor into a surge condition. The highest of the compressor throughput flow signals from the units is used as a feedback signal to the central controller. Since the load sharing controller for each unit has the same input setpoint from the central controller, the units are caused to share the total load equally. This equal sharing of the total load creates a balanced operation and increases the efficiency of all the units.