High Viscosity: Polymer Measurement Challenges and Solutions

One of the challenges in the measurement of products used in wastewater and water treatment arises when the fluid being measured is very viscous. Case in point: polymers used in wastewater treatment plants.

Ace Krohne 1405ww
Ace Krohne 1405ww
Krohne's OPTIMASS 7000 single straight tube meter is available in tube sizes from 1/4 inch to 3 inches.

One of the challenges in the measurement of products used in wastewater and water treatment arises when the fluid being measured is very viscous. Case in point: polymers used in wastewater treatment plants. Costly polymers are mixed with water and used to coagulate (thicken) suspended solids and produce what is referred to as floc (large curds of solid materials). This process aids in speeding up the settlement of solids. There are generally two types of polymers: dry solids, which are mixed as needed; and emulsions, which are in liquid form and later further diluted with water to achieve the desired mixture. An emulsion can have flowing viscosities as high as 40,000 cPs.

When the correct amount of polymer is dosed into the treatment process, the time for settlement of solids is optimized and is most beneficial to the plant throughput. This means the plant can run at optimal flows, with all the processes operating at peak performance so that no energy, chemicals or time is wasted. Some facilities depend on the metering pumps to ensure the correct amount of polymer is being fed into the mixer and then dosed into the process. However, a growing number of facilities are utilizing the high accuracies of Coriolis mass flow meters to measure the polymer. The accuracy of a Coriolis mass flow meter can be as high as 0.12% of the mass flow. This allows users to measure exact amounts of polymer for flocculation.

The use of Coriolis mass flow meters allows the facility to adjust the dosing of polymers in real time and with great accuracy. There are two challenges with this measurement: the viscosity of the polymers and the flow rate of the dosing. The higher the viscosity, the higher the pressure drop; the higher the pressure drop, the more energy needed, and so on.

Most Coriolis mass flow meters use a bent tube arrangement. By design, this type of tube has a pressure loss built-in due to the tube being bent. The pressure drop will vary by manufacturer, as some tubes have large radius bends and others have smaller radius bends. The smaller the radius bend, the larger the pressure drop.

The dosage flow rates also play a role in the choice of Coriolis mass flow meters because the normal flow rates are very low, sometimes measured in GPH not GPM. Some Coriolis mass flow meters have dual bent tubes and others have a single bent tube. The meters with dual bent tubes suffer even more pressure loss, as the tubes are normally 1/2 or less the diameter of the mating process piping. High viscosity coupled with small- diameter tubes equals large pressure drops. These pressure drops can be overcome with the use of pumps designed for low capacity and higher pressures, but these pumps are expensive. It would seem to follow that removing the bends and the dual tubes from the meter would be ideal for use in these high-viscosity applications. KROHNE has done just that with the OPTIMASS 7000 single straight tube meter, available in tube sizes from 1/4 inch to 3 inches. The pressure loss is equivalent to a straight piece of pipe in the same diameter. A comparison of pressure loss in a 1/4-inch-diameter single straight tube meter and a 1/3-inch-diameter dual bent tube meter, based on a viscosity of 2,000 cP, a pressure of 60 PSI gauge and a flow rate of 4 GPH, shows a pressure loss of 33% less in single straight tube meter. So a quality measurement with less pressure loss and high accuracy is available for high-viscosity, low-flow-rate products used in the wastewater industry.

Krohne is exhibiting at AWWA's ACE14 expo in Booth 1769.

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