Protective Equipment Reference

Combination Air Valve (CAV)—is installed at local high points to allow air to come into the system during periods when the head drops below the pipe elevation and expels air from the system when water columns begin to rejoin. The presence of air in the line limits subatmospheric pressures in the vicinity of the valve and for some distance to either side, as shown on HAMMER profile graphs. Air can also reduce high transient pressures if it is compressed enough to slow the water columns prior to impact. This valve requires the following parameters:
Initial Air Volume near the valve at the start of the simulation. The default value is zero. If there is an initial air volume, pressure at the valve must be equal to zero at the start of the simulation.
Small Outflow Diameter is the size of the opening that releases air from the system when the volume of air is less than the Transition Volume. This diameter is typically small enough to throttle air flow, compressing any air remaining in the system.
Transitional Volume is the threshold volume of air at which the outflow diameter changes between the smaller and bigger size. The default value of this parameter is zero.
Outflow Diameter is the size of the opening that releases air from the system when the volume of air is greater than, or equal to, the Transition Volume. This diameter is typically larger than the Small Outflow Diameter. Because it is rare for this to throttle, the default value of this diameter is considered to be infinite.
Inflow Diameter is the size of the opening that lets air enter the system. This diameter is typically large to allow the free entry of air without throttling. By default, this diameter is considered infinite in HAMMER.
Air Valve (Slow-Closing) between 2 Pipes—allows air into the system freely when the head drops to below the pipe elevation and releases air and/or fluid from the pipe when head increases again. Also known as a downsurge relief valve. Unlike a CAV, the large outlet closes over a preset time period. This valve requires the following parameters:
Time to close the valve. Valve starts to close only when air begins to exit the pipe. If air reenters, then the valve opens fully again.
Diameter is the size of the valve opening for inflow and outflow.
SAV/SRV at End of 1 Pipe—represents 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. These valves require the following parameters:
Type of Valve(s) provides three possible valve types: SAV, SRV, and SAV+SRV.
Diameter of Orifice/ Throat for the liquid discharged by the valve.
Parameters for SRV
Diameter is the opening available to release fluid from the system.
Threshold Pressure is the critical pressure at which the SRV opens. This may be controlled by a spring, piloting, or other mechanism.
Spring Constant represents the restoring force of the return spring per unit lift off the valve seat. A typical value of this constant is 150 lb/in (26.27 N/mm).
Parameters for SAV:
Diameter is not used by HAMMER but useful for display. Flow through the valve is determined based on the Cv at Full Opening and valve type. It is assumed that the percent of open-area curve for each valve type corresponds to its Cv curve.
Threshold Pressure is the critical pressure below which the SAV opens.
Type of SAV provides five options: Needle, Circular Gate, Globe, Ball, and Butterfly.
Time to Open is the time required to open the SAV fully upon activation.
Open Time is the time the SAV remains fully open (i.e., the time between the valve's opening and closing phases).
Time to Close is the time required to close the SAV fully. SAV must be closed as soon as pressures are relieved to avoid developing too high a return-flow velocity. SAV may not be able to close against extremely high reverse-flow velocities for certain pilot configurations.
CV at Full Opening refers to the valve coefficient, which is a function of flow through the valve and the corresponding pressure drop across it.
SAV/SRV between 2 Pipes—operates in the same way and requires the same parameters as the SAV/SRV at End of 1 Pipe hydraulic element described previously.

Note: In rare circumstances when the pressure is zero or negative at the SAV, in reality air would be sucked into the pipeline through the valve. However air inflow is not modeled by WaterCAD . Instead, this condition is modeled by not adding negative inflows, but retaining the negative flow that is predicted.