A break pressure tank or reservoir is often used when the topography causes pipeline pressure to be too high. It’s a good and safe solution, but it has some disadvantages:
- They’re expensive and bulky.
- The level and flow control mechanisms are vulnerable to tampering and damage.
- Pressure is reduced all the way down to atmospheric pressure. This makes supplying areas immediately downstream at sufficient pressure difficult.
Standard pressure reducing valves could be used instead of a break pressure tank if it were not for the risks of failure due to:
- Tampering with the pilot settings.
- Breakages or malfunctioning of the pilots or main valve.
- Reaction delay causing an increased pressure downstream.
While it is true that precautions can be taken to reduce these risks, they are not enough to allow the engineer to feel completely safe.
The Ultra Ratio Reducing valve operates on a relatively new concept which makes it suitable for use in a failsafe pressure reducing station.
The valve works on the principal of pressure forces on different piston areas which keep the piston in balance. No pilot valves are needed.
These areas are sized to provide a certain ratio between inlet and outlet which will in turn reduce the pressure in a chosen ratio. Thus, if you wish to reduce pressure from 12 bar to 4 bar the areas are sized to provide a 3:1 ratio.
This offers several advantages for use in a pressure reducing station:
- Immediate reaction to pressure or demand changes without the typical lag in pilot operated control valves.
- Wide rangeability enabling the valve to cope with a wide range of flows without the typical instability found in pilot operated PRVs at low flows.
- Immune to dirty water with particles in it as the valve has no small orifices which can clog up with dirt.
- Simplicity of operation without pilots or external controls. In fact, the valve looks like a pipe spool piece and will not even be recognized as a valve.
The most significant feature is that there are no conditions in which the valve could fail in the open position. It is inherently fail safe. This means it can confidently be specified instead of a break pressure tank.
A typical failsafe pressure reducing station would consist of the following items:
- Upstream and downstream isolating gate valves. We recommend gate valves instead of butterfly valves as the downstream valve would be required to slowly fill the line after maintenance shutdown and gate valves are better suited to cope with high differential pressures generated during this action.
- Strainer. Although the ratio reducing valve is not vulnerable to particles in the water, large objects could prevent the valve from closing completely.
- Ratio reducing valve. These are currently available in sizes up to 300mm and 100 bar pressure rating.
- Relief valve. Always recommended even though with the ratio reducing valve there is less need for this. The relief valve can be sized to ensure either complete protection in the very unlikely case of catastrophic failure of the ratio reducing valve, or to just cope with leaking seals of the ratio reducing valve which would prevent its complete tight closure.
- Maric flow control valve. This is a good idea to prevent run-away flows in case of downstream pipe failure. This unique flow control valve is also inherently fail safe and needs no pilot valves.
- Air release valve. Air comes out of solution after any pressure reducing device. An air release valve downstream of a PRV station helps remove the air at source to ensure stable control.
The above items are considered essential in to provide a completely safe PRV station. We recommend this approach to save money and space when providing a break in pressure gradient in any pipeline.
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.