A PVDF impeller in combination with a
lower cost polypropylene casing provided
a cost-effective, durable pump for
handling a corrosive/abrasive
hydrofluoric acid glass etchant.
Critical selection factors for thermoplastic pumps
CHEM-GARD Horizontal Centrifugal Pump, FLEX-I-LINER
Sealless Self-Priming Peristaltic Pumps, Nonmetallic Tank
Pump Systems, SUMP-GARD Thermoplastic Vertical Pump
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Reprinted from Chemical Processing
Dan Besic, Chief Engineer, Vanton Pump & Equipment Corp.
Non-metallics offer broad abrasion and superior
Although the use of thermoplastic pumps has become so common that
some designs are now being carried on the shelves in distribution
stocks, for those applications involving highly corrosive or abrasive
fluids, selection of the right pump and the materials of construction
should be made with care.
A recent article by a group of engineers at Bechtel, San Francisco, CA,
states, "Plastic has yet to realize its potential as a material of
construction for the chemical process industries. The reason for this
under-utilization is a lack of confidence in plastic, usually resulting from
Despite the fact that individual thermoplastics can tolerate a broader
range of corrosive and abrasive conditions than metals, misapplication
is certainly the major reason for pump failure.
The primary limitation on the use of thermoplastics is temperature.
A good guide to upper temperature limits for those materials most
frequently specified in thermoplastic pump construction is given in
Table 1. For use at higher temperatures, complete data on the service
conditions should be discussed with the pump manufacturer.
Because pumps are purchased for specified flow requirements and
heads, these parameters present no problems. Pump manufacturers
provide performance curves for their designs, and these can be readily
checked against a user's requirements.
The most critical criterion is suitability of the material selected for the
fluid to be pumped. This is particularly significant as corrosion and
erosion directly affect maintenance, repair and downtime costs. In
specifying or purchasing a plastic pump, it is important to make sure
that all wetted components are made of the proper material.
Metals have a fixed rate of corrosion in any given fluid. These rates are
listed in available handbooks. For many applications, the rate is so low
that it is insignificant. However, where metallic corrosion is a factor
resulting in excessive maintenance, expensive repair, pump failure, or
product contamination, engineers ten to look closely at the selected
materials. For many of these applications, stainless steel types 304 and
316 are standard.
Engineered thermoplastics compete favorably in price with stainless
steel, and thus, their chemical inertness has become a major reason to
consider them. With installations requiring high alloys or exotic metals,
thermoplastics offer significant savings in both initial equipment costs
and upkeep. With the precaution noted previously about temperature limitations, existing suitability tables for thermoplastics are a good place
to start the evaluation.
There are many applications for which thermoplastic pumps are the only
reasonable choice. These include handling such corrosives as bromine
and strong oxidizing acids. Installations that cannot tolerate metallic
contamination (e.g., ultrapure water, reagent-grade chemicals and
pharmaceutical and electronics industry fluids) are also ideal candidates
for thermoplastic pumps.
Another area where thermoplastic pumps are mandatory is the handling
of waste streams with unknown chemical compositions or where the
composition fluctuates across the full pH spectrum.
Sanitary-design peristaltic pump handles
liquid protein and water at a
pharmaceutical company. The casing is
made of ultra-high molecular weight
(UHMW) polyethylene, and flexible liner
The metal clamping plates holding the
casing and cover were also coated with
ECTFE. The threads were then protected
with specially engineered PVDF sealing
The significant seven
Thermoplastics are so named because they are composed of linear
molecular chains that flow over each other and separate when heated,
then solidify into predetermined shapes upon cooling. They can be
reformed without significant property change upon reheating.
Because of their homogeneous structure, thermoplastics offer high
resistance or complete inertness to many aggressive fluids. They
physical properties of the seven thermoplastics most frequently used
for aggressive fluids are show in Table 2.
Manufacturers of plastics often attempt to convince the user that one
brand or formulation is better than another, but the common perception
that plastics are plastics still prevails. Just as there is a variation in the
characteristics of individual metals, there are significant differences
between one plastic and another, and between filled homogeneous and
virgin materials. The pump manufacturer's selection of the material for
each component is another important performance factor.
