Preventing Choke and Surge in a Compressor

Turbo Compressors are used to increase the pressure of a gas, which are required in propulsion systems like a gas turbine, as well as many production processes in the energy sectors, and various other important industries such as the oil and gas, chemical industries, and many more.

Such compressors are highly specific to the working fluid used (gas) and the specific operating conditions of the processes for which they are designed. This makes them very expensive. Thus, such turbo compressors should be designed and operate with high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.

Axial Compressor and Centrifugal Compressor in AxSTREAM
Figure 1 (A) Axial Compressor (B) Centrifugal Compressor in AxSTREAM®

Turbo Compressor Characteristic Curves

The characteristic curves of any turbo compressor define the operating zone for the compressor at different speed lines and is limited by the two phenomenon called choke and surge. These two opposing constraints can be seen in Figure 2.

Choke conditions occurs when a compressor operates at the maximum mass flow rate. Maximum flow happens as the Mach number reaches to unity at some part of the compressor, i.e. as it reaches sonic velocity, the flow is said to be choked. In other words, the maximum volume flow rate in compressor passage is limited by limited size of the throat region.  Generally, this calculation is important for applications where high molecular weight fluids are involved in the compression process.

Surge is the characteristic behavior of a turbo compressor at low flow rate conditions where a complete breakdown of steady flow occurs. Due to a surge, the outlet pressure of the compressor is reduced drastically, and results in flow reversal from discharge to suction. It is an undesirable phenomenon that can create high vibrations, damage the rotor bearings, rotor seals, compressor driver and affect the entire cycle operation.

Compressor Performance Curve
Figure 2 Compressor Performance Curve

Preventing Choke and Surge Conditions

Both choke conditions and surge conditions are undesirable for optimal operation of a turbo compressor.  Each condition must be considered during design to ensure these conditions are prevented.

Choke Prevention

To prevent the compressor from operating in choke region, one can maintain the minimum flow resistance to the fluid flow by providing Anti-choke valves at discharge which closes to restrict the flow and hence prevents choking.

Choke mass flow can also be increased while designing the compressor wheel by adapting different methodologies like using an impeller with splitters, or by modifying geometrical dimensions etc. Figure 3 shows the performance characteristics curves for an impeller with and without splitters which have the same effective number of blades. As shown, the impeller with splitters has a much higher choke mass flow compared to the impeller without splitters.

To calculate the effective number of impeller blades use:

Effective number of blades = Zm + Splitter length ratio * Zsp Where: Zm = number of main blades; Zsp = number of splitter blades; Splitter length ratio = splitter length/main blade length.

Figure 3 Impeller performances map
Figure 3 Impeller performances maps: 1 – with splitter; 2 – full blades without splitter

Surge Prevention

Surge can be prevented by providing an anti-surge valve, which allows more flow to be recycled back to the suction side and enables moving the compressor operating point away from the surge line. In axial compressors, designers also use casing treatment to recirculate the flow from discharge to suction, shown in figure 4 (C) and (D). In turbocharger compressors, generally a ported shroud mechanism (figure 4B) is used to recirculate the flow in order to enhance the surge margin and enables the compressor to handle significantly low mass flow rate.

Figure 4 (A) Anti-surge Control System, (B) Ported Shroud, (C) Bleed Slot, (D) Casing Treatment


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