The Vane Passing Syndrome Cavitation 1-3.5
Cavitation means that cavities or bubbles are forming in the liquid that we’re pumping. These cavities form at the low pressure or suction side of the pump, causing several things to happen all at once:
- The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration, and damage to many of the components.
- We experience a loss in capacity.
- The pump can no longer build the same head (pressure)
- The pump’s efficiency drops.
This type of cavitation damage is caused when the OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, lowering the fluid pressure and causing local vaporization. The bubbles then collapse at the higher pressure just beyond the cutwater. This is where you should look for volute damage. You’ll need a flashlight and mirror to see the damage, unless it has penetrated to the outside of the volute.
The damage is limited to the center of the impeller vane. If it’s a closed impeller, the damage will not extend into the shrouds. You can prevent this problem, if you keep a minimum impeller tip to cutwater clearance of 4% of the impeller diameter in the smaller impeller sizes (less than 14′ or 355 mm.) and a 6% clearance in the larger impeller sizes (greater than 14″ or 355 mm.).
To prevent excessive shaft movement, some manufacturers install bulkhead rings in the suction eye. At the discharge side, rings can be manufactured to extend from the walls to the impeller shrouds.