Michigan’s Pittsfield Township bridges the cities of Ann Arbor and Saline, serving 39,000 industrial, commercial, urban, and rural customers within 28 square miles.
Average daily water usage is 3.5 mgd but can rise to 7 mgd during summer. The utilities department contracts with a neighboring community, Ypsilanti, to buy up to 5,000 gallons per minute, as necessary. That water flows into a booster pump station/storage tank built in 2004, where a plug valve with electric actuator regulates velocity based on a meter in the system and programmable logic controller.
That was the plan, anyway.
The problem was that Ypsilanti’s water pressure ranges from 60 psi to 70 psi and the storage tank is about 7 psi (reservoir tank levels range from empty to 16 feet). Whenever the pressure ratio across a plug valve is greater than 3 to 1, as this was, a destructive process called cavitation can occur (see online sidebar).
Ultimately, two ductile iron reducers were destroyed. Cavitation ate away at the fitting’s cement lining and eroded into its walls, forming pin holes that became leaks. Damage to the second reducer wasn’t limited to just the fitting; it had begun eating into the seat of the plug valve as well.
The department had to eliminate cavitation without changing processes or system dynamics.
Closing upstream valves to help create pressure losses prior to the fill valve would have required sacrificing additional valves. Kennedy Industries of New Hudson, Mich., a company that distributes and repairs pumps and valves and had worked with the township on other projects, suggested installing a single rolling diaphragm (SRD) actuated control valve with anti-cavitation trim. Manufactured by Canada’s Singer Valve Inc., the rolling diaphragm enables a control valve to operate throughout the entire flow range with very stable control. This eliminates the need to install a low-flow bypass valve and prevents hunting, which can produce pressure waves, at lower flows. Singer is the only company that offers a rolling diaphragm that can be used with the anti-cavitation trim.
“It was the best solution for this scenario,” says Kennedy Industries’ Jennifer Zelski, who specializes in municipal control valve applications in Eastern Lower Peninsula Michigan.
The company determined actual flow ranges as well as inlet pressure ranges and the required outlet pressure. Singer Valve’s engineering team then modeled that performance and selected a drilling pattern for the multiple orifices in the dual anti-cav cages.
“The secret is to supply orifices that can manage maximum flow while creating enough backpressure in the cage to prevent the microscopic vapor bubbles from escaping,” says Singer Valve Business Development and Marketing Manager Mark Gimson. The dual anti-cavitation cages are then included in the new control valve.
The 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 putting on the pipe wrap and some minor concrete work to modify the pipe supports under the valve. The new valve was integrated with SCADA controls without modifying existing programming. Kennedy Industries performed the start-up and training to ensure a smooth transition.
Pittsfield has a very proactive maintenance program and checks this station weekly. The strainers are always inspected, but have yet to be found fouled up after a year of operation. No maintenance has been required. “The valve is working fantastically,” says Pittsfield Township Utilities Superintendent Billy Weirich. “It’s a simple and efficient solution with minimal maintenance.”
Mark Gimson ([email protected]) is business development and marketing manager for Singer Valve Inc.; visit www.singervalve.com. Jennifer Zelski ([email protected]) is sales representative for Kennedy Industries; visit www.kennedyind.com.
How to avoid cavitation
Anywhere there’s a high-pressure drop ratio across a valve—typically 3 to 1 where the Sigma value is greater than 0.8—water may vaporize.
Vapor bubbles migrate to the downstream side of the valve and/or the downstream pipe. There, water velocity slows, increasing pressure on the bubbles until they implode with incredibly destructive force.
The imploding bubbles erode any coatings on the valve and cast or ductile iron pipe, creating a porous, pock marked surface. This typically occurs around the seat area and on the downstream bridge wall of the valve.
Cavitation usually sounds like small rocks rolling around within a valve but in extreme cases sounds like an explosion. Consequences include strong vibrations, loud noise, choked flow, erosion of valve components, destruction of the actual valve, erosion and destruction of downstream piping, and even plant or distribution system shutdown.