Although many people harbor the idea that all plastics are alike or very
similar, or have had a bad experience with an off-the-shelf plastic pump
in a difficult application and have ruled out the choice of a plastic pump,
it is important to keep in mind the statement made by the Bechtel
engineers about misapplication as the prime reason for plastic pump
Table 2 clearly illustrates the differences among the seven most
commonly used thermoplastics. Taken in conjunction with the
temperature ratings for each material, they are a good guide to
selection — but nothing is as significant as direct experience — the user
and the pump manufacturer's.
A second misconception is that the use of plastic pumps can cut initial
capital outlay. The "plastics are cheap" concept is a carry over from the
Japanese toy syndrome of yesteryear. Consider the use of
thermoplastic pumps for a given application because they may prove to
be better in terms of eliminating corrosion, resisting abrasion, lowering
maintenance and reducing spare parts inventory.
The perception that one might replace trouble-free metal pumps with
"cheap" thermoplastic ones is not valid. Engineered thermoplastic
pumps made of virgin, homogeneous molded and extruded shapes are
quality products competitively priced with stainless-steel pumps.
Another misconception is that plastic pumps are simply metal pumps
made out of plastics. This was true to some extent a number of years
ago when pumps made of various thermoset materials were
introduced. These fiberglass-reinforced polyester/glass-reinforced
plastic (FRP/GRP) pumps are very similar to metal pumps in design and
They can successfully handle many corrosive materials at temperatures
to 250°F. Because of their composite fiber/resin construction, however,
they are subject to absorption, wicking and bleeding out of the
absorbed chemicals. This can cause contamination of the pumped
fluid, as well as deterioration of pump components. They are severely
limited for use with abrasive fluids.
Thermoplastic pumps, on the other hand, are not metal pump clones.
They are engineered to take advantage of the unique characteristic of
plastics. The use of molded shapes provides for smooth interior
contours and surfaces, minimizing friction and turbulence.
Identifying plastic pumps, on the other hand, with metal pump
characteristics can lead to troublesome and costly conclusions. A good
example is the simplified L3/D4 ratio for shaft deflection. This
abbreviated version of the full formula (Fig. 1) for shaft deflection used
by mechanical engineers works well when comparing two metal pumps
of similar materials.
When used to compare a metal pump with a plastic one, however, the
lighter weight of the plastic impeller is not taken into consideration in
determining the downward vertical force. Because a maximum ratio of
50 is preferred, the plastic pump is often not even considered for the
application. Thermoplastic pumps, with the lightweight impeller and
smaller diameter shaft, are frequently designated "not suitable" when
the L3/D4 ratio is used, despite the many advantages they offer.
Thermoplastic pumps are engineered to take advantage of the unique
characteristic of plastics. They use of molded shapes provides for
smooth interior contours and surfaces, minimizing friction and
Another common misconception is that plastics are for small pumps. It
is true that many small pumps are specified in plastics, but thee is
nothing small or lightweight about polypropylene thermoplastic sump
pumps that stand 25 ft. tall and weigh more than 2,000 lb. that have
been built for handling corrosive waste and stormwater runoffs, or the
large 6 X 4 centrifugals that handle scrubbing liquids for hydrochloric
acid fumes at large galvanizing and plating facilities.
Figure 6 - This magnetic-drive,
close-coupled pump is shown with wet end
opened and the solid,
molded-thermoplastic casing, impeller
and bearing housing exposed. All
components are produced from
polypropylene or polyvinylidene
fluoride. In this design, no metal
comes in contact with the pumped fluid.
The fact is that hundreds of thousands of engineered thermoplastic
pumps are now handling corrosive, abrasive, hazardous and ultrapure
fluids in process lines, laboratories, transfer operations and an endless
list of chemical, industrial, electronic and municipal waste applications.
Selected case histories below highlight some of the unusual applications
associated with the unique characteristics of thermoplastic materials.
They should make it clear that whenever there is a danger of corrosion
or abrasion in the pumping of acidic, caustic, hazardous, toxic or
noxious fluids, or those that cannot tolerate metallic contamination,
thermoplastics should be considered.
Mobile unit for liquid protein
A large pharmaceutical manufacturer required a metering pump of
sanitary design that could readily be taken to a variety of plant locations.
