Oil surge tank is an H2S service vessel designed to store liquid hydrocarbons after separation. The surge tank is used to measure liquid flow rates, as well as the combined shrinkage and meter factor. It can also be used as a second stage separator and hold a constant backpressure by using its automatic pressure control valve on the gas outlet.

The surge tank usually consists of a single or double compartment vessel and a level measuring system with sight glasses or magnetic levels. To prevent overpressure and overfilling, the surge tank is fitted with a pressure-relief valve and a high- and low level alarm system.

Surge tanks are designed with a diverter, a vortex breaker, and stiffening rings capable of withstanding a vacuum in the vessel. They are fitted with sampling, pressure, and temperature ports. A bypass manifold is also included.

All surge tanks are shock-protected by a frame, and operate in the vertical position, even they are transported in a horizontal position.