INSTRUMENTATION: Flowmeters Can Be Diabolical
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FLOWMETERS are invaluable in process environments. But choosing the correct one, especially so for magnetic flowmeters, can be a diabolic problem.
Consider the following: the robust measuring tubes of magnetic flowmeters are almost always made from steel or stainless steel. Such a magnetic flowmeter will not work, however, without a non-conductive lining in order to use the principle of electro-magnetic induction.
“Most manufacturers of magnetic flowmeters offer different linings to cope with the huge variety of applications; these linings not only have different material properties but also different prices. The selection of the lining therefore depends on the medium and process conditions such as temperature and pressure as well as on the price. But care is necessary. Not every manufacturer offers every lining,” says Rob Oliaro, regional manager in the Western Cape for Yokogawa SA.
But there are technical limits which cannot be exceeded for certain linings.
The most common linings are the all-rounder, Teflon, and its derivative PFA, Oliaro points out.
Teflon is chemically extremely non-reactive and therefore very corrosion resistant.
Compared with normal thermoplastics, PTFE is relatively heat-resistant; it retains its electrical, chemical and physical properties up to temperatures over 300 °C. In addition, Teflon’s behaviour when in contact with foodstuffs complies with the requirements of the American Food and Drug Association.
It is therefore almost inevitable that this material is used for cladding the measuring tubes of magnetic flow-meters.
Perfluoralkoxy (PFA) is a thermoplastic and has a higher diffusion impermeability as well as improved workability. Further on, different to PTFE, the PFA reacts elastic to pressure effects.
The poor diffusion permeability or, to put it in ordinary language, the relatively large, open pores of the materials, cause problems under certain circumstances. Small molecules such as water, hydrochloric acid or sulphuric acid, can penetrate the material under certain circumstances (fluid permeation which occurs particularly in the case of high temperature gradients between the fluid and its environment via the measuring tube, when the condensation point of the fluid is inside the steel tube).
In magnetic flowmeters, this phenomenon leads to the accumulation of conductive liquid between the steel of the tube wall and the plastic lining; over a period of time this leads to short-circuits between the electrodes or to false readings.
The multiplicity of tube linings for magnetic flowmeters covers most requirements.
“In a few exceptional cases the flowmeters have to be considered as disposable items as the media attack the lining to such an extent that the instrument’s working life is systematically limited. In such cases the question often occurs whether it is more economic to develop new lining materials for these applications or just to replace an instrument that is at the end of its more or less long life cycle with a new one.”
“It should also be remembered that in the European Union, flowmeters must generally bear the CE mark as required by the Pressure Equipment Directive.”
“If the pressure hull has been eroded by corrosion or abrasion, it may no longer comply with the directive,” Oliaro explains.
But of course the nature of the lining is not the only criterion for the selection of a flowmeter’s measuring tube. The properties of the electrodes or the grounding rings, depending on the measuring require-ments, are of great importance.
“If one examines the wide Yokogawa product range for example, it is possible to gain an impression of their multiple uses. The electronic components are also an important parameter for the selection process; the market offers many versions, ranging from the very simple to the sophisticated,” according to Oliaro.
In the case of the ADMAG AXF, Yokogawa offers a function which automatically recognises deposits in the measuring tube as standard, as well as many other features.
Users are also placing increased importance on the simple and reliable integration of the instrument into process environments, a requirement which this instrument meets by using HART or BRAIN communications and the Foundation Fieldbus as well as current output and pulse output.
“Although the correct design of a flowmeter is not a diabolic miracle, it is more than worthwhile to consult professional advice, as with all important field devices,” Oliaro suggests.
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