Photo 1. Cut-a-way view of an ANSI
thermoplastic end suction centrifugal
pump for flows to 1450 gpm (5,488 lpm),
heads to 400 feet (122 m) and
temperatures to 275°F (135°C). These
pumps, which meet ANSI B73.1 process
pump standards, incorporate the wide
open seal area and retractable front
bearings to simplify maintenance and
provide ample room for most types of
commercially available single and double
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Reprinted from Pumps and Processes
By George Black, Materials Engineering Consultant, Vanton Pump & Equipment Corp.
A fresh look at recent design developments
Every research study on pump usage clearly indicates that horizontal
centrifugal pumps are the most widely used pump type for industrial,
chemical and municipal processing, and waste treatment operations.
Many years ago, the oft-quoted professor Harold Woodhouse of
Stevens Institute of Technology, made this statement in his mechanical
engineer's Guide to Selecting Centrifugal Pumps: "There is probably no
other piece of mechanical equipment so deceptively simple in
construction and yet so complicated in application... perhaps, no other
piece of equipment is made in so many styles and designs, a number of
which are the result of evolutionary experience which has caused a
departure from theoretical design rules. Still misapplications occur.
Systems fail to operate as expected, and as required."
There appear to be two major reasons for the misapplications referred to
by Professor Woodhouse. The first, and perhaps the most significant, is
the tendency to automatically replace a failed pump with a new one of
the same design. This is the easiest approach because it requires the
lowest level of expenditure approval. Unless the service rendered by
the failed pump is economically intolerable, companies learn to live with
"acceptable" repetitive maintenance costs. In most manufacturing
operations, it is much easier to get authorization for a direct product
replacement than an OK to install something new, particularly if the
reason for the change might embarrass the individual responsible for
the original pump selection, or if the suggested change requires an
unbudgeted capital expenditure.
The second reason is tied to a number of causes such as the limited
awareness of product upgrades in terms of design, and more recently,
adequate knowledge about the availability and potential use of new and
modified materials of construction (Table 1). Unfortunately, both of
these get short shrift in the engineering curriculum at most universities,
and to some extent, even in trade magazine editorial coverage. When I
was young and twenty, we couldn't wait for the next issue of Product
Engineering or Materials and Methods — two publications that
concentrated on design changes to secure higher and more cost
effective productivity. Unfortunately, both of these magazines are no
longer being published, and the Internet has not begun to fill the void.
THE LATEST DEVELOPMENTS
Horizontal centrifugal pumps have been around for a long time, but
design and variations in configuration and material selection have not
stood still. Let's take a look at some of the developments in
thermoplastic pump design that have grown out of "evolutionary
experience," and which directly affect performance and maintenance,
and the ability to minimize what Professor Woodhouse referred to as
One area that has had and continues to have extensive coverage in both
advertising and editorial is the subject of fluid leakage and related seal
maintenance. The obvious answer has been the move to sealless
horizontal centrifugal pumps. These designs automatically prevent fluid
leakage and reduce maintenance because they eliminate the use of
shaft seals. Magnetically coupled end suction centrifugal thermoplastic
pumps, for example, are now readily available from many dependable
manufacturers in a broad range of chemically-inert thermoplastics.
These should be thoroughly explored and carefully considered in terms
of conditions of service and cost. But let us not ignore or overlook the
availability of improved seal materials, creative seal construction and
placement, as well as back pull-out thermoplastic pump designs. All of
these have helped to significantly reduce the cost of seal maintenance
in conventional centrifugal pumps and deserve close attention.
Higher purity of chemically-inert thermoplastics suitable for use at
elevated temperatures and offering superior resistance to corrosion and
abrasion than the stainless steels have recently become available.
These have greatly extended the suitability of thermoplastic pumps for
applications not previously considered feasible. Consider the recent
studies in the semiconductor and pharmaceutical industries, which
indicate quite strongly that the thermoplastics, when supplied in their
homogeneous, natural state, ensure greater electronic product reliability
so vital to the former, and higher degrees of chemical and water purity
demanded by the latter. This relatively new knowledge has opened
many new avenues for consideration by system designers and should
be high on the "what's new" list for pump users and specifiers.
Unfortunately, the hesitancy to accept change continues to be a difficult
hurdle to jump.
The leaching differences between stainless steel and PVDF, for example,
illustrate why semiconductor plants have opted to use PVDF and why
even a biotech facility concerned about producing the highest purity
water possible would opt for PVDF or other fluoropolymer materials.
Companies that process or utilize high purity acids in the semiconductor
industry have settled upon fluoropolymers as a material of construction
because they can be manufactured in such a manner that no foreign
additives, that later could become extractables, are needed.
Pharmaceutical plants need piping and other fluid-handling components
that can also withstand exposure to hot water or steam used in
sterilization. It is important that pipes, pumps, valves and other wetted
surfaces not promote microbial activity. An equally strong concern is
that these components not contain extractable substances that will
leach into and contaminate high purity water. The leaching differences
between stainless steel and PVDF are shown in nanograms per milliliter
in Table 2.
