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by Ted Farnon of Micronics UK
Balancing the need for improved monitoring and metering within tight financial limits requires innovative management and the use of technology with lower installation and lifetime maintenance costs. This is the position of many companies, and in the area of metering and monitoring of process parameters or waste discharges, there is scope for savings in liquid systems: the latest ultrasonic technology can offer some attractive solutions.
The non-invasive character of some ultrasonic devices for both level measurement and closed pipe flow metering provides cost-effectiveness in installation and maintenance, whilst the accuracy and initial costs of the instruments compare favourably with those using other technologies.
When compared with technologies requiring in-line installation, cost savings of up to 90 percent on installation are achievable, with additional 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.
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 by the cross-sectional area of the pipe, a measure of flow rate is obtained.
Doppler flow meters use a pair of transducers, mounted to one side of a pipe. The liquid must contain entrained bubbles or solids that will sonically reflect the signal, and flow of the liquid alters the frequency of the signal reflected back to the receiver. The frequency shift is proportional to the flow velocity and calculated as for transit-time instruments.
Ultrasonic level measuring instruments transmit a signal to the air/liquid interface and analyse the reflected signal. Level is determined from calculations of the time-of-flight of the signal echo through either media.
Typical Configurations.
Clamp-on designs in flow meters enable them to be employed with minimal installation cost and time, and ensures that corrosion and other effects from the liquid being measured will minimally impact the sensors and electronics. The clamp-on ultrasonic flow meter designs are often portable and can be used readily to validate results from existing meters or to take measurements where no meters exist. Fixed flow meters generally incorporate serial communications for integration into plant-wide control systems. Ultrasonic liquid level measuring instruments are tank mounted or, in the case of open channel applications, are mounted at some point above the channel or at the bottom below the liquid/air interface.
Applications.
Ultrasonic liquid-level and flow measurement and monitoring technology is applied across a wide range of process control applications. Transit-time and Doppler flow meters both require full pipes, for true flow rate measurements, in closed pipe applications. Dual mode flow meters are also available to provide transit-time and Doppler technologies to assure accurate results regardless of fluid characteristics.
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 in conjunction with flow velocity measurements to calculate the flow rate. Typical applications for ultrasonic instruments in industrial process applications include: process liquid flow measurements, clean water flow metering, flow confirmation, effluent discharge measurements, local and remote liquid level measurements, leak detection and thermal energy flow measurements.
Some examples of process plant applications.
Prelude Engineering has applied the Ultraflo 2000 clamp-on flow meter to many applications in pharmaceutical plants around the world. They find they solve previous problems by eliminating any possibility of contamination and saving a great deal of expense. At one GlaxoSmithKline plant Clamp-On meters provide flow measurement for variable speed control of 3 pumps to maintain constant flow rate with varying demand. The meters provide control signals for the pump inverter drives on the ultra pure water supply, resulting in more efficient operation and control of these vital facilities with significant installation and operational cost savings.
On another site a Micronics - Portaflow 300 portable flow meter was used to analyse flow rates in an existing 75 kW motor pump circuit, which led to replacement of the oversized pump with an 11 kW unit and significant cost savings. At an automotive plant near Derby multiple Doppler flow meters have been installed to measure paint 'over-spray' collected by water curtains, which is re-circulated to filter out the paint particles. The liquid flow rate is monitored to detect the need for a condition based filter change.
Performance improvements in Non-invasive.
Ultrasonic meters have made them comparable and in some applications superior to traditional in-line meters. Improved reliability and having the advantage of vastly reduced installation costs and overall life time costs it is not difficult to see why Ultrasonic instruments and meters can offer the process industry the solutions they require to effect further improvements in performance and efficiency.
This review article w as contributed by Ted Farnon of Micronics UK.
For further information call Micronics +44 (0)1628 810 456 or visit www.micronicsltd.co.uk.
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Editor Note: For further information regarding this release or a supporting picture please contact David Leigh on 01579 321750 or email micronics@leighandersonassociates.com
