The vertical surge tank is a vessel, which functions of secondary separator, and storage vessel for the produced liquids and gases coming from the test separator.

The units contain the required piping and manifolds for inlet/bypass, control devices, and safety relief systems. Surge tank can be classified as single-compartment and twin compartment based on the requirements of well testing operator. Surge vessel usually consists of tank body, sight glasses (level indicators), pressure and temperature indicators, relief valves, level alarm system, and skid for holding whole unit.

Advantage of the surge tank is that the design allows the operator to keep backpressure on transfer pumps for production disposal.