Pump ‘Cavitation’ Might Not Be
Jane Alexander | October 10, 2017
The word “cavitation” should be quite familiar to those who work around pumps.
As Stan Riddle of VibrAlign Inc. (vibralign.com, Richmond, VA) points out, it’s often used as a catch-all term for any flow-related pump vibration. It’s also frequently reported by vibration analysts when:
• pump noise level is high
• pump vibration is erratic, with lots of noise in the spectrum.
Riddle cautions, however, that of the several flow-related pump-vibration problems, only one is truly cavitation. All of those problems, though, share similar vibration characteristics that can lead to inefficient pumping systems and, if severe, damaged pumps. Given the fact that different solutions are required for dealing with them, it’s important to determine if a pump is truly cavitating or experiencing another issue.
In a recent blog post on VibrAlign’s website (Aug. 2, 2017), Riddle discussed the various causes of pump noise and how to distinguish them—and the problems they reflect—from each other.
True cavitation occurs when “cavities,” i.e., low-pressure vapor bubbles, form in pumped liquid on the unit’s suction side. This can be a result of:
• clogged suction strainers or plugging on the suction side
• poor piping design
• a pump that’s running too far to the left of its curve—a condition becoming more common with the increasing popularity of variable-frequency drives (VFDs).
When the bubbles get into the impeller section, the pressure balances and they implode, often onto the impeller surface. This can damage the impeller and housing. Cavitation is often very loud, typically sounding like gravel going through the pump.
Aeration, or Air Entrapment
Air entrapment happens when bubbles appear in a pumped liquid before it reaches the impeller. This can happen when:
• the liquid falls into an open tank system, or is aerated near the pump inlet, due to the liquid level being lower than the top of the pipe inlet
• de-foaming agents aren’t or can’t be used
• liquid is near its boiling point, as in a condensate pump.
While air entrapment isn’t always as serious (or as loud) as cavitation, left unchecked, it can damage an impeller.
Recirculation is most often caused by low flow. Two types can occur:
• Suction recirculation is reversal of flow at the impeller eye, causing low-pressure bubbles to form, very much like cavitation. This type of recirculation is just as damaging to the suction side of the impeller.
• Discharge recirculation is reversal of flow at the discharge side of the pump and can also damage the pressure side of the impeller.
Determining the actual cause
Vibration analysis is good at detecting flow-related pump problems but, by itself, may not be capable of distinguishing, if at all, the actual cause of a noisy pump. A method for determining the type of problem is to slowly throttle the discharge valve closed. As the valve is closing:
• if noise and vibration improve, expect cavitation
• if noise and vibration stay the same, expect air entrapment
• if noise and vibration worsen, expect recirculation.
Throttling the discharge valve may seem like a straightforward fix, but it is merely a diagnostic technique. If any of the above issues are occurring, you have either an operational or design problem—which must be addressed by engineering. EP
For more information on vibration and other plant-floor issues, visit VibrAlign’s “Long Live the Machine” blog at vibralign.com/blog.