Cavitation: Water Pump Killer

The tiny “bubbles” of cavitation from the water pump can kill the pump. Although you’ll never actually see them, you can see the cavitation damage that looks like termite eaten metal.

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That’s because they’re not really bubbles. As for cavitation in pumps, they are vacuums. Inside these tiny voids is superheated steam that can erode metal and crack plastic. Cavitation in pumps is usually not the fault of the water pump itself; instead it falls victim to other issues with the coolant and other components.

Water pump cavitation voids are generated by the movement of the pump impeller against the coolant. There are two types of pressure that determine whether cavitation in pumps will occur. The first is the vapor pressure which is related to the system pressure. The second is the pressure and suction generated by the pump.

For example, if you remove the radiator cap from a hot system, the coolant is suddenly above its vapor pressure and vapor is formed. The same can happen inside the water pump. As the wheel spins, its coolant is pushed one way and pulled the other by the blades. If the negative pressure is large enough, vacuums form. Superheated steam inside these voids can damage the turbine and housing.

Condition of coolant and cavitation in pumps

Coolant condition has a direct relationship to cavitation in pumps. The specific gravity of the coolant depends on the dilution of the coolant with water. If the specific gravity is too high, it will change the operation of the pump.

Up to a 70% mixture of antifreeze can be used in extremely cold climates to lower the freezing point of the coolant, but the tradeoff is reduced cooling efficiency because ethylene glycol transports heat less efficiently. If the concentration of antifreeze in the water is too high in hot weather, it can increase the risk of engine overheating and make cavitation in the pumps more likely to occur.

Coolant additive packages contain wetting agents or surfactants that reduce surface tension and allow the coolant to transfer heat more efficiently. Diesel coolant includes additives that prevent cavitation erosion from the water pump around the cylinder liners.

The new coolant includes buffers that can control its pH. Acidic coolant cavitation is even more damaging to plastic composite wheels because the vapor is now acidic.

Water pump cavitation culprits

The health of the overall cooling system can contribute to water pump cavitation. If a system is not able to transfer heat efficiently, it will cause hot spots and increase pressures in the system. Leaks can also cause cavitation in pumps because the vapor point of the coolant is lower due to lack of pressure. This is why pressure testing is important.

Restrictions in the system can cause changes in pressure and vacuum which can lead to water pump cavitation issues due to the change in coolant flow over the impeller. Restrictions can be caused by a collapsed pipe or a clogged radiator. Installing a new water pump on a system with a restriction may limit the life of the new pump.

Signs of cavitation in water pumps

There is no outward visual evidence of water pump cavitation. Typical symptoms include overheating and possible leakage from the water pump weep hole. In some cases, the impeller will separate from the shaft. It is not until the pump is removed that the extent of the damage is revealed.

Cavitation in the pumps can attack not only the water pump, but also the part of the housing contained in the block, the front cover or the cylinder head. These issues must be addressed if the new pump is going to last.