Although viewed by some as a questionable principle, ultrasonic Doppler meters have been used to good effect by many in the correct application. As non-invasive clamp-on meters their portability and ease of use makes them desirable. What, you may ask, is the correct application? In order to answer this question we need to discuss the principles briefly.
Frequency shifts
Doppler flowmeters operate by transmitting a continuous ultrasonic wave inclined at an angle into the flow. The sound is ‘scattered’ from a core group of particles in the fluid and is received back by the instrument. The difference in transmitted frequency and received frequency is measured. This frequency difference is directly proportional to the velocity of the particles. The Doppler flowmeter is essentially then, a velocity measuring flowmeter. Once the instrument has processed this data, the area of the pipe is multiplied with the velocity of the particles in the fluid to produce the volumetric flow value.
Operational guidelines
Sensors must be fitted in a manner which excludes air from the space between the sensor and the pipe surface. This surface needs to be clean and free of loose coatings, which might trap air; coupling grease such as silicone is also required. The temperature of the fluid needs to be in the operating range of the instrument. The sensor needs to be strapped on tightly to form the best contact between sensor and pipe surface. Welding seams are to be avoided in sensor placement. The flow profile must be considered as disturbed flow conditions will cause the source particles to either move faster or slower than the actual flow of the carrier fluid. Bends, valves and reducers need to be as far as possible from the measuring site (a rule of thumb is 10 pipe diameters upstream and five downstream of the site).
Application
By understanding the operating principle and following standard flow measurement ‘rules’ on applications that fall within the manufacturers’ specification, many industry operators are easily and quickly solving their flow measurement challenges. Industries where good application examples commonly exist are mining (slurry processing), pulp and paper mills, sugar mills, juice and food processors and water and wastewater treatment. At a relatively low cost a locally produced instrument becomes a sound prospect indeed.
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