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TL-121

 

Common materials for the bodies and

liners of plastic pumps are, clockwise

starting at top left: polyethylene

body, Viton liner; reinforced Teflon

body, Hypalon liner; polypropylene body,

natural rubber liner; Teflon body,

Nordel liner, carbon-filled Teflon body,

Neoprene liner; and polypropylene body,

Buna-N liner.

Cut-away view of centrifugal pump shows

construction using Halar ECTFE

fluoropolymers, which extends service

life in extremely corrosive and abrasive

applications.

Application Possibilities

Grow for Plastic Pumps

INDUSTRY:

ENTITY:

SOLUTION(S) PUMPED:

PUMP TYPE(S):

Wastewater Municipal

 

Groundwater

Non-metallic Tank Pump Systems, CHEM-GARD Horizontal

Centrifugal Pump, FLEX-I-LINER Sealless Self-Priming

Peristaltic Pumps, SUMP-GARD Thermoplastic Vertical

Pump

 

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Reprinted from WATER/Engineering & Management

By Ken Comerford

 

GROUNDWATER TREATMENT

 

No matter what design solutions engineers and construction contractors

devise to contend with the growing number of water and air pollution

regulations, they can't do very much without pumps. Gravity sometimes

may play a role in water and wastewater systems but pumps are,

without a doubt, the prime movers in the business of fluid handling.

And as water pollution control technology continues to develop to meet

the needs, it is becoming more evident that non-metallic pumps are

particularly suitable for many applications in the field.

 

Unfortunately, the quantity of published information covering the design

features, materials of construction, and applied experience of

non-metallic pumps used for water pollution control purposes is

meager compared with what is available in published form on metallic

pumps and other equipment. Plastic pumps have been around for

decades (our company developed thermoplastic pumps in 1950), but

the bulk of experience with them has been acquired in the chemical and

other process industries, where they have been applied in numerous

cases for handling corrosive and hazardous fluids.

 

As a result of government regulations affecting water quality and waste

disposal, municipal facilities now are seen as chemical operations

requiring the same degree of knowledge and sophistication associated

with manufacturing and processing plants. With this in mind, a review

of significant information on plastic pumps, and their design and

application, should be of interest and value. Three important aspects

are covered.

 

• The non-metallic materials in widest use for pumps handling corrosive

and abrasive wastewater, water and wastewater treatment chemicals,

and corrosive fumes. Which materials for which service?

• Design aspects unique to non-metallic pumps in water pollution

control service, and how metallic contact with pumped fluids can be

avoided.

• The expanding use of non-metallic tank/pump systems to handle a

variety of wastewater streams.

An additional area of increasing importance to engineers and managers

in the water/wastewater field is the control of undesirable atmospheric

emissions. The incorporation of thermoplastic pumps and other

components in the design of scrubbing and odor control systems has

solved a number of problems, and will be the subject of a future article

in WEM.

 

Materials of Construction

Let's look first at materials of construction. Several years ago a study

aimed at knowledgeable engineers and others involved with wastewater

treatment revealed that a large majority had used non-metallic pumps

for some purpose. But generally they were unaware of the range of

potential applications for them, and the specific reasons for selecting

non-metallic rather than metallic pumps.

Corrosion resistance was identified as the major attribute of non-metallic

pumps. But their abrasion resistance and ability to avoid metallic

contamination of the product they are pumping, thus preserving its

purity, also are important characteristics. Of even greater significance is

the inert chemical nature of the thermoplastics, which are suitable for

use over the full pH range. This property simplifies the choice of the

specific material, and extends the usefulness and service life of a given

pump in a variety of applications.

Several other attributes of plastic pumps, in addition to their being

chemically inert and resistant to abrasion, are worth taking into account.

For instance, they are light, being 25 to 50 percent the weight of the

metallic items they can replace. Since the plastic parts will not rust or

seize, they are easy to service, and their initial cost is lower than pumps

fabricated with exotic alloys.

The plastic materials of construction for rigid wet end components such

as casings and impellers, which received the most mentions in the

study were:

1. PVC/CPVC-polyvinyl chloride and chlorinated polyvinyl chloride.

These relatively low-cost thermoplastics are widely used for acids,

caustics and salts. PVC has an upper temperature limit of 140°F, but

CPVC can be used at temperatures to 210°F. Neither material is suitable

for solvents.

