No one wants to experience compressor surge. It’s not only a strain, but it’s dangerous if you are not fully aware of the effects. Read further to learn more!
Understanding what compressor surge is will help in surge control to get the system fully operational again.
The same applies to terms such as mild surge, violent surge, stall, flow separation, rotating stall, and lots more since these are closely related to compressor surge.
Moreover, knowing the impacts of surge on a compressor can help to manage the unit better each time instability occurs.
What is Compressor Surge?
Compressor surge can be linked to the interaction between the piping system and the compressor.
In this case, there is instability which causes the intermittent reversal of gas flow throughout the compressor.
Instead of the gas flowing normally from the suction nozzle to the discharge nozzle, it reverses and moves from the discharge nozzle to the suction nozzle.
This flow creates large forces that can destroy the compressor’s seals, bearings, and other rotating elements.
Types of Compressor Surge
A compressor can experience a violent or mild surge. The type of surge that is evident in the compressor depends on the piping system.
The geometry of the piping system upstream and downstream of the compressor also determines the frequency of flow.
Violent Surge:
A violent force is where flow reversal leads to damage in the compressor’s rotating elements.
Mild Surge:
A mild surge occurs when the gas flow has not reversed fully.
What is Stability?
It is worth differentiating between stability and instability of flow in the compressor and how the latter helps to prevent surge.
Accordingly, stability occurs when there is a finite flow fluctuation in the system, however, the compressor does not amplify this flow fluctuation to lead to a surge.
For instance, a compressor map featuring a negative slope of the head-flow curve tends to react to a small reduction in flow with an increase in the head.
As such, this counteracts the disturbance.
In contrast, if the compressor map has a positive slope, the machine would reduce head, which could heighten the disturbance and lead to flow reversal.
What is a Stall?
Stall is often evident in one stage of a compressor and it might occur even before surge does.
However, a surge may not occur in all cases since multistage compressors are capable of showing increased head at a reduction of flow in the face of a stall in one of its stages.
Stall is an aerodynamic phenomenon in the compressor and this stall is often detected by the increased vibration of the compressor’s rotor.
Here, a reduction in the constant speed of gas flow through the compressor increases losses in all aerodynamic components.
At some point, the flow in one of the aerodynamic components such as the diffuser or impeller inlet may separate.
Stall in the Diffuser or Impeller Inlet
Stall that occurs either in the vaned diffuser or impeller inlet can be attributed to the incoming flow’s direction changing with the rate of flow through the compressor.
Vaned diffusers would often reduce the operating range of a stage, which is quite different from a vaneless diffuser.
Consequently, a reduction in flow causes an increased difference between the direction of the incoming flow, which the impeller was designed for and the actual direction of the incoming flow.
This difference in directions can increase significantly thereby causing a breakdown of the flow through to the impeller.
Flow Separation in a Vaneless Diffuser
Flow separation in a vaneless diffuser describes a process where part or all the flow fails to exit the diffuser on its discharge end.
Rather, there will be areas where the flow remains stagnant and may reverse back to the inlet of the diffuser.
What’s more, flow separation can adopt the features of a rotating stall. The diffuser’s parts experience flow separations once the flow through the compressor stage is reduced.
Rotating Stall
Rotating stall takes place when the regions of flow separation are not fixed, but can transport in the direction of the rotating impeller.
This stall can be detected from an increase in vibration signatures in the sub-synchronous region.
However, the beginning of a stall may not constitute an operating limit of the compressor. There are cases where the flow can be reduced even before the actual stability limit has been attained.
Why Compressor Stall is Dangerous
Compressor stall is dangerous for the following reason:
1. Affects Compressor’s Operation
Reduced flow can increase the compressor’s head and thereby help to bring about flow stability.
The flow can also be reduced even before the actual stability limit has been reached.
However, the practical operation of the compressor is impossible at flows that are lower than the flow at stability limit.
The compressor may also be unable to have the same head as at the stability limit.
When this happens, the compressor will not be able to overcome the pressure differential between the suction and discharge side.
2. Affects Pressure
When the gas volumes at the compressor discharge has attained a higher pressure compared to what the compressor can achieve, the gas will tend to flow from higher to lower pressure.
Also, the flow through the compressor will be reversed thereby causing the system pressure at the discharge side to be reduced over time.
At some point, the compressor will be able to overcome the pressure on the discharge side again. However, the inability to take corrective action could see that the compressor operates to the left of the stability limit.
The compressor surge causes strong oscillations of pressure and flows in the compression system. This surge can be tied to the interaction between the piping system and the compressor.
Conclusion
Compressor surge can affect the operation of this machine. Tackling it begins with having a good understanding of the term or contacting a manufacturer to learn more.
It focuses on gas flow reversal in a compressor from the discharge nozzle to the suction nozzle.
The signs of this surge can be detected through vibrations, which give the operator an idea that there could be instability in the system.
