A compressor going into stall is like jumping off a cliff with a bungee cord. If the bungee cord has no losses to dampen the oscillation, we have something akin to surge. A 0.5% drop in efficiency can occur for each surge cycle. Several surge cycles can occur due to delays and lags in high temperature, thrust, and vibration shutdown systems. In some compressors the damage is so severe after multiple surge cycles that rotors and seals need to be replaced. The cost of process downtime can be significant particularly when a compressor feeds parallel trains of equipment. The restart of exothermic fluidized bed reactors in the petrochemical industry may be the most hazardous mode of operation.
A precipitous drop in flow occurs in less than 0.05 seconds at the start of full surge. The oscillation period of 1 – 2 seconds is too fast for recovery by closed loop control. An open loop backup is needed to prevent a compressor trip. The culprit is what you don’t see on a compressor map.
Compressor maps typically show only the positive flow negative slope portion of the characteristic curve possibly including the zero slope point. To the left of the zero slope point is a positive slope portion preceded by a negative flow negative slope as depicted in Figure 3-1 in the excerpt "Description of Surge" from the Momentum Press book Axial and Centrifugal Compressor Control
The negative flow negative slope can be measured by compressor manufacturers by feeding gas backwards from the discharge of the compressor. The positive slope section may simply be a 3rd order polynomial fit between the negative flow negative slope and positive flow zero slope point of the characteristic curves. Users typically don’t get to see the curve to the left of the zero slope point that is the source of dynamic instability.
The precipitous drop in flow occurs when a discharge valve is closing and the operating point reaches the zero slope point. The positive slope provides positive feedback so fast the flow jumps horizontally on the compressor map to the negative slope in the negative flow region. The operating point then walks down the negative flow negative slope and at the start of the positive slope jumps horizontally to a point of positive flow negative slope. The result is a surge cycle shown in Figures 3-5a and 3-5b of the book excerpt.
How do we deal with these exciting dynamics? Besides the vibration and temperature shutdown systems we need a surge controller and an open loop backup. The open loop back up has historically been triggered by a precipitous drop in flow to step open a fail open surge valve (e.g. vent or recycle valve). The open loop back holds its output or decays its output to give time for the feedback surge controller to take over smoothly. A frequent question is how fast does the feedback control loop need to be? Companies have built a business on saying the control system must have an analog controller or a high speed microprocessor with an execution time of 0.05 seconds or faster.
If the surge control system has an air actuated surge valve, even with volume boosters the pre-stroke deadtime is 0.1 sec with 2nd order rate limited exponential lags of 0.2 sec for large compressors. The transmitter has a minimum lag of 0.1 sec. The minimum control loop reset time is about 1 sec. A 0.1 sec PID execution time adds 10% to the integrated error. The ultimate period is about 1 sec as well. A surge oscillation of 1-2 seconds is too fast to be attenuated so the job of the surge controller is to keep the compressor from getting close to the zero slope point. Feedforward from the downstream user feed flows dropping provides a helpful preemptive action for shutdowns in a parallel train. The surge controller should have minimal overshoot but not prematurely open the surge valve. The surge valve should be fast opening and slow closing. Dynamic reset limit, high speed readback of actual position, and an analog output (AO) closing setpoint velocity limit can be helpful if properly setup and tested. An open loop back up must kick in if the surge control can’t stop the operating point from reaching the zero slope point and be fast enough to prevent a shutdown on high thrust or vibration. If configured properly the open loop backup can be done in a module with a 0.1 sec execution time. Thus today’s DCS can be used for surge detection and prevention eliminating the need for special systems.
The key to an open loop back up system preventing even the start of surge is the use of a deadtime block to create a fast train of rates of change of flow and pressure to predict a future potential crossing of the surge curve as noted in the June 28 2012 post “Future PV Values are the Future”
If the rate of change of flow is negative, the open back up takes preemptive action when the rate of change of pressure approaches zero indicating the zero slope point on the operating curve is imminent or a future value of the operating point is too close to the surge curve. In this case an incremental opening of the surge valve can be used causing less disruption to downstream users.
The feedback surge control system can do a better job of getting off its output limit to open the surge valve sooner for a decrease in downstream user flow by an enhancement to the PID to increase the effective reset time. Additionally the future value of the operating point and associated rate of change can be used to adapt the reset time to make sure the contribution of the proportional mode exceeds the contribution of the integral mode so the operating point does not overshoot the surge setpoint.
