Greg: I learned early in my career that compressor surge is the fastest phenomenon that occurs in process industries around turbomachinery, and with the most disastrous consequences. We had huge axial air compressors whose acceleration upon surge during tests was measured to be 2,000 rpm/sec from the unloading during flow reversal. We used a derivative module to trip the compressor in less than a second. While this compressor was undoubtedly special, the fact remains that when a compressor goes into surge, the compressor flow can reverse direction in less than 0.02 seconds. It’s like running up a hill and just as you get to the top, you come to a cliff, can’t stop in time and go over the edge.
This corresponds to an operating point moving up the compressor characteristic curve to the point of maximum pressure rise, then coming to a sudden reversal in the sign of the slope of the characteristic. Not typically shown on compressor maps is that the slope of the performance curve becomes positive after reaching a maximum, so that as the flow decreases, the pressure rise from suction to discharge also decreases, creating positive feedback.
Another factor contributing to developing surge is the rotating stall phenomenon, which can push the operating point into full surge. The overall result is that the operation of the compressor becomes unstable, with self-sustaining oscillations, as the operating point jumps to a negative flow region, then rapidly recovers as the pressure ratio across the compressor drops. The surge cycle then repeats itself. The surge cycle period varies from about 0.1 seconds in benchtop tests of laboratory-scale units to about 1-3 seconds in large industrial installations. My book, "Centrifugal and Axial Compressor Control," reveals the path of the operating point on the unseen compressor map to the left of the surge curve and the resulting oscillations in suction flow and discharge pressure, as well as the dynamic model that will show this behavior. Naum Staroselsky was the person who triggered many of my thoughts and was the ultimate expert in my book, literally and figuratively, as exemplified by his article “Improved Surge Control for Centrifugal Compressors,” Chemical Engineering, May 1979.
Stan: Naum Staroselsky was the first person to emphasize the precipitous drop in flow during surge, the exceptional requirements as to how fast the automation control system needed to respond, and that feedback control by itself often could not get a system out of surge because the oscillations were so severe and fast.
Greg: I had the privilege of talking with Naum several times back in the 1980s. He developed the essential speed-of-response requirements of each component in the system, as well as algorithms for dealing with these fast transients. He also implemented strategies for compressors operating in series to increase the total pressure rise and in parallel to increase the total flow capacity. Here we have the opportunity to get a review and update with Naum’s son, Serge Staroselsky, who is the chief technical officer for Compressor Controls Corporation.
Stan: Which types of compressors have the most severe surge cycles and damage?
Serge: Axial compressors are more susceptible to damage due to their design. The issue of protection from damage caused by surge has always been very important to anyone operating turbomachinery. Emphasizing this point, the latest edition of API Standard 670, “Machinery Protection Systems” (5th Edition), has a dedicated section on surge protection, which mandates a separate surge detector for axial compressors and recommends segregated detection for critical centrifugal machines deemed susceptible to surge damage, such as compressors operating at very high pressures (e.g., reinjection service). If you have a redundant system, you can integrate the detector into the compressor control system, but the preference is for a segregated system. The detection system protects the compressor, in case the primary antisurge system fails to do so, by either opening the antisurge valve via a trip solenoid, actually tripping the unit, or both.
Almost every compressor will eventually suffer damage from excessive surge cycles. In most cases, it’s continuous rather than individual surge cycles that cause mechanical damage. Our recommendation is to always prevent three or more successive surge cycles in the time period on the order of 10 seconds, whereas the guidelines by some manufacturers may be more conservative, and use three or more surge cycles in a time period as long as half an hour. In many cases, even if there's no damage to the compressor due to surge, there is a severe process upset that may lead to lost production. The financial implications increase if you're talking about more than one surge cycle, since the process may not be able to recover, resulting in equipment tripping and a subsequent lengthy startup period.
Greg: I once heard that there is a loss in efficiency for each surge cycle, albeit quite small for a single cycle. What do you see as a loss in efficiency from surge?
Serge: I have not seen much data on this. We'll be offering performance monitoring software that includes the ability to calculate the efficiency online, which will allow us to evaluate the loss of performance over time.