2. PP-polypropylene. This is the lightest of the thermoplastics and is

recommended for acids, caustics and organic solvents to temperatures

of 185°F. It is not suitable for use with strong oxidizing acids, chlorinated

hydrocarbons or aromatics.

3. PVDF-polyvinylidene fluoride, most commonly known as Kynar, a

product of Elf Atochem. Fluoropolymers such as PVDF and a very

similar thermoplastic, ECTFE or ethylene chlorotrifluoroethylene, are

tough, abrasion-resistant materials which retain their mechanical

properties in the temperature range -40°F to 275°F. They are chemically

inert to most solvents, acids, and caustics, as well as to chlorine,

bromine and other halogens. Also, they are recommended for use with

ultrapure water and reagent grade chemicals–in fact, wherever freedom

from contamination is a key consideration.

4. PTFE-polytetrafluoroethylene, which is DuPont's Teflon. This also is

used for a variety of pump components, which must be chemically

inert, and withstand temperatures up to 500°F.

5. FRP/GRP-polyester, vinyl and epoxy resins reinforced with glass or

other fibers. These thermosetting materials are more like metals in

structural properties. They represent a group of composite materials,

which offer higher strength than thermoplastics, but limited corrosion or

abrasion resistance. Also, their use is not recommended above 240°F.

Clearly, choosing a material of construction for specialized water or

wastewater treatment pumping applications should be based on

checking the corrosion resistance of the material in terms of the fluids to

be handled and the anticipated temperatures. In many cases, a pump

manufacturer may have experience in dealing with identical or similar

service conditions. When it comes to handling waste streams

containing unknown or varying chemicals and concentrations,

engineering and operating personnel can be confident in the ability of

thermoplastic pumps to be up to the task because the wetted parts are

so chemically inert.

 

Spotlight on Pump Design

 

Some so-called plastic pumps on the market are misnamed. They are

basically metallic pumps with non-metallic casings and impellers. For

best results, plastic pumps should be designed to take maximum

advantage of the unique properties of the plastic material. If the

application requires non-metaIlic parts to be in contact with the fluids

being pumped, the following points are important:

 

1. Make sure the pump shaft is completely sleeved in a thermoplastic

material inert to the fluids. In a horizontal centrifugal design, the

thick-sectioned sleeve need only isolate the short section of the shaft

within the pump head.

2. In a vertical sump pump, the encapsulating sleeve should run the

entire submerged length and through the cover plate. A vapor seal

where the sleeved shaft penetrates the cover plate is required to protect

the external bearings and motor from corrosive fumes.

3. Horizontal and vertical pump designs should have an O-ring seal

between the thermoplastic sleeve and the impeller, and between the

impeller and the lock nut, to prevent metal-to-fluid contact.

4. In horizontal centrifugal pumps, the design should permit installing the

mechanical seal so that its non-metallic face is in contact with the fluid.

This reverse mounting avoids the use of expensive seaIs with exotic

alloy retainer assemblies or cages, and ensures no metal is exposed to

the fluid.

5. Since there is a significant expansion differential between a sump

pump's metal shaft and its thermoplastic column, the design should

incorporate a self-adjusting mechanism to compensate for this

differential if service conditions involve sudden or extreme temperature

changes. Without such a device there is a danger of impeller binding.

6. To provide a positive drive and prevent damage from reverse rotation,

thermoplastic impellers should be key driven. Another advantage is to

use an impeller with the metal key molded in. This offers additional

rigidity at higher temperatures and pressures. Also, plastic impellers

should be dynamically and hydraulically balanced at the factory.

7. For maximum service life, vertical pump designs should be furnished

with chemically inert sleeve bearings in the submerged area. Best

results appear to be achieved with ultrapure ceramic inner sleeves, and

silicon carbide, reinforced Teflon, or Vanite outer bearings.

8. Various sealless pump designs are being selected in response to the

tighter regulatory requirements, three types in particular.

