Process pumps and valves with PFA linings

Is the new fluoroplastic PFA-P a highly permeation-resistant revolutionary material? On many plants, PFA-P will lead to a significant prolongation of the service life of the pumps and valves used

Is the new fluoroplastic PFA-P a highly permeation-resistant revolutionary material? This assessment would perhaps be somewhat presumptuous, but for many plant operators PFA-P will lead to an equally significant prolongation of the service life of the pumps and valves used.

Linings made of the well-known thermoplastic fluoroplastic PFA (perfluoroalkoxy) have been used since roughly 1980 as an alternative to high-alloy, expensive metals for pumps, valves, control valves, containers etc.

PFA has superseded PTFE, which is processed in a pressure sintering operation, to a large extent.

The PFA success is due to several important advantages over PTFE.

PFA is processed in a transfer moulding operation and as a result the lining wall thicknesses can be accurately defined and reproduced.

PFA is almost transparent and therefore permits much more reliable quality control.

Thanks to its dense molecular structure, PFA has generally much lower permeation rates than PTFE with the same wall thicknesses, but it has the same chemical and thermal resistance properties.

Porosity owing to production fluctuations are excluded.

PFA is justifiably the material of first choice in applications with permeating media, as innumerable installations worldwide prove.

The barrier effect of pure PFA.

This is the key issue: even the good barrier effect of pure PFA with the usual wall thickness of +/- 3 mm is often not sufficient to achieve satisfactory service lives of the units with media which especially tend to permeate, such as chlorine, bromine or fluorine compounds.

If the chemical penetrates the lining, this may cause corrosion on the pressure-bearing metal body and perhaps even failure of the unit.

Experience shows that an increase in the PFA lining wall thickness to 5-6 mm produces substantially longer service lives.

This is implemented, for example in ball and globe control valve bodies.

However, in the case of internal wetted components, such as shut-off elements, stems and pump shafts, inner magnetic assembly linings and cans, the wall thicknesses cannot be increased, or only to a very limited extent, for functional reasons.

This is exactly where the weaknesses of the unit are.

The new development of PFA-P.

The aim was therefore to provide a lining material even more resistant to permeation.

The material engineers concentrated their efforts on creating a PFA variation which could be subjected to thermoplastic processing but with a much higher permeation resistance and with the same chemical resistance and temperature resistance up to 200C.

With the compound PFA-P (the "-P" stands for permeation), a material is now available which satisfies these general conditions.

Long-term tests in a laboratory and in practice confirm the high expectations.

Chlorine laboratory tests.

As an example, the permeation of chlorine gas was examined under operating conditions.

In this test, samples made of PFA-P and pure PFA with different thicknesses were compared with each other at temperatures of up to 150C and pressure differences of up to 7 bar.

The result: The permeation through PFA-P is roughly half of that compared with pure PFA.

This effect is particularly noticeable with the critical elevated operating temperatures.

Tests with the extremely mobile helium as a test medium also showed a similar reduction in permeation.

Examples from practice.

* MCA, monochloroacetic acid, at approx 150C: The ball valve KN/F-P, DN 25 with a PFA-P lining, was tested as an alternative to special valves made of special materials and after 1.5 years of continuous use still shows no signs of failure but is considerably cheaper.

* FHC-H2SO4-HF mixture, 180C: After max 1 year in each case the PFA-lined magnetic drive pump failed owing to core corrosion on all wetted components.

Its pump housing could well have been used for roughly another year thanks to the thick-walled lining.

The MNK 50-32-160 now in service, completely PFA-P-lined, is still operating after 2 years without any repairs.

* H2SO4-HF mixture, 125C: The fluid diffused through the standard inner can made of modified TFM-PTFE and dissolved some resin out of the outer, pressure-bearing CFRP can of the magnetic drive pump.

This effect was audible after a few months of service because the resin particles which had hardened in the drive section of the pump were swirled around.

The can was replaced with one made of PFA-P.

Examinations after another 6 and 12 months showed that this problem had been rectified.

Apart from the ball valves and pumps described here, control valves, sight glasses, sampling valves, boiler drain valves and safety valves can also be produced with a PFA-P lining.

The operational experience gained confirms the positive properties.

General specification of the PFA-P.

The carrier polymer PFA is enriched with an extremely corrosion-resistant filler.

This filler extends the diffusion paths and acts as a diffusion barrier.

As a lining material, it covers the same pressure and temperature ranges as pure PFA for valves, globe control valves, pumps etc, ie from -60C to + 200C and from vacuum up to 25 bar.

Both the PFA base material and all fillers and auxiliary agents used in the production process are FDA-compliant.

The extraordinarily good suitability for use with highly permeating media makes PFA-P-lined pumps and valves interesting for a large circle of users.

Moreover, the chemical and physical properties relevant to applications permit the use of PFA-P where pure PFA has proved successful.

In many cases it will already be sufficient to line individual components with PFA-P instead of with PFA or PTFE while the main part of the valve still remains lined with standard pure PFA.

A general "all or nothing" policy is not advisable: The problem in question governs whether a PFA-P lining is necessary for the entire unit or only for critical components.

Definitions of permeation and diffusion.

Permeation is the term used for the transport of substances through solid materials at molecular level.

Permeation always runs in the direction of the lower partial pressure.

Polymers are already permeable at lower temperatures whereas metals only become appreciably permeable at elevated temperatures.

The process of permeation is divided into three phases.

* Adsorption: Gases, vapours or chemicals dissolved in liquids or suspended substances are absorbed on the surface of the plastic component.

* Diffusion: The adsorbate diffuses at molecular level in and through the plastic.

* Desorption: The adsorbate escapes as gas on the other side of the plastic body.

Diffusion is a compensation process in which molecular particles try to achieve a concentration or density balance and in doing so flow from the chemical into the solid surface of the plastic part.

Diffusion is therefore one phase in the permeation process.

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