FLOW CONTROL VALVE
A Flow Control Valve (FCV) is an instrument that is installed in the
hydraulic network to regulate the flow of water. They are typically
used to limit flow, and hence water velocity which could otherwise
cause water quality issues due to turbulence stirring up sediments.
An FCV can be used as an altitude valve on a tank, allowing it to fill
or drain at a controlled rate while shutting off at high or low level.
While the valve does not need an external power source, water pressure is often enough to provide the control actions required, it is more typical to have some form of instrumentation and electrical actuation on an FCV as it usually works in series with a flow-meter and a PID loop controller device. A valve with a good part of its operating range that has a linear relationship between flow and valve position is preferred.
FCV's can be used to control flow while maintaining various protections including:
Simple Flow Control
The valve maintains flow to a setpoint value, opening and closing over a wide operating range as it does so. Proper sizing of the valve is essential to ensure that the valve operates over a linear part of its position/flow characteristic curve for typical flows and pipeline pressures. Damping of the operation is useful to prevent quick valve movements that could lead to pressure shocks in the pipelines.
Flow Modulating with Secondary Pressure Control Valves
This control method involves a more complex controller (the flow modulated controller) which provides greater flexibility and control than that offered by the simpler time modulated controller. Flow modulating pressure is the best type of control for areas with changing conditions, variable head loss, and high fire flow requirements. Flow modulating pressure reduction provides advanced control of outlet pressure related to demand. The valve controller reads flow from a meter and uses this input to control the position of the valve, the greater the flow, the more the valve opens and thus the higher the pressure. The controller is normally supplied with a local data logger and optional remote communications. Water pressure can be controlled with a preset profile related to the changing relationship of demand and head loss in a zone.
Improved flow control is very useful in operations optimised around mass-balance, such as energy management. By controlling the flows through critical network points, and setting different flow rates for different times of the day, more efficient control of both production and distribution can be achieved. An FCV can also be used as a flow meter, where the manufacturer uses valve position, the valve characteristic curve and upstream and downstream pressure instruments to calculate flow through the valve and report this back to SCADA as if it were a flow meter.
While less commonly implemented than PRVs the installation requirements are similar. Sizing and site selection are important to make sure that the valve will operate effectively and will not materially affect the performance of instrumentation in its proximity, especially flow meters which prefer undisturbed flow. A valve bypass line is good practise allowing both maintenance of the FCV and the ability to have much higher flows for pipeline flushing and hydrant testing.