Say “pressure reducing valve” to someone in the water industry and they’ll assume you’re referring to a complex pilot-operated valve, something like the one in the picture below.
Pilot operated pressure reducing valves allow for bolting on controls for remote operation, automatic flow rate limitation and upstream pressure sustaining. You can end up with something like the picture below.
My experience has been that these complex installations don’t work as expected in the long term. They’re too complicated.
Often the operators won’t have been trained on how they are supposed to work, and preventative maintenance doesn’t always get done on schedule.
I’ve seen incidents of broken pipes, huge water losses and areas being without water as a result of field technicians tinkering with controls.
Ratio reducing valves are a bullet-proof alternative.
A ratio reducing valve is very simple. It has no pilots and consists of a piston with different inlet and outlet areas.
Some engineers have pointed out that ratio reducing valves are not adjustable. They are right, but, in most cases it doesn’t matter.
For instance you could use a 4:1 ratio reducing valve to reduce pressure from 12 bar to 3 bar. If the upstream pressure rose to 16 bar (say at night) the downstream pressure would go from 3 bar to 4 bar. That’s not a likely to be problem except for a handful of very specific scenarios.
The advantages of ratio reducing valve outweigh the the lack of adjustability.
- They’re tamper proof. There are no settings, and there is nothing to steal.
- They can act in series without instability. This compares well with pilot-operated pressure reducing valves which have to be carefully tuned during commission ,and are vulnerable to instability from changes in demand or pressure.
- They’re inherently failure proof. Pilot controlled valves can fail when any part of the pilot circuit is damaged. They generally fail into the open position, often with disastrous results. This is impossible with a ratio reducing valve. It contains no parts to fail. If the main seal is damaged water will leak through a breather hole but the valve will continue to reduce pressure, albeit not drop-tight.
- The axial flow path provides good resistance to cavitation. This enables high pressure drop ratios. Generally up to 5:1 but specialist valves can go as high as 12:1.
- They’re not vulnerable to dirt particles.
We’ve seen ratio reducing valves used with good effect in a range of installations.
In high-rise buildings where pressure reducing valves often have to act in series, something which can’t be done easily with pilot-operated valves.
For water-saving projects. I recently worked with team of consultants in the Eastern Cape on a successful water-saving project. We used simple ratio reducing valves instead of the more sophisticated electronic ‘“smart PRV” for all the above reasons.
Another advantage of ratio reducing valves for water-saving projects is that no complicated valve sizing is required to ensure stable operation. You simply fit a ratio reducing valve of the same size as the line.
Pump bypass control. Demand can be variable in many pumping applications. This means you need bypass from the delivery side back to the suction side (usually the reservoir). But, the downstream pressure is usually close to atmospheric pressure. This leads to the risk of cavitation.
Pilot-operated pressure sustaining valves are normally used in these applications and most of them can handle a max of 4:1 pressure drop ratio. So, anything above a pump delivery pressure of 4bar would need a pressure sustaining valve to handle this to prevent cavitation.
A ratio reducing valve provides an ideal solution. If the pressure is anything up to 20 bar, a 5:1 valve can be installed upstream of the pressure sustaining valve. If the pressure is above 20 bar one can install 2 x ratio reducing valves upstream.
Below is a typical installation drawing.
Pressure reducing stations for critical applications. There are certain applications where pressure reducing valve failure would cause disastrous results. Below are 2 applications which come to mind Off take from a high-pressure supply line into a low-pressure pipeline. In place of break pressure tanks/reservoirs. Break pressure tanks are expensive and vulnerable to tampering/malfunction. They also need space on the surface which might not be easy to secure.
Below is a schematic of an failsafe pressure reducing station.