Electricity and Control April 2023

MEASUREMENT + INSTRUMENTATION

The benefits of simple pressure reducing valves PeterTelle of Ultra Control Valves says, for most people involved in the water industry, a ‘pressure reducing valve’ is usually understood to be the complex pilot-operated valve, as is typically used in the industry.Telle argues rather for the use of simple ratio reducing valves to manage water flow in pipelines effectively and consistently. ƒ They are tamper-proof. There are no settings, and nothing can be stolen.

P ilot-operated pressure reducing valves allow for the bolting on of controls for remote operation, automat ic flow rate limitation and sustaining upstream pres sure. With these various add-on devices, they can become complex configurations. “In my experience, these complex installations don’t work as expected in the long term,” Telle says. “They’re too complicated. “ Often the operators won’t have been trained on how the instruments are supposed to work, and preventive mainte nance does not always get done on schedule. I have seen incidents of broken pipes, huge water losses and areas be ing without water, as a result of field technicians tinkering with controls.” Ratio reducing valves Telle argues for simple, ratio reducing valves as a more ap propriate solution for most pressure reducing requirements in water pipelines. A ratio reducing valve has no pilots and consists simply of a piston with different inlet and outlet areas. “Some engineers point out that ratio reducing valves are not adjustable. That is true, but in most cases, it does not matter,” according to Telle. “For instance, a 4:1 ratio reducing valve can be used to reduce pressure from 12 bar to 3 bar. If the upstream pres sure rises to 16 bar (say, at night) the downstream pres sure would go from 3 bar to 4 bar. That is not likely to be a problem except in a handful of very specific scenarios,” he says. He suggests that the advantages of the ratio reducing valve outweigh the lack of adjustability.

ƒ They can act in series without instability. This com pares well with pilot-operated pressure reducing valves which have to be carefully tuned during com missioning and are vulnerable to instability from changes in demand or pressure. ƒ They are inherently failure-proof. Pilot controlled valves can fail when any part of the pilot circuit is damaged – and they generally fail into the open po sition, often with serious results. This cannot happen with a ratio reducing valve as 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, although it will not be drop-tight. ƒ The axial flow path provides good resistance to cav itation. This enables high pressure drop ratios: gen erally up to 5:1, but specialist valves can go as high as 12:1. ƒ They are not vulnerable to dirt particles. Telle says ratio reducing valves have been used with good effect in a range of installations. He cites several ex amples. In high-rise buildings where pressure reducing valves often have to act in series – this is something which cannot be done easily with pilot-operated valves. For water-saving projects – “I recently worked with a team of consultants in the Eastern Cape on a successful water-saving project where 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 compli cated valve sizing is required to ensure stable operation. Simply, a ratio reducing valve of the same size as the line is fitted. For pump bypass control – Demand can be variable in many pumping applications. This

means a bypass is needed from the delivery side back to the suc tion 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 normal­

Standard pilot operated pressure reducing

A standard pilot-operated valve, as typically used in the water industry.

valves can become complex configurations.

A simple ratio reducing valve.

20 Electricity + Control APRIL 2023