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№ 5 (May 2012)

Improving Flow Measurement in the Water and Wastewater Industry

   Water and wastewater industry has continued to gain attention globally due to climate change issues. Some industry professional now see it as the new oil. In regions that witness seasonal climates and were there is limited water resource, wastewater processing is very essential.

By Chikezie Nwaoha

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   Thereby serving as a means of curbing water scarcity. All these combined together has increased the challenge in its wastewater measurement. To accomplish this, correct measurement of the wastewater is pivotal as it also enhances water quality control. In other cases, inaccurate flow measurements or failure to take measurements can cause serious or disastrous results.

   In good process operations, flow through out the process must be regulated near their required values with only a small variability. The job of the measurement personnel is to maximize the return on investment while utilizing the equipment in place. The main objective of this article is to discuss the major flow meters that serve the water and wastewater industry, to ensure accurate flow measurement results. There are different types of flow meters used to measure the flow profile of wastewaters. They are: turbine flow meters, ultrasonic flow meters, electromagnetic flow meters, and positive displacement flow meters.

Turbine Meters
   The turbine meter is a volumetric measurement device. It functions by sensing the linear velocity of the fluid passing through the known cross sectional area of the meter housing to determine the volumetric flow rate. The fluid, as it passes through the meter, imparts an angular velocity to the rotor, which is proportional to the linear velocity of the flowing fluid. Since the linear velocity of the flowing fluid through a given area is directly proportional to the volumetric flow rate, it follows that the speed of rotation of the rotor is directly proportional to the volumetric rate. The principle of turbine flow meter operation has not changed, but how each segment of the turbine meter design contributes to the accuracy of the instrument has changed considerably.

   Even though liquid turbine flowmeters have existed for several decades, there have been many enhancements, such as dual-rotor technology, bearing materials, embedded flow computers and improved methods of mechanical installation of supports and rotors, which offer impressive flow performance. These enhancements enable the modern turbine flowmeter to accomplish applications that were not possible in the past, while offering additional critical flow data and performance. The modern precision turbine flowmeter remains one of the most reliable and accurate flow sensor devices for today’s critical flow measurements in the water and wastewater industry.

  Turbine flow meter applications in the water and wastewater industries include, chilled water, high purity water, HVAC systems (heating, ventilating, and cooling).

Ultrasonic Flow Meter
   Ultrasonic flow meter like the turbine flow meter is an inferential meter. Ultrasonic meters are sub-divided into two types: Doppler meters and time-of-travel meters. In Doppler fl ow meters, two transducers are mounted in a case attached to one side of the pipeline, while the time-of-travel flow meters transducers are mounted on each side of the pipeline. Ultrasonic flow meters send a signal of known frequency across the flow stream, and measure how the flow modifies it. This value is used to determine the flow rate. The liquid being measured must be relatively free of entrained solids or gas to reduce scattering of signal. However, with doppler technology, the signal bounces off particles in the flow stream instead of the other side of the pipe. The flow particles are travelling at the same speed as the flow. Ultrasonic flow meters have the advantage in that it works across a high range and can handle a variety of flow rates, and has no moving parts.

   Its applications in the water and wastewater industries include measuring of chilled water, brine and salt slurry streams, and water flow.

Electromagnetic Flow Meters
   Electromagnetic flowmeters, also known as magmeters, are a popular choice among instrument engineers, making up about 20 percent of flowmeter installations. Faraday’s law says that a conductor moving through a magnetic field produces an electric signal. In this case the fluid is the conductor and electromagnetic coils surrounding the meter body generate the magnetic field. If an electrical conductor is moved in a magnetic field, which is perpendicular to the direction of motion and to the conductor, an electrical voltage is induced in the conductor whose magnitude is proportional to the magnetic field strength and the velocity of the movement.

   In order to utilize the operating principle, it is imperative that a magnetic field exist within the pipe and that the induced voltages can be measured without any form of interference. Two coils generate the magnetic field that extends through the pipe only when if it is not shunted by permeable pipe materials. For instance austenitic steel does not hinder the magnetic field; therefore it is the most commonly used material for the meter pipe in the electromagnetic flow meter. To prevent shorting out the induced signal voltage, the inner surface of the metering pipe must be electrically insulating. The signal voltage is measured at two electrodes which are in galvanic contact with the fluid. An additional requirement for the operation has already been mentioned, namely the fact that the fluid must be an electrical conductor. Therefore a minimum conductivity of 20; 5; 0.05 µS/cm is required. This is very dependent on the type of flowmeter.
Electromagnetic flow meters are used in water and wastewater industry due to its ability to measure almost all electrically conducting liquids, pastes, slurries, and emulsions with excellent long term stability and accuracy. Temperature, density, pressure, and viscosity have no major influence on measurement using this type of flow meter. Electromagnetic flow meters have rangeability up to 1,000-to-1, depending on the maximum tolerable measurement error. For flow velocities of 0.5 to 50 ft/sec, accuracies are usually stated as a percent of rate, for lower velocities, accuracies are stated as a percent of span. Typical velocities measured range from three to 15 ft/sec for water and clean chemicals, three to six ft/sec for abrasive fluids, and six to 12 ft/sec for coatings and liquids with entrained air.

   Electromagnetic flow meter applications in the water and wastewater industries include, wastewater feed, sewage, demineralised water.

Positive Displacement Meters
   Positive displacement meters take a physically enclosed volume of fluid and move it from upstream to downstream of the metering point. The sum of these operations is an indication of the amount of liquid which is moved over a period of time. An expected accuracy of 0.25 percent for a positive displacement (PD) meter can be attained under proper conditions. Positive displacement meters measure discrete quantities of the flowing fluid. The rotating element is mechanically coupled to a transmitter or counter which integrates or totals the counts to provide an indication in units of gallons, liters, cubic feet, etc.
Some common types are: rotating vane, bi-rotor, rotating paddle, oscillating piston, and oval gear meters. Positive Displacement (PD) meters are useful for large quantities of liquid. At the same time, a measured quantity of liquid for every revolution is moved from the inlet to the outletof the meter. These are accurate instruments, which are suitable for flow rates of between 200 and 40,000 cu.m/hour. Typical applications in water and watewater industry are measuring water and process cooling, water in plumbing systems etc   

References
Frenzel, F., ET AL “Industrial Flow Measurement Practice” ABB Automation Products, Germany.
Gas Processors Suppliers Association Engineering Data Book SI Version, 11th Edition Volume 1, Section 4.
Ron Madison, “A Report on Modern Turbine Flow Meter Enhancements”.
Nwaoha, C., “Flow Meters: Minimizing Flow Measurement Setbacks”, PetroMin, (March/April 2009), pp. 42-46.

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