Previously we discussed the top 6 pump vibration problems, with cavitation as the most prevalent. Now it seems appropriate to discuss cavitation further including its causes and prevention methods. In short, pump cavitation is the formation of cavities or bubbles in the pumped media that develop in areas of low pressure around the impeller. When these cavities collapse, they create shockwaves inside the pump that can significantly damage the pump housing or even the impeller.
The good news is that many forms of pump cavitation are accompanied by symptoms that are easily detected. They include but are not limited to:
Excess noise such as popping, crunching, rumbling, or the sound of marbles rolling
There are many types of pump cavitation, but the four major ones include:
Suction Cavitation – This type of cavitation is caused when the fluid’s turbulence releases entrained gases into the suction piping. This often occurs when a pump is under low pressure or high vacuum conditions. The pump does not receive enough flow, which allows cavities or bubbles to form at the eye of the impeller. As they carry over to the pump’s discharge side, the bubbles are compressed causing them to implode against the impeller.
Vaporization Cavitation – This occurs when the liquid within the pump vaporizes as the result of a loss of suction head or when the pressure becomes too low or too high. All centrifugal pumps have a required suction pressure operating range to prevent vaporization. However, this requirement is often calculated with the assumption that it operates with fresh water at 68 degrees Fahrenheit. When this is not the case, vaporization can happen.
Vane Passing Syndrome – When the OD of the impeller passes too close to the pump cutwater, it can cause damage. The liquid velocity increases as it flows through the small passage, lowering the pressure and causing vaporization. Bubbles are formed by this collapse just beyond the cutwater. Damage is typically limited to the center of the impeller’s vane and does not extend into the shrouds on a closed impeller.
Discharge Cavitation – If the discharge pressure of a pump is too high or runs at less than 10% of its best efficiency point, it can cause discharge cavitation. The high pressure can keep fluid from flowing out of the pump, instead causing it to circulate in the pump. The resulting vacuum at the housing wall supports the formation of cavities or bubbles, which cause shockwaves when they implode.
There are many ways to prevent and treat pump cavitation, some of which include:
If you have a pump that is showing signs of cavitation, contact us. HDS operates a modern repair service center that is strategically located in southeast Houston and is capable of handling all your repair needs.