Over the past 10 years, ultrasonic flowmeters, both for gases and liquids, have gained wide acceptance. The main reasons for this are their high repeatability combined with zero pressure loss and extended diagnostic features. During this time, meters with differing path configurations have been launched, each of them trying to obtain the highest accuracy. Numerous papers have been published on their performance at the calibration laboratories. Many of them showed (often after multipoint linearisation) almost ideal straight lines with errors close to the repeatability of the lab.
What happens to a gas flowmeter under real-life conditions – the effects of fouling
For users, however, the key question is not how good a meter is at the lab, but what is the accuracy after installation in the field and how to be sure that the meter’s performance is not deteriorating (by fouling) over time.
Fouling is one of the major causes of additional uncertainty in the field. This is not only a problem for ultrasonic meters, but affects all installed flowmeters, the strength of ultrasonic flowmeters being their powerful built-in diagnostic capabilities designed to cope with the problem. Looking at the impact on ultrasonic meters, fouling can be classified into five categories; each of which affects the measurement differently.
These categories are:
1. Fouling as a small flow on the bottom of a pipe.
2. Fouling intermittently sticking to the pipe wall.
3. Fouling which is as an evenly distributed coating on the inside of the pipe.
4. Fouling as dirt build-up on transducers (especially on those facing upstream).
5. Fouling as liquid build-up in gas transducer pockets.
For ultrasonic flowmeters, the major effects of fouling on the measurement are:
* Reduction in the cross-sectional area.
* Increased wall roughness.
* Shortening of the acoustic path.
* Attenuation of the acoustic signal through the reduction of the reflection coefficient.
* Absorbance of the ultrasonic signal by a fouling layer on the transducer.
* Increased cross-talk by liquid in the transducer pockets.
Using dedicated diagnostics hardware and appropriate algorithms it is possible to analyse the signals from the various transducers and compare these to standard and logged data. Deviations beyond set thresholds will raise an alarm and permit early intervention.
According to Krohne, figures show that just 0,5 mm of bottom fouling in a DN300 system can result in measurement error to the tune of 1,6 million Rand.
The basis for this brief article was the road map for the development of the Altosonic V12 ultrasonic flowmeter which features diagnostics for fouling and meter integrity and a unique multi-plane bouncing path design for neutralising the effects of swirl.
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