Avoiding Cavitation Damage

Avoiding Cavitation Damage

By April Hirsch / Published May 2017

 

Long before most of us heard of cavitation, we exploited cavitation as child’s play. We (or our fraternal siblings) strove for ghastly noises by trying to suck a miniscule amount of remaining fluid through a straw. (The most ingenious noise makers even blew into the liquid in a glass and created bubbles before sucking through the straw.)

Today, we know that ghastly noises coming from a pump are likely to signal cavitation. In simplest terms, there are gas bubbles or pockets of air in the water in our pump. Cavitation is the formation and collapse of vapor bubbles in a flowing liquid.

Given big enough pockets and strong enough collapse, water can be driven with such force that it causes damage to the pump—notably pitting of some component. (The phenomenon of cavitation is not restricted to pumps per se. The hydrofoil and propeller can sustain cavitation damage, for example.)

Because significant cavitation causes vibration, a pump that is affected by cavitation becomes noisy. What causes cavitation? Well, think back to the straw. Even in a full glass of liquid, it’s possible to experience cavitation if an ice chip lodges in the straw. Or, if the straw is damaged by a slit. And so on. All of these layman straw experiences have analogs in a pump.

In other words, cavitation can result from either a suction problem or a discharge problem. If a filter or strainer is clogged, it’s the equivalent of getting an ice chip (or glob of ice cream) lodged in a straw.

Now, we could bring up the Bernoulli equation for an incompressible flow—okay, we just did, but it’s easier to analogize than to delve into physics and math, given that our goal is to avoid cavitation damage. So let’s stay with the straw analogy.

If the straw is very, very long, the suction pump (mouth) may be too distant to maintain a steady flow. If someone couples two straws together by cutting a slit in one and sliding it over the other, that coarse fitting will not work well. It’s all beginning to be familiar, is it not?

For flow to be sustained and uninterrupted by pesky voids, a pipe (straw) cannot be too narrow in diameter or too long or have too many fittings and so on. Manufacturers and distributors endeavor at every juncture to make certain that their end users will have the specifications for optimal configuration. And when the optimal configuration is in place—and routine maintenance is scrupulously followed—cavitation should be a rarity. Yet, it’s too common and more can be done to avoid it.

Cavitation in some settings can be so serious that it truly creates a life-or-death situation. Consider a bypass machine for the heart. Or, recall decompression sickness (the bends) experienced by divers.

Perhaps end users of pressure washers grant themselves some leniency because they do not expect to damage a pump if they make a one-time exception. Long before there is catastrophic damage to a pump, though, violating specifications and allowing cavitation to continue can result in higher consumption of energy and less than optimal flow (because of pressure reduction).

Cavitation can also cause seals or bearings to wear out sooner than their well-maintained life-span would dictate. Finally, serious cavitation damage can destroy the pump (e.g., housing, impeller).

Perspectives

Again, proper maintenance and proper use prevent cavitation. Because of differences in water quality (hardness, for example) across the country, cavitation threat varies somewhat. Differences in municipal water supply are also many. Consequently, perspectives on how to use or not use the municipal supply differ, too.

Scott M. Christopherson, service department manager at General Pump in Mendota Heights, MN, estimates that “approximately 33 percent” of pressure washers/pumps reach the service center because of cavitation damage. There are some basic things that contractors can do to avoid such damage, he says.

“Pressure feed the pump with the proper sized inlet plumbing—hoses, fittings, filtration, etc.,” says Christo-pherson. “And avoid elbows or restrictions that could cause turbulence. Also, make sure filtration/screens are kept clean.”

Using the water from a residential home is generally safe, says Christo-pherson. He sees an advantage to using water from the residential tap. Is it better than using a mobile tank, for instance?

“Yes, actually it is better to utilize the home’s water supply since it is pressurized as long as proper filtration is used,” says Christopherson. “If the pump is fed from a tank supply, many times it is operating in a vacuum state, especially as water level decreases.”

Dave Hildebrand, vice-president at Barens Inc. in Seneca, PA, shares his perspective on cavitation damage. “We have not found cavitation to be a major problem with the pumps we have sold over the years,” he says. “Is it a problem? Yes, but I’m not sure how great.”

On the subject of whether or not to use a municipal residential tap or a tank, Hildebrand offers that it’s a matter of evaluating the water quality and the flow. “Most of the water problems we find are with poor quality of water causing calcium, etc. to build up and destroying check valves, packings, and plungers,” explains Hildebrand.

“I’m not sure using water from a tank over water from a constant ‘city’ source is a good recommendation,” says Hildebrand. “No matter how old the pipes in the house, the flow and pressure will be greater than from a water tank, plus you would need a large tank if the job lasts for any length of time. If the machine is a 4 gpm, you would need a 240 gallon tank for one hour of cleaning.”

An end user of a pump should always keep in mind just how much force the collapse of bubbles in a liquid can muster, either directly or by propelling liquid jets as they collapse. Those jets can pit surfaces.

Another way to look at it is this: Without an optimally-functioning pump, a contractor does not have an optimally-functioning pressure washer.

Cavitation Q&A

Jeffrey Paulding, president of Dirt Killer Pressure Washers Inc. in Baltimore, MD, like all manufacturers, distributors, and suppliers of pressure washers, wants end users to know as much as possible about cavitation and how to avoid it. Optimally-functioning pumps keep the industry strong. In the Q&A that follows, Paulding answers several questions for us.

Cleaner Times [CT]: Do you have any estimate of how many pressure washers/pumps reach a service center because of cavitation damage?

Paulding: More than 50 percent show cavitation damage.

CT: What is the top piece of advice you give a contractor regarding how to avoid cavitation (and cavitation damage)?

Paulding: There are many reasons for cavitation. Not feeding the pump the required amount of water, [such as] feeding a 4 gpm pump 3.7 gpm. Having too long a garden hose. Restrictions in the inlet fittings or inlet fittings not being sufficiently tight so that there are leaks that will allow in air. Leaving the pressure washer in bypass too long so that the water heats up in the pump, causing the water to start to decompose and releasing air that causes cavitation. Contractors should try to avoid all these things.

CT: In a residential setting, it seems that a contractor might avoid some of the likely causes of cavitation by using a water supply that he carries with him instead of tapping the water supply of the homeowner, especially when it’s an older home with water pipes that have not been replaced in decades. Is that a naïve (lay writer) concept?

Paulding: No. Bringing your own water is smart, especially in an area where the local water source is from wells and may not provide enough gpm for the contractor’s pressure washer. We build truck, van, and trailer units with water tanks all the time for professional cleaning contractors for this reason.

CT: Do you recommend specific tools to a contractor to help thwart the possibility of cavitation—e.g., a particular type of pressure gauge or filters?

Paulding: The best tool is a five gallon bucket. Measure the amount of time it takes to fill the five gallon bucket from your water source to see if you are going to get sufficient water for the pressure washer you plan to use from that water source. Use heavy duty oversize garden hoses that will provide sufficient water from the source to the pressure washer and will resist kinks that can cause restrictions in the water line. Use oversize fittings. Clean your filters regularly to prevent clogs or flow restrictions. Use your ears. If your pressure washer makes a hammering sound, or the pressure is surging, you are probably cavitating your pump and you need to stop and figure why rather than proceeding with the job.

CT: What should we have asked you?

Paulding: Many contractors know about cavitation and the things to do to avoid it. The problem is that their workers may not, or may not care. See this link to a video explaining cavitation at www.youtube.com/watch?v=2DZ8JFSXloU