Richard Bateman, Product Marketing Manager at RWC, explains how pressure reducing valves (PRVs) make homes safer, and what to consider when specifying them.
With working from home the new normal, and more and more of us are confining ourselves to our properties 24/7, water systems have been in high demand. This has lead to more fluctuations in water pressures, creating more stress on pipes, valves and fittings.
These significant changes in the water pressure can lead to pipe bursts, flooded homes, and people being left without water. This is why ensuring a PRV is installed at the mains outlet of a property is vital.
How do pressure reducing valves work?
A pressure reducing valve takes a high inlet pressure and reduces it to a lower outlet pressure. When it does this under both flow and no-flow conditions, the type of control is known as ‘drop tight’.
Under no flow conditions, the downstream pressure puts force on the seat and diaphragm of the valve, which in turn overcomes the spring pressure. This means the seat moves up, forcing it to seal against the diaphragm, therefore preventing the downstream pressure from increasing. Under flow conditions the back pressure against the seat is reduced, thus allowing the seat to open and water to flow through the valve.
Reliance Valves’ PRVs use a balanced spring and diaphragm to control the downstream pressure. This ‘drop tight’ feature is one of the most important features for any PRV, as this stops the pressure from ‘creeping’ – a term which is used when an increase in the downstream pressure occurs under no flow conditions.
A valve that allows this ‘creep’ cannot be classified as ‘drop tight’, or in fact a true PRV, as it will eventually allow the downstream pressure to creep up to equal the upstream pressure. This can cause significant problems and essentially negate the point of using a PRV in the first place.
An additional safeguard
For domestic properties, typically, water authorities will install their own PRVs in the road to regulate the pressure for each home. This is because the water supply pressure in the UK can vary from 1bar to 20bar (or even higher in some low usage areas), so it needs to be lowered for safe usage. And usually, the mains pressure coming into the home will be between 1bar and 3bar.
The water supply pressure will also tend to vary through the day. For instance, at high usage times (typically mornings and late afternoons) the pressure may drop by comparison to low water usage times (throughout the night), when it may increase dramatically.
All of this could lead to significant and sudden changes in the water pressure that can blow fittings apart and cause pipe bursts, especially if the PRVs in the road fails for any reason.
To prevent these circumstances, it is highly recommended that an additional PRV is installed to protect homes from any failures in the water authority’s pipework. These valves should be fitted at the point where the mains pipework first enters the property, and usually, these lead into the kitchen.
Choosing the right PRV
PRVs are sized by DN (Diameter Nominal). To select the right sized PRV, it is always important to consider four key factors:
- The type of building (domestic, commercial or industrial)
- The required flow rate (this is the volume of water needed to flow through the outlet to operate effectively and is measured in litres per minute (lpm)
- The required outlet pressure range
- The flow velocity
Flow rate is the most important factor for sizing a pressure reducing valve. To work out the flow rate you must work out how many outlets are required and what the combined maximum flow rate for these will be.A hypothetical example is a property containing just a single basin (4lpm), a toilet (6lpm), a bath (12lpm) and a kitchen tap (10lpm). This gives a total flow rate required of 32lpm, if all outlets were opened at once. Therefore, the PRV should be sized at 20DN.
For domestic properties, a general rule of thumb is to size the valve to fit the mains water pipe entering the property. This should provide enough flow for all normal activity in a dwelling.
It is crucial for installers to ensure that PRVs are sized correctly, as otherwise they can cause several problems. If oversized and the flow rate is constantly low, the valve seat can erode over time. This means the PRV will not be ‘drop tight’ and would allow the downstream pressure to ‘creep’.
On the flip side, if undersized, the valve can become very noisy every time outlets are opened and shut. This is due to the large volume of water squeezing through the valve, causing the internal spring to vibrate under strain, and also creating a ‘water hammer’ effect in the pipework.
A final point to consider when choosing PRVs is how easy they are to install, commission and service. For domestic-use, Reliance Valves’ 312 Compact Series PRV is most suitable. Adjustable to pressures between 1.5bar – 6.0bar, the valve’s compact design makes it quick and easy to install and test. WRAS approved, the valve features a single cartridge design for efficient servicing, and two test points either side to test the downstream pressure. It also features an integral strainer that protects the seat from corrosion and can be serviced to ensure optimum performance.
This valve offers great installation flexibility too. It can be fitted in any orientation to suit the space requirements, and is also available with JG Speedfit push-fit ends to enable fast, simple and tool-free connections in tight spaces.
It needs no saying that we are all heavily reliant on our water systems. Keeping them safe and stable is paramount for protecting homeowners from costly remedial works such as pipe bursts and flooding.
For more about RWC and pressure reducing valves (PRVs) visit https://www.rwc.com/