PEDESTAL POWER FRAMES
More than 20 years ago, in response to customer demands for longer
seal life and lower seal maintenance, pump engineers created a
pedestal power frame construction design that simplified seal
inspection in the field (Photo 1). This design simultaneously permits
seal inspection and repositioning of the inboard shaft bearing closer to
the impeller, keeping shaft overhang and deflection at a minimum. This
inspection and adjustment is accomplished without removing the pump
and without affecting shaft alignment. It is made possible by a series of
rigid bars parallel to the axis of the shaft, on which the flanged ball
bearing assembly can easily be moved, repositioned and locked to the
shaft. It also permits the bearing assembly to "float" parallel to the axis
of the shaft as it automatically compensates for the differential in
thermal expansion between the stainless or other alloy steel shaft and
the cast iron pedestal (Photos 2a and 2b).
This unique sliding bar bearing assembly construction received the
coveted Vaaler design award on its announcement, but it took many
years before it became an accepted standard on conventional end
suction horizontal thermoplastic pumps. If your centrifugals are giving
you excessive seal maintenance problems, check to see if they
incorporate the sliding bar pedestal or something similar. As centrifugal
pump designs with back pull-out construction became popular, the
sliding bar pedestal design was even incorporated into the ANSI line of
thermoplastic pumps because this construction also provides for a
much larger, more open seal area than conventional power frame
designs. It also permits pumps with this design to accept most
commercially available single and double mechanical seals. This
attribute enables you to select from a greater selection of suitable
mechanical seals and opens an additional approach to reducing seal
leakage as well as maintenance costs.
MOUNTING THE SEALS
Another simple concept made feasible by the sliding bar design is
directly related to reducing initial seal costs and repetitive maintenance.
This is the "reverse mounting" of seals so that the nonmetallic seal
component — rather than the metal one — is in contact with the
corrosive fluid. Since the nonmetallic materials tend to have greater
chemical resistance, seal life is automatically extended. In addition, seal
reversal also makes it unnecessary to utilize high cost exotic metal
Since the literature is replete with competitive information about seal
selection and design, no attempt will be made here to duplicated the
recommendations so readily available. I would suggest, however, that
time spent with a seal specialist is well worth the investment.
Maintaining accurate records on seal maintenance and discussing the
data with your pump supplier can be very rewarding. In selecting
centrifugal pumps, don't overlook design differences that provide you
with the greatest variety of seal choices and arrangements, and base
your decision on construction arrangements that permit seal selection
that best fits your own application. Consider your choices among single
seal with water jacket, single seal with water flushface for fluids that
leave crystal deposits, single seal with direct product or water flush,
double mechanical seal with water jacket, and balanced mechanical
If your application calls for a sealless thermoplastic centrifugal pump,
consider available offerings of magnetically-driven designs conforming
to ANSI B73.1 end suction process pump specifications and Hydraulic
Institute standards. Most of these design factors are critical for all
pumps handling corrosive, abrasive and other aggressive fluids. A few
specifically pertain to magnetically-driven designs.
1. All fluid-contact components of conventional or mag drive pumps
should be furnished in solid, nonmetallic materials inert to the specific
chemicals. (Photo 3).
2. The high performance rare earth inner magnet rotor assembly should
be encapsulated in thermoplastics and isolated from the fluid to avoid
troublesome eddy currents that reduce pump efficiency by loss of
3. The stainless or other alloy steel shaft should be sleeved and the inner
magnet rotor assembly completely encapsulated in the appropriate
thermoplastic to isolate it from the fluid.
4. Metal armor provides structural protection to the thermoplastic
molded casings. ANSI-conforming designs should permit pumps to
withstand the same nozzle loadings as ANSI metal pumps (Photo 4).
5. The pump design should incorporate fresh water flushing capability
and wide open fluid passages to enable continuous cooling and
capability for handling slurries and viscous fluids (Fig. 1).
Europe appears to have led the move toward extensive use of
close-coupled thermoplastic centrifugal pumps. Recently, however the
interest has grown in the United States because of the demand by
system designers and OEMs for space saving and unit cost reduction.
As a result, pump specifiers and users now have a choice of high
quality, end suction nonmetallic pumps with footprints approximately
20% shorter than standard foot-mounted designs (Photo 5).
Pumps with this compact configuration are now available in
polypropylene and polyvinylidene fluoride (PVDF), and in a choice of
tangential as well as centerline discharge. Both designs offer easy
access for seal maintenance, and for removal without disturbing
Where space is critical and cost savings important — and where
dependable transfer of aggressive or ultrapure fluids is a must, these
close-coupled designs should be looked at. They are designed to
accommodate standard C-face motors and most commercial
Photos 2a and 2b. The sliding bar design
lets users pull the bearing back for
easy inspection and maintenance.
Photo 3. Open pump showing solid molded
thermoplastic casing and impeller.
Photo 4. Metal armor provides structural
protection and permits this ANSI mag
drive pump to withstand the same nozzle
loadings as metal pumps.
Photo 5. The comparative size difference
between standard and close-coupled
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