 

Peristaltic pump

 

This pump group with fluid contact parts limited to non-metallics

includes tube type and flexible liner type which trap the fluid

temporarily between an elastomeric member and a thermoplastic

housing. The latter type has been in industrial and municipal service

since the 1950s. Fluid contact is limited to two parts: the thick

thermoplastic body available in such materials as high molecular weight

polyethylene, polypropylene and Teflon; and the liner furnished in pure

rubber or an assortment of synthetics from neoprene to various Dupont

elastomers like Nordel, Hypalon and Viton (see photograph showing six

versions). The Flex-i-liner pump design makes liner changes easy to

accommodate a variety of chemicals.

 

Diaphragm pumps

 

These pumps isolate the pumped fluid so that there is only non-metallic

contact. Carefully chosen for a specific application, and closely

monitored, they provide good service, but have three drawbacks. First,

they are noisy devices, being driven with compressed air, and indoor

application can present problems for nearby workers. Second,

diaphragm failure can lead to difficult-to-handle spills. If hazardous or

toxic fluids are being pumped, this is a serious concern. Third,

atmospheric oil emissions can be troublesome, but manufacturers are

attacking this shortcoming.

 

Magnetically driven non-metallic centrifugal pumps

These pumps have a number of attractive characteristics. In addition to

being inherently sealless, they permit the use of devices for leakage

monitoring, and avoid the emission of hazardous and toxic fumes.

Polypropylene, PVDF and Teflon are commonly used in these designs.

When severely corrosive, hazardous or toxic chemicals are present,

Teflon appears to be the material of choice for the containment can in

direct contact with the fluids, since it offers the broadest range of

chemical resistance. One design approach has a dual containment

system, with one can of Teflon for fluid contact, and a secondary can of

a high strength thermoset composite. The non-metallic materials

provide corrosion protection, and the design permits incorporation of

leak and temperature monitoring devices if called for.

 

Non-Metallic Pump/Tank Systems

 

The traditional below-grade concrete sump with a mounted pump of

some style is no longer the best way to contain and deal with hazardous

liquid wastes. Regulations now require these to be lined with

corrosion-resistant coatings to prevent chemicals, oils and other

materials from leaching into adjacent groundwater. But concrete sumps

are difficult to seal completely, and keep sealed. Coatings are typically

125 mils thick since sump service is considered immersion service, and

anything less might not last long and also be inadequate. Regular

inspections are necessary to ensure continued integrity of the coating.

 

When chemicals penetrate it through constant immersion, patching is

possible but difficult, and seldom acceptable. In many cases the coating

must be completely stripped off and the concrete surfaces recoated.

Packaged non-metallic pump-tank units now available often can provide

economical solutions to some of the problems described above. These

are standard or customer-engineered self-contained tanks containing

not only pumps, but level controls, control panels and related piping as

well. Welled parts machined or fabricated from a number of

thermoplastic or thermosetting materials, for instance the five families

of plastic compounds discussed earlier, make the systems suitable for

handling a broad range of corrosive or otherwise hazardous materials

up to temperatures of 275°F. Installation usually involves only electrical

and influent/effluent connections. In most cases they are free-standing

and require only a concrete pad, but some have been installed in

existing concrete basins.

 

To sum up, plastic pumps and plastic sump systems have established a

dependable service record over the last few decades that points to their

suitability for many fluid-handling duties. They are particularly capable

where corrosive and hazardous liquid wastes or chemicals have to be

contained and pumped. As a result they are being applied increasingly

for such tasks in the water/wastewater field. Their development

continues.

 

Definitions:

 

A thermoplastic resin will repeatedly soften when heated and harden

when cooled. Decomposition occurs only at higher temperatures.

A thermosetting resin cannot be melted or remolded without changed

its chemical structure.

Each of these three Vanton Polypropylene

pumps delivers 20 GPM of 50% caustic at

108°F against a 46-ft. total dynamic

head.

The sulfuric acid and sodium hydroxide

required for the ion exchange

purification process is stored in these

large tanks outside the processing

building. Note the concrete containment

vault to catch any spillage.

This vertical centrifugal pump fabricated

from plastic materials is equipped with

a proprietary seal which prevents the

escape of corrosive or toxic fumes and

liquids from pressurized tanks and sumps.

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