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A pressure reducing valve (PRV) is a valve that reduces a higher upstream pressure to a lower downstream pressure under flow and no-flow conditions.
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Mains water pressure throughout the UK varies from below 1 bar (insufficient for most dwellings) to above 10 bar. There are extreme areas where pressures of 18 bar have been supplied to domestic dwellings and pressures above 20 bar to commercial property (motorway services are a case in point).
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The average supply pressure is 3 bar during the day (it may be considerably higher at night) with approximately 20% of the country above this at some time. These areas would benefit from the use of pressure reducing valves. | |
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Under no flow conditions the down stream (outlet) pressure acts on the diaphragm and overcomes the spring pressure. The diaphragm moves up and the linkage that joins the diaphragm to the seat holds the seat closed so down stream pressure cannot increase. |
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Under flow conditions the downstream (outlet) pressure decreases until the spring can overcome the pressure.
The diaphragm moves the linkage down and so opens the seat and water flows through the valve. When the outlet is closed, pressure builds up until the spring pressure is overcome and the seat is closed again. |
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Balanced pressure valves operate in basically the same way except there is an additional "piston" of the same area as the main seat. This gives better control under low and high flow conditions. |
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As UK plumbing systems become more advanced and plumbing equipment becomes more sophisticated, mains pressure is being increasingly used in situations that used to be the domain of the low pressure gravity systems: ie unvented hot water heaters, continental style multi outlet showers, electronic controls. As the use of such systems and specialised fittings becomes more common, so too is the use of pressure reducing valves in order to control and regulate pressure and flow to the terminal fittings.
In the UK, water pressure from the mains can vary from between 1 bar to 20 bar, and in some places at low usage times the pressure can be even higher than that. The water pressure is also variable throughout the day, with pressures increasing at night and in the middle of the day but dropping at high usage times: early in the morning and late in the afternoon. Generally, for most domestic applications a pressure of around 3 bar is sufficient if the internal plumbing system is designed and installed correctly. Water pressures above 3 bar can start to cause problems such as excessive noise from the high flow velocity, water hammer from ceramic disc taps, solenoid valves on dishwashers and washing machines or other quick-closing valves; it is also wasteful as higher pressures mean higher flow rates and ultimately higher water bills. One simple way to prevent these difficulties and protect the plumbing system from over pressurisation is to install a pressure reducing valve at the entrance to the property which will limit and maintain the incoming mains pressure to the desired level.
Reliance pressure reducing valves are balanced spring and diaphragm type valves that will limit the downstream pressure to a pre-determined maximum under flow and no-flow conditions. A valve that works under both flow and no-flow conditions is described as ‘drop tight’. The most important criterion for a pressure reducing valve is that it must be 'drop tight', i.e. pressure does not increase downstream of the valve under no-flow conditions. It must be emphasised that 99% of pressure reducing valve applications require the pressure to be limited to a maximum predetermined level under flow and no-flow conditions. Another term that is used in relation to the operation of a pressure reducing valve is ‘creep’. This is a slow or small increase in downstream pressure under no-flow conditions. A valve that allows this can not be considered to be drop tight or in fact a true pressure reducing valve as what will happen is a slow build up in outlet pressure until it equals the inlet pressure.
There are many different applications for PRVs. Basically, they can be used just about wherever water pressure needs to be controlled. There are however certain types of PRV in the Reliance range which have been developed with particular applications in mind.
Commercial Sector
The 315 HC series PRV is very popular for the commercial market and is used extensively in areas such as high rise property developments, hotels, sports stadiums, schools, prisons, leisure centres etc. Several features make the 315 HC series attractive to the commercial market:
1. It is a dial-up type PRV. This means the valve has an indicator dial on the valve which will move as the adjustment knob is turned. This indicator will show the downstream pressure that the valve is set at, to within 0.2 bar. This feature can save a significant amount of time and cost when commissioning a large system.
2. The valve is full cartridge type construction which means only one spare part needs to be stocked to service the valve fully. It also saves significantly on downtime if repairs are ever required as a full cartridge change can be done in less than a minute.
3. The 315HC series is suitable for cold and hot water up to 90°C, meaning that only one valve needs to be specified and only one range of spares needs to be stocked.
4. As this valve is targeted specifically at the commercial sector, which can have significantly higher peak time demand for flow rates, the valve has been designed to give maximum flow with the absolute minimum pressure loss.
5. It complies with all aspects of EN1567 as certified by DVGW. This is significant as the acoustic requirements of the standard demand low noise levels: below 20Db. This is a particularly important feature for high rise flats and office blocks.
6. It is fully WRAS approved to the UK water regulations
Domestic Sector
Reliance’s new 320 Easiset PRV is the latest addition to the range of pressure reducing valves for the domestic market. Because issues such as servicing and maintenance are just as important in the domestic property as they are in a commercial one, the 320 Easiset range also incorporates our patented one piece cartridge type construction and the valve is available for temperatures up to 90°C. The biggest innovation though is the pull and twist adjustment mechanism which eliminates the need for cover caps and difficult-to-reach adjustment screws. When the cap is pulled up and turned, the pressure is adjusted. When finished, the cap is pushed back down and automatically locks in position. This new range is available in ½”, ¾”, 15mm and 22mm compression sizes and in hot or cold water versions. All versions comply with EN1567 and are WRAS approved to the UK water regulations.
Tenant Valve Assembly
The concept of the tenant valve assembly emerged because most multi-unit buildings, whether residential or commercial, require that an isolating valve and a double check valve be fitted at the entrance to the unit. And in a high proportion of cases it is common practice to install pressure reducing valves and secondary water meters at this point as well. The idea behind the development of the TVA was simple: if all these valves are being fitted anyway, why not incorporate them into one compact unit?
Although in most installations an isolating valve and a double check valve will be fitted and a very high proportion also have a pressure reducing valve, secondary water meters are not so widely needed in certain applications. Feedback from the market has shown that only around 30 % of the installations require a water meter fitted as standard although 100 % of customers like the idea of being able to fit one as an option. With this in mind, Reliance has designed the TVA with the option of fitting a water meter and capped off the meter connection point. Should a meter be required then the cap can be removed and an optional precalibrated water meter cartridge inserted. If a pulse output meter is preferred, then an optional clip-on pulse output cap can be retrofitted to the meter. It is now also possible to fit a class “D” primary charging meter supplied from the local water authority by using a class “D” connection kit. | 
