HOW CAVITATION MURDERS PUMPS
It differs little if it is a pump on an engine, a hydraulic system, or any other application. Fact is, a broken pump can bring your entire operation to a standstill. Though pumps can fail due to age and use, the reality is that most are murdered, snatched from their prime with much life left in them. The culprit is cavitation, and it sends warning signs (excessive vibration, hammering, groaning, and whistling) but most are ignored.
Pump cavitation describes the formation of bubbles or cavities in the bulk fluid being moved that usually develops around a low-pressure area. This is the result of either an entrained gas in the liquid from the vapor pressure being exceeded or from a lack of flow. When the vapor bubbles collapse or implode, they strike at the speed of sound, creating the noise and vibration. This collision will erode the surfaces of the pump and impeller, and will attack the bearing, shaft alignment, and seal.
When you examine a failed pump, you will notice an appearance that resembles a sponge-like texture or missing material. Depending on the pump design and operating characteristics, the bearing may fall victim first, allowing excessive shaft movement and a collision of the impeller with the housing. Minor cavitation will result in decreased output or pressure. It is imperative that you are cognizant of the sound and pressure/flow characteristic of your pumps. Cavitation caught in the early stages will have minimal to no impact on pump life.
types of cavitation
Suction side cavitation occurs when a pump is under low pressure or excessive vacuum. The pump is being starved of liquid and is not being fed enough flow. At that time, bubbles form at the eye of the impeller (where it connects to the shaft). As these bubbles move over to the discharge region, the fluid condition is altered and the bubbles are compressed into a liquid, causing them to implode against the face of the impeller. An im- peller subjected to suction side cavitation will have pieces of material missing.
Discharge cavitation is the result of the discharge pressure being extremely high, so that it is difficult for the liquid to vacate the pump. It then circulates around the impeller and housing causing a very high vacuum at the wall and the formation of bubbles. Discharge cavitation allows the imploding bubbles to create intense shock waves, removing material from the housing and impeller. In extreme discharge cavitation cases, the impeller shaft may even break.
The most common cause is a flow restriction or running the pump at a speed that is out of its operating range. The flow issue can be either on the suction or pressure side, or a cumulative effect of both. Proper and timely maintenance of filters and screens goes a long way in preventing cavitation. Keep in mind that on a sprayer or other application with aged rubber hoses, they can be collapsing slightly and limiting suction performance and evoking cavitation.
Plumbing design such as pipe diameter and the amount of turns and the sharpness of them will potentially create either suction or discharge cavitation. You may have upgraded to a larger pump and now the factory piping cannot support it. No fluid likes to make turns; this will result in a flow restriction, both on the feed and discharge sides.
If a pump does fail, you need to take it apart and determine if it was the result of cavitation. On an engine, the coolant (water) pump seal can fail prematurely if the rpm is brought too high while the thermostat is closed. During that time, the coolant is being forced through a small bypass hose or passage. Excessive engine speed even under no load will cause the suction side of the pump to experience a very high vacuum and, over time, violate the pump seal and leak from the weep hole. On any engine, the rpm should be moderated when the coolant is below the temperature of the thermostat opening point.