Flow metering now reduces maintenance to a trickle

Launched ten years ago, the Flo-Dar has remained the only non-contact area velocity flowmeter and has now evolved into several different systems for a wide range of applications, says Flowline director Mark Davis

Developed and manufactured in the USA and introduced after more than five years of research and testing by manufacturers Marsh McBirney, the Flo-Dar system was their response to the problems associated with "conventional" wetted type flow sensors, which, until that time, were the principal method used in area/velocity metering.

Wetted sensors of all technologies - ultrasonic, electromagnetic and so forth - require on-going maintenance. It's a simple fact of life 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. It should be noted that conventional non-contact
open-channel flow meters are available but they only measure level and rely on a primary device such as a flume or simply calculate flow based on level measurements and inferred velocity.

Marsh McBirney's solution was to develop a radar/ultrasonic based system, measuring both the speed (radar) and the level (ultrasonic) of the flow so it can be installed in existing channels and pipes, without the need for a primary device.

This greatly simplifies installation and usually eliminates civil costs, thereby
significantly reducing capital outlay. The system consists of a single field-mounted sensor 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 that uses very little power and requires no user licence. Level is measured using an ultrasonic sensor. A remote-mounted control unit combines the sensor data with the preprogrammed site data (channel width, shape) and calculates a volumetric flow reading, which is displayed, logged and/or transmitted.

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

Upgrades on the control unit added a built-in data logger, four-screen display and four analogue signal outputs, while the latest control units allow up to four sensors to be monitored by one unit, which can also be web enabled.

The sensor, while remaining outwardly similar to the original, has also been enhanced principally to improve reliability and serviceability through the use of modular construction. This permits easy fault diagnosis and repair, with other enhancements, including better signal processing, allowing the system to be used in non-ideal installations.

Most Flo-Dar systems have been installed in existing chambers and channels often as part of a plant upgrade, but also so users can 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, installed into a 900mm dia pipe placed into a small river channel, flow has a PH of 1
  • Hot brine discharge from salt processing plant, atmosphere attacks all metals including stainless steel
  • Effluent discharge from chemical plant, as well as being classified as ATEX Zone 1 the atmosphere in the chambers is toxic with entry into the area prohibited.

Sewer flow motivation
The user base for the system is split equally between water companies and industrial dischargers. Water companies mainly use the meter for control purposes. It is in sewer flow measurement and control that the benefit of maintenance-free flow meters can really be appreciated. The main spur to develop the Flo-Dar was to measure raw sewage flows and it is for this that it has been most widely used.

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

The sensors used ultrasonic beams that reflected from particles suspended in the flow. When first launched over 20 years ago, these systems were of huge benefit to survey and water companies, sewer design consultants and regulatory bodies.

The downside of wetted sensors is maintenance. Typically, a survey team will have to visit a site every two weeks to download data and check the sensor. This can involve hours of driving followed by entry into a confined space by a qualified team.

In contrast, non-contact sensors never rag up, which means the flow data does not require any processing and customers can directly access their own data through remote monitoring.

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 new ATEX certified internet enabled systems log data and transfer it at periods to suit
a customers' requirements. This data is displayed on a web page and can be downloaded in spreadsheet format. The web site itself is user configurable and can show data from over 30 different flow meters on a single page, making it
ideal for network monitoring.

This ability to measure real time data is of massive benefit to users who need to control sewer flows automatically and many systems have been installed and work 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.

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