Treating municipal water

Ted Farnon from Micronics reviews the potential for ultrasonic level and liquid flow technology in water and wastewater treatment

The refinement of ultrasonic liquid measurement technologies has created an opportunity for cost savings in municipal water and wastewater treatment. Recent improvements in performance and reliability of clamp-on ultrasonic meters, coupled with the inherent efficiencies of ultrasonic devices, have drawn significant attention in the marketplace and resulted in a global sales growth rate of circa 18% for ultrasonic flowmeters.

The non-invasive character of some ultrasonic devices for both level and closed pipe flow metering equates to cost-effectiveness in installation and maintenance, while the accuracy and initial costs of the instruments compare favourably with those using other technologies. When compared with technologies requiring in-line installation, for example, cost savings of up to 90% of installation costs can be realised, with additional operational benefits at installation time and beyond because these 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 flowmeters use either transit-time or Doppler measuring principles, or both. Transit-time (Figure 1) flowmeters employ a pair of transducers that are clamped to the outside of a closed pipe at a specific distance from each other and operate as both ultrasonic transmitters and receivers, alternately sending and receiving ultrasonic pulses. The pulses, generated by piezoelectric transducers, 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 that velocity is multiplied with the cross-sectional area of the pipe, a measure of the flow rate is obtained. Doppler flowmeters (Figure 2) use a pair of transducers, mounted to one side of a pipe, to send and receive a pulse through the liquid. Liquid flowing through the pipe must contain discontinuities (for example, entrained bubbles or solids) that will sonically reflect the signal, and any flow of the liquid alters the frequency of the signal reflected back from the discontinuities to the receiver. The frequency shift is proportional to the flow velocity and, as for transit-time instruments, the velocity multiplied with the cross-sectional area of the pipe will yield a flow rate measurement. Ultrasonic level measuring instruments use a transducer that transmits a signal, generated by a piezoelectric element, to the air/liquid interface and receives the reflected energy after its passage up through the liquid or down to the liquid surface. Level is determined from calculations of the time-of-flight of the signal echo through either media. A number of configurations and approaches are employed in the designs of these devices, and their specific application determines the most appropriate instrument configuration.

Clamp-on designs in flowmeters enable them to be employed readily, with minimal installation cost and time, and their stand-off capability 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. Hand-held microprocessor-based converters provide the means to enter site-specific parameters and offer graphic displays of flow data. Most converters have rechargeable batteries and AC adapters, and can thus be mounted permanently or used as portable devices. Flowmeters designed strictly for permanent installation use converters that are designed for rack or wall mounting and generally incorporate serial communications for integration into plant-wide control systems. Ultrasonic flowmeters are available as single or multi-channel devices and some manufacturers offer instruments designed for hazardous environments.

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 channel bottom below the liquid/air interface. Instrument configurations consist of a single transducer plus microprocessor-based converters.

Applications

Ultrasonic liquid level and flow measuring and monitoring technology is widely applied in municipal water and wastewater treatment process control applications. The specifics of each technology is a major determinant in selection, but properly applied, ultrasonic devices provide cost-effective solutions to water and wastewater treatment process control requirements. Transit-time and Doppler flowmeters both require full pipes, in closed pipe applications, for true flow rate measurements, though a Doppler device will continue to provide proper readings if both transducers are located below the liquid level of the pipe. Consideration must be given to the nature of the liquid to be measured.

Doppler meters rely upon sonic reflectors in the liquid to obtain accurate measurements, so they should be used only when solids or bubbles exist in the liquid. Some manufacturers claim that newer units operating at frequencies above 1MHz provide reliable readings in clean liquids as well, with reflections from fluid eddies providing necessary echoes to the receiver.

Dual mode flowmeters that are available from some manufacturers provide transit-time and Doppler technologies to assure accurate results regardless of fluid characteristics. Ultrasonic flowmeters are also used for open-channel flow rate measurements. In this application, a transducer is mounted above the channel or at the channel bottom 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 in the channel that is, in turn, used in conjunction with flow velocity measurements to calculate the flow rate. With an operational battery life of up to four years, these devices are ideal for long-term environmental evaluation of effluent outfalls and streams and sewers in remote locations without having to rely upon a main power source.

Typical applications for ultrasonic instruments in municipal water and wastewater treatment plants include: clean liquid flow measurements, fresh water flow metering, effluent discharge measurements, sewage and sludge flow measurements, local and remote liquid level measurements and detection, leak detection, liquid interface detection in closed pipes, concentration and density measurements, liquid type and quality identification and thermal energy flow measurements. Some examples of ultrasonic liquid level and flow applications in UK water and wastewater treatment facilities are provided in the following paragraphs.

Southern Water Services, under its Remote Control of Water Sites (RCWS) project, is using ultrasonic liquid level sensing to monitor reservoir levels at more than 200 sites. Reservoir level data is transmitted to a central location for monitoring and control and the company reports that it has successfully reduced costs through this programme.

Micronics reports the use of its Portaflow 300 portable flowmeter to analyse flow rates in an existing 75kW motor pump circuit led to replacement of the oversized pump with an 11kW unit and significant cost savings. The use of the flowmeter, at a capitol cost of £1,500, facilitated a reduction in the cost of annual energy consumption from £5,698 to £926.

An ultrasonic differential level monitoring system has saved Thames Water thousands of pounds on a flow meter refurbishment project in North London. Ultrasonic level meters and dual input controllers are used to monitor mixed liquor flows to 16 final settlement tanks. Thames Water is also using ultrasonic flowmeters to measure open channel influent and effluent volumes at its wastewater treatment sites. Coupled with a flow logger unit, the instrument measures instantaneous flow and records daily totals for up to 60 days at reported savings of 30% less than alternative technology products.

Dunstable WwTW uses ultrasonic level sensors to control the rotating screen spiral in the plant's inlet flow. The ultrasonic device operates the screen for the duration necessary to clean the screen and convey solids to the discharge chute, ready for dewatering and removal.

Performance improvements in ultrasonic meters have made them comparable and, in some applications, superior to traditional in line meters. Improved reliability and the advantage of vastly reduced installation costs and overall lifetime costs, it is not difficult to see why ultrasonic meters can offer the water industry the meter it requires to effect further improvements in performance and efficiency.


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