In this article, we will see the centrifugal compressor surge-Basics, mechanism and also we will see how the surge takes place in centrifugal compressor with an example.
Surge is a characteristic behaviour of a centrifugal compressor that can occur when inlet flow is reduced such that the head developed by the compressor is insufficient to overcome the pressure at the discharge of the compressor. Once surge occurs, the output pressure of the compressor is drastically reduced, resulting in flow reversal within the compressor.
When a centrifugal compressor surges, there is an actual reversal of gas flow through the compressor impeller. The surge usually starts in one stage of a multistage compressor and can occur very rapidly.
Now we will see how the surge starts in a compressor. The surge is starting with the instant flow reverses, shown in the above figure. Consider the Point “A” is the actual flow developed by the compressor during normal operation. Due to the decrease in flow the operating point shift from “A” to “B”. The compressed gas actually rushes backwards through the impeller from the discharge to the inlet. The release of compressed gas from the discharge side results in the pressure drop from “B” to “C”. The reduction in pressure allows the flow to be re-established in the positive direction “C” to “D” and increase the discharge pressure from “D” to “A”. If nothing in the system change, then the surge cycle will continuous. The increase in duration of the surge cycle results in damage to the compressor.
How Surge takes place in Centrifugal Compressor
Now we will see the surge phenomena in centrifugal compressor with the example compressor performance curve
Now consider the inlet flow decreases due to the suction valve throttling (they are many reasons will cause the compressor inlet flow rate decreases). Consider the compressor operating at the following conditions
Flow rate = 5500 kg/hr
Discharge pressure = 20 bar.
Speed = 6000 rpm
Due to suction valve throttling the inlet flow rate of the compressor is decreased from operating point (A) to the new operating point (B) (Refer above figure). At the new operating point (B) the compressor flow is reduced on the other hand the discharge pressure will rise further. Due to the rise of pressure and decrease flow will cause the Surge cycle.
Discharge valve throttling
Similarly, if the discharge side system resistance will increase due to discharge valve throttling. The pressure developed by the compressor will increase and flow rate will start decreases. The same phenomena will happen, that is the operating point “A” shifted to the new operating point of “B”. This will cause the surge in the centrifugal compressor.
Change in Speed
An increase in operating speed of the compressor also causes compressor surge. Now consider the operating speed of the compressor is 6000 rpm. If the speed increase to 7500 rpm. The Operating point of the compressor will shift from “A” to “B”. (Refer below figure)
At new operating point “B”, the discharge pressure of the compressor will increase. Due to the increase in pressure. The point “B” fall in the surge line. Due to this surge phenomena will occur in the compressor.
Other reasons will cause Surge in Centrifugal Compressor
Inlet Filter Chocking
Due to dirty particles present in inlet filter will decrease the flow rate and reduce the suction pressure. Due to the reduce flow rate, the operating point will move toward the surge line. Once the operating point touches the surge line then Surge occur in the compressor.
Driver Input Speed
In the case of the compressor is driven by Turbine or Variable speed drives. Sometimes the increase in speed may cause operating will shift to surge limit line and surge will occur.
Change in Compressed gas Property
The change in operating gas can cause compressor surge.
Surge result in centrifugal compressor
As we seen the surge is due to the flow reversal in the compressor. As a result of the surge in the compressor, it may lead to damage of compressor or compressor system. The following are some of the resulting due to surging.
- During the surge, a significant mass gas will flow in the reverse direction. As a result of a large dynamic force act on the impeller or blading within the compressor. Due to this the components of the compressor (such as thrust bearings, bearing, casing) exposed to large changes in axial force on the rotor. If the surge is not controlled it may result in fatigue damage to compressor or piping components.
- During the surge, the reversal of flow within the compressor results in hot compressed gas returning to the compressor inlet. If the surge is not controlled, as a result the temperature at compressor inlet will increase and leads to a potential rubbing of close clearance components. Due to the differential thermal expansion of components within the compressor.
- The vibration level of the compressor is very high during surging.
- A reversal of flow may lead to process-related problems that could shut down a plant.