Let's say that instrumentation starts to become boring and you want to add a little excitement to your life. Volunteer to work on the surge controls for the next compressor project. There is nothing quite like the experience of standing next to a compressor going into surge. Noise from the collapsing voids within the unit and flexing of the intake duct during flow reversals have caused even the oldest and bravest control system designers to set records for the 60-yard dash. And operators always appreciate the exercise they get acknowledging radial vibration and axial thrust alarms; of course, if a unit stays in surge for about 20 seconds, the alarms will stop - at which time, iron will have been relocated on the plot plan.
Part of the thrill of the surge experience is that you often cannot watch it coming. Forward flow reverses direction in about 30 milliseconds. A second or so later, the reverse flow changes to forward again and the cycle repeats itself. Moving up the characteristic curve to the point of slope reversal and surge is like flooring the gas pedal of a car on the way to a precipice at the top of a mountain.
Surge dynamics carries good news and bad news. The good news is that the sudden flow transient can be used as a surge indicator. The bad news is that once the compressor reaches the point of zero slope on the characteristic curve, nothing can be done to prevent the surge. If you try to use feedback to get out of surge, you will find that the flow oscillation period is much shorter than the controller settling time. The oscillations therefore seem like uncontrollable
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