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All About Pressure Reducing Valves – Hidden Treasures

The water systems in domestic, commercial and industrial properties can present unique problems and challenges as plumbing fittings become ever more sophisticated.

The water supply pressure in the UK can vary from 1bar to 20bar (or even higher in some low usage areas). The water 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 with low water usage times (throughout the night), when the pressure may increase dramatically.

Such periods of high pressure can cause several problems: excessive noise from high flow velocities, water hammer from quick closing taps or solenoid valves, plus the risk of water wastage is particularly great because higher pressure means higher flow rates.

The water supply usually enters domestic dwellings beneath the kitchen sink so the first effect of high pressure is often experienced at that point; when the cold tap is turned on too quickly or too far, this creates a gush of water which hits the bottom of the sink and bounces back, soaking the user and creating a wet mess!

The best way to control high pressure is by installing a pressure reducing valve. These take a high pressure at the inlet, then the valve reduces it to a lower pressure at the outlet as desired, under both flow and no-flow conditions.

How do PRVs work?

A pressure reducing valve is a valve which 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’. Reliance’s pressure reducing valves use a balanced spring and diaphragm to control the downstream pressure. This ‘drop tight’ feature is one of the most important criteria for any pressure reducing valves, 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 which will allow this ‘creep’ cannot be known as ‘drop tight’ or in fact a true pressure reducing valve, as it will eventually allow the pressure to creep up to equal the upstream pressure, which can cause significant problems and essentially negates the point of using a pressure reducing valve in the first place.

The diaphragm effectively separates all of the water contact parts and the pressure from the water supply away from the control spring and associated mechanism. The body is then protected from debris by the use of a stainless-steel strainer.

Under no flow conditions the downstream pressure puts back-pressure 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 not allowing the downstream pressure to increase.

Under flow conditions the back pressure against the seat is reduced thus allowing the seat to open and water to flow through the valve.

How to size a PRV

This is predominantly based on two different criteria: application and flow rates. Application describes the type of property the valve is to be used in: whether it is commercial/industrial or a domestic installation.

Flow rate is the most important factor for sizing a pressure reducing valve. Sizing a valve incorrectly can cause several problems; if oversized the valve seat may open for a very small flow rate, which may occasionally be acceptable but over a long period of time can result in a wire drawing across the valve seat. A wire drawing occurs when the valve disc and seat position operate close to the shut-off point of the valve for extended periods of time. This then means the water flow scores a pathway in the seat material which remains when the valve closes tight to the shut-off position and allows a little flow and pressure to creep through the valve.

To calculate the flow rate you must work out how many outlets are required and what the combined maximum flow rate for these will be. You can then use the provided table to ascertain which size valve is required:

For larger commercial applications various sizing solutions can be used: for example, if the flow rate is lower at some times than at others then using several smaller size PRVs in parallel may be more practical, or use one smaller valve as a bypass thus allowing water to flow easily through the valve when the flow rate is lower than normal, without causing either wire drawings or creating noise across the valve, as previously mentioned.

Reliance UK PRVs

All Reliance UK pressure reducing valves are WRAS approved: this means that they have undergone independent third-party testing to ensure that they comply with the current UK water regulations for pressure reducing valves. It also ensures that all materials used within the make-up of the valves have been verified as safe for potable water systems.

You can find out more about our PRV range in the ‘Flow Control’ section of our website’s product portfolio, by contacting our team via the below form, or by speaking to your local sales representative.

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