Pumps & Systems, May 2007
If pump cavitation is not detected and quickly eliminated, significant damage can occur to the impeller and other internal components. Since cavitation (the formation and collapse of vapor bubbles in the pumped fluid as it passes through the pump impeller) is often temporary or even induced by the process, timely knowledge of the condition is extremely important so operators can act to alleviate the problem and prevent damage.
Intermittent pump cavitation can go on for weeks without detection, because actually knowing when a pump cavitates can be difficult and thus hard to correct, unless someone is near enough to hear those unmistakable sounds. However, only by reducing the time a pump is in that condition can operators extend its life and minimize repair costs.
Many times, pump cavitation can be corrected by adjusting the flow or a tank level. In other cases, it can be eliminated only by changing the piping or pump design.
Essentially, cavitation occurs when the net positive suction head (NPSH) pressure at the pump inlet falls below the fluid's vapor pressure, which is the minimum pressure required to keep a liquid in that state and prevent it from turning into vapor. In simple terms, the NPSH is the actual pressure that the fluid experiences within the pump. If the NPSH falls below the vapor pressure for that liquid, vapor bubbles form. As the bubbles flow through the impeller, the pressure on the fluid increases to a point above the fluid's vapor pressure, causing the bubbles to collapse violently. This action attacks the surfaces inside the pump, and over time results in significant pump damage, such as loss of hydraulic performance due to impeller wear.
Anyone standing beside a cavitating pump can fairly easily distinguish cavitation from other faults, because it actually sounds like rocks going through the pump or popcorn popping. However, plants with large numbers of pumps in critical applications can't position personnel near them all in order to identify intermittent cavitation, so an alternative way to detect cavitation is by monitoring vibration levels.
Role of Vibration
Along with making that loud popping sound, the collapsing vapor bubbles create certain vibration patterns - and the vibration analysis rules for detecting cavitation are fairly well understood. The ability to detect high frequency energy at a pump and determine whether this energy is increasing or decreasing is one key to recognizing cavitation. If the high frequency energy is increasing, the cavitation is getting worse, and the impeller and seals are probably sustaining damage. Increasing vibration can also place added stress on the pump bearings, causing accelerated wear on these components as well.
Figure 1 shows vibration data collected from a pump experiencing cavitation. The similarity in the data on both pump bearings should be expected, as pump cavitation vibration tends to be present at both bearings, whereas bearing problems are generally isolated on one specific bearing. If energy has increased on just one of the bearings, the vibration is probably related to a bearing fault or lack of lubrication and not the result of cavitation.
The Key is When
Cavitation can be somewhat transient since it is frequently process dependant. Changes in the flow rates, tank levels, and valve positions can all contribute to cavitation within a pump. However, the hard part of detecting cavitation is catching it when it happens. Operators can only do something if they hear that sound when walking by a pump, or by capturing vibration data when the pump is cavitating. Telltale vibration readings can be very elusive when data is collected only once every 30 days or so. A pump could be cavitating for several days within a week, but due to changing process conditions, may not exhibit that condition at the moment a technician comes by to collect vibration data.
What many companies need is a permanently connected device capable of continuously collecting and analyzing vibration data right there in the field. Such a device can scan for changes in a pump's vibration signature as a sign that the unit is cavitating, and then issue appropriate warnings for operating and maintenance personnel.
In fact, automated monitoring of pumps already exists. Software embedded in a machinery health monitoring device collects vibration data continuously and analyzes the data results automatically. The use of embedded analysis logic of the type portrayed in Figure 2 enables the monitoring system to do more than just present high vibration readings, which do not really tell an operator what he or she needs to know to alter the process.
However, a smart machinery health device can deliver a message indicating that a pump is experiencing cavitation right now (see Figure 3), which really empowers that operator to correct the problem immediately.
Additional details can also be provided about what to do while the condition is occurring in order to alter the circumstances that cause the pump to cavitate. By following these suggestions, an operator can extend the life of the pump and minimize repair costs.
This is a real-life solution that enables operators to make positive changes in the process condition to eliminate the cavitation. It may involve opening a valve, which affects the NPSH, or adjusting the process fluid's temperature, which affects the vapor pressure. In certain cases, process design changes may even need to be addressed to eliminate the recurrence of cavitation.
A smart machinery health device can continually measure the operating parameters of key motor-pump machine trains in the field and report a consolidated analysis based on current operating conditions. This technology is capable of making in-the-field assessments and automatically warning plant personnel whenever the possibility of cavitation exists.
Automated diagnostics packages can augment the work of maintenance personnel by giving the analysis directly to operators in time for them to make appropriate adjustments. When combined with information from other sources, such as lubricant analysis and infrared imaging, a true picture emerges of the operating condition of an asset and its potential for failure.