What is surge and how is it prevented?

• A. Surge occurs when discharge pressure is low and flow is high; it is prevented by spill back.
• B. Surge occurs when discharge pressure is high and flow is low, causing gas to flow backwards; it is prevented by spill back to avoid dead head.
• C. Surge occurs during startup and is prevented by warming the gas.
• D. Surge is a normal condition that requires no action.

Answer: (B)

Surge is an unstable operating condition of a compressor where the flow through the machine becomes very low for the pressure it’s trying to overcome, and the gas can even reverse in the discharge path. This happens when the system resistance is high and the discharge pressure is elevated, so the compressor’s head is too large for the available flow. In this region, small disturbances grow instead of dampening, causing pressure and flow to oscillate and sometimes reverse direction back toward the compressor.

Preventing surge relies on keeping the compressor operating on its stable part of the performance curve. A spill back path recirculates some of the discharge gas to the inlet, raising the flow through the compressor and reducing the effective head it must develop. This moved operating point away from the surge region and away from dead head, maintaining smooth, forward flow.