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TL-132

 

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

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 Chemical Processing

Dan Besic, Chief Engineer, Vanton Pump & Equipment Corp.

 

Non-metallics offer broad abrasion and superior

corrosion resistance

 

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

its misapplication."

 

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

of neoprene.

The metal clamping plates holding the

casing and cover were also coated with

ECTFE. The threads were then protected

with specially engineered PVDF sealing

nuts.

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.

 

Common misconceptions

 

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

failure.

 

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

performance.

 

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

turbulence.

 

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.

Satisfied users

 

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

multiply.

 

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

85°F.

 

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

thermoplastic design.

 

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).

 

An overview

 

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.

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