Control Valve Waterhammer Theory

We can use the methods for known flow or pressure to calculate the conditions at a control valve. AFT Impulse flow and pressure control valves allow the control setting to be changed with time. Whether constant or changing, the flow or pressure is known and can be used to solve the equations.

Flow Control Valves

With the known flowrate, one can substitute into Equation 1 and Equation 2 and solve for the upstream and downstream pressures, respectively, at the valve. These can then be compared to the Action if Setpoint not Achievable settings to see if loss of control occurs. For instance, if the upstream pressure was less than the downstream, that would indicate loss of control.

Once the valve loses control, it acts like a regular valve. However, it is possible for the control valve to later regain control if the upstream pressure raises sufficiently.

Pressure Control Valves

Pressure control can be specified on the upstream or downstream side of the valve. A discussion of one these control locations will apply to the other. Let's consider the downstream pressure control case – a Pressure Reducing Valve (PRV).

With the known pressure downstream, one can substitute the known pressure into Equation 2 and solve for the flowrate through the valve. This flowrate can then be substituted into Equation 1 to solve for the upstream pressure. The upstream pressure can then be compared to the control pressure and Action if Setpoint Not Achievable settings to see if loss of control occurs. For instance, if the upstream pressure was less than the control pressure, that would indicate loss of control.

Once the valve loses control, it acts like a regular valve. However, it is possible for the control valve to later regain control if the upstream pressure raises sufficiently.

Pressure Drop Control Valves

The flow through a constant pressure drop is obtained by subtracting Equation 1 and Equation 2 and solving for flowrate.