Monitoring and metering water flow

27 October, 2009

Accurate monitoring of water supplies is becoming an essential requirement, as is the need to ensure water is of an acceptable quality. To ensure these requirements are met, flow meters are being adopted by many companies.

Take drinking water as one example, where purification is needed. Slow sand filters are commonly used in this process. These use a biological filter known as a Schmutzdecke, in addition to the physical filter of the sand, which needs to be maintained in optimal condition to prevent bacteria and algae in the water.

If a filter produces water of unacceptable quality it will need to be taken out of commission for at least eight weeks. In addition, there may be fines from the Drinking Water Inspectorate; and a drop in production could mean it becomes impossible to supply 100% of the distribution network, resulting in compensation costs and the need to provide an alternative supply.

With slow sand filters, however, low flow can result in reduced levels of dissolved oxygen that would affect the Schmutzdecke and high flow can mean excessive turbidity. The target is to establish a flow rate that is equivalent to a drop in water level in the filter of at least two inches per hour.

At one major UK water treatment works, engineers were conscious that flow needed to be measured continuously, but there were a number of problems: the feed channel from the filter had a square cross-section; the water supply pipe contained a natural venturi; and the pipe was only 2m long. This meant a suitable flow path would have to be constructed before a traditional flow meter could be installed – work estimated to cost £27,000 per filter.

As an alternative, YSI Hydrodata recommended the installation of the Argonaut-SW, a small Doppler flow meter. So, two of the meters were installed within the outlets of the two slow sand filters by simply attaching them to blocks made of the same material as the channel in which they were located. A professional diver placed the meters in the pipes.

The Argonaut-SW has three acoustic beams, one pointing straight up which measures water level, the other two pointing up/down stream at a 45° angle to measure the water velocity in two dimensions via the acoustic Doppler method. They can measure both forward and reverse flow.

The Argonauts scan and store data continuously, with two-minute average data transferred to the company’s SCADA system which controls the flow rate through the filter. Following initial installation, a drop test was conducted in which a known volume of water was passed through the beds and compared with the Argonaut, which demonstrated accuracy of better than 99.9%, claims the company.

Nick Martin from YSI commented:?“If this water company had chosen to install traditional flow meters an extra capital cost of over £54,000 would have been incurred and the filters would have been out of action for at least a week. Instead, we were able to install the Argonauts in less than an hour with minimal cost and no interruption to supply.”

Standpipe meters

Monitoring the amount of water used is also becoming increasingly more important. Standpipe flow meters, for example, are popular when such users as Fire Brigades and Water Authorities need to accurately calculate and regulate their water useage.

While mechanical meters were often used for standpipe metering applications, the accuracy tended to deteriorate over time due to component wear. Furthermore, the iron and metal particles that are found in water would often stick to the magnets, causing blockages and also affecting accuracy.

To provide a more accurate and reliable solution, Langham Industrial Controls has adapted ABB’s AquaMaster electromagnetic flow meter to fit the 57mm standpipes.

Standpipes that are horizontally positioned can result in inaccurate readings when the flow tube is not full. The new standpipe, however, can be vertically positioned. With vertical pipes, the flow pipe is full when readings are taken as the water moves up and through the pipe, giving enhanced accuracy.

The new flow meters can also cope with higher velocities – with the older models the extreme velocity of water can destroy the paddle wheel, but this has been removed thereby opening up more possibilities for use.

To fit the standpipe, the AquaMaster’s sensor section has been remodelled, with the connection flanges removed and replaced with a ‘welded’ fitting to allow it to be fixed into the standpipe upright. These modifications have also reduced the overall weight of the standpipe to 15kg. Another benefit of the meter is its battery-powered transmitter, which means the standpipe can easily be adapted into a portable device. To ensure the collected flow data is easier to read, the meter’s transmitter is also mounted at an angle. It can also use an in-built data logger to enable flow and pressure readings to be recorded for further analysis.

With the assistance of Anglian Water Services, the new unit has accurately recorded flow rates of less than 0.1 l/sec.