Exploiting the benefits of non-invasive liquid measurement

Doppler device
Fixed clamp-on meters such as this portable Doppler device find application for in-line metering.
TED FARNON reviews the potential for the non-invasive measurement of liquid flows, levels and energy flow using ultrasonic devices.Balancing the need for improved monitoring and metering of modern building services within tight financial limits requires innovative management and the use of technology with lower installation and lifetime maintenance costs. Finding funds to improve performance for metering and monitoring is often difficult. Ultrasonic flow and level technology can provide attractive solutions to this dilemma and a fast return on capital invested. Improvements in the performance and reliability of ultrasonic meters have resulted in sales growing by around 18% year. The non-invasive character of some ultrasonic devices for both level and clamp-on closed pipe flow metering can deliver cost-effective installation and maintenance, whilst the accuracy and initial costs of the instruments compare favourably with those using other technologies. Installation cost savings of up to 90% are achievable compared with technologies requiring in-line installation, and there are other operational benefits at installation time and beyond. Non-invasive installations obviate the need for system downtime in closed-pipe pumping operations and further facilitate energy savings, as no hardware is inserted into the line to create any unrecoverable pressure drop. Operating principles Ultrasonic flow meters use either transit-time or Doppler measuring principles, or both. Transit-time flow meters employ a pair of transducers that are clamped to the outside of a closed pipe and operate as ultrasonic transceivers. The pulses are first transmitted in the direction of the fluid flow and then against the flow. The difference in transit-time of the signals is proportional to the flow velocity and, when multiplied with the cross-sectional area of the pipe, provide a measure of the flow rate. Doppler flow meters use sound transmitted through the pipe wall from a single transducer mounted on the outside surface. Sound bursts are reflected from entrained particles or air bubbles at a different frequency. This frequency change is directly proportional to the flow velocity and facilitates the calculation of volumetric flow. Instruments for measuring the level of liquid in a tank or flow in an open channel use the transmission time of sound pulses reflected from the liquid surface back to the transducer to calculate liquid height. Newly introduced open-channel instruments use a submerged ultrasonic sensor that measures flow velocity and height to calculate flow volume. Typical configurations Clamp-on designs minimise the cost and time of installing meters and ensure that corrosion or contaminating effects from the liquid have no impact on performance. Portable designs are often battery powered and suitable for system commissioning and balancing, maintenance, surveys, emergency meter application and to validate results from existing meters. Fixed clamp-on meters offer a viable, cost-effective alternative for many in-line meter applications. Both portable and fixed meters generally incorporate local displays, graphics options and industry-standard serial communications for integration with a monitoring-and-targeting system or building-energy-management system. Ultrasonic instruments for measuring liquid level can be used for open or closed channels and tanks. In the case of open-channel applications they may be mounted above a fixed weir or flume or at the bottom below the liquid/air interface to calculate volumetric flow in conjunction with a flow velocity sensor. Applications Ultrasonic liquid-level and flow measurement and monitoring technology is applied across a wide range of building-services applications. Transit-time and Doppler flow meters both require full pipes to give a true measurement of flow rate. Dual-mode flow meters are also available, combining transit time and Doppler technologies to assure accurate results regardless of fluid contamination. In ultrasonic open-channel flow measurements, a transducer is mounted above or at the bottom of the channel and transmits a signal to the surface of the liquid. The transit time of the reflected signal is used to determine the level of the liquid, which is used with flow velocity measurements to calculate the flow rate. Applications for clamp-on ultrasonic meters include thermal energy flow measurements (both heating and cooling), secondary water-flow metering, system commissioning and balancing, flow confirmation, consumption monitoring, effluent discharge monitoring, local and remote liquid level measurements and leak detection. Metering thermal energy consumption in building and process applications is a key tool for monitoring and managing energy — but is often considered as unviable due to the disruption and costs associated with in-line metering. Clamp-on ultrasonic meters can provide a viable solution, which has been adopted by organisations ranging from Luton Borough Council to the ExCeL exhibition centre in London and major airport terminals. Traditionally clamp-on solutions have been used on larger pipe sizes. However, recent developments have led to cost-effective solutions across a wide range of building-services pipework. They include heating and cooling for energy management and automatic billing in refurbished buildings with multi-tenant occupation.
This fixed ultrasonic transit-time meter has industry-standard serial communications.
For short-term investigations, monitoring and commissioning of wet systems to identify waste and scope for improvement, products such as the Micronics Portaflow 300 portable flow meter and logger can provide valuable insight. One has been used to analyse flow rates in a circuit with a pump having a 75 kW motor, leading the oversized pump being replaced with an 11 kW unit and significant cost savings. Performance improvements in clamp-on and other non-invasive ultrasonic meters have made them comparable and, in some applications, superior to traditional in-line meters. Their improved reliability and vastly reduced installation costs and overall life-time costs offer potential users the solutions they require to effect improvements in performance and efficiency. Ted Farnon is chairman of Micronics Ltd, Unit B3, Beech Business Centre, Davies Way, Loudwater, High Wycombe, Bucks HP10 9QR
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