Reliable flow measurement under demanding conditions

The long-distance water network of the supra-regional drinking water supplier Fernwasser Elbaue-Ostharz (FEO) extends over a total length of around 800 kilometres. Large-diameter pipes up to DN 1200 transport around 220 million litres of drinking water to the central German region every day. One of the three waterworks in the network, the Wienrode waterworks, is connected to the Rappbode Dam operated by Talsperrenbetrieb Sachsen-Anhalt AöR. After the raw water has been treated, this waterworks supplies around 40 per cent of the drinking water required in Saxony-Anhalt.

Part of the infrastructure at the Wienrode waterworks, operated by FEO, comprises a bypass structure including control valves (2x DN 800 ring piston valves) and the subsequent supply pipes to the waterworks. Two robust KATflow 150 ultrasonic flowmeters are installed in the waterworks’ bypass structure to regulate two separate flow rates. These consist of two DN 800 steel pipes bypassing a turbine operated by Avacon-Natur GmbH in the inflow to the Wienrode waterworks.

A key factor in the success of ultrasonic measurement is the availability of sufficient inlet and outlet sections to ensure an axisymmetric flow profile. Before deciding on a fixed-installation unit from the manufacturer Katronic, FEO carried out three tests at various locations using minimum and maximum process peaks to identify the optimal position for the measurement. The decisive factors in the purchase of the two dual-channel flowmeters were the successful preliminary tests and the associated prospect of being able to reliably measure large volumes of water in a turbulent environment using the KATflow 150. 

The primary requirement – ‘water flow rate’ – comes from the dam’s waterworks, which supplies drinking water to the region. This pipeline operates round the clock. A typical water flow rate requested by the waterworks is approximately 6,000 m³/h. In the bypass pipelines, this water flow is usually distributed equally between the two pipes at 3,000 m³/h. The bypasses are used whenever the turbine fails or is unable to guarantee the inflow required by the waterworks.

Both KATflow 150 units are configured as two-channel measuring devices, each equipped with an analogue 4–20 mA process output and a Profibus PA interface. The control system receives four measured values simultaneously, which are processed as part of a plausibility check – a form of quality control. The key criterion is signal redundancy. If one signal fails, the system switches over to the remaining available signal. If the current output signal drops to 0 mA, the programmable logic controller (PLC) switches to a substitute value and deactivates the control system.

Although the installation conditions for clamp-on flowmeters are not ideal due to internal fittings and shut-off valves, both permanently installed devices have proved their worth in this application within subsequently defined limits.

The flow rates at both measuring points are turbulent and axially symmetrically offset. Whilst the control range of the ring-piston valves can be utilised in full, the flow rate can no longer be measured correctly. The PLC therefore limits the flow rate and the opening degree of these control valves via software. Despite the circumstances described, the measurement quality is sufficiently good to allow both lines to be controlled remotely via the two ring-piston valves. With these supplementary measures, the measurement data is 98 per cent usable, even under the challenging flow conditions.

Thanks to clamp-on technology, the KATflow 150 flow meter can be installed whilst the process is running. Configuration and positioning are carried out via the flow meter’s menu, which can be operated intuitively whilst the process is running.