To do this, you need to know gas composition and use real gas equations; otherwise, the monitoring system can’t provide sufficient reliability. For many compressors experiencing wear-and-tear due to surging for several years, the performance curves (compressor pressure rise versus suction flow curves for various speeds) tend to move down and to the left, indicating internal recirculation. The compressor produces a lower pressure at the same flow. Of course, continuous and frequent surging can result in loss of performance within weeks or even days; it just depends on how often the machine surges and for how long.
Surge causes high radial vibration and high dynamic axial thrust forces. Radial vibration can damage the journal bearing due to breakdown of the flow pattern around the impeller, causing unbalanced aerodynamic forces acting on the rotor and moving it relative to the radial bearing. Surge cycles cause changes in the direction of the flow, and therefore result in rapidly changing axial forces that can damage the thrust bearing due to axial movement of the rotor beyond design tolerances. Changing forces also adversely affect the tight clearances and inter-stage seals in a typical multi-wheel machine, which are designed to prevent backward flow. The result is internal recirculation of flow from the compressor discharge to suction during normal operation and a subsequent loss in efficiency and machine capacity.
While the loss in efficiency after a single cycle may be too small to measure, the ability to trend the cumulative effect of surging and degradation due to fouling and other causes can be useful in scheduling maintenance and in educating us on the importance of proper surge control, providing justification for better control system design, implementation and maintenance.
Stan: How do you get an accurate surge curve?
Serge: Our engineers start with the performance curves from the original equipment manufacturer (OEM) of the compressor. It's critical that we have a flow measurement with good accuracy and repeatability. A venturi tube can provide an excellent low-loss measurement of compressor flow given sufficient straight runs of pipe. Orifice plates, V-cones, and Lo-loss tubes also can provide sufficiently accurate and reliable flow measurement, provided they're installed following good engineering practices.
We highly recommend testing the compressor in the field. The differences between field and shop surge curves can be greater than 10% due to inherent flow measurement inaccuracies, as well as uncertainty in the pressure and temperature measurements at different operating conditions, and limitations in the accuracy of the off-design performance predictions of the OEM.
In some cases, typically for axial compressors and very large, new centrifugal machines, the OEM or the end user may opt out of doing field testing for surge due to the perceived risk. In this case, we recommend at least reducing the flow to 10-12% above the expected surge flow to prove that surging doesn't occur within the expected operating envelope of the machine. We have a technique to detect the start of surge and recover within one cycle, which in a great majority of cases is not dangerous for the machine. An additional benefit of doing tests to identify the surge limit is verification of the ability of the control system to provide a fast and complete recovery.
Before we even do a surge test, we test for proper response of the overall system, which includes the antisurge valve and the compressor with associated vessels and piping. After generating a step command, you should be able to see a discernable change in compressor flow within 1 second from a change in the command signal to the antisurge valve for the system to be fast enough to take the machine out of surge within a single cycle.
We like to do three surge tests at different operating conditions to build the surge limit curve. If there's a major discrepancy between field and OEM curves, we try to find out why. Maybe the conversion of the square root of the differential pressure (dP) signal to flow is incorrect or there is a measurement error. Our new diagnostic system will provide an indication of whether the operating point matches expected performance and, in case of mismatch, indicate probable causes, such as measurement error. In any case, we report any discrepancies to the user and OEM, and work with them to identify the causes.
We try to make our recommendations regarding proper selection and installation of instrumentation as early as possible. In many retrofit jobs, we do a site walk through the existing installation before even starting the design of the surge control system. Sometimes, there are obvious problems with the dP impulse lines, such as accumulation of condensation in the lines from pockets and lack of slope. There may be excessive length due to the desire to make the location of the dP convenient for maintenance. The length from dP transmitter to process connections should not be greater than 10 feet to keep the delay less than 0.1 sec for pressure changes to be sensed in the transmitter. To measure compressor pressure rise, the pressure connections should be close to the inlet and outlet flanges of the compressor. It's surprising how often good engineering practices are not followed, creating excessive delay, noise and erratic behavior in the measurements.