Pumps are at the heart of most industrial processes and the second most common machine in the world (after the electric motor). Because they are so common, pumps are often overlooked as a potential source of improved productivity or a cause of excess costs if not operated properly.
As with any machine, some problems can occur. A common problem in pumping systems is pump cavitation. Pump cavitation causes a number of issues including excess noise, vibration and energy usage, not to mention serious damage to the pump itself.
What Causes Pump Cavitation?
Imagine pinching a water hose and the water moves faster coming out but it is aerated and spread out. That is what happens with pump cavitation. It is a physical phenomenon that occurs when the pressure of the liquid incoming becomes lower than the vapor pressure of the liquid. A reduction in pressure is typically caused by increased speed of the fluid. When fluid pressure on the trailing side of the impeller blade (opposite the pump intake) falls below the vaporization point of the fluid, vapor bubbles begin to form. As the bubbles/cavities travel to the discharge side of the pump, moving to a high pressure area, the cavities implode. The imploding or collapsing of these bubbles trigger intense shockwaves inside the pump, causing damage to the impeller, vibration and excess noise.
These shock waves can cause mechanical damage to the impeller and pump, only increasing in severity over time and leading to potential pump failure. Factors that impact the degree of compression for the vapor bubbles are the speed and shape of the impeller.
When dealing with liquids of higher temperatures, the risk of this occurrence is increased due to increased vapor pressure.
What Is the Difference Between Dead Head & Cavitation?
Pump cavitation and dead head are similar concepts but far from the same.
When a pump operates with no flow through the pump due to a closed discharge valve or line blockage, a dead head has occurred. The pump recirculates the same water, causing water temperature to continually rise. If the pump continues to run in a dead-headed condition for too long, excessive heating can damage expensive seals and reduce the life of the pump.
Dead heading in a centrifugal pump can lead to explosions due to the energy being put into the liquid in the pump. Hydraulic overpressure and possible chemical reactions in the pump can also be caused by the overexertion of pressure. The same results can be caused by running the pump dry for an extended period, which can lead to cavitation.
Dead head means the outflow valve is open so the pump continues to circulate the same liquid over and over, which can damage the pump motor because the liquid can get too hot. The low power protection will work to detect this condition and trip to protect the motor.
Cavitation is similar in that the low power protection will detect it and trip and protect the motor as well. But this condition means that the pump is running dry and no liquid is there to pump. That too will cause the motor to overheat.
How to Know if a Pump Is Cavitating
When dealing with pump cavitation, be aware that technology and techniques are available to detect, monitor and prevent pump cavitation.
Cavitation damage can shorten the life of the pump impeller, mechanical seals, bearings and possibly other pump components. In other words, it will reduce the mean time between failure (MTBF), which will increase maintenance costs and pump downtime.
The most common way to identify pump cavitation is through sound or vibration. There is an audible sound similar to crackling that can be heard when this is occurring. Due to the bursting of the vapor bubbles, there is increased vibration experienced by the pump, which can also be observed.
Other ways to identify pump cavitation are temperature or power detection at the outboard motor. Pump cavitation means there is little to no liquid to pump while the pump itself continues. Pumping dry in this fashion, with building vapor pressure, will cause the motor to overheat—reducing motor life and potentially causing damage to the impeller and the pump itself.
Alternatively, the outboard motor of a pump that is cavitating would experience a low power event.
Once an event is identified, it is important to identify the cause of the pressure drop and correct it. In most cases, the problem can be solved by simplifying the suction pipework, removing as many bends and valves as possible:
- Move the pump closer to the fluid source.
- Clean pipework and remove any blockage.
- Increase the diameter of the suction pipework so it reduces the inlet velocity.
Routine maintenance can be performed to avoid a future occurrence of cavitating. Suggested maintenance includes regular review and cleaning of the filters and strainers leading into the pipework.
How Does Monitoring Avoid Pump Cavitation?
Most industries use large, highly complex pumps. Failure of these critical pumps could result in downtime costs that exceed $200,000 per day in operations cost alone, not counting the cost of equipment replacement due to damage. Therefore, users tend to closely monitor these pumps. Since cavitation can implode a pump and result in a major safety risk, users are always seeking reliable solutions that allow them to predict and prevent it.
As the shockwaves from the vapor bubbles bursting continue, the temperature will increase to the outboard motor. Motor management relays monitor and trip based on the temperature thresholds. Protections can range from the incoming power source feeding the motor to the individual pump or load. Motor management relays provide a high level of monitoring accuracy and power protection for the entire power system while maximizing uptime.
Always listen to Ben Franklin: “An ounce of prevention is worth a pound of cure.”
By preventing cavitation, users will increase the efficiency and life span of the pump. Remember, prevention is worth the time, so take the time to carry out a thorough maintenance program and it will save users in the long run.
