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Pump showing molded thermoplastic casing,

impeller and bearing housing.

PLASTICS or METAL?

Which Material is RIGHT

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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

corrosion resistance.

 

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

properties?

 

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

thermoplastics.

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.

Copyright 2016 - Vanton Pumps (Europe) Ltd - All rights reserved

About Us

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

Royle Street

Congleton CW12 1JJ

UNITED KINGDOM

www.vantonpump.com