Selecting Materials for Aggressive Media Flow Meter Applications

For the purposes of this feature I have defined aggressive media to include corrosive and abrasive flows and also measurements made at higher temperatures. Corrosive materials are commonly encountered in flow measurement and they present a series of problems. Typically corrosion resistance changes with temperature, a material which offers good chemical resistance at 20°C may be severely attacked at 100°C .

So when selecting the best flow meter material for your corrosive media application consideration should be made not only for the normal running conditions but the extremes under an “unusual event”.

What if you have a fluid that has all three of the “aggressive” characteristics listed above? Flow meter manufacturers try to use the most widely chemical resistance materials for their standard products to maximise the applications for which they can be used. Elastomeric seals are usually interchangeable but the flowmeter body is normally metal or polymer. Below is a chart detailing some common flowmeter body and seal materials, their maximum operating temperature and fluids for which they offer acceptable durability as well as fluids they should not be used with.

Material
(Max operating temperature)

Acceptable fluids

Not recommended for use with
316 Stainless steel (400°C)A good general all rounder, good for most oils, solvents, glycol, neutral aqueous solutions, hydrogen peroxide, food and medical products.Attacked by strong acids (hydrochloric, hydrofluoric, phosphoric, sulfuric ), plating solutions,ginger oil, bromine.
PTFE (200°C)A good material for many chemicals but has poor mechanical properties.
PVC (60°C)For lower temperature use only. Good with aqueous solutions, some organic solvents and mild acids and bases.Very unhappy with aromatic hydrocarbon based liquids and some strong acids.
PVDF (100°C)Available in food and drug approved grades. Good with most solvents and many acids.Severely affected by basic materials, acetone, ethyl acetate, methyl acetone, nitrous oxide.
PPS (200°C)Good all-rounder for aqueous solutions, hydrocarbons, weaker acids and bases, oils and ketones.Attacked by chlorine, hydraulic oil, hydrochloric acid and sulfuric acid.
PEEK (250°C)Good all-rounder with a very long list of compatible liquids.Attacked by some concentrated acids such as sulfuric and nitric.
Seals – EPDM (130°C)Offers good weather resistance and can be used with a wide range of fluids.Severely affected by hydrocarbons, oils, fats and some acids.
– Fluorine rubber (Viton© 150°C)A good general all-rounder, resistant to many hydrocarbon based fluids and chlorinated solvents.Although a good general seal material it is attacked by benzene, carbon tetrachloride, diesel fuel, hydrofluoric acid, many oils and sulfuric acid.
– Nitrile (90°C)Offers limited use for certain hydrocarbons, many alcohols, some fluorocarbons and hydraulic oil.Attacked by carbon tetrachloride, ether, Freon 11, hydrofluoric acid, MEK, nitric acid & many natural oils.
– Neoprene (100°C)For lower temperature fluids only. Good with aqueous solutions, organic solvents, most acids and bases.Severely affected by aromatic hydrocarbon based liquids, some oils and strong acids.
– Perfluorinated elastomers

          
(Simrex©, Kalrez© etc. 260°C)

Another great all-rounder but at a cost. These seals are expensive and are challenged by only a few chemicals.

Disclaimer: the above chart is a rough guideline only. Full consultation should be undertaken with all component suppliers when aggressive chemicals are being handled.

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