PRESSURE REDUCING VALVE
A Pressure Reducing Valve (PRV) is an instrument installed at
strategic points in the network to reduce or maintain network
pressure at a set level. The valve maintains the pre-set
downstream pressure regardless of the upstream pressure or
flow-rate fluctuations. PRVs can further ensure the minimum
required pressure at each pressure zone’s critical point, enabling
continuous readjustment of delivery pressure. This allows the
system to work at a lower average pressure.
PRV's can reduce pressure in the water network through the following methods:
Pressure Reducing Valve (Fixed Outlet)
The traditional method of control is via a hydraulically operated control valve. This is effective for areas with low pressure head losses, demand that does not greatly vary due to seasons, and areas that have uniform supply characteristics.. The valve reduces the pressure of the incoming water to a predetermined outlet value. Therefore, as the upstream pressure on the valve fluctuates due to different needs (e.g. varying levels within reservoirs and pump switching etc.), the downstream pressure remains constant.
Modulated Pressure Reduction Valve (Multi-Point Control)
Time-based modulation is the simplest form of Advanced Pressure Control and uses a controller with an internal timer. Pressure is controlled in time bands according to demand profiles. This method is suitable for areas with stable demand profiles and head losses and is usually used where pressure management is needed but cost is a significant consideration.
Flow Modulating Pressure Reduction Valves (Flow-Based Dynamic Modulation)
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 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 pre-set profile related to the changing relationship of demand and head loss in a zone.
Flow Compensated Pressure Reducing Valve (Flow-Related Fully Dynamic Hydraulic Regulation)
This is a pressure management solution that will reduce the downstream pressure dynamically within a pre-set range in relation to the flow rate passing through the valve. This fully self operated hydro-mechanical method automatically and continuously optimises downstream pressure, correlating pilot setting with demand. This method requires no electric power or control and no additional pipe line accessories, making for simple installation, commissioning and maintenance.
Improved pressure control presents the dual benefit of reducing leakage and stabilising system pressures, which increase asset life. Most pipe bursts occur not because of high pressure but due to on-going pressure fluctuations that force the pipe to continually expand and contract, resulting in stress fractures. Installing a pressure control device, such as a PRV, helps reduce pressure throughout the day, stabilise fluctuations, and reduce stress on pipes.
Establishing various Pressure Metered Areas (PMAs) or zones is one of the most common ways to achieve balance in water transmission and distribution networks. PRVs are usually sited within a PMA and should be downstream of the meter so that turbulence from the valve does not affect the meter’s accuracy. It is good practice to install the PRV on a bypass pipe to enable future major maintenance works. To activate a lower pressure at night and during periods of low demand and to further reduce leakage levels, water utilities should install a timer device with two set levels—one for daytime when customers need water and the second for night-time when demand is low.