A two-way surge tank controls transients by converting stored potential energy in the elevated water body inside the tank into kinetic energy, which supplements flow in the piping system at critical times (or vise versa, for pipe flow into the tank) during periods of rapid flow variation. The tank is normally located at the pumping station or at a high point in the system.

A differential orifice may be installed at the riser of the tank to throttle reverse flow from the system to the tank, but create very little loss for flow leaving the tank. If an overflow and drain is provided, the tank can also act as a foolproof overpressure device that can overflow in a controlled manner.

One of the main concerns is the stability problem inside the tank. A rapid rise or drop in water level in the tank should be avoided. Usually, the surface area of the tank should be significantly larger than that of the pipeline. In a high-head water system or a sanitary forcemain, a two-way surge tank may not be economically feasible because of height or odor problems. A sample Bentley HAMMER CONNECT run extracted from a case study is shown in the following figure.

Figure 14-16: Output of Bentley HAMMER CONNECT Run for a Two-Way Surge Tank