Cavitation Elimination

Caviation DamagePittsfield Township is approximately 28 square miles and bridges the territory between the City of Ann Arbor and the City of Saline in the State of Michigan. The township provides water to about 39,000 people with needs varying from industrial, commercial to urban and rural. The average daily water usage is 3.5 MDG (million gallons per day) and increases up to 7 MGD during summer days.

Pittsfield has a contract with the City of Ypsilanti, Mich., a neighboring community, to purchase up to 5,000 GPM (gallons per minute) of water that runs through the city?s Textile Road Booster Pump Station. This station, built in 2004, used a plug valve with electric actuator to modulate rate of flow into the tank based on a meter in the system and a programmable logic controller. Water pressure in Ypsilanti is typically between 60 to 70 psi, whereas the maximum tank elevation ranges between empty and 16 ft (approximately 7 psi). Generally, if there is a high pressure drop ratio 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 as 70 psi to 7 psi is much greater than 3:1. This resulted in significant cavitation of the plug valve.

Cavitation is the formation of vapor bubbles which are created anywhere there is a local pressure low enough to allow the water to vaporize. These vapor bubbles migrate to the downstream side of the valve and/or the downstream pipe, where the velocity of the water slows down and the resultant pressure increase allows these vapor bubbles to implode with incredible destructive force. The sound is similar to small rocks rolling around within a valve and in extreme cases, it can sound like an explosion. These imploding vapor bubbles will literally erode any coatings on the valve and the cast or ductile iron, creating a porous, pock marked surface. This typically occurs around the seat area and on the downstream bridge wall of the valve.

Anti-cavitation trimThe 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 finally, plant or distribution system shutdown. For Pittsfield, the downstream eccentric reducer failed first and the cavitation destroyed the cement lining in the ductile iron reducer and eventually eroded into the reducer walls. Pin holes formed, leaks sprung and the township had to replace the fitting. During the second reducer replacement, the cavitation damage was not reserved to just the fitting, it had begun eating into the seat of the plug valve as well. The township needed a solution to stop the cavitation damage immediately as well as maintain the current valve functions with their control programming.

After two reducers were destroyed, it was clear that the plug valve was not working in this application. It was recommended that Singer Valve?s Single Rolling Diaphragm (SRD) actuated control valve with anti-cavitation (anti-cav) trim be implemented because it was the only plausible solution for this scenario. Pittsfield Township?s engineer evaluated options with other valves as well as looking at closing upstream valves to help create pressure losses prior to the fill valve. All parties agreed that the SRD with anti-cav trim would provide the all-around solution needed to eliminate cavitation damage without sacrificing other valves, changing processes or system dynamics.

Orifices in the dual anti-cav cagesWorking together with Pittsfield Township, Kennedy Industries determined the actual flow ranges as well as inlet pressure ranges and the required outlet pressure. Then, the Singer Valve engineering team modeled that performance and selected a drilling pattern for the multiple orifices in the dual anti-cav cages, specific to this application. The goal in such a situation is 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 are then included 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 putting on the pipe wrap and some minor concrete work to modify the pipe supports under the valve. The new valve was integrated with the township controls without modification of the 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 in this first year of operation. Other than these checks, there has been no need for any maintenance.

Completed installation of the SRD actuated control valveThe SRD control valve with anti-cavitation trim operates quietly and smoothly without cavitation. ?The Singer Valve is working fantastically,? said Billy Weirich, Pittsfield Township Utilities Superintendent. ?It is a simple and efficient solution with minimal maintenance.? The valve continues to consistently modulate the flow into the tank without any shut down, so that the residents of Pittsfield can rely on a consistent water flow.

Jennifer Zelski is a sales representative for Kennedy Industries and Mark Gimson is business development and marketing manager for Singer Valve, Inc.

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