By Jennifer Zelski and Mark Gimson
Pittsfield Charter Township, located in the city of Ann Arbor, Mich., provides water to about 39,000 people with an average daily water usage of 3.5 million gallons per day (MGD) and increases up to 7.0 MGD during summer days.
Pittsfield purchases up to 5,000 gallons per minute (GPM) of water from a neighboring community and runs it through its Textile Road Booster Pump Station. The water pressure is typically between 60 and 70 psi, whereas the maximum tank elevation ranges between empty and 16 feet (approximately 7 psi). The station was built in 2004 and used a plug valve with an electric actuator to modulate rate of flow into the tank based on a meter in its system and a programmable logic controller.
Anywhere that a high pressure drop ratio occurs across a valve (typically a 3-to-1 ratio or more in pressure, where the Sigma value is greater than 0.8), cavitation can be an issue. In this case, the Township was well above that ratio at 70-to-7 psi -- much greater than the 3-to-1 ratio -- which ultimately resulted in significant cavitation of the plug valve.
The consequences of cavitation can be strong vibrations, loud noise, choked flow, erosion of valve components, destruction of the actual valve, erosion and destruction of downstream piping, and plant or distribution system shutdown.
To solve the problem, Pittsfield installed Singer Valve's Single Rolling Diaphragm (SRD) actuated control valve with anti-cavitation trim to eliminate cavitation damage without sacrificing other valves or changing processes or system dynamics.
Once the actual flow ranges and inlet/outlet pressure ranges were measured, the factory engineered a custom drilling pattern for multiple orifices in the dual anti-cavitation cages specific to this application. The goal was to supply orifices that can manage maximum flow while creating enough backpressure in the cage to prevent the microscopic vapor bubbles from escaping. The dual anti-cavitation cages were then fitted in the new control valve.
Singer's control valve operates hydraulically with system line pressure as well as with dual solenoids and an interface controller that communicates with the Township's SCADA. The SRD allows the valve to operate steadily at both high and low flows, and the pilot controls are built to be self-flushing for reduced maintenance.
Installation took three days, most of which was installing the pipe wrap and conducting minor concrete work to modify the pipe supports under the valve. The new valve was integrated with the Township controls without modification of their existing programming. Kennedy Industries performed the startup and training to ensure a smooth transition.
Pittsfield has a proactive maintenance program and checks this station weekly. The strainers are always inspected but have yet to be found fouled in its first year of operation. Other than these checks, there has been no need for any maintenance.
The SRD control valve with anti-cavitation trim operates quietly and smoothly without cavitation. "It is a simple and efficient solution with minimal maintenance," said Billy Weirich, Pittsfield Township Utilities superintendent. The valve continues to consistently modulate the flow into the tank without any shutdown so that the residents of Pittsfield can rely on a consistent water flow.
Singer Valve is an exhibitor at WEFTEC.14 and can be found at Booth 7313. For more information, visit www.singervalve.com.
