TL-123

 

Overhead view of four thermoplastic

centrifugal pumps handling discharges

from the production area of a major

silicon wafer facility in the U.K.. The

process fluids include hydrochloric,

nitric, sulfuric, hydrofluoric and

phosphoric acids as well as hydrogen

peroxide, caustic soda and other

chemicals in unknown quantities.

Gaylord H•SO— fume scrubbers

usethermoplastic pumps and thermoset

tanks to resist corrosives. Teflon

rotary pump with Hypalon flexible liner

transfers 50% NaOH to the fiberglass

tank. The caustic in the tank is

circulated by a polypropylene sump pump

with cantilevered integral pump/motor

shaft sleeved in plastic.

Non-metallic pumps

withstand a wide range of

severe service applications

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 SYSTEMS

By Dan Besic

 

Non-metallic pumps withstand a wide range of

severe service applications

 

A recent research program conducted jointly by Pollution Engineering

magazine and the customer service division of Vanton Pump &

Equipment Corporation queried pump specifiers for the factors

contributing to their selection of non-metallic pumps.

 

Although many advantages of non-metallic pumps were indicated,

corrosion resistance was clearly the number one factor – receiving 58%

of the responses. This chemical inertness permits the safe application

of a single pump for the complete pH range from 1-14 as well as for

salts, halogens, solvents and other troublesome chemicals.

 

Abrasion resistance, particularly when coupled with resistance to

corrosion, was also frequently cited as grounds for selecting a

non-metallic pump. Lower maintenance, longer service life, prevention

of metallic contamination, lighter weight, and lower cost when related

to high alloy and exotic metal pumps were also mentioned.

 

When specifying a non-metallic material for the wet-end of the pump or

other fluid contact components, and following factors should be

considered:

 

1. specific chemical(s) to be pumped and the possible range of process

fluids

2. degree of abrasiveness

3. temperature range

4. pH range

5. importance of metal contamination avoidance

6. field experience with similar installations

The non-metallic materials most often specified are listed below:

Thermoplastics

• Polyvinyl chloride (PVC)

• Chlorinated polyvinyl chloride (CPVC)

• Polyethylene (PE)

• Polypropylene (PP)

• Polyvinylidene fluoride (PVDF)

• Ethylene chlorotrifluoroethylene (ECTFE)

• Polytetrafluoroethylene (PTFE)

Thermosets

• Fiber reinforced plastics (FRP/GRP)

 

To select the most cost effective construction material for a given

application, it is also important to understand the basic characteristics

of thermosetting and thermoplastic materials.

 

Thermosets cannot be remelted or reshaped once they have been cured
by heat or chemical means. The thermoset materials most commonly

specified for pump components consist of vinyl or epoxy resins

reinforced by glass fiber. This composite structure provides relatively

high mechanical properties, but lessens resistance to corrosive and

abrasive chemicals.

 

Thermoplastics, on the other hand, are homogeneous polymers which

can be remelted, remolded and reshaped. Their uniform structure

provides increased chemical resistance over a broad pH range, freedom

from product contamination and excellent abrasion resistance.

 

CLOSE-UP ON THERMOSETS

 

The glass fibers employed to reinforce the plastic resins are inert to

most chemicals and the resistance of the composite is generally limited

by the specific resistance of the initial and/or added polymers, as well as

by the characteristics of the composite construction.

 

Pumps manufactured with thermoset structural components have

strengths similar to metal pumps. These pumps do not usually require

metal armor unless they are installed in an area subject to falling metal

objects. Although high nozzle loads do not generally result in nozzle

failure of thermoset pumps, they can cause shaft misalignment. And

these pumps should only be exposed to nozzle leads at 1/2-2/3 that of a

metal pump of corresponding size. With respect to corrosion,

thermoset pumps can successfully handle a wide range of acids,

caustics, solvents and salts at operating temperatures to 250°F.

Horizontal centrifugal FRP/GRP pumps are available for flows to 5000

gpm and heads to 400 ft. Vertical sump pump designs are available for

flows to 4500 gpm and heads to 275 ft.

 

The vinyl ester resin composite pumps are generally recommended for

their corrosion resistance, whereas the epoxy resin materials offer

superior resistance to solvents. Special composite formulations are

available for handling mildly abrasive solutions.

 

Since the fiber reinforced composite material is non-conductive, no

electrochemical corrosion can occur. But the resin, fiber and other

components in the composite can degrade. Such degradation, if it

occurs, will tend to occur rapidly. An exception to this rule is the

reaction with chlorine and oxidizing chemicals, which can slowly attack

resin fiber composites in a manner similar to metal.

 

The interface between the fiber and the resin may be a source of trouble.

If the process fluid is absorbed into the interstices by capillary action,

the interface resin material may subsequently bleed out and

contaminate the fluid being pumped. The wicking or bleeding action is

particularly significant when a single pump is employed for a variety of

fluids. The leaching out of the resin by corrosive attacks, which occurs

when thermosets have been misapplied, may also cause impeller

imbalance.

 

Another concern with thermoset composites is the softening of the

surface by alkaline salt slurries. This condition, which has been referred

to as "cor-brasion," tends to limit the use of composite to non-abrasive

applications and also limits their use with caustics.

