A surge tank increases the storage capacity of boiler feed system. It performs similar function to a boiler feed unit except it feeds a deaerator instead of a boiler. A surge tank is typically used on systems having more than 20% hot condensate returns. Surge tanks are used to measure liquid flow rate and the combined shrinkage and meter factor. They are also used as a second stage separator, and hold a constant back-pressure by using an automatic pressure control valve on the gas outlet. Surge tanks are also used for storage.