Driving away bubble trouble

24-Sep-2021 - Last updated on 24-Sep-2021 at 09:01 GMT

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Brith Isaksson, ABB global segment manager food & beverage, told Dairy Reporter the causes of cavitation and explains how it can be prevented with the latest variable speed drives (VSDs).

Pumping systems used in the dairy industry to transport raw and finished liquid products can sometimes be affected by the troublesome issue of cavitation. It reduces the life of equipment, requiring expensive maintenance and replacement, it can also damage milk-fat globules, resulting in reduced product quality.

Cavitation happens when the pumping system subjects a liquid to rapid local changes in static pressure, creating bubbles or voids. The point when this liquid-to-bubble transition occurs is when the pumped liquid falls below its vapor pressure.

For example, at normal atmospheric pressure, liquid water turns into water vapor (steam) at its 100°C boiling point. But as the pressure is reduced, then so is the vapor point. The transition can even occur at room temperature should the pressure fall to a vacuum. In addition to milk and water, the same transition applies to some other fluids with similar characteristics such as during a clean-in-place (CIP) process.

Dairy plants mainly use centrifugal pumps. As the pump impeller spins, high pressure is created on the front side of the blades. At the same time, there is low pressure on the back of the blades. In some conditions, the liquid vaporizes, creating bubbles as shown in the main image.

When these vapor bubbles reach areas of high pressure, they collapse. The implosions produce shockwaves. This creates a characteristic rumbling or cracking noise that sounds like rocks passing through the pump.

Isaksson said the cumulative effect of many tiny implosions can over time have a significant impact on the pump performance well as affecting product quality. Cavitation will eventually damage the pump impeller, housing and other pumping system components through wear and metal surface fatigue.

Cavitation can reduce pump life by as much as 50%, and in the most extreme cases it might destroy a pump in minutes. Since surface fatigue causes metal particles to be released from the impeller blades, product safety and quality can also be at stake, Isaksson added.

To avoid cavitation, system designers and maintenance engineers need to look at ways to detect the onset of cavitation and modifying the pump operation accordingly.

Isaksson said one possibility is to use standalone sensors to monitor the changes in pressure that accompany cavitation. However, there is now a more cost-effective and straightforward option. That is to use the extended capabilities of the new generation of intelligent VSDs.

In addition to the benefits of energy efficiency, some VSDs, such as ABB's industrial drives, now incorporate anti-cavitation software. This makes it possible to prevent cavitation without the additional cost and complexity of external sensors.

Algorithms that measure pump torque and speed are incorporated into the dedicated anti-cavitation software. This enables the VSD to monitor the pumping process constantly for the specific patterns that indicate cavitation is taking place, Isaksson said.

There is no latency in detection because the measurements are taken directly from the pump shaft. That means the response is virtually instantaneous. When it detects cavitation, the VSD adjusts the pump speed automatically to react to the change in pressure. It will then resume normal operation as soon as the pump has stopped cavitating.

Anti-cavitation operation is especially beneficial for the dairy industry as it enables liquids to be pumped at the optimum flow rate. At the same time, the pump can adjust automatically and immediately to any change in the flow or draining vortex which might cause cavitation to occur. For example, this enables a tank to be pumped out quickly and efficiently, right to the bottom, without causing bubbles and foam that damage product quality.

Pump cavitation is a significant risk for the dairy industry. It can damage the finished product, cause significant disruption to pumping operations and may even require pumps to be repaired and replaced.

With the new generation of intelligent VSDs, Isaksson said, it is possible to address cavitation issues locally, within the drive, in real time. There is no need for sensors and additional controls. The only additional work required is to set the operating parameters.

The result is precise, instantaneous control that keeps pumping systems operating with maximum efficiency and reliability while preserving the quality of the finished dairy product.

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