Set in the foothills of Canada’s Rocky Mountains is the city of Airdrie. Without a water supply, Airdrie purchases its water from neighboring Calgary. Every night, Calgary pumps enough water to fill two reservoirs in Airdrie (a total of four million gallons).
Then Airdrie pumps water into its distribution system so the city’s 38,000 people have water throughout the day. Calgary supplies water at 5,300 gpm (335 l/s) at 60 psi (4.1 bar) to the inlet of the reservoir fill valve. Outlet pressure of the fill valve is at or close to atmospheric pressure.
Pressure Drop Is a Problem
Anytime there is a high-pressure drop ratio across a valve, (typically a three to one ratio or more in absolute pressure) cavitation can be an issue. This is a common occurrence with reservoir feed valves as well as relief valves when they are designed to continuously relieve higher, upstream pressure to atmospheric pressure. People have tried to resolve this issue by using two valves in series: the first reducing the pressure partly, and the second valve reducing the pressure to the required outlet pressure, which eliminates the three to one pressure drop within one valve. This approach can work. However, it requires extra space, extra piping components and two valves. So, this is often not the best solution due to cost and space constraints.
Cavitation is a serious problem when a valve reduces pressure with a ratio of three to one (or greater) of the absolute inlet pressure. From 60 psi (4.1 bar) to atmospheric pressure may not sound overly dramatic, but it is deep in the cavitation zone. Typically, if the inlet pressure into a reservoir fill valve is 20 psi (1.3 bar) or less, cavitation should not be an issue; however, when pressure at the valve inlet exceeds 20 psi (1.3 bar), some form of cavitation control should be considered. Kelly McKague, Airdrie’s facility operator discovered this the hard way.
“We inspected the reservoir fill valve every year for damage due to cavitation,” says McKague. “The valve was completely eaten away, so we had to replace it every 18 months because of cavitation damage.” Cavitation is the formation of vapor bubbles that are created anywhere there is a local pressure low enough to allow the water to vaporize. These 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. It sounds similar to small rocks rolling around within a valve. These imploding vapor bubbles will erode any coatings on the valve and the cast or ductile iron, creating a porous, pock-marked surface. This occurs most frequently around the seat area and on the downstream bridge of the valve.
McKague attended a trade show where he was fascinated to learn about an anti-cavitation trim. “On display was the exact valve we had in the reservoir,” says McKague. “That caught our attention; so, the conversation led to the anti-cavitation trim technology.” After further consultation, the City of Airdrie decided to purchase this new technology. The valve was then customized so that the anti-cavitation trim would be specific to Airdrie’s application.
The Cause of Cavitation
In customization, the specific application must be considered, as well as the capacity of the valve to prevent the supply side from being reduced to unacceptable levels. So, Airdrie provided the company with the actual flow ranges as well as inlet pressure ranges and the required outlet pressure. With this information, the company’s engineering team could model that performance and select a drilling pattern for the multiple orifices specific to Airdrie’s application. The trick is to supply orifices that can manage maximum flow while creating enough backpressure within the cage to prevent the microscopic vapor bubbles from escaping.
The anti-cavitation cage is not a sacrificial lamb and does not cavitate, as all the destructive forces are kept in the middle of the cage and the opposing vapor bubbles collide and implode against each other, not against a metal surface. Newer entries to the anti-cavitation market tend to use elongated grooves and take a position that one size fits all. The problem with this is that when the pressure drop is extreme, not enough back pressure is created, and orifice plates may be required as extra insurance, which complicates the entire installation and does create a sacrificial lamb.
The challenge in designing this kind of anti-cavitation trim is to control the pressure inside the anti-cavitation cage so that through the full stroke, the pressure remains low enough to prevent cavitation when flowing out of the cage, and yet high enough to assure that the valve opens. The actual main valve used by the company is different from the traditional main valves to ensure a smooth constant flow around the entire diameter of the anti-cavitation cage.
This solution is by no means a simple task and is the result of five years of R & D from the company’s engineering team. For guaranteed results in solving a cavitation problem, you need to have an engineered solution as each application is different and requires some customization. A one size fits all approach is not the best approach.
After Airdrie’s anti-cavitation trim had been operating for six months, McKague was curious to check the valves performance. “We could have boxed the valve and resold it,” says McKague. “There was absolutely no sign of cavitation damage, not even on the coating. We couldn’t believe it.”