Flow Measurement & Control


Precise dosing of chemical additive solutions

August 2002 Flow Measurement & Control

Two distinct features of the Profiflux IFS 5000 make Krohne’s magmeter a popular choice for the dosing of chemical solutions: Firstly, it’s Krohne’s most accurate electromagnetic flowmeter; secondly, the device exhibits notable chemical and abrasion resistance.

The instrument features a high density fused - aluminium oxide (Al2O3) ceramic flow tube and platinum electrodes, offering long-term stability. This combination ensures chemical corrosion, abrasion - and resistance to harsh, reactive substances, such as ClO2 and FeCl3 solutions, high concentrations of acids, suspensions and slurries up to TiO2 and ZrO2 hydrochloric acid and caustic and even to hydrofluoric acid. The hardness of the ceramic of the flow tube ensures that it maintains its dimensions, remaining accurate and stable - even under severe pressure variations.

Dosing processes

For a precise volumetric ratio of water or sewage water and an additive, both flow rates need to be accurately measured. The flow rate of the main stream is usually measured with ALTOFLUX IFS 4000. The flow rate of the additive solution, very small compared to the main stream, must be kept strictly proportional to the main stream flow rate.

Traditionally, the flow rate of an additive solution is controlled by regulating the speed of a diaphragm pump. The pumps were chosen because of their wide span of flow adjustment by controlling the speed and stroke length. These days, much higher demands are set on accuracy, beyond the pump capabilities, following the ecological and economical requirements set for these processes - the Profiflux IFS 5000 offers the required accuracy. The special flow sensor design of the IFS 5000 streamlines the flow profile, for example when used for applications with pulsating flows causing asymmetric distortions. Most Krohne signal converters are able to measure pulsating flows up to 200 strokes per minute.

Two practical dosing examples where accurate flow metering has been used are disinfection and flocculation - to remove solids:

* Disinfecting potable water

A chlorine dioxide solution is dosed online to potable water after the usual purification treatment for disinfecting. In order to achieve a constant concentration of chlorine dioxide and consequently chlorine in the water, the dosing rate of chlorine dioxide needs to be strictly proportional to the flow rate of the water to be disinfected.

* Flocculation

Iron-chloride and aluminium sulphate are often used as a flocculate to increase the settling speed of solids (sludge) in sewage treatment or of carbonates in water purification. By means of precise volumetric dosing FeCl3 is mixed into the mainstream of water or sewage.

For more information: Gordon Duff, Krohne, 011 315 2685, [email protected]



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