Surge occurs in a turbo compressor when discharge head cannot be sustained at the available suction flow.Surge occurs at specific combinations of head and flow, as defined by the compressor manufacturer’s performance curves. One or more of the following can result from surge:
- Unstable operation
- Partial or total flow reversal through the compressor
- Disrupted process
- Mechanical damage to the compressor
Surge is usually accompanied by the following:
- Increase in discharge temperature
- Reduction in discharge pressure
- Increase in vibration
- Sharp rise in inlet temperature dependent on the volume flow at the suction.
In the gas compressor section, surge can be avoided by recycling a controlled portion of the discharge flow back to the suction through a recycle valve. Recycling raises the suction pressure and lowers the discharge pressure, which increases flow and moves the operation away from surge. Raising speed also moves the compressor away from surge. This is a temporary solution because it also raises Pd and lowers Ps, which tends to drive the machine back towards surge. In the air compressor section (Figure), a blow off valve is used to vent the compressor discharge to atmosphere. This does not affect the suction conditions, but it reduces discharge pressure and increases flow, which moves the operating point away from surge.
This method of surge control uses the ratio of compressor pressure rise to inlet flow rate to set the flow in by-bass loop. When the suction pressure drops and discharge shoots up, the compressor starts surging.