Electricity + Control July 2019

CONTROL VALVES + ACTUATORS

- Ratios up to 5:1 are available – cavitation is prevented - Instant reaction time to pressure/demand changes - They can be used in series without instability - They offer high rangeability and can handle very low flows. Ratio pressure reducing valves require little sizing input from engineers – a line-size valve can simply be installed where pressure needs to be reduced. RPRVs have been in use in Australia for many years, mainly in high-rise buildings. In South Africa, locally designed RPRVs are used successfully in the mining industry. Ultravalves has now introduced its own RPRV which is available in sizes up to 300 mm and to handle pressure of 100 bar. Ratio pressure reducing valves can be used in various ways to reduce water losses: - As a simple PRV without any potential instability issues - As an upstream PRV to a pilot operated PRV, to avoid cavitation without instability issues - Fitted with a solenoid valve the PRV can be used to achieve a lower setting of downstream pressure during off-peak times without straining the pilot operated PRV. Flow control at usage points A further means to reduce water losses is to install flow control valves before usage points. The Maric flow control valve is a simple device to control flow rate. It has the advantage of being tamperproof, self-cleaning and long-lasting, and has a simple operating mechanism. By controlling the flow rate to end-use points significant water savings can be achieved.

Most standard POPRVs can handle a pressure drop ratio of 3:1 – so if upstream pressure during the day is 12 bar, the valve can reduce pressure down to 4 bar without cavitation. If during the night upstream pressure increases to say, 15 bar (due to lower pipe friction losses), and downstream pressure is reduced to say, 2 bar by the smart POPRV, the reduction ratio is now 7.5 – which would lead to serious cavitation and much reduced valve life. This issue can be resolved by the installation of two POPRVs in series, but they are difficult to set up and tend to become unstable leading to faulty operation. 2) Low flow – POPRVs have a limit to the low flow they can handle and become very unstable when required to operate below this level, which often happens at night. This can be solved by adding a smaller bypass PRV, but this adds to the complexity of the system and often leads to faulty operation and instability. Although POPRV manufacturers have developed ‘trims’ to address both the above issues, they do not solve the problems completely and they increase the cost and complexity of the valves. None of these factors support their suitability for operating conditions in Africa. A simple concept The ratio pressure reducing valve (RPRV) reduces pressure in ratio. RPRVs offer a number of key features: - They do not require any adjustments and are therefore tamperproof - There are no pilots which are vulnerable to breakage and theft

Where sophisticated pilot operated

PRVs can lead to instability in the system, especially at low flow levels, ratio pressure reducing valves reduce pressure in ratio and therefore offer greater stability.

Flow control valves installed before end-use points can achieve further water savings.

Ratio pressure reducing valves are designed as simpler valves to suit local operating conditions.

Simple RPRVs can be used in different ways, shown here (below) fitted with a solenoid valve and without (above).

Electricity + Control

JULY 2019

27