Ask Greg McMillan - What role do you see dynamic simulation playing in the future of best understanding of compressor open-loop backup?

Greg's Response:

Even a fast feedback controller is unable to get a compressor out of severe surge because of the huge jumps in flow. What is needed is an open-loop back up that forces the surge valves to immediately open and holds them open for sufficient time to sustain operating point stability before allowing the feedback controller to start to close the surge valves. The open-loop backup is triggered by a large predicted overshoot of the surge setpoint to prevent surge, or a precipitous drop in flow indicating an actual surge.

An innovation uses a predicted overshoot via a fast future value that is generated by the rate of change of a decreasing flow, with a good signal-to-noise ratio multiplied by the total loop dead time, with updates every controller execution. The open-loop backup simply puts the feedback controller into a remote output mode that is seen by operators. The remote output is immediately stepped up to a position that typically prevents surge, but is incremented every execution until the future value stabilizes, putting the surge controller bumplessly back in cascade with the surge setpoint computed to sustain an offset from the surge curve.

The key component for both recovery and prediction is a deadtime block to create a low noise reliable rate of change signal that can be updated as fast as the PID execution rate. For recovery from surge, a high rate of change triggers the backup that is the input minus the output of the deadtime block divided by the block deadtime that is typically just large enough to provide a good signal to noise ratio. For prevention of surge, a predicted flow one deadtime into the future that is too close to the surge curve triggers the open loop backup. The flow future value is simply the input to a deadtime block plus the change that is the block input minus its output with the block deadtime set equal to the total loop deadtime.

For the latest details on advances in compressor modeling and control see my 2019 AIChE Spring Meeting Presentation “Compressor Surge Modeling and Control” and the Control Global article "Compressor surge control: Deeper understanding, simulation can eliminate instabilities"

For much more knowledge, you can download for FREE the Momentum Press book Centrifugal and Axial Compressor Control.