Incipient Surge in a Centrifugal Compressor

Incipient surge is a flow disturbance in the impeller of a centrifugal compressor that occurs at low flows. However, calling this phenomenon “surge” is a bit of a misnomer. Since the disturbance occurs at low flows and will precede an actual surge, there is a tendency for some in the turbomachinery business to treat both of these operating disturbances as similar problems. There are some characteristics that surge and incipient surge share: they both occur at low flow rates, they both can cause shaft vibration and they can both be prevented with a minimum flow controller. Despite these similarities, their causes and effects are different.

What is incipient surge?
Incipient surge is a flow disturbance in the blade passage of the impeller when the flow is very low, but still “going forward”. If there is not enough gas flow to fully fill the blade passages in the impeller, boundary layer separation occurs on the trailing side of the blade. When the boundary layer separates, eddy currents form, similar to what happens on the top of an airplane wing just before the wing stalls. The shearing energy that creates the eddy currents becomes velocity energy in the eddy currents. These eddy currents escape the blade passage and move radially outward into the diffuser. Once in the diffuser, they fall behind the impeller and move back into one of the following blade passages. When they land in one of the blade passages, they impart their energy into that passage as a pressure pulse. This pulse acts normal to the shaft. The frequency of these pulsations (due to the movements of the eddy currents around the impeller) is about ½ running speed.

What is the relation between incipient surge and “real” surge?
The main relation between surge and incipient surge is their name. They are otherwise quite different events.
A centrifugal compressor raises the pressure of a flow stream by accelerating the gas to a very high velocity and then converting the high velocity into pressure. Surge in a compressor occurs when the pressure across the compressor exceeds the pressure that the compressor can impart to the gas by the velocity conversion process. At the point the system pressure exceeds the compressor’s pressure energy, the flow reverses through the compressor. The flow reversal causes the discharge pressure to drop and the suction pressure to rise. The drop in the system pressure differential allows the compressor flow to return to normal forward flow. This completes one surge cycle. If the conditions that provoked the original surge event have not changed, the surge cycles will repeat.The cyclic changes of the pressure difference across the compressor causes pressure pulses acting in the axial direction. These pressure pulses cause the thrust forces on the rotor to reverse. The thrust reversals, if large enough, can lead to a failure of the thrust bearings, and excessive axial displacement.

Repeated thrust reversals of sufficient magnitude can damage the thrust bearing, inter-stage seals, shaft-end seals and shaft sleeves. The pressure change across the impeller due to a full surge is approximately:

where ρ is the density of the gas and U is the impeller tip speed. Unlike incipient surge pulsations (that act radially on the rotor), these pulsations act axially on the rotor.

What does incipient surge do to the compressor?
Depending on the gas density and rotor sensitivity, it is possible to operate in incipient surge with no symptoms. In fact, most compressors show no evidence of incipient surge. High pressure and high molecular weight services such as injection machines and CO2 compressors tend to be the machines that are more likely to react to incipient surge. In general, if a compressor is affected by operation in incipient surge, it will manifest itself as increased radial vibration. If the vibration is excessive, the compressor can sustain bearing and seal damage.

What does incipient surge do to the process?
Unlike actual surge, incipient surge has no direct affect on the process.

How is incipient surge detected?
Incipient surge does not always occur with sufficient magnitude to allow detection. The pressure pulsations that accompany incipient surge act in the radial direction and have a magnitude of approximately:

These pressure pulsations will create torsional oscillations that are possible to detect if the driver is a motor. A special circuit designed to pass motor current noise in the frequency between about 0.4N and 0.6N can be used to detect incipient surge. This same noise detection circuit can be employed to pick up aerodynamic noise in the gas stream with limited success. Detecting the noise in the flow stream requires measuring the flow very close to the compressor suction with a fast, sensitive analog flow transmitter.

Even with these measures, the efficacy of this technique is not guaranteed. If the flow is measured in the discharge, the incipient surge disturbances are attenuated as they pass through the compressor to the discharge. Since the pulsations act radially, they can become a forcing function for radial vibration.

Most flexible shaft machines have a natural frequency that is in the same frequency range as the pulsation frequency of incipient surge. The coincidence of the incipient surge frequency and the rotor critical speed can lead to an increase in radial vibration if the compressor operates for an extended period in incipient surge. Increases in radial vibration when operating at low flows could be an indication of incipient surge.

How can incipient surge be prevented?
The same technique used to prevent surging the compressor is used to avoid incipient surge: recycle. If a compressor shows sensitivity to incipient surge, the compressor safety margin must be increased to prevent operation in this region. Just like real surge, incipient surge can be accurately predicted if the proper prediction algorithm is used.

Some compressor control system vendors incorporate a technique that increases the surge controller safety margin if high vibration is detected at low flows. This may or may not be an appropriate response since there are numerous causes of increased radial vibration. In general, the rotor sensitivity does not change significantly over time and if a compressor installation is sensitive to incipient surge, the change in vibration at low flows due to flow instabilities should be evident during the commissioning of the surge controls. On compressors that show symptoms of incipient surge (flow instabilities, increased radial vibration), the surge line should be set to the right of this area of instability.

Unlike in the case of actual surge, since occasional short term operation in the incipient surge region is not immediately detrimental to the compressor, using vibration as an indicator for automatic adjustment of the compressor minimum flow line can lead to unnecessary recycle or venting. High radial vibration of the compressor is an operational condition that should be evaluated when it appears on a case-by-case basis to assure that the corrective measure is appropriate.

Does incipient surge lead to “real” surge?
Incipient surge will precede “real” surge as the flow drops, but operating in incipient surge doesn’t lead to a surge. As long as the flow does not drop below the actual surge line, the compressor will not surge, regardless of how long it operates in incipient surge. As mentioned above, most compressors will not exhibit any symptoms when operating in incipient surge. Some compressors don’t experience significant boundary layer separation before surge occurs and some compressors are not excited by pulsations associated with incipient surge.

What does incipient surge do to the process?
In general, if incipient surge is of sufficient magnitude to create problems for the compressor, it will show up as high vibration. If there are no symptoms, there is no real value in attempting to detect incipient surge. Some control system vendors use incipient surge detection as a means of backing up a non-redundant control system. The problem with this approach is it basically provides a notification that the compressor operating point is too low. The output from the detector is not useful for controller action. The incipient surge detector might protect the compressor, but the action of the detector will likely create a process upset. If the compressor application is critical, it makes more sense to use a redundant controller with redundant transmitters. As long as a reliable surge prediction algorithm is employed, a redundant system will not only provide a more reliable and predictable protection system, it allows online repair without compromising the protection of the compressor.