An unstable operating condition when the flow through a compressor is decreased to the point that momentary flow reversals can occur. This can lead to major damage of the compressor. Surge control is therefore used to prevent this and uses a surge tank which is a storage vessel, drum, or reservoir used to absorb unexpected rises in flow or pressure. It can also be used to provide additional fluid in the event of a drop in pressure or flow. Surge tanks are used in hydroelectric power stations to provide additional capacity and used to mitigate pressure variations due to rapid changes in the velocity of water.