There are many destructive forces in centrifugal compressors and these machines are more prone to failure when operated at off-design conditions. The purpose of this article is to present a case study of an impeller failure that is suspected to have been caused by operating at incipient surge, coupled with insufficient margin between the diffuser excitation frequency and an impeller natural frequency. References
This case study pertains to a 3-stage integrally geared centrifugal compressor driven by a 1,780 rpm, 5,000 hp induction motor. The gearbox consists of a bullgear and two rotors. The low speed (LS) rotor consists of a pinion with overhung impellers mounted at both ends. The high speed (HS) rotor consists of a pinion with an overhung impeller mounted at one end. All the impellers are a semi-open type. In this design the front side of the impeller is open and the vanes run against a close clearance, non-contacting stationary shroud. See Figure 1. There also are vaned discharge diffusers on every stage.
The compressor performance map is shown in Figure 2. These are a series of performance curves at different inlet guide vane (IGV) angles. For each IGV position there is a curve of discharge pressure vs. flow. As shown, for each curve the discharge pressure increases as flow is reduced. The highest pressure, lowest flow point on each curve is the surge point. Surge is an unstable operating condition in which flow reverses direction somewhere in the flow path. When this happens, the discharge pressure drops and flow reverts back to the normal direction. This cycle of oscillating flow continues until the low flow, high pressure condition is corrected. Although surge starts in one stage in a multi-stage compressor, the flow reversal affects the entire machine. Surge causes high noise, high rotor vibration, and an increase in inlet and interstage suction temperatures. The pressure fluctuation results in significant changes in rotor thrust load and direction, which can cause thrust collar and/or thrust bearing failures. While surge is characteristic of all turbocompressors, it is a condition that, if not corrected via process changes, will lead to machine damage and possible component failure.
Variable inlet guide vanes are an efficient method to control the flow of a centrifugal compressor.
IGVs pre-whirl the gas entering an impeller, which reduces the flow and head (pressure rise) of the compressor stage with little loss in efficiency. While some machines incorporate IGVs on every stage, it is more common to have an IGV on only the first stage. The compressor described in this article included an IGV on the first stage only. The compressor performance curve in Figure 2 also shows how the flow and pressure change as the IGV is closed. Note how the surge point changes with IGV position. The line connecting the surge points for the different IGV positions is commonly called the surge line.
Stall is a localized flow reversal that occurs in a compressor stage. Stall cells typically start to form close to the surge point, and so stall is sometimes referred to as incipient surge. Stall is not severe enough to cause a complete flow reversal. Although stall does not generally have as much destructive energy as surge, stall is a potential source of excitation, which also can cause an impeller failure. It is more difficult to detect than surge because the overall machine performance and vibration typically remain stable. Although stall cells start forming close to the surge point, the margin and magnitude vary from one design to another.
Compressor control system
The control system for the compressor described in this article is shown in Figure 3. Besides the first stage IGV, the other primary components included a flow meter, a pressure transmitter and a discharge vent valve. In this application, the compressor capacity was controlled with the IGV and the machine discharge pressure was a function of the system back pressure. A DCS (Digital Control System) was used to operate and control the compressor. Based on the measured discharge pressure, the surge flow was determined by the DCS and compared with the measured flow. A flow margin to the actual surge flow was maintained, which established a surge control line. If the compressor flow approached the surge control line, the control system would start to open the discharge vent valve to prevent the machine from surging.