While public safety issues such as fire fighting - with specific laws and design criteria, as well as customer service concerns are key components to determining the pressure/s of a distribution system, pressure management is also an effective way to control the amount of water lost in a system. This can be implemented without effecting service levels when activated during low demand periods such as late night and early morning. It can also reduce consumption in networks with no intermediate storage. A small reduction in pressure can mean a significant reduction in real losses through leaks. It is important though, to comprehensively evaluate an area and gain an understanding of its background losses, before or alongside introducing pressure control.   The effect of pressure control on real losses can then be quantified.

Managing system pressure

This can be controlled in two ways using PRV’s

• Use of Pressure Reducing Valves (PRVs) which regulate the pressure to a constant outlet pressure independent of the inlet pressure.

• Flow modulating PRVs, increasing pressure with increasing flow.

Pressure management is best carried out in small definable areas with only one in feed and it works extremely well when combined with a district metering area.

There are a variety of methods for controlling pressure. In its simplest form a Pressure Reducing Valve (PRV) is used. The valve reduces the pressure of the incoming water to a pre-determined outlet value.

Although the upstream pressure on the valve may fluctuate on the valve due to varying needs (varying levels within reservoirs and pump switching etc.) the downstream pressure remains constant.

A development of the standard PRV is the flow modulating PRV. This operates by increasing pressure when the flow is increased and decreasing pressure as the flow is reduced. This has the effect of only supplying pressure to the network when there is a demand on the system. When there is little or no demand eg. At late night/early morning, the pressure is reduced and thus the background leakage levels falls. Installing a flow modulating PRV is dependent on setting an Average Zone Pressure Point (AZPP) and monitoring it. It is also essential to know the location of the Critical Pressure Point of the system.

Pressure control can enable substantial savings of water in an area. Pressure reduction is best carried out in small District Metered Area, supplied by mains that can be reduced to accept a PRV not in excess of 150 mm in diameter. PRVs and flow modulating PRVs become disproportionately expensive above that size.