Cavitation is one of the biggest problems faced by pump owners. Here, in this guide, we show you how to identify and diagnose cavitation troubles so that you can keep your pump operating smoothly for years to come.
1. What is cavitation?
To explain it in the simplest terms, cavitation is used to describe the presence of air, vacuum, or bubbles in a liquid. When a flowing liquid goes below its vapour pressure, it leads to the formation of vapour bubbles. When the fluid moves through a high-pressure chamber, these bubbles explode, causing damage and cavitation.
2. What are the types of cavitation?
Pump cavitation can be classified into two types – inertial and non-inertial.
- Inertial cavitation – Also known as transient cavitation, it is the process when the bubble/void explodes quickly, resulting in a shock wave.
- Non-inertial cavitation – It occurs when the bubble oscillates in shape and size but doesn't explode.
3. What are the causes of cavitation?
The primary cause of cavitation in a pump is due to low NPSH (Net Positive Suction Head) on the suction end of the pump. The NPSH is the energy that pushes the liquid via the pump. When the NPSH falls down, cavitation occurs.
There are several reasons for a drop in NPSH from the expected values. The various scenarios are:
- When the diameter of the suction pipe is too small
- When the suction pipe is at a long distance from the pump
- When the pump is positioned at a high place above the fluid source
- When the displaced liquid has a low vapour pressure
- When the pump operating speed is too fast
Now that we've seen the various causes of cavitation let's take a look at the consequences.
4. What happens when cavitation occurs?
There are several possible consequences.
- Reduction in the pumping capacity of the pump. When the pump’s speed drops, it cannot keep up with the incoming flow, leading to an overflowing situation.
- Damage to the pump. Cavitation can cause excessive vibration, which leads to excessive wear and tear of
the impeller, wear rings, wear plates, etc.
- Premature failure of the bearings and seals.
When cavitation is not rectified, it damages critical pump components, thereby severely reducing the pump's life.
5. Are there any warning signs to look out for?
Yes, there are specific warning signals that help you identify cavitation. When the pump experiences cavitation, you can notice excessive vibration than usual. Additionally, the pump will emit loud noises that sound similar to marbles rattling inside the pump. The sound generally starts low and increases in intensity as cavitation wears down the various parts inside the pump.
6. Is Cavitation same as Air Entrainment?
No, they are a different phenomenon. Let's understand the differences between both.
|This occurs when air enters the pump from the suction end and moves to the impeller eye.||Cavitation is the presence of air, vacuum, or bubbles in a liquid.|
|It reduces the flow of the pump.||It causes a reduction in the capacity of the pump.|
|It causes some amount of damage to bearings and seals.||It causes excessive damage to bearings and seals.|
|Easy to identify and fix.||Difficult to identify and rectify.|
One interesting point to note here is that one way to rectify cavitation is by purposefully injecting small amounts of air into pumps. By injecting air into a cavitating pump, we can prevent the air bubbles formed due to cavitation from exploding. This effectively reduces the severity of cavitation.
However, we do not encourage customers to attempt this remedy on their own, as too much air inside the pump can lead to other problems like priming and a further reduction in pump capacity.
7. What are some best practices to prevent cavitation?
Keeping track of the NPSH values can help you avoid cavitation. Calculate the NPSHa (Net Positive Suction Head available). Compare this value with the NPSHr (Net Positive Suction Head required). To avoid cavitation, the NPSHa should be one or two feet more than the NPSHr.
Remember that the NPSHr is determined by the pump's design, and it cannot be altered. The NPSHa, on the other hand, is dependent on the system parameters and can be altered. The NPSHa value is determined by several factors like:
- Steady atmospheric pressure – the atmospheric pressure is dependent on the altitude. At higher elevations, the pressure is less, and there is insufficient energy to push the liquid.
- Vapour pressure of the pumped liquid – depends on the viscosity and temperature of the pumped fluid. If the liquid is at room temperature, it can be pumped easier than high-temperature liquids.
- Static height between the pump and water level – By moving the pump closer to the liquid level, you can increase the NPSHa.
- Friction losses – Friction losses can be reduced by using pumps of large diameters.
By altering these factors, the pump user can increase the NPSHa closer to the NPSHr, thereby preventing cavitation.
8. Can a user use devices to diagnose and effectively eliminate pump cavitation?
The easiest and most effective solution is to listen to the pump’s flow. The flow can be easily tracked by using flowmeters. Flowmeters are readily available in the market. Choose the right one based on the type of pumped liquid.
Alternatively, trained pump technicians can also listen to the pump without using an external device. These technicians can spot changes in vibrations, thereby diagnosing cavitation.
9. Which pump applications are more susceptible to cavitation troubles?
Cavitation is most common in pump applications that have high suction lifts and little-to-no discharge heads. For example, pumps used in bypassing sewage from manholes are among the most common occurrences of cavitation. Another example is pumping heated liquids that are close to their boiling points.
10. Can you suggest the best way to avoid pump cavitation?
The easiest way to avoid cavitation and a lot of other pump troubles is to use the right pump best suited for your application. At Sintech, leading industrial pump manufacturers and exporters in India, we have a wide array of pumps customised for various industries.
To find the best pumps for your application, get in touch with our pump experts today.
Cavitation is an Avoidable Outcome
With the right know-how and understanding of the pump, you can easily avoid cavitation. Use the tips listed here to prevent this trouble, and ensure the long life of your pumps.