Yarra Valley Water (YVW) is the largest of Melbourne, Australia’s three water corporations providing water supply and sewerage services to over 1.7 million people and over 50,000 businesses in the northern and eastern suburbs of Melbourne. The district covers approximately 4000 square kilometers from as far north as Wallan and extending to Warburton in the east. They maintain over 5500 miles (9000 kilometers) of water mains and nearly the same distance of sewer mains.
The Wandin North site had three pressure reducing valves (PRVs) of differing sizes. The 2-inch (50 mm) valve was used during periods of low demand in the zone whilst the 4-inch (100 mm) valve supplied water during periods of high demand which left the 6-inch (150 mm) valve to supply water during extremely high demand periods, such as firefighting. In early 2012, YVW’s field service technicians observed severe erosion within the 2-inch valve during routine maintenance at this site, so they contacted the engineers at Metaval to investigate.
A NEW DISCOVERY
When arriving at the site, Metaval’s Steven Hill noted that the noise from the valves could clearly be heard from the in-ground concrete valve chamber with the concrete lid on. The three valves at the site were not fitted with Anti-Cavitation Trim (Anti-Cav), and, at the time of installation, the design did not require it. Typically, an Anti-Cav Trim is needed when the inlet pressure is three times higher than the required outlet pressure or when the cavitation index Sigma is below 0.8. Inspection showed the site’s upstream (inlet) pressure transmitters to be 68 m and downstream (outlet) to be 6.6 m.
This meant the downstream pressure (desired setpoint) that feeds into residential homes was fluctuating due to the cavitating valves. These valves were struggling to provide a stable downstream pressure. Excessive noise along with damage to the valve and downstream pipeline were consequences of the cavitating valve; therefore, it was clear that an Anti-Cav Trim was needed to halt the damage being done.
The reinstallation would also require the damaged 2-inch valve to be replaced. However, if only the impaired valve was replaced with an added Anti-Cav Trim, then this would still leave the other two valves vulnerable to cavitation. Smaller diameter valves can maintain control at lower flow rates, but they offer limited capacity for high flow rates, hence why YVW needed the additional valves in the system.
As luck would have it, though, Singer Valve had just added new sizes in their single rolling diaphragm (SRD), starting at 6 inches. This technology offers an advantage over a flat diaphragm operated valve in that it does not need to be operated between 20 and 80 percent open. Therefore, a larger diameter valve can be used to control low and high flows in the same valve.
GOING AGAINST THE GRAIN
Most distribution systems have a combination of extremely low flow and high pressure periods. Traditional automatic control valves often experience seat chatter under these conditions. As a result, a smaller bypass valve is needed to control the lower flows. With the SRD technology, the moulded diaphragm provides a constant surface area no matter the valve position and avoids injecting small pressure pulses into the piping. By doing this, the valve eliminates seat chatter at low flows helping to prevent water loss and leakage while providing smooth precisely controlled flow.
“It goes against the ‘rule of thumb’ to remove a smaller diameter valve from a system and expect a larger diameter valve to control pressure during low and high flows,” said Steven Hill. “But the SRD worked like a charm.”
By replacing the 2-inch valve with a 6-inch SRD fitted with Anti-Cav Trim, YWW removed the need to protect the other two valves with Anti-Cav Trim as the one valve can now do it all.
THE DANGERS OF CAVITATION
Cavitation can be an extremely damaging force with loud noise, excessive vibration, choked flow, destruction and erosion of control valves and their components, which results in disruption of water distribution or plant shutdown. Singer’s Anti-Cavitation technology contains two heavy stainless steel sliding cages that maximize the full flow capacity. The first cage directs and contains the cavitation recovery, allowing it to dissipate harmlessly, while the second cage allows further control to a level as low as atmospheric pressure downstream. The cages are engineered to meet the flow / pressure differential of each application.
By installing a valve with both the SRD and Anti-Cav Trim features, the 6-inch S106-PR-AC valve addressed the past cavitation problems and, today, provides better stability with having one valve in control. “Another advantage that is significant over time is that we now have fewer assets requiring less maintenance,” said Fiore DiPietro, specialist technician for operations at Yarra Valley Water. With an organization that requires 650 people to maintain operations, finding ways to reduce maintenance time and costs can make a big difference.
The Yarra Valley customers are back to enjoying constant stable water pressure without cavitation noise, and YVW has extended the life time of its pipeline.
MODERN PUMPING TODAY, November 2012