Control valves for Pressure Management
Pressure management has become a very important element in the fight to reduce unaccounted-for water losses in pipe networks. It’s a “quick fix” solution, which usually brings instant results at relatively low costs. The theory of pressure management is that if you reduce the pressure in a network below the normal value, existing leaks will be reduced as well. The possibility of pipe breaks are also reduced, as the whole system is operating at the lowest practical pressure. To achieve the ultimate low pressure during the complete demand cycle, pressures would have to be adjusted all the time to match flow demands. Battery driven electronic controllers have been developed to adjust pressures in two different ways:
- Time modulated, whereby the controller is programmed to adjust the valve’s pilot to two settings; normal setting during high demand periods, and lower setting during low demand periods.
- Flow modulated, whereby the valve’s pressure setting is continuously adjusted as flow demand changes. This method is more expensive and complicated as a flow meter is required with the PRV installation. Savings using this method are optimized.
How does all this affect the control valve selection process?
Considerations in control valve selection are as follows:
Every pressure reducing valve has a particular pressure drop ratio it’s able to handle without cavitation occurring. For most standard control valves this ratio is 3:1, i.e. if the incoming pressure is 9bar, the valve can comfortably handle an outlet pressure of 3bar. In a lot of cases it would be possible to reduce the pressure to, say, 2.5bar (which would make a great improvement to savings), but it would mean that the control valve is operating in its cavitation zone. This will seriously reduce the life of the valve. The other problem with cavitation is that air is released with a lot of inherent instability, which could result in pipe breaks, and which would have the opposite effect to the end goal: saving water. Pressure reduction ratios need to be carefully checked to ensure that critical ratios are not exceeded. Most standard control valves with max 3:1 capability would have to install two valves in series in the above pressure scenario. Putting two valves in series increases the cost and complexity of the installation, as well as increasing potential instability, which can also lead to pipe breaks. The Ultra ACV control valve can handle a 4:1 ratio, which would allow the downstream pressure to be set down to 2.25bar.
The other problem area to investigate is the valve’s ability to handle low flow during night-time demand. All standard control valves have a minimum flow rate at which ensures stable control. If flows go below this value, the valve becomes unstable, and is likely to cause pipe breaks. Standard control valves would have to overcome this problem by installing a smaller valve in parallel to automatically handle the low flow during night time demands. Again this adds complexity, cost, and possible instability. Some manufacturers have a V-ported valve which can handle lower flows, but these valves still have a lower limit. Also, the V-port brings other problems such as lower pressure drop capability with high wear rates of the V, which often leads to the valve jamming – which, in turn, can lead to broken pipes! The V-port also drastically decreases the capacity of the valve.
The C-Valve range is available, which can handle up to 10:1 pressure drop ratio, and has a low flow capability down to zero flow.
The important point to be considered is that pressure management can only be achieved by installing pressure reducing valves with associated controllers. Care should be taken to ensure that the control valve chosen is able to handle the more arduous conditions imposed on the valve. If the correct control valve choice is not made, all the proposed benefits of pressure management can have the opposite effect, and result in great water losses due to pipe breakages.
Ultra Control Valves supply three different types of control valves:
- Ultra ACV diaphragm operated control valve. This valve is similar to the standard competition’s range, although it has significant benefits such as higher pressure drop capability, and longer life due to robustness and internals which are easier to service than other valves. This valve is based on the Muesco Baker/Watts design, and has a worldwide reputation of excellent design.
- Ultra ACV Ratio Reducing valve. This valve reduces pressure in a ratio and does not use any pilots. As such, this is the simplest form of pressure reducing valve.
In water loss control by pressure management, the ratio reducing valve has the following uses:
- As a simple pressure reducing valve. By installing this valve in any system where the pressure is higher than it needs to be, the simple reduction of the pressure envelope will result in water savings.
- Where the reduction ratio is higher than the Ultra ACV pilot operated valve can handle, placing a ratio reducing valve upstream of the pilot operated valves will solve any cavitation problem. Whereas two pilot operated valves in series can cause instability problems, the ratio reducing valve just acts like a passive device, immediately following what the pilot operated valve is doing, without any instability.
- C-Valve inline axial flow control valve. This valve is a totally different design which has unique benefits such as 10:1 pressure drop capability, and low flow control down to zero flow rate! Where pressure management parameters could require more complicated control systems, the C-valve is able to handle the applications with one valve, keeping the cost down and the installation much simpler.
As a major manufacturer and supplier of control valves in South Africa, Ultra pride themselves on their application knowledge and field service of control valves. Together with associate companies, the complete service of water loss management can be offered including:
- Leak Detection
- Data Logging
- Compiling a report of requirements for a network to optimize pressure settings and minimize losses.
- All related field service and commissioning.
In most parts of the South African water industry, maintenance is not performed. Ultra offer a service to ensure that all its pressure reducing valves are regularly checked and perform well. Just this simple strategy will already have a huge impact on water saving, without any costly and complicated electronic equipment.