The Theory Behind Ultrasonic Flowmeters
End users and flow metering specialists all over the world utilise non-invasive techniques to establish flowrates utilising clamp on flow meters or ultrasonic flowmeters. The two most common technologies used are that of transit time (or time of flight) and doppler. Transit time clamp on meters use the difference in time for an ultrasonic pulse to travel with the direction of flow and against it. When the velocity is zero, the transit time of an ultrasonic pulse is the same in both upstream and downstream directions. With flow, the upstream sonic pulse will travel at a slower rate than that of the downstream pulse. When flowrates increase further, this transit time differential increases linearly. As the cross-sectional area is already known, the product of the velocity (transit-time differential) and area equates to a volumetric flow. It is widely accepted that transit-time flow measurement devices are more accurate than clamp on doppler equivalents.
Unlike doppler flow meters which require entrained particles or solids, transit time ultrasonic flow meters cannot measure velocities accurately as particles interfere with the ultrasonic path. Whereas doppler flowmeter have a combined transmission/receiver unit, transit time flow meters have two which can be configured in different ways to suit the measurement application.
As ultrasonic technology does not obstruct flow, there is no pressure loss or indeed any requirement to break into the pipeline to obtain flowrates. It is therefore possible to use clamp-on transducers to measure liquids with corrosive and abrsaive properties without having to used wetted transducers. Permanent or temporary flowmeters can be used in all situations where suitable pipework can be exposed or utilised. In theory, the minimum distance required to measure liquid flow is 10 diameters upstream and 5 downstream from the nearest pipe disturbance but this should be increased where there are significant numbers of bends or fittings or where bends are in two planes. Where suitable distances of straight pipe cannot be located, mult-path devices can negate the effects of the disturbed flow.
Transducer Setup
The basic configuration is the 'Z' configuration where the transducers are placed on opposite sides of the pipe (typically between 3 and 9 o'clock), with one at a set distance downstream from the other. This distance is generally half the diameter of the pipe but can vary according to pipe wall and lining materials. The 'Z' configuration is generally only used for difficult situations where signal strength is poor (high turbidity or poor pipe condition), typically dirty abstraction water or sewage. For most applications the 'V' traverse configuration is used, whereby the transducers are located on the same side of the pipe at a designated space apart. The 'V' configuration has many advantages over the 'Z' configuration including, increased sonic path length, less inaccuracies of spacing and locating sensors, and shorter setup times. A third configuration known as the 'W' setup involves a double reflex sonic path but is only used on smaller pipes (below 2" or 50mm) to increase the path length.
Transducer Types: Wetted and Dry
Dry clamp-on transducers can be mounted on the outside of any pipe in reasonably good condition: clean, smooth and with good sonic properties - typically steels, plastics and irons. A sonically conductive couplant is also required to provide a sonic bond between the exterior of the pipe wall and the transducer.
Where it is not possible to locate clamp on transducers, spool pieces or wetted transducers can be installed. Wetted transducers can be installed by drilling and tapping the pipe at the point of measurement. Although the pipe may have to be drained, signal quallity and hence accuracy are increased. However, the associated installation and purchase costs of wetted transducers & spool pieces can be much higher than dry clamp on devices.
Applications for Ultrasound including Gas
One of the reasons for the rapid development and success of ultrasonic flowmeters is that they are suitable for the measurement of most liquids including water, hydrocarbons, corrosive and abrasive liquids. The only prerequisites for successful measurement are that the pipe is full, the media conducts sonic energy and that the pipe wall is in good condition. Doppler meters also require there to be enough entrained material to reflect the sonic transmission off. Whereas transit time flowmeters work well between 0-5%, doppler flowmeter work well in the 0.1-10% band.
Clamp on technology has been extensively developed to measure gas applications. Gas clamp on flowmeters from Panametrics and Controlotron are now capable of measuring mass gas flow in pipes above 1" with minimum pressures of 6 bar.