What is the compressor surge?
Surge is a characteristic action that occur in the centrifugal compressor when the inlet flow is decreased so that the compressor’s head is insufficient to overcome the pressure at the compressor’s discharge. Once surge occurs, the output pressure of the compressor is drastically reduced, resulting in flow reversal within the compressor.
Surge occurs at the different head and flows combinations as specified by the performance curves of the compressor manufacturer. A surge can result in one or more of the following:
- Unstable operation
- Partial or total flow reversal through the compressor
- Disrupted process
- Mechanical damage to the compressor
- Increase in discharge temperature
- Reduction in discharge pressure
- Increase in vibration
- Sharp rise in inlet temperature dependent on the volume flow at the suction.
Surge mechanism:
There is a significant reversal of gas flow through the compressor impeller when a centrifugal compressor surges. In one stage of a multi-stage compressor, the surge usually begins and can happen very rapidly.
The surge begins with the reversal of the instant flood, shown in the figure above. Consider point “A” as the actual flow during normal operation developed by the compressor. The operating point change from “A” to “B” due to the decrease in flow In fact, the compressed gas rushes backward from the discharge to the inlet through the impeller. The release from the discharge side of compressed gas.
The stress reduction allows the flow to be restored in the positive direction “C” to “D” and the discharge force to be decreased from “D” to “A.” If nothing changes in the process, the cycle of the surge will continue. As the duration of the surge period increases, the compressor is impaired.
How to avoid surges?
A surge can be prevented in the gas compressor segment by recycling a regulated portion of the discharge flow back through a recycling valve to the suction. Recycling increases the pressure of suction and reduces the pressure of discharge, which increases the flow and eliminates the activity from the surge.
The speed increase also eliminates the compressor from the surge. This is a temporary solution as it also increases Pd and decreases Ps, which tends to push the machine back upward.
A blow off valve is used in the air compressor section to vent the discharge of the compressor into the atmosphere. This does not change the conditions of suction, but it decreases the pressure of discharge and raises the flow, thereby eliminating the operating point from the surge.
The ratio of compressor pressure increase to inlet flow rate is used by this method of surge control to set the flow in by-bass loop. The compressor starts to surge when the suction pressure drops and discharge shoots up.
How surge affects the compressor?
The ratio of compressor pressure increase to inlet flow rate is used by this method of surge control to set the flow in by-bass loop. The compressor starts to surge when the suction pressure drops and discharge shoots up.
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A large mass gas may flow in the opposite direction during the surge. Because of a strong dynamic force, the compressor works on the impeller or blading. Because of this, the compressor components (such as thrust bearings, bearings, casing) are exposed to major axial force changes on the rotor. If the surge is not managed, the compressor or piping com can suffer fatigue damage.
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The reversal of flow within the compressor during the surge results in the return of warm compressed gas to the inlet of the compressor. If the surge is not regulated, the temperature at the inlet of the compressor may increase, resulting in a potential rubbing of components with near clearance. Because of the compressor’s differential thermal expansion of materials.
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The compressor’s vibration rate is very high during the surge.
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A reversal of flow may lead to process-related problems that could shut down a plant.