Flow Measurement & Control


The case for electromagnetic flowmeters

April 2002 Flow Measurement & Control

Electromagnetic flowmeters (magmeters) have proven to be adaptable, accurate and reliable. The magmeter operates by measuring the volume flow of electrically conductive liquids.

Ultimately, the signal is processed by the magmeter's electronic converter which generates a standard 4-20 mA and pulse signal outputs used for indication or control.

Selection of a magmeter

Process engineers often prefer magmeters because they are obstructionless devices that produce virtually no pressure drop affecting flow rates. The absence of moving parts in the measuring section also means little or no maintenance is required over the life of the instrument. Moreover, in the wastewater treatment plant where virtually all flows contain water and are therefore conductive, the preference for a magmeter can be even higher because the instrument can be utilised to match the process conditions at each treatment stage.

For instance, consider the primary treatment stage of a utility wastewater plant. Several large diameter meters, measuring anywhere from 600 mm to 2,25 m, channel incoming streams containing human and kitchen waste and city stormwater. The collection process is similar at industrial sites. However, the influent lines measure 150 to 400 mm because of lower flow rates and less solids content.

As many of these meters are installed below grade, they should feature all-welded construction and sealed connection cable boxes to protect against accidental submergence. The flowtube liner can be polyurethane, Teflon or hard rubber, depending upon the severity of chemical content and the percentage of abrasive solids. Signal capturing electrodes should be manufactured from corrosion resistant metals such as 316 stainless steel or Hastelloy C.

Following the influent collection stage, the water enters the intricate removal process for suspended and dissolved solids. Several mechanical filtering systems remove papers, plastics etc. Dissolved solids, however, are microscopic and difficult to remove by simple filtration. Lime, aluminium or ferric chloride solutions are often used to trap dissolved particles and form insoluble precipitates that settle at the bottom of the process tank.

At this stage, selection of a magmeter is more critical due to the presence of chemicals that are highly corrosive when mixed with water. Teflon liners are recommended for handling corrosive liquids and a thick, smooth Teflon liner can protect against abrasion and coating. This is essential as flowtube coating can distort the measurement and eventually block the meter. Lime solutions can be very abrasive and are best handled by ceramic flowtubes. Aluminium and ferric chloride call for zirconium or tantalum electrodes because of strong corrosive action, even in small concentrations.

Industrial wastewater plants frequently use specially formulated polymers to increase the settling rate of dissolved solids. Polymer flows are small and, therefore, require fractional size magmeters. Chemical characteristics can cause polymer solutions to stick to rubber and polyurethane magmeter liners, therefore high quality (gloss finish) Teflon liners and ceramic tubes are preferred for handling concentrated polymers. Moreover, ceramic flowtubes have a venturi shape inlet-outlet tube to maintain accuracy of small flows.

Settled sludge is usually mixed with micro-organisms to form activated sludge. Hard rubber or Teflon lined magmeters, 12 to 48" in diameter, are often chosen to handle sludge mixtures. Many of these meters are fitted with hot tap removable electrodes that can be cleaned or replaced under normal pipe pressure conditions.

Free of solids, organic and inorganic matter, the wastewater stream will enter the pH adjustment phase. Industrial plants have to adjust using caustic soda or lime. As caustic soda is very corrosive, even in small concentrations, the preferred magmeter configuration should be the ceramic flowtube with cermet electrodes. The cermet electrode is a mixture of ceramic and platinum. This technology allows the electrode to form an integrated seal with the flowtube to prevent caustic seepage that will damage the meter and contaminate the environment.

Ceramic flowtubes are recommended for pH balancing with lime due to their resistance to abrasive solutions.

Disinfection is the final stage for treatment before the effluent enters natural water sources. Hypochloric acid (chlorine) is most widely used for disinfection because it is relatively inexpensive, fast and easy to handle. The solution is mildly corrosive and it can be easily handled by a magmeter with Teflon liner and Hastelloy C electrodes.

Caustic solutions are also used to disinfect wastewater. For example, sulphur hydroxide takes a little longer to kill bacteria and any remaining micro-organisms, but is less toxic than chlorine. Magmeters with ceramic or Teflon liners and platinum electrodes are best for caustic chemicals.

The two-wire revolution

Water companies are continually striving to push the boundaries of performance, reliability and cost reduction, by use of digital technology. The two-wire revolution promises not only a reduction in the costs of installation, cabling and maintenance, it will also reduce revenue costs by enabling quicker and easier diagnosis at the plant level. It also facilitates the flow of information, leading to increased control and process efficiency.

In response, some manufacturers of flow and level instrumentation are developing two-wire flow and level measurement devices which are compatible with most fieldbus communications protocols and which can be easily integrated into sophisticated process control systems.

Conclusion

Magmeter specification can be important to the successful operation of any wastewater treatment plant because of the considerations involving the wide range of sizes, construction, materials and options required.

It is crucial to develop a strong relationship with a manufacturer who will work closely with specifying engineers and designers to ensure the correct systems are installed.

Gordon Duff, Krohne

011 315 2685

gordon.duff@krohnesa.co.za





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