A surge tank (also known as a stand pipe) typically has a relatively small volume and is located such that its normal water level is typically equal to the hydraulic grade line at steady state. When low transient pressures occur, the tank feeds water into the system by gravity to avoid subatmospheric pressure at the tank connection and vicinity.

There are two different surge tank types, as defined in the attribute called Surge Tank Type.

Simple Surge Tanks

This node can operate in three distinct modes during a transient analysis: normal (level between the top and the connecting pipe(s) at the bottom); weir overflow (level at the top) with the cumulative volume being tracked and printed in the output log; and drainage (level at the elevation of the connecting branch(es)).

If equipped with an optional check valve, it becomes a one-way surge tank which supplies the pipeline with liquid whenever the adjacent head is sufficiently low (the refilling operation is a slow process which is not represented in HAMMER). During normal operation, the continuity equation applied to this node is dHT / dt = Q / A, where HT is the tank level, A is the tank's cross-sectional area and Q =

Qi is the net inflow to the tank. At the mouth of the tank, there is a differential orifice with head loss

, where the subscripts T and or refer to the tank and orifice, respectively, b is the head loss coefficient and d = di for inflow (Q > 0) and -1 for outflow (Q < 0). By definition, d (known as the Ratio of Losses in HAMMER) asserts that head losses are di times greater for inflow than for outflow. A typical value of di is 2.5.

A user can optionally choose a Section type for the Simple Surge Tank. The choices are: a). Circular - so a tank diameter is required; b). non-circular - so an equivalent cross-sectional area is required; or c). variable area - where the cross-sectional area is provided in a table as a function of elevation. Note that for variable area tanks there is no facility for a check valve to preclude inflow to the tank.

Differential Surge Tanks

There are numerous modes of operation for differential surge tanks ranging from drainage, with the entry of air into the pipeline, to overflow from the tank. Other modes are distinguished by the riser level relative to the orifice elevation and the tank level versus the top of the riser. For "normal" operation, the tank level is between the orifice and the top of the riser. During a powerful upsurge, the upper riser will overflow into the tank to complement the orifice flow.