TL-114

 

Cut-away of Vanton Chem-Gard®

polypropylene pump with PVDF impeller

and PVDF thick sectioned shaft sleeve.

These horizontal centrifugal pumps

helped solve the severe

corrosion/abrasion problem created by

the pumping of an etching solution of

hydrofluoric acid and a proprietary

powder compound. Both metal and FRP

pumps previously specified for this

service resulted in repeated failures

and costly maintenance.

How to Select a

Non-Metallic Pump

INDUSTRY:

ENTITY:

SOLUTION(S) PUMPED:

PUMP TYPE(S):

Pollution control, Wastewater Industrial, Wastewater

Various

Wastewater

CHEM-GARD Horizontal Centrifugal Pump, FLEX-I-LINER

Sealless Self-Priming Peristaltic Pumps, Nonmetallic Tank

Pump Systems, SUMP-GARD Thermoplastic Vertical Pump

Reprinted from POLLUTION ENGINEERING

By Edward Margus and Kenneth Comerford

 

Pumps constructed of various non-metallic materials

are widely used in wastewater and other pollution

control applications.

 

The use of non-metallic pumps for applications involving water and

wastewater treatment, pollution control and other related services has

been expanding at a dramatic rate for a variety of reasons. Some of

these reasons are related to the growing awareness of the need to

protect our environment and natural resources. Others are mandated by

government regulations. Still others are associated with the

development of new and superior engineered plastics and the

corresponding economic need to extend equipment life and improve

performance.

 

The information presented here is based on the answers to a

questionnaire sent to consulting engineers, system designers, plant

operators and plant managers, querying them on pump types,

construction materials and applications in the environmental industry.

 

Non-metallic pumps for environmental use

 

Although pumps trace their origin to early civilization, plastic designs for

industrial and similar heavy-duty service are relative newcomers. The

first such pumps were developed in the post World War II period,

coincident with the advent of synthetic materials. Because of their

chemical inertness, the pumps were widely used in the chemical

process and manufacturing industries.

 

Where are non-metallic pumps used in the environmental industry? The

overwhelming majority, more than 97 percent of the respondents,

indicated non-metallic pumps are used in their facilities in water,

wastewater treatment and other environmental applications.

Respondents also were asked what pump types are used at their

facilities. Using partially aided recall, the questionnaire listed three basic

pump types — centrifugal, vertical (sump) and rotary. Several "other"

pump types also were identified by respondents (Table I).

 

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Non-metallic materials of construction

To simplify response to the question about which non-metallic materials

of construction were used for the wet-end components of pumps used

in wastewater and related service, the seven materials most commonly

used were listed. The participants in the study doubled the number of

materials listed (Table 2).

Selecting the most effective material for a given application is extremely

important. There is no substitute for experience in material selection.

Engineered plastics have greater tolerance for a broader range of

corrosives. Unlike metals, which have a corrosion rate for given

chemicals, engineered plastics are usually either totally resistant or

incompatible.

 

Detailed information on the usefulness of individual plastics is available

from the materials manufacturers. This information, normally resulting

from static immersion tests, should be consulted for new applications.

However, data available from pump manufacturers is very significant

since it represents a dynamic test, and the wet end of the pump is

where the material gets its most severe exposure.

 

When dealing with plastics, one of the major parameters is service

temperature. Manufacturers tend to be conservative when it comes to

the working temperatures for which they will recommend their pumps.

The plastic components must be able to retain their shape and maintain

the mechanical strength requirements at the temperature level.

 

Several materials are used in engineered pumps for water, wastewater

and related environmental applications. Some of the most common are:

POLYVINYL CHLORIDE (PVC). Perhaps the most common thermoplastic

material used in pump construction, PVC is a relatively low cost material

characterized by high physical properties and resistance to corrosion

and chemical attack from a broad range of acids, caustics and salt

solutions. PVC tends to be attacked, however, by solvents such as

ketones, chlorinated hydrocarbons and aromatics. PVC is generally

used at ambient temperatures and is not recommended for pump

service above 140°F.

 

CHLORINATED POLYVINYL CHLORIDE (CPVC). CPVC has physical

properties and chemical resistance similar to PVC. The major difference

is its higher useful temperature limit, which for pumps is 210°F, and its

correspondingly higher pressure ratings at these elevated temperatures.

CPVC is suitable for corrosive liquids at elevated temperatures as well

as for low temperature liquids.

 

POLYPROPYLENE (PP). Polypropylene is the lightest of the industrial

grade plastics, having a specific gravity or density of 0.90. This popular

polymer offers an excellent strength-to-weight ratio and is

recommended for pump service at temperatures to 185°F. In addition to

resisting acids and alkalies, polypropylene is also highly resistant to

organic solvents. Polypropylene is not recommended for use with

strong oxidizing acids, chlorinated hydrocarbons or aromatics.

POLYVINYLIDENE FLUORIDE (PVDF). PVDF is a strong, tough and

abrasion resistant fluorocarbon material. Because of its high density, it

resists distortion and retains its strength at elevated temperatures. PVDF

can be used over a wide range of temperatures, from -40°F to 300°F. In

terms of chemical resistance, PVDF is inert to most solvents, acids and

alkalies, as well as wet and dry chlorine, bromine and other halogens.

 

Its high hardness and low coefficient of friction make it ideal for

abrasion resistant applications. PVDF is recommended for use with

ultrapure water and reagent grade chemicals, as well as other

applications where freedom from contamination is important.

POLYETHYLENE (PE). This ultra high molecular weight (UHMW) material

is impermeable to water and resistant to organic solvents, acids and