Pump showing molded thermoplastic casing,
impeller and bearing housing.
PLASTICS or METAL?
Which Material is RIGHT
for your pump?
Bromine, Halogens, Calcium, Chlorine, Corrosives,
CHEM-GARD Horizontal Centrifugal Pump, FLEX-I-LINER
Sealless Self-Priming Peristaltic Pumps, Non-metallic Tank
Pump Systems, SUMP-GARD Thermoplastic Vertical Pump
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PLASTICS or METAL?
Reprinted from CHEM! Materials, Processing & Control
By George Black, Communications Consultant
During the first half of the last century the decision concerning which
material to use for a fluid handling application was relatively simple.
Choices were limited to determining which metal would prove most
cost effective for the service conditions. Corrosion was the main culprit
and all metals corrode. Plastics were not in the equation.
Engineers involved in material selection have extensive laboratory test
data to guide them in the selection of the most cost effective metal for
pumping chemicals. Once this decision was made by materials
engineers, pump selection can be based on a variety of service related
items such as product availability, dependability, published product
performance, initial cost and an analysis of various maintenance factors.
Before long the 18-8 chromium/nickel stainless steel became the
industry standard because of their resistance to corrosive fluids. Pumps
made with alloys of these materials offered the chemical resistance
required for many applications. But there were problems.
Chemicals such as bromine and other halogens can destroy metals,
even upgraded alloys and special metals like nickel. Capital costs for
more exotic metals are unacceptable. In addition, world demands are
changing drastically, with processors demanding even stronger
Chemical inertness, not just low corrosion rates, has become critical.
This adds a new dimension to the material selection decision. The
electronic, computer, semiconductor, pharmaceutical, biochemical and
medical industries cannot tolerate any metallic contamination
whatsoever. This requirement for high product purity, plus the
increased need for materials with greater versatility in handling
extremely aggressive fluids and mixed chemical waste streams is
creating new markets for non-metallics. The standard stainless alloys,
the more exotic alloys and special materials such as titanium can no
longer satisfy all the needs of industry.
Designers are responding to these critical needs by spearheading the
development of pumps made with thermoplastic components. A recent
study indicates that plastic pumps — in a variety of configurations and in
capacities from fractional metering to 1,450 gallons per minute — are
now seeing service in more than 60% of industrial companies.
Despite evidence that shows plastic fluid flow systems have economic
and quality advantages over stainless steel, habit inhibits actions. In
fact, the pharmaceutical and biotechnology industries spend millions of
dollars to compensate for the shortcomings of materials of construction
(i.e. stainless steel).
Technical information is critical when determining which non-metallics
material to use.
1. How do they compare in significant mechanical
In terms of tensile strength, hardness, and impact resistance there is no
contest. Metals are superior. In vertical pumps and peristaltic rotary
pumps these strength factors are not significant, but they can be critical
in horizontal centrifugal pump designs. For this reason plastic
horizontal centrifugal pumps have molded plastic casings and flanges
protected by metal armor to prevent physical damage. This armor
offers an additional advantage. It allows thermoplastic ANSI pumps to
withstand the same nozzle loadings as metal pumps. (Table 1)
Table 1 - Strength of Nonmetallics
Avoid this! This couldn't happen with
In the fifties we found the ideal
corrosion resistant metal wasn't metal.
2. How do they compare in the handling of abrasive fluids?
This is often a surprise to most plant operating personnel who have not
had experience with plastic pumps. Abrasion resistance, as measured
by weight loss in milligrams using standard Taber laboratory abrasion
test equipment, shows that most of the thermoplastics commonly used
in pump construction are superior to stainless steel. Only FRP
fiberglass reinforced plastic and PTFE show less resistance to abrasion
than stainless steel.
3. How do they compare in corrosion resistance?
Tabular data comparing stainless steel with the various available plastics
is available in many textbooks and corrosion tables furnished by the
various product manufacturers. These tables are helpful guides, but
they are no substitute for experience. As stated earlier, all metals
corrode and the rates at which they corrode are carefully recorded. For
the most part, these tests are based on immersion under static
conditions. Conditions in the real world are very different, and the
dynamic action within a pump can seriously change the result. The
thermoplastics, on the other hand, are either inert to the chemicals or
they are not recommended. Application data based on installation
reports, not laboratory tests, are invaluable. When in doubt check the
experience of the pump supplier.
4. How do they compare at elevated temperatures?
Temperature parameters are not critical for metals, but they are for
plastics. As a general rule polyvinyl chloride, polypropylene, and
polyethylene cannot be used above boiling, but the fluoropolymers can
handle boiling temperatures and higher. (Table 2)
Table 2 - Materials of Construction
5. How do they compare in the avoidance of
leaching when handling high purity water?
This is a question raised by the pharmaceutical industry because
leaching of piping materials into ultrapure water must be kept to a
minimum. This comparison between harmful leaching from type 316
stainless steel and the fluoropolymer PVDF clearly demonstrates the
advantages of using plastics rather than metal. (Table 3)
Table 3 - Leaching Comparison Between PVDF and316LSS (ng./mL)
6. How do they compare with respect to cost?
The initial cost of a stainless steel piping system and a PVDF system are
about the same. A polypropylene system will cost substantially less,
assuming it can satisfy the service conditions. But initial cost is only
part of the picture. The lighter weight of the plastic units simplified
installation, reduces the time involved and minimizes downtime with
respect to future maintenance. Threaded joints of plastic components
will not seize. Disassembly for any purpose is easier and faster.
Author's Note: Material selection for pumps and related equipment
becomes more difficult as the conditions of service become more
demanding. Recent studies have shown that when consideration must
be given to the problems involved in handling corrosive chemicals at
varying concentrations of the full pH range, or to secure better
resistance to abrasion, or to avoiding product contamination, or to
reducing the high cost of maintenance and production downtime,
thermoplastic pumps are preferred and more commonly being
specified by consulting engineering firms. In response to
thermoplastics increased acceptance by industrial manufacturing
companies, processing firms and utilities, thermoplastic pumps have
become readily available on short delivery schedules in standard and
custom designs in a wide range of chemically inert materials.
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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Unit 4, Royle Park
Congleton CW12 1JJ