Mobility was significant, so was the necessity of avoiding any metallic
components in contact with the liquid protein. The engineers insisted
on a thermoplastic material and a design that eliminated internal
crevices, dead areas and threads where bacteria might hang up and
The answer was found with a skid-mounted lightweight peristaltic-type
rotary pump with its casing made of ultra-high molecular weight
(UHMW) polyethylene and its flexible liner made of neoprene. The
interior surfaces of the pump casing were flame polished, and the
suction and discharge quick-disconnect fittings were spin-welded into
position to eliminate threads. Fluid traps and threaded connections
were thus avoided.
Pumping is accomplished by means of a precision-molded phenolic
rotor mounted on an eccentric shaft. The oscillating motion of the rotor
against the inner surface of the flexible liner creates a progressive
squeegee action on the liquid trapped between the outer surface of the
flexible liner and the inner surface of the casing or pump body. Only
the liner and casing contact the fluid. The pump is self-priming and
sealless, with metering controlled by a variable speed drive motor.
Ultrapurity for hydrogen peroxide production
To assure maximum purity for the production of hydrogen peroxide, and
international producer of this universal chemical decided to specify
centrifugal pumps constructed of ECTFE.
Specifications for the centrifugal pumps called for this virgin, unpigmented,
homogeneous fluoropolymer to be used for the casing, impeller and shaft
sleeve. The seal rings were specified in Viton® fluoroelastomer, and the
casing gasket in Teflon® PTFE. The rotating face of the mechanical seal was
furnished in Teflon and the fixed face in ultrapure ceramic. External metal
parts were to be epoxy coated. The pumps were required to handle 70%
H•0• at 50 gpm against a total dynamic head of 80 ft at a temperature of
Glass etching with HF
When metal horizontal centrifugal pumps failed repeatedly, causing high
replacement costs and extensive downtime, a major manufacturer of
decorative glass objects switched to thermoset pumps to handle the
etching solution. This highly corrosive/erosive hydrofluoric acid (HF)
and abrasive grit mixture proved destructive to the fiberglass-reinforced
composite material, and severe production losses resulted. A decision
was made to test an all-PVDF pump in this service because this
fluoropolymer has superior abrasion resistance in addition to its
resistance to HF.
The test installation was successful — but the high cost of the PVDF
pump was a cause for concern. At a meeting between the pump
manufacturer and the maintenance chief, it was decided to test a
second pump, utilizing PP for the casing and flanges, and limiting the
use of PVDF to the impeller, the part subject to the most erosion from
the abrasive mixture. The compromise worked. As the other pumps in
the system failed, they were all replaced by the customized
Fluoropolymers for bromine
Handling bromine, which rapidly attacks most metals, can be a major
headache. In a case of sump pumps handing the corrosive halogen, the
problem was solved by using polyvinylidene fluoride (PVDF) for all
structural components in the pumps. The stainless-steel shaft was
completely isolated from the liquid by a heavy sectioned PVDF sleeve,
welded to a PVDF impeller.
To retain the hazardous bromine vapors, a specially designed PVDF
stuffing box was packed with woven polytetrafluoroethylene (PTFE).
Because of the heavy weight of bromine (specific gravity = 3.11), solid
PVDF casing and flange bolts would not be able to withstand the high
pressures, so metal bolts had to be used. The exposed metal created a
problem, which was solved by coating each of the metal bots with 50
mils of ethylene chlorotrifluoroethylene (ECTFE).
Engineered thermoplastic pumps are playing an increasingly significant
role in the handling of fluids which aggressively attack most metals.
Because the choice of materials is relatively large, as it is with metals, it
is important to understand how variations in composition and
manufacture can affect performance. Additives are often incorporated
to simplify molding or increase strength, and various pigments may be
added to identify a particular type of material or the manufacturer.
For most applications, these additives prove helpful or harmless. Where
ultrapure fluids are being pumped, however, or in applications which
cannot tolerate any contamination, virgin, homogeneous, unpigmented
thermoplastics are required.
In the 1950, Vanton developed a revolutionary all-plastic pump for use in conjunction with the first heart-lung device. The design limited fluid contact to only two non-metallic parts: a plastic body block and a flexible liner. This was the birth of our Flex-I-Liner rotary pump. Its self-priming sealless design made it an industry standard for the handling of corrosive, abrasive and viscous fluids as well as those that must be transferred without contaminating the product. Vanton now offers the most comprehensive line of thermoplastic pumps in the industry.
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(+44) 01260 277040
Vanton Pumps (Europe) Ltd.
Unit 4, Royle Park
Congleton CW12 1JJ