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Designed for continuous use
Valves and pumps with a permeation-resistant lining
Author: Manfred Kluge, ITT Richter Chemie-Technik
For many plant operators PFA-P leads to a significant prolongation of
the service life when used as a lining in valves and pumps. This is
attributable to the strong resistance of the material to highly
permeating media. The chemical and physical properties relevant to
applications permit the use of PFA-P where pure PFA has proved
successful.
Linings made of the well-known thermoplastic fluoroplastic PFA
(perfluoroalkoxy) have been used 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has superseded PTFE, which is processed in a pressure sintering
process, in these applications to a large extent. PFA’s success is due
to several important advantages over PTFE. PFA is processed in a
transfer moulding process 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. Moreover, 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.
Barrier effect of PFA
With media which especially tend to permeate, such as chlorine,
bromine or fluorine compounds 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 equipment. 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 56 mm already produces
substantially longer service lives. This is implemented, for example, in
ball and globe control valve bodies. There is a quadratic relationship
between the breakthrough time, i.e. the time the medium takes to
permeate through the plastic and emerge again, and the wall thickness.
In most applications the breakthrough time is higher than the useful
life of the units because the temperature and pressure gradients driving
the permeation process decrease significantly towards the outside. This
is also seen in pump housings where the PFA linings of 5-6 mm normally
are particularly thick-walled anyway.
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 permeation
can cause an too early failure of the unit.
Higher permeation resistance
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 a temperature resistance from
-60 to +200°C (-75 to 400°F). With the compound PFA-P (the “-P” stands
for permeation), a material is now available which satisfies these
general conditions. The carrier polymer PFA is enriched with an
extremely corrosion-resistant filler. This filler extends the diffusion
paths and acts as a diffusion barrier. The surface resistance of PFA-P
as regards electric conductivity corresponds to that of PFA. As a lining
material, it covers the same pressure and temperature ranges as pure PFA
for valves, globe control valves, pumps etc., i.e. from -60 to + 200°C
(-75 to 400°F)and from vacuum up to 25 bar (360 psi). Both the PFA base
material and all fillers and auxiliary agents used in the production
process are FDA-compliant.
In order to confirm the permeation resistance, the permeation of
chlorine gas was examined under operating conditions as an example. In
this test, samples made of PFA-P and pure PFA with different thicknesses
were compared with each other at temperatures of up to 150°C (300°F) and
pressure differences of up to 7 bar (100 psi).
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. The permeation rates of solvents tend to be considerably
lower but experience shows that they follow the same trend and so PFA-P
should produce substantial improvements here, too.
Examples from practice
A ball valve KN/F-P, DN 25, lined with PFA-P, was tested in an
application in which monochloroacetic acid was present at approx. 150°C
(300°F), as an alternative to special valves made of special materials.
Even after 1.5 years of continuous use this ball valve showed no signs
of failure. And it is considerably cheaper than the special valve
previously used. The ball valve of the KN series lined with PFA-P covers
a broad operating spectrum with a temperature range of -60 to +200°C
(-75 to +400°F) and a pressure range from 1 mbar up to 16 bar (0,015 to
235 psi). The KN modular system comprises nominal widths of DN 15 to 200
and ½” to 8” with face-to-face dimensions and flanges to ISO
5752-1/7005-2 and ANSI B 16.10/B 16.5 Cl. 150. PFA, FEP, antistatic
PFA-L and PFA-P are available as lining materials. ITT Richter decides
on the basis of the problem in question whether a
PFA-P lining is necessary for the entire unit or only for critical
components. In many cases it is 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 PFA.
The valve is type-tested, complies with the German Clean Air Act and is
also approved to GGVSE/ADR/RID/TRT24 for the transport of hazardous
goods in tanks. Depending on the application, the KN can be equipped
with a PFA or Al2O3 shut-off ball or TF-ball. With the V-control ball
and the special hysteresis-free coupling between the ball valve stem and
the actuator, the KN becomes a complete control valve for kv 0.8 to 400
(Cv 0,7 to 345).
Application also in pumps
The PFA-P linings can also contribute to substantial extensions to
the service life of pumps. In a specific case the normal service life
of a PFA-lined magnetic drive pump for conveying trifluoroacetic acid
(50°C = 120°F) was roughly three months. The pump failed because the
internals were heavily swollen by permeation. The metallic base material
was attacked, the rotating unit stuck. The PFA-P-lined MNK pump now in
use has been running for 12 months without any indications of a change
in the material. The MNK series produces flow rates of 1 – 375 m³/h (4.4
to 1,650 US gpm resp. 3.7 to 1375 Imp gpm) and delivery heads of up to
145 m (475 ft). The pumps are available in foot-mounted and
close-coupled designs for temperatures from -60 to +200°C (-75 to 400°F)
and operating pressures of up to 16 bar (235 psi), with accessories even
up to 25 bar (360 psi). PFA/PTFE, antistatic PFA/PTFE, the new PFA-P and
PP/PE-UHMW can be used as the lining material. The wetted surfaces are
completely metal-free.
Thanks to the non-metallic, eddy-current-free separating can the MNK may
also be used for media close to the boiling temperature. For example, a
PFA-lined magnetic drive pump used in a FHC-H2SO4-HF mixture at 180°C
(360°F) failed after max. 1 year in each case owing to core corrosion on
all wetted components. The MNK 50-32-160 now in service, completely
PFA-P-lined, is still operating after 2 years without any repairs. In
another application involving an H2SO4-HF mixture (125°C = 260°F) the
fluid diffused through the standard inner can made of modified TFM-PTFE
and dissolved some resin out of the outer, pressure-bearing CFRP
separating can of the magnetic drive pump MNK. 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 as a result. Apart
from the ball valves and pumps described here, control valves, sight
glasses, sampling valves, vessel drain valves and safety valves can also
be produced with a PFA-P lining. <<
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