Surge Valve elements represent a surge-anticipator valve (SAV), a surge relief valve (SRV), or both of them combined. A SAV opens on low pressure in anticipation of a subsequent high pressure. A SRV opens when pressure exceeds a threshold value.

The following attributes describe the surge-anticipator valve behavior:

• Threshold Pressure (SAV): Pressure below which the SAV opens.
• SAV Closure Trigger: The closure of an open/opening SAV is initiated either by time (Time SAV Stays Fully Open attribute) or the threshold pressure (Threshold Pressure attribute), but not both. When based on pressure, the SAV will begin to close when the pressure rises back above the specified Threshold Pressure (SAV) value, which may occur before the SAV has fully opened.
• Time for SAV to Open: Amount of time that the SAV takes to fully open after being triggered.
• Time SAV Stays Fully Open: Amount of time that the SAV remains fully open (i.e., the time between the end of opening phase and the start of the closing phase).
• Time for SAV to Close: Amount of time for the SAV to close fully, measured from the time that it was completely open.

There are three optional valve configurations as defined by the attribute SAV/SRV type: (1) Surge Anticipator Valve, (2) Surge Relief Valve, and (3) Surge Anticipator & Relief Valve.

For the SAV, at full opening it's capacity is represented by the discharge coefficient Cv, while the valve characteristics at partial openings are provided by the valve curves discussed in Closing Characteristics of Valves (note that there is no user-specified valve currently provided for the SAV).

The SRV is modelled as being comprised of a vertical-lift plate which is resisted by a compressed spring. At the threshold pressure, there is an equilibrium between the compressive force exerted by the valve's spring on the movable plate and the counter force applied by the pressure of the liquid. For a linear spring, the lift x is given by the equation: A (P - P0) = k x, where A is the pipe area, P is the instantaneous pressure, P0 is the threshold pressure, and k is the spring constant. In this formulation, the acceleration of the spring and plate system is ignored. As the plate lifts away from the pipe due to the excess pressure, more flow can be vented to atmosphere to a maximum value at 0.937 times the pipe diameter.