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The benefits of clamp-on ultrasonic flowmeters in the battle against water leakage

The benefits of clamp on ultrasonic flowmeters in the battle against water leakage

The benefits of clamp on ultrasonic flowmeters in the battle against water leakage

With unacceptable levels of water loss making the headlines, and the need to meet progressively stricter regulations, water companies throughout the UK are increasingly discovering the benefits of clamp-on ultrasonic flowmeters in the battle against disruption to customers. 

Unable to manage leakage or unaccounted losses without measuring flows and volumes, clamp-on ultrasonic metering offers a cost-effective, non-invasive solution, allowing installation without supply interruption, unlike conventional in-line metering, which can be somewhat challenging in this respect. 

How does clamp-on ultrasonic metering work?

In simple terms, when it comes to measurement, clamp-on flowmeters work by sending two ultrasonic pulses through the medium – one in the flow direction and a second one against it – with the transducers alternately working as an emitter and a receiver. The transit time of the ultrasonic signal propagating in the direction of flow is shorter than the transit time of the signal propagating against the flow and the time difference is directly proportional to the flow rate. 

Measuring to manage

Recently appearing at the prestigious, tenth annual international Water, Wastewater and Environmental Monitoring event (WWEM) in Telford, FLEXIM had the chance to demonstrate in collaboration with Anglian Water, one of the UK’s largest water companies with over six million customers, why FLEXIM ultrasonic clamp-on flowmeters are now their metering technology of choice. 

With the pressing need to comply with AMP7 objectives high on the agenda, Anglian would need to commit to reducing leakage by 16%, decrease supply interruption by 41%, and reduce pollution incidents by 30% in order to meet the 2025 deadline for water companies in England and Wales to achieve their PR19 Outcome Delivery Incentives. 

The challenge to grow real-world confidence in clamp-on capabilities.

Anglian were well aware of the importance of measuring flows and volumes, in order to have any chance of effectively managing leakages or unknown losses. Other obstacles in their way included a lack of appropriate network metering and the excessive costs involved with the use of intrusive meters. Civil, chambering and reinstatement installation work is capable of exceeding £100,000 for large-bore meters, whereas clamp-on flowmeters, with no need for bypasses or valves for maintenance, would cost considerably less, for example from ~£5K rising to ~£30,000 with a basic chamber. 

While there was definite interest in the capabilities of ultrasonic clamp-on flowmeters, and a recognition that they lend themselves well to installation without supply interruption, Anglian was still hesitant as there were a number of preconceptions about the technology that led to some concerns. 

With ultrasonic clamp-on metering garnering a variable reputation over the past three decades, water companies need real-world confidence that these versatile flowmeters can truly deliver robust and reliable solutions. Isn’t clamp-on just a temporary solution? Aren’t they tricky to install? Aren’t measurements or even installations unreliable or intermittent? And there was more than a little scepticism of the readings – are they really completely accurate? These were some of the worries Anglian had to overcome.

FLEXIM was first tasked with proving the effectiveness of clamp-on technology for Anglian Water in 2017, beginning by providing sewage pumping station monitoring, including buried installations. MCERTS-accredited meters followed, and during this period, not one meter failed. With opinion shifting and confidence gained through experience, FLEXIM was asked to begin work for Anglian on raw and potable water network monitoring in 2020, delivering accurate and reliable data solutions in often awkward, difficult to access locations, including the replacement of long-dead inline electromagnetic metering. 

Commitment to research and development takes ultrasonic measurement to a whole new level.

With the capabilities of clamp-on technology improving dramatically in recent years, we take a look at the accuracies of ultrasonic, clamp-on measurement.

In the past, doppler ultrasonic flow measurement has been frequently recommended for wastewater flow measurement, as performance is typically stated as having an accuracy of ±2% of readings on liquids with entrained solids or gases. However, performance at this level of uncertainty is rarely the case in practice. Despite specification claims, entrained air or suspended solids percentages within wastewater vary constantly and doppler rarely achieves measurement uncertainty better than ±10% in the field. 

Ultrasonic transit time measurement, on the other hand, uses transducers that typically operate in the 0.2-2 MHz frequencies, and works by sending two ultrasonic pulses through the medium – one in the direction of flow, and a second one against it. The transducers are alternately working as a transmitter and a receiver; ultrasound sent with the flow direction speeds up, whilst against the flow slows down, and the time difference, Δt, is directly proportionate to the flow rate.

So, what accuracies can be expected? The following table gives an indication:

Installed accuracy with recommended straight lengths, for example 10Ø up/3Ø down after disturbance such as a 90-degree elbow

Meter accuracy (lab or field calibrated)

±0.3% of reading, ±0.01 m/s

Installed accuracy

±1.0% of reading, ±0.01 m/s


0.15% of reading, ±0.01 m/s

Maximum pipe diameter

Up to 6.5m


Because it’s not always possible to achieve optimum installation away from valves, bends and other inline disturbances, FLEXIM’s automatic flow disturbance correction feature is highly advantageous. 

Developed in collaboration with the highly respected PTB, the National Metrology Institute of Germany, research focussed on the exact determination of non-disturbed and disturbed flow profiles, with disturbance correction factors derived from the ratio of the two results.

In situations where recommended straight runs can be difficult to find, the (usually) required 10D upstream straight length to achieve ±1.0% installed accuracy is reduced to a much more achievable 2D upstream and 3D downstream, optimised for 90° bends and double bends, both in and out of plane. Other disturbance types are also improved. 

For even greater measurement confidence, advanced meter verification can be carried out annually. This involves instrument conditioning monitoring, health check and predictive maintenance, with the diagnostic data ‘fingerprint’ being stored at the time of installation, forming the verification reference. Because diagnostic values may change over time due to pipe condition deterioration or even internal pipe build-up, the reference diagnostics can be used for an accurate comparison, and a verification report is generated automatically to provide evidence for predictive maintenance and quality management systems. This negates the need for periodic meter recalibration, offering considerable operational expenditure savings and no measurement interruption.  

Developing a standard approach within potable water networks

Working where other clamp-on flowmeters had failed, Anglian were beginning to realise that FLEXIM’s clamp-on ultrasonic metering could help them achieve their AMP7 objectives. However, they still needed to be reassured that clamp-on flowmeters could be deployed at scale in clean networks with zero network supply interruption, and at optimal total expenditure. This is why ‘outside the box’ thinking would be needed to deliver the most cost-effective deployment.

The ’coffin chamber’ technique

Needing to deploy more meters at scale to address network visibility and drive down leakage without supply interruption, a truly cost-effective flowmeter deployment methodology was developed that would dramatically reduce the investment usually required on site civil construction work. The ‘coffin chamber’ involves excavating the pipe, before installing the ultrasonic clamp-on meter on the outside of the pipe. Next, a durable plastic chamber is built around the pipe and flowmeter, backfilled with pea shingle and capped off. Finally, the coffin chamber is backfilled and restored to grade. 

Allowing installation of ultrasonic flowmeters on HPPE, carbon steel, ductile iron, grey cast iron or even cement pipes, in sizes from 150mm to 2m or potentially larger still, advantages of the coffin chamber method of flowmeter deployment are wide-ranging. These include protection of both the pipe and flowmeter during excavation, in the event of any necessary meter maintenance, as well as improved or reduced planned outages, lowest cost chamber and civil work, and improved meter uptime leading to effective network monitoring. 

To find out more about non-invasive ultrasonic flow measurement in the water industry, contact Andy Hammond – | | +44 (0)1606 781 420