The measurement of accurate flow rates is an integral component of many industrial and commercial processes. Flow meters are designed to measure the volume or rate of a transient fluid- either a gas or liquid- in a closed or open conduit. Whether they are required to measure fuel usage, precise quantities of ingredients in manufacturing or ensuring sufficient cooling of equipment, different flow meter technologies have been created to meet such variable working conditions. Nowhere is this more apparent than dealing with extreme temperatures. Not every flow meter is suitable for dealing with temperature boundaries. Flow meters that deal with extreme temperatures are required in petrochemical refineries, atmospheric installations, vacuum distillation units(VDUs), solar field heat transfer circuits and power blocks.
Which type of flow meter is considered as an extreme temperature flow meter?
The answer is Ultrasonic Flow Meters. This type of flow meter uses sound waves to measure the velocity of a liquid. The volumetric flow rate can then be calculated. The advantage of using ultrasonic flow meters is that they do not have movable parts. Therefore they are more accurate, reliable and provide maintenance-free operation.
It is well-known that ultrasonic signals are capable of penetrating solid materials; thus transducers may be mounted on the outside of the piping so that a completely non-invasive measurement takes place. This eliminates pressure restrictions, pressure loss and compatibility issues.
Ultrasonic meters are influenced by a fluid’s acoustic properties and are therefore impacted by density, viscosity, suspended particulates and temperature. If the fluid is homogenous and advanced signaling is used, many issues related to chemical liquid variations and noise can be eliminated.
When it comes to ultrasonic flow meters, there are two versions:
1) Transit Time
This type measures travel time of two simultaneous sound waves. One wave will travel in the direction of the flow and other will travel against the flow. During zero flow, both waves are received by the sensor (no transit time delay present). As the liquid begins to move, it takes the downstream wave a longer time to arrive at the upstream sensor. This “transit time difference” is proportional to the velocity of the liquid and to the flow volume. Transit time flow meters must be located in a full and closed piping system and be free from gas bubbles or suspended solids in order to operate efficiently.
This type of flow meter operates on the notion that the approaching wavelength of sound is shorter that the wavelength which is moving away; both originating from the same source. A transducer will emit sound waves which reflect off of bubbles or entrained particles and return it back to the transducer. The difference in wavelengths between the reflected signal and the transmitted signal is measured and deemed as the process’ velocity.
Doppler-shift flow meters are used for liquids with bubbles, gasses with sound-reflecting particles or slurries. They may be adapted for usage in open channels using a level transmitter.