Ultrasonic flowmeters have strong potential

Ted Farnon of Micronics UK reviews the potential for non-invasive ultrasonic level and liquid flow technology in the process industry.

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.

The non-invasive character of some ultrasonic devices for both level measurement and closed pipe flow metering provides an attractive solution.

When compared with technologies requiring in-line installation, cost savings of up to 90 per cent 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.

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 that are mounted to one side of a pipe.

The liquid must contain entrained bubbles or solids that will sonically reflect the signal.

The 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.

Clamp-on designs in flow meters enable them to be employed with minimal installation cost and time.

The design also ensures that corrosion and other effects from the liquid being measured will minimally impact the sensors and electronics.

The clamp-on ultrasonic flowmeter 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.

Ultrasonic liquid level and flow measurement and monitoring technology is applied across a wide range of process control applications.

Transit-time and Doppler flowmeters 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.

At one Glaxosmithkline plant, clamp-on meters provide flow measurement for variable speed control of three 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 75kW motor pump circuit, which led to replacement of the oversized pump with an 11kW 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 recirculated to filter out the paint particles.

The liquid flow rate is monitored to detect the need for a condition based filter change.

With improved reliability and reduced installation and maintenance 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.

Back to top