Reducing pressure during periods of low demand has long been a strategy to reduce water loss. Pilot operated PRVs (POPRVs) with electronic controllers that set working pressure by time of day are a quick solution. They have reduced unaccounted for water losses significantly.
But these so-called Smart PRVs suffer several problems:
- They’re expensive – upwards of R 50K in addition to the cost of the valve.
- They’re complicated and require a sophisticated operator to use them correctly.
- They need maintenance. We’ve seen them simply disconnected when something goes wrong.
- They can force control valves to operate beyond their design parameters. This results in cavitation damage and instability.
Standard control valves are unsuitable for this application.
They suffer cavitation at pressure ranges regularly experienced in a network. Most standard POPRVs are designed to handle a pressure drop ratio of 3:1. If upstream pressure during the day is 12 bar the valve can safely reduce pressure to 4 bar without cavitation.
A smart PRV might reasonably be set to reduce downstream pressure to say 2 bar at night. It’s quite possible that the upstream pressure could increase to 15 bar at night because of lower friction losses. The ratio between upstream and downstream pressure has now increased from the safe 3:1 to 7.5:1. This leads to cavitation and consequent early failure.
Attempts to fix this by installing two pressure operated POPRVs in series are seldom successful. They’re difficult to set up and tend to become unstable.
Also, POPRVs become unstable at flow rates lower than they’re designed for. Again, these low flow rates are not unusual. Attempts to work around this by adding a smaller bypass PRV add to the complexity and often cause instability.
Unstable operation often leads to broken pipes. This happens mostly at night and results in huge losses – exactly what the smart PRV was to prevent.
Attempts to solve these issues with “trims” add to the cost and complexity without completely solving the problem.
There is an alternative which is much better suited to our conditions here in Africa: the ratio pressure reducing valve – the RPRV.
- It’s tamperproof. There are no operator adjustments.
- There are no pilots to be broken or stolen.
- Safe (no cavitation) pressure ratios of up to 5:1 are available.
- They react instantly to pressure and demand changes.
- They can be used in series without instability.
- They handle very low flows without instability.
The RPRV requires very little sizing input from engineers. Simply install a line-size valve where pressure needs to be reduced.
RPRVs have been tried and tested in Australia, mainly in in high-rise buildings. They’re also used extensively in the South African mining industry.
They have real application in water loss prevention.
- They can be used as a simple PRV without the risk of potential instability issues.
- They can be installed upstream from a POPRV to protect it from cavitation, again without the risk of instability.
- They can be installed downstream of a POPRV and fitted with a solenoid valve to lower downstream pressure during off-peak times. This reduces the strain on the POPRV.
More information on ratio pressure reducing valve can be downloaded here.