Hydrodynamic cavitation monitoring system

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July 2009 Sensors & Transducers

In continuous industrial processes, such as in the petrochemical or wastewater treatment sectors, flowing materials might cause various phenomena inside pipes, valves, or pumps that are damaging to the process equipment. One such phenomenon is hydrodynamic cavitation.

Cavitation occurs in a flowing liquid when the pressure of the liquid falls below its vapour pressure. It appears as vapour bubbles, formed as a result of the low pressure, and creates unfavourable conditions for materials that are exposed to it. Such material might be subject to erosion problems and damage.

Nonetheless cavitation is also useful for applications such as ultrasonic cleaning. Under the auspices of the UK National Physics Laboratory, scientists from the London-based Imperial College made use of their experiences in ultrasonic cleaning to develop a system for cavitation monitoring.

Cavitation sensor

The original sensor design was based on a cylindrical receiver, which is placed in the fluid to be monitored. A sensitive piezoelectric polyvinylidene fluoride (PVDF) film detects broadband acoustic signals produced by bubbles collapsing within the cylindrical volume.

Signals detected by the sensor can be monitored using a custom-designed analogue electronics module, which produces an averaged reading of the signals across a broad frequency range. The detection system has been used in commercial cleaning vessels and demonstrates good correlations with patterns of cavitation erosion produced on aluminium foil, showing that it relates to damaging forms of cavitation.

Applications

The detection and monitoring system is now being applied to pumps and flow systems using similar sensors, which are mounted externally. Early measurements have shown that high-frequency signals are detected that correlate with flow characteristics in which cavitation is expected.

The system should be able to detect cavitation occurring in new and existing installations. Frost & Sullivan expects that diagnosing cavitating systems can lead to design improvements, avoiding damage to equipment, and lowering operational downtime. There are also implications for energy consumption, since cavitating pumps do not run at optimum efficiency.

Development continues

Further development work continues in optimising the design of the external device. Simultaneously, researchers are seeking commercial partners to define market focused design.

For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)18 468 2315, [email protected], www.frost.com

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