Introduction
There are two types of ultrasonic flow measurement:
– Transit time measurement
– Doppler effect
The fundamental difference is that the transit-time method should be used for clean fluids, while the Doppler reflection type used for dirty, slurry type flows.
1.Transit time
The transit-time flowmeter device sends pulses of ultrasonic energy diagonally across the pipe. The transit-time is measured from when the transmitter sends the pulse to when the receiver detects the pulse. Each location contains a transmitter and receiver. The pulses are sent alternatively upstream and downstream and the velocity of the flow is calculated from the time difference between the two directions.
Installation Techniques
Designs are available that allow installation and removal of the transducers without interrupting the process flow. However there are three main options available:
– Fitted section of pipe
– Clamp on
– Transducers installed in-situ
The first option is where the manufacturer supplies a section of pipe fitted with the transducers factory mounted. These units have the advantage of being calibrated by the manufacturer to meet specifications. Allowances need to be made when fitting this section of pipe – something that can complicate existing installations.
Clamp-on transducers have the added advantage of being easy to install. They are mounted outside of the existing pipe. Since no section of pipe need to be installed then this type of flowmeter is easily retrofitted onto an existing system. They can be installed on metal, plastic and ceramic pipes
Because they are portable and non-intrusive, clamp-on devices provide a good means of determining flowrates of unknown flows in existing installations. A cheaper option is to install the transducers into the pipework. This does require tapping into the pipe and care needs to be taken to ensure the correct angles and tolerances are adhered to. This method generally requires calibration by the user once installed.
Application Limitations
Clamp-on designs are limited because of the differing mediums in which the ultrasonics signals pass through. For optimum results, a sound-conductive path is required between the transducer and the process fluid inside the pipe. Couplings are available for reducing these effects but are quite expensive.
Typical Applications
Transit-time ultrasonic flow measurement is suited for clean fluids. Some of the more common process fluids consist of water, liquefied gases and natural gas.
2.Doppler Effect
The Doppler effect device relies on objects with varying density in the flowstream to return the ultrasonic energy. With the Doppler effect meter, a beam of ultrasonic energy is transmitted diagonally through the pipe. Portions of this ultrasonic energy are reflected back from particles in the stream of varying density. Since the objects are moving, the reflected ultrasonic energy has a different frequency. The amount of difference between the original and returned signals is proportional to the flow velocity.
It is quite common for only one sensor to be used. This contains both the transmitter and receiver. These can also be mounted outside of the pipe
Application Limitations
As the Doppler flowmeter relies on reflections from the flowstream, it therefore requires sufficiently sized solids or bubbles. It is also required that the flow be fast enough to maintain a suitable quantity of solids and bubbles in suspension. Higher frequency Doppler flowmeters are available, but are limited to cleaner fluids.
Most ultrasonic flowmeters are mounted on the outside of the pipe and thus operate without coming in contact with the fluid. Apart from not obstructing the flow, they are not affected by corrosion, erosion or viscosity. Most ultrasonic flowmeters are bi-directional, and sense flow in either direction.
Advantages
– Suitable for large diameter pipes.
– No obstructions, no pressure loss.
– No moving parts, long operating life.
– Fast response.
– Installed on existing installations.
– Not affected by fluid properties
Disadvantages
– Accuracy is dependent on flow profile.
– Fluid must be acoustically transparent.
– Errors cause by build up in pipe.
– Only possible in limited applications.
– Expensive.
– Pipeline must be full
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