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TL-162

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

mechanical seals.

What's New with

Thermoplastic Centrifugal

Pumps?

INDUSTRY:

ENTITY:

SOLUTION(S) PUMPED:

PUMP TYPE(S):

General

Various

Various

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 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

"misapplications."

 

SEALING OPTIONS

 

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.

 

NEW MATERIALS

 

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

seals.

 

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

seals.

 

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

magnetic force.

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).

 

CLOSE-COUPLED PUMPS

 

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

existing piping.

 

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

mechanical seals.

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

designs.

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.

 

 

Stay in touch

mail@vantonpump.com

(+44) 01260 277040

Vanton Pumps (Europe) Ltd.

Unit 4, Royle Park

Royle Street

Congleton CW12 1JJ

UNITED KINGDOM

www.vantonpump.com