How to Select and Size a Pressure Reducing Valve

A pressure reducing valve holds a set downstream pressure regardless of how the upstream main fluctuates. In water distribution, this typically means maintaining a district meter area inlet at minimum acceptable service pressure while pumping cycles and demand variation drive the trunk main pressure up and down. In treatment works and building services, PRVs protect sensitive instrumentation and downstream equipment from pressure spikes that can damage seals, burst fittings and trigger false alarms on pressure-monitored systems.

The operating principle is hydraulic. A spring-loaded pilot senses downstream pressure through a small-bore sensing line. When downstream pressure falls below the set point, the pilot allows water to drain from the valve cover chamber, reducing the force holding the disc closed and allowing flow to increase. When downstream pressure rises above the set point, the pilot closes, cover pressure rises and the disc moves toward closed. The disc never reaches full open or full close under normal conditions: it modulates continuously around the set point. This is normal behaviour and does not indicate a fault.

Sizing for flow

The standard sizing parameter for control valves is the flow coefficient: Kv in European practice, Cv in US-derived specifications. Both express the flow that will pass through the fully open valve at a differential pressure of 1 bar (Kv) or 1 psi (Cv). The relationship is Q = Kv × √dP, where Q is in m³/h and dP is in bar.

For water distribution PRVs, the practical internal velocity limit is 2 to 6 m/s under normal operating flow. The lower end is appropriate for continuous duty; short peak flows up to 20% above this are generally acceptable. A valve selected to pass maximum flow at or below 6 m/s will typically operate between 40% and 80% open under normal demand, which is the correct range for accurate pressure control and long seat life.

Oversizing is the more common mistake. A DN200 body selected for convenience on a DN200 pipe may pass design flow at only 10 to 15% open. At near-closed positions, the disc creates a high-velocity jet that erodes the seat rapidly. Within months the valve will no longer hold set pressure at low flows, and the only remedy is replacement. A correctly sized DN150 or even DN100 valve in a reducer spool will outlast it.

The cavitation limit

Cavitation occurs when local pressure within the throttling zone of the valve drops below the vapour pressure of water. Vapour bubbles form and then collapse violently as pressure recovers downstream, eroding metal surfaces. In severe cases, cavitation can perforate a valve body within weeks of commissioning.

For diaphragm-actuated globe control valves, the practical pressure ratio limit is 3:1. With 12 bar upstream, the valve must discharge to at least 4 bar back pressure. Some modified flow-path designs achieve 4:1, but 3:1 should be assumed unless the manufacturer can provide test data for the specific model.

Where the available differential exceeds the safe ratio, two options are available. The first is to install a pressure-reducing stage upstream: a second PRV or a fixed orifice plate to bring the differential across the main control valve within the safe ratio. The second is to use a valve specifically designed for high-differential applications with a modified internal flow path. Orifice plates are a lower-cost option, but they produce a fixed pressure drop at one specific flow rate. Away from that design point the differential across the downstream valve changes, and cavitation protection may be lost.

Fixed outlet, time-modulated and flow-modulated control

The simplest PRV pilot holds a fixed outlet pressure set by a spring adjustment. This is reliable and requires no external power, but it sets pressure for the worst case: high demand at the critical point of the zone. During off-peak periods, typically from midnight to 5 am, the full zone pressure is maintained even though demand may be a fraction of daytime flow. On networks where background leakage is a concern, this represents a significant volume of unnecessary loss.

Time-modulated pilots reduce the outlet pressure set point during programmed low-demand periods, typically by 0.5 to 1.5 bar. The schedule is programmed into a small timer unit in the pilot circuit. The reduction during the night period, applied across all service connections in the zone, reduces background leakage significantly without affecting daytime service pressure.

Flow-modulated pilots adjust outlet pressure in real time based on a signal from a flow meter at the DMA inlet. As demand rises, the pilot automatically increases outlet pressure to maintain adequate service pressure at the critical point; as demand falls, outlet pressure reduces. This approach requires a flow signal input but adapts automatically to seasonal and diurnal demand patterns. On high-leakage networks, it typically delivers a further 5 to 10% leakage reduction over time-modulated control.

Installation requirements

A strainer upstream of the PRV is essential, not optional. Debris that enters the pilot orifices will cause the valve to open fully or fail to open at all. A Y-strainer of appropriate mesh size should be installed one to two pipe diameters upstream, with isolation valves on both sides for cleaning without system shutdown.

Install isolation valves upstream and downstream of the PRV to allow inspection and maintenance without draining the system. A bypass assembly, consisting of a parallel pipe with a manual globe or gate valve, allows the zone to remain pressurised during maintenance. Size the bypass valve at least one size smaller than the main PRV to prevent inadvertent overpressure if it is opened fully.

Pressure gauges upstream and downstream of the valve, with isolating cocks, allow operators to verify set pressure without calibrated instruments. Include a commissioning needle valve downstream of the pilot sensing connection to allow set-pressure adjustment under live flow conditions.

Worked example

A DMA inlet serves 850 service connections with a peak demand of 28 l/s (100.8 m³/h) and an average night-line demand of 3.2 l/s. Upstream pressure varies between 6.5 and 9.5 bar depending on the pumping cycle. The required outlet pressure is 2.8 bar, established by the critical point elevation within the zone.

The maximum pressure ratio is 9.5 bar inlet to 2.8 bar outlet: a ratio of 3.4:1. This marginally exceeds the 3:1 cavitation limit, so either a modified flow-path valve should be specified, or a fixed orifice is installed upstream to reduce inlet pressure to 8.4 bar under worst-case conditions.

For sizing at peak flow with a differential of 6.7 bar (9.5 minus 2.8), the required Kv is 100.8 / √6.7 = 38.9. A DN100 valve with a fully open Kv of 65 will pass this flow at approximately 60% open — within the correct operating range. A DN150 valve with Kv 150 would pass the same flow at around 25% open, operating in the erosion-prone near-closed range during average demand conditions.