| A typical low-pressure-style PRV. |
Low Pressure PRVs
If very low pressures are encountered during peak demand periods, standard PRVs can not supply the required flow. If inlet pressures drop below 10 psi (.6 bar), the differential pressure across the valve is not enough to maintain the valve in the open position. The valve capacity is reduced and downstream customers may not have enough water. This issue can be overcome by using two pilots. The first pilot is a standard PRV pilot that is used to control or reduce pressure at non-peak usage periods. The second pilot is a modified altitude pilot that allows the main valve bonnet to vent to atmosphere at a predetermined low pressure (usually just below the downstream set-point). When the inlet pressure drops below the low pressure setpoint, the main valve opens fully, minimizing pressure loss through the fully open valve at peak demand periods. The valve can open fully even with pressures as low as 2 psi (.13 bar).
A standard PRV typically has the pilot set for the lowest pressure required at peak demand to make sure that all users have enough pressure to satisfy their needs. Remember that the outlet pressure setting remains constant, regardless of variances in inlet pressure or flow rate. At non-peak usage periods (night), there is less demand on the system so pressure loss though the distribution and service mains is much less, resulting in higher downstream pressures especially in areas remote from the PRV. At non-peak times, the increased pressure has two negative outcomes:
1) Leakage rates increase
2) Pipe bursts tend to correspond to higher pressure periods
The above scenario can be overcome in part by using two standard PRV pilots on the main valve. One PRV pilot is set for the high demand pressure (day time) while the other pilot is set for low demand pressure (night time). A solenoid valve is supplied and a basic timing controller decides which of the two pilots is used based on the time. Depending on availability of power, a standard solenoid with a locally sourced timing device may be used or, alternately, if no power is available, a battery operated timer (submersible) combined with a latching solenoid valve using minimal power may be used. The above options can be very economical with excellent results. One consideration is the fire department minimum pressure requirements.
Pressure Reducing Valves – Self Adjusting Based on Flow (Pressure/Flow Control)
Combination pressure flow control valves can be a very interesting option. This style of valve senses flow through pressure differential across either a correctly sized orifice plate or partially open gate valve down stream of the PRV. The standard PRV pilot has a large secondary diaphragm operator that adjusts the pilot setpoint based on flow. The PRV pilot is set for minimum (night) pressure. The secondary operator then increases the setpoint as the flow increases. Minimum setpoint and pressure increase are determined to assure adequate pressure in all parts of the DMA at peak demand. Limitation of this style of valve is that it must be a single source to a DMA. Multiple valves feeding a DMA will not function correctly.
