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Since its introduction in 1999, the Flo-Dar instrument from Flowline is claimed to be the only non-contact area velocity flowmeter available that has evolved into several different systems.

Initially developed and manufactured in the US and launched after more than five years of research and testing by manufacturers Marsh McBirney, the system was a response to the problems associated with ‘conventional’ wetted-type flow sensors, which until that time, were the principal methods used in area/velocity metering.

Wetted sensors of all technologies – ultrasonic, electromagnetic and so on – all require ongoing maintenance.

When dealing with sewage and effluent flows, channels will silt up, ragging and fat will accumulate on anything in contact with the flow medium.

The issue, especially now, is that maintenance has become a lower priority, however, removing the sensor from the flow eliminates the problem.

Conventional non-contact open-channel flowmeters only measure level and rely on a primary device such as a flume or simply calculate flow based on-level measurements and inferred velocity.

The Marsh McBirnie solution was to develop a radar/ultrasonic based system, measuring both the speed (radar) and the level (ultrasonic) of the flow, which enables it to be installed in existing channels and pipes, without the need for a primary device.

This simplifies installation and usually eliminates civil costs, thereby reducing capital outlay.

The system consists of a single field-mounted sensor, the sensor is common to all the Flo-Dar models both fixed and portable, with the sensing head mounted above the flow.

Ideally it is mounted in a location where possible surcharged or peak flows would not reach it, although surcharged flows can also be measured.

Flow velocity is measured using a Doppler radar and the radar itself uses very low power and requires no user licence.

Level is measured using an ultrasonic sensor and the data from the sensor is transmitted in digital format to the control unit.

The remote mounted control unit ‘combines’ the sensor data with the pre-programmed site data (channel width, shape) and calculates a volumetric flow reading; this is displayed, logged and/or transmitted.

With a 10-year history and more than 4,000 installed systems, the Flo-Dar has evolved over time.

The first models were only available as fixed AC powered units with a small local display and no data logging capability.

However, a portable battery-powered version was launched in 2001 with the first systems being sold to Scottish Water for survey use on small remote treatment works.

By virtue of its ‘no civils’ installation requirement, most Flo-Dar systems have been installed in existing chambers and channels often as part of a plant upgrade, but also to allow users to conform to stricter environmental regulations.

This retrofitting ability has often seen the system used in very challenging applications, such as: dewatering discharge from a quarry; hot brine discharge from salt processing plant; and effluent discharge from chemical plant.

The user base for the system is split roughly 50:50 between water companies and industrial dischargers, with water companies mainly using the meter for control purposes, whereas consent monitoring is the typical application for industrial users.

However, it is in the area of sewer flow measurement and control that the benefit of maintenance-free flowmeters can really be appreciated.

It could be said that the main spur to develop the Flo-Dar was to measure raw sewage flows and it is for this application that the system has been most widely used.

Until the arrival of radar-based technology the only area velocity meters available for sewer flow monitoring and surveying used a wetted ‘mouse’, usually placed in the invert of the pipe.

The sensors used ultrasonic velocity measurement, either Doppler or correlation type, with both types using ultrasonic beams that reflected from particles suspended in the flow.

When first launched more than 20 years ago these systems were of huge benefit to survey and water companies, sewer design consultants and regulatory bodies.

Although still using wetted sensors, the biggest advances seen with these systems has been with the electronics, most notably signal processing, which has expanded their usefulness and allowed them to be used in applications where they would not previously have been suitable.

Some systems use multiple sensors, which can help to compensate for the effects of ragging, silting and fat build up and they can also be used where sewers surcharge.

There are hundreds of these types of meters used in the survey industry, principally due to their low cost.

The downside of using wetted sensors for survey work is maintenance.

Typically, a survey team will have to visit a site every two weeks to download data and check the sensor has not become ‘ragged up’, this can involve hours of driving followed by an entry into a confined space by a qualified team.

If a sensor has become fouled then velocity data will be lost, the flow data will then have to be reconstructed from just the level readings combined with periods where the velocity data was available.

Reconstructing data takes time and skill and delays getting survey results to customers.

In contrast, non-contact sensors never rag up, which means the flow data does not require any processing prior to releasing to a customer, this also means that customers can directly access their own data.

Data transfer to the internet via the GSM/GPRS network has existed for many years and is now very much an off-the-shelf solution, however, because of the need to ‘clean up’ data this technology was not easily applied to sewer monitors.

With Flo-Dar this has now become a reality.

The Atex-certified web-enabled systems log data and transfer it at periods to suit a customer’s requirements.

This data is displayed on a web page and can be downloaded in spreadsheet format.

The website itself is user configurable and can show data from more than 30 different flowmeters on a single page, making it suitable for network monitoring.

This ability to measure real-time data is of benefit to users who need to control sewer flows automatically and many systems have been installed in a closed-loop control function to automatically regulate flows in sewers and CSOs.

With new housing schemes putting additional pressure on existing networks, automatic control of flows will become more of a necessity as will automatic measurement and billing of discharges into sewer networks.

The primary benefit of non-contact technology is long-term reliability.

For example, at a sewage treatment works in Fazackerly, near Liverpool, a Flo-Dar meter that is installed onsite has required no maintenance, no servicing and no problems during 10 years of constant service.

The Flo-Dar system is a rugged, general-purpose flowmeter for use in most open channels such as sanitary sewers, storm water sewers and other man-made channels including aqueducts and ‘natural’ small streams.

It measures open-channel velocity and depth by non-contact means, virtually eliminating the need to periodically clean the sensors – as required by all submerged type sensors.

Data recovery from Flo-Dar deployments ranges between 98 to 100 per cent even under site conditions that render most submerged systems inoperable.

The accuracy of the system, both under open-channel conditions as well as surcharge (submerged) conditions is more than adequate for the most demanding of metering applications, including open-channel billing applications.

Flowline are an independent company based in North London serving the entire UK. We have been in the flow metering business for over 20 years and our staff have many years experience of providing flow metering solutions for even the most challenging applications. We are ISO:9001 approved.

We offer a full range of metering technologies from the worlds leading flow meter manufacturers. Because we are independent we can offer the best technical solution for your flow application, this means a working solution first time, every time. We specify and supply flowmeters for all meter applications and also offer a full installation, commissioning and maintenance service including confined space work and complete turnkey solutions.

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