Ratio reducing valves
Ratio Reducing Valves … The Next Evolution in Pressure Reducing Valves
Pressure reducing valves have evolved, over the last 50 years, to incorporate many new shapes and features. One of the emerging concepts is a ratio reducing valve (RRV), which has been available, for many years, from an Australian manufacturer in the building and mining industry. This RRV has, however, never successfully been developed or marketed in the water industry. RRVs have also, to date, only been available in limited sizes and pressure ratings. Some South African manufacturers have developed versions of this concept for the high pressure mining industry.
What exactly is a ratio reducing valve?
A ratio reducing valve is a simple form of pressure reducing valve, based on a ratio between upstream and downstream pressures. In order to reduce the pressure, it uses a simple piston inside the body of the valve. This piston is driven by pressure forces, which impact onto the upstream and downstream piston faces. These varying pressures on the different piston areas cause the pistons to always remain in balance, as calculated by the design for a particular size and ratio of RRV. In turn, this keeps the ratio between upstream and downstream pressure constant, regardless of varying flows or pressures.
Example: if you want to reduce pressure from 12bar to 4bar you use a 12:4 or 3:1 ratio valve. Simple as can be!
Applications are many, as follows:
- Straight pressure reducing valve to control a downstream pressure in a distribution network. The RRV is the ideal PRV, as it has an immediate reaction time, unlike a pilot operated PRV. The latter has a delayed reaction time to demand changes, and can sometimes create problems in maintaining “dead-end” control of downstream pressures.
- As secondary PRV to reduce pressure before a pilot operated PRV (POPRV) if the pressure drop is too high to handle by one valve. This is the ideal solution to install upstream, as it reacts instantaneously to the position changes of the POPRV. It also eliminates any instability problems, which are common when using 2 x POPRVs in series.
- As “Energy Dissipator” upstream of a Level Control valve when the dynamic head is in excess of 5bar. This works to overcome cavitation problems.
- In pressure management applications. By installing a RRV, fitted with a solenoid pilot, on the downstream side of a POPRV, the following operation can be easily achieved:
- During day time flows, the solenoid valve ensures that the RRV is wide open, and all control is by the POPRV.
- During low nighttime flows, the solenoid is set to activate the RRV. This reduces the set-pressure of the POPRV by a further ratio, to be determined by an analysis of the distribution system.
This method provides an extremely user-friendly and stable system to maintain lower pressures at night, and make sure no cavitation occurs. Its axial flow design ensures the RRV can also cope with very low flows.
The RRV is an extremely simple and robust pressure reducing valve. It has no external control piping or pilots, which can be vulnerable to damage, theft or tampering. It is ideal for use in rural water distribution systems, where knowledge and maintenance procedures are often non-existent.
Questions are asked about the fact that downstream pressure cannot be accurately set or adjusted, and variations in upstream pressure will cause a variation in downstream pressure. Let’s examine this:
In the above example of reducing pressure from 12bar to 4bar, if the upstream pressure increases to 15bar at low flow times due to decreased friction losses in the supply line, the downstream pressure will increase to 5bar, instead of 4bar. Is this really a problem? The writer thinks not … at least not in 80% of normal applications!
The Ultra Ratio Reducing Valve is available in sizes from 50mm to 250mm, and pressure rating up to 100bar. Although currently imported, local manufacture is being planned and the “Proudly South African” product will be available during 2015.
Is this concept the next stage in the evolution of pressure reducing valves? Watch this space…