 

CLOSE-UP ON THERMOPLASTICS

 

Choosing the specific thermoplastic for a given pump application begins

with consideration of the operating temperature range. Thermoplastic

pumps are generally recommended for service over a range from -20°F

to 275°F. These pumps can handle flows to 1450 gpm and heads to 280

ft. Even though some of the materials can be exposed to temperatures

from cryogenic to 500°F, the various elastomeric components employed

as seals for centrifugal pumps and flexible liners for rotary (peristaltic)

pumps limit the pump application to the indicated range. Table 1 gives

physical property data, including maximum service temperatures, for

the thermoplastics most frequently used in pumps.

 

• Vinyls

Vinyls, such as PVC and CPVC, are widely used for their relative low

cost, good physical properties and broad chemical resistance. For

temperatures to 140°F, the low cost PVC gets the nod. If temperatures

to 210°F are anticipated, CPVC should be specified. Both materials resist

acids, alkalis, salt solutions, aliphatic hydrocarbons and oils, but they

tend to swell in ketones, esters and aromatic hydrocarbons. CPVC offers

greater impact strength and abrasion resistance and is often

recommended when higher mechanical strength is required at elevated

temperatures.

• Polypropylene

Polypropylene is the lightest of the thermoplastics in general use for

pump components. Its specific gravity of 0.91 is significantly less than

the densities of the vinyl polymers as shown in Table 1. Its tensile

strength is slightly lower than the vinyls, and its heat resistance falls in

between, with a maximum operating temperature of 185°F. Because of

its low cost, high strength-to-weight ratio and good resistance to a

broad range of solvents, acids, alkalis and salts, polypropylene has

become the industry standard for general corrosion service. It is not,

however, recommended for use with strong acids, chlorinated

hydrocarbons or aromatics. Polypropylene is especially suited to

handling wastewater effluents and waste treatment chemicals used by

industrial and municipal plants and laboratories. The petroleum industry

is also a large user of polypropylene pumps because the polymer

resists sulfur-bearing compounds. Since unpigmented, natural

polypropylene is affected by UV light, it is frequently pigmented or

otherwise stabilized when specified for outdoor use.

• Polyethylene

Although polyethylene is widely employed for industrial applications in

Europe, it is used on a limited basis in the U.S. because the ultra high

molecular weight (UHMW) formulation cannot be processed by

conventional injection molding methods. Polyethylene has properties

very similar to polypropylene. The specific gravity is slightly higher, and

the tensile strength is slightly lower (Table 1). Its heat resistance is

suitable for applications to 200°F. However, polyethylene is more readily

attacked by oxidizing acids. Thick-sectioned, machined pump bodies,

such as those used in rotary pumps, are frequently furnished in

polyethylene because the material is relatively low in cost and offers

excellent abrasion resistance.

• Polyvinylidene fluoride

When higher corrosion, abrasion and heat resistance are required, the

material of choice is polyvinylidene fluoride (PVDF). Although this

fluoropolymer has a melt point of 352°F, the commercial product known

as Kynar® is most generally recommended for pump components that

will see continuous service in the range from -40°F to 275°F. PVFD is a

strong, tough, relatively heavy material with a specific gravity of 1.75

and tensile and compressive strengths similar to CPVC. Its high density

enables it to resist distortion and retain its strength at relatively high

temperatures. PVDF offers superior resistance to abrasion and is

frequently employed for pump impellers even when the casings are

specified in polypropylene. It is chemically inert to most acids, alkalis

(except sodium hydroxide), organic solvents, wet or dry chlorine,

bromine and other halogens. PVDF cannot be used with fuming acids,

polar solvents, amines, ketones or esters. Kynar PVDF pumps are

widely applied in electronic product manufacturing and laboratories

where pumps are required to handle ultrapure water or reagent grade

chemicals that cannot tolerate contamination of any sort.

• Ethylene Chlorotrifluoroethylene

A similar fluoropolymer, ethylene chlorotrifluoroethylene (ECTFE), offers

equal density, slightly higher strength and temperature resistance,

along with broader resistance to strong oxidizing acids, chlorides,

alkalis and organic solvents. The commercial grade of ECTFE, Halar®, is

recommended for use at temperatures as low as -105°F and for

continuous service to 300°F.

• Polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) is the most inert plastic material currently

in commercial use. Commonly referred to as Teflon®, it has extremely

high density and is resistant to weak and strong acids, alkalis, salts and

organic solvents. Although its impact strength is high, its coefficient of

friction is exceptionally low. As a result, its abrasion resistance is much

poorer than polyethylene, PVDF, the vinyls or polypropylene. It can be

employed, however, for sliding surfaces.

 

DESIGN CONSIDERATIONS

 

To offset the lower impact resistance of the thermoplastics, the designs

of the thermoplastic horizontal centrifugal pumps incorporate metal

armor, which provides protection from falling objects or careless plant

operation. Recent ANSI designs completely reinforce the flange so that

the thermoplastic pumps can accommodate the nozzle loading levels of

the steel pumps these plastics units replace. In terms of unit cost, the

average thermoplastic pump for corrosive/abrasive service is about the

same price as a quality 316 stainless steel pump and appreciably lower

than pumps of high alloy or exotic material construction. Thermoset

and thermoplastic pumps are available in conventional and magnetically

coupled horizontal centrifugal configurations, in standard and

cantilevered shaft vertical pumps designs, and in numerous rotary

pumps involving gears, vanes or flexible liners.

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