Vibration – It’s not always Cavitation
Excessive vibration is a sure-sign indicator that something is wrong with your pump. Find out how to identify if the vibrations are due to cavitation or indicate other troubles with the pump system.
Vibration is a common occurrence in all pumps. However, it becomes a problem when it goes beyond normal levels. A common question raised by pump owners is, "Why is my pump vibrating so much? Is it because of cavitation, or is it something else? How do I correct it?”
There are many reasons for vibrational troubles – you must identify the underlying problem so that you can correct it. Before we look at the potential causes of excessive vibration problems, let's understand why vibration occurs.
Why do excessive vibrations occur in pumps?
Vibration in all machinery – including pumps – arises when there is an interaction between the hydraulic or mechanical force and the associated hydraulic/structural resonance frequencies.
Generally, vibration troubles arise in new installations due to improper installation i.e. misalignment of pump and motor coupling, tension of pipeline on pump's suction/discharge nozzles, pump installed at lower head than rated etc. However, they may also occur on existing installations due to changes in the system like – the addition of a new variable speed drive, changes to the pump piping, etc.
How do I know if the excessive vibration is because of cavitation?
Most pump engineers and operators are familiar with the term cavitation. However, it has come to become a catch-all phrase to denote all vibration-related issues occurring in a pump.
Generally, pump operators assume a cavitation issue, if the following signs occur:
The noise level emitting from the pump is high
The vibration in the pump is erratic with plenty of noise
However, there are several reasons for flow-related troubles and excessive vibration in a pump. Cavitation is not always the culprit. All these problems cause excessive vibration and reduce the efficiency of the pump.
How to identify the root causes of vibration in a pump?
Let’s take a look at how to identify the reasons for excessive vibration and prevent it:
The vibration is due to Cavitation, If:
The vibration is quite loud and feels like gravel being thrown around the insides of the pump.
Cavitation occurs when "cavities – low pressure bubbles" form in the displaced liquid at the pump's suction end. Cavitation can occur due to:
Poor design of the pipes
Clogged suction end
Pump operating far too left to the pump curve – resulting from VFDs (Variable Frequency Drives)
When the low-pressure bubbles enter the pump's impeller end, it breaks/implodes due to a pressure imbalance. The breaking bubbles rise to the surface of the impeller damaging the housing and impeller.
The vibration is due to Air Entrapment (or Aeration), If:
The vibration is not excessively loud.
Air entrapment occurs in the pump when bubbles are created in the liquid before reaching the impeller. It occurs when,
Defoaming agents are not used
When the liquid falls inside an open tank system
The liquid level is lower than the top of the inlet pipe
The displaced liquid is near its boiling point
Air entrapment is not a severe issue like cavitation. However, it can certainly damage the impeller, if not rectified.
The vibration is due to Re-circulation, If:
You notice a reversal of flow at the impeller/discharge end.
Re-circulation is a result of low flow. It can be categorised into two main types:
Suction Re-circulation – the flow gets reversed at the impeller's eye, leading to the formation of low-pressure bubbles. The issue is similar to cavitation and causes significant damage to the impeller.
Discharge Re-circulation – – the flow gets reversed at the discharge end of the pump. Similar to suction re-circulation, but the damage here is on the pressure end of the impeller.
Keep Track of Vibrations to Monitor the Health of the Pump
Vibration analysis is an effective method to determine if your pump is operating as intended. If you notice any of these troubles occurring in your pump or notice excessive vibrations all of a sudden, make sure to call the pump engineer to rectify the issue before it leads to major consequences.
If you need help troubleshooting any pump issues, reach out to our team of top pump suppliers in India at Sintech, India's leading industrial pump manufacturer and exporter. We offer extensive after-sales support to our customers to help them keep their pumps in top shape for years to come.
Easy Method to Determine the Cause of Excessive Vibrations
While not accurate, this method can act as a first step in troubleshooting the underlying issue. Slowly throttle the discharge valve. Close it and notice what happens:
If the noise and vibration reduce – then, the problem is mostly cavitation.
If the noise and vibration remain the same – then, the problem is due to air entrapment.
If the noise and vibration get worse – then, the problem is due to re-circulation.
Note that this method is not an accurate solution. It should be used just for the initial diagnosis. For a detailed understanding of what happens, you need to call in the pump engineer.