Thread: Pump stator blowout
10-17-2010, 09:44 PM #1
Pump stator blowoutOriginally Posted by beerdart
10-18-2010, 12:41 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
Cavitation and Hydrolock damage in jet pumpsOriginally Posted by beerdart
I suppose if you had a damaged/bent/dinged impeller blade or stator vane, cavitation erosion could occur even with modest engine power.
An very high power PWC, both cavitation and hydrolock can damage stator and impeller blades.
Cavitation is the well known erosion of metal when the localized pressure of the water drops below the boiling point. While one face of the impeller blade is creating tremendous positive pressure against the water flow (which is what creates the thrust), the back side of the same blade is creating lower and sometimes negative pressure. Negative pressure is another way of saying it creates bubbles of vacuum within the water.
Those vacuum bubbles promptly collapse as soon as the water flows into an area of higher pressure. The bubble collapse is violent. Often the pressure rise that allows the vacuum bubbles to collapse is where the water flow meets the stator blade, and the violent cavitation bubble collapsing literally tears tiny chunks of metal from the surface. This metal erosion is the primary fingerprint of cavitation damage.
Water flow through a jet pump is very complex. The water does not simply flow in a straight line from before the impeller to after the stator. The impeller is spinning (obviously) and the blades have a complex curved shape. Water accelerates as it pass through the impeller.
Since water can not compress, differences in pressure caused by the blade rotation, hub taper and blade curvature create regions of higher and lower pressure, and the water flow spirals outward as it passes through the impeller.
At every point throughout the jet pump, the volume of water per second passing through is exactly the same. It must, since the water cannot expand nor contract. The impeller hub is a tapered shape. That means the water must accelerate as it passes the hub, just to maintain the same volume per second.
The water nearest the outside of the impeller is under tremendous force, as this is where the bulk of the thrust is imposed by the moving blades.
When that accelerated and highly pressurized water leaves the trailing edge of the impeller blade, it promptly encounters the leading edge of the stator vanes. When the engine power is high enough the jet pump design (mostly the impeller shape and spacing between impeller and stator) can create an extremely high water pressure pulses where the water hits the stator vane leading edge. This is a form of dynamic hydrolock (hydraulic locking).
As the impeller blade trailing edge passes by the stator vane, the water pressure can spike high enough to tear small chunks of metal from the vanes. The pressure is not relieved by flowing elsewhere since everywhere else there is also water.
Effectively there is a water pressure pulse or spike every time each impeller blade passes each stator vane. If the pressure pulse is strong enough, it can damage the metal. It blows out the metal that is in the way of the water pulse.
10-18-2010, 01:24 PM #3
I find it hard to believe that a 650 would have the power to cause this type of failure, but regardless, on a Polaris, the fix is easy...Install an OEM stainless 6 vane stator...end of blown stator vanes. The Yamis have crappy aluminum stators, with no cheap stainless replacements available. An aftermarket Skat pump is over $1000.
10-18-2010, 10:04 PM #4
I wonder if any Polaris stators have had anything like this going on? Like a Modified race pump or a set back pump. 6, 8, 12,14 vane? I wonder what kind of horsepower it would take in a Pro785 to rip apart a vane SS stator. That sounds like fun.lol
10-20-2010, 07:50 AM #5
10-20-2010, 08:21 AM #6
In James's 1st post, there a link to another thread on this subject.. aren't those stainless steel pumps that are getting the blades broken off? Hard to tell in the picture.. although they are pushing way more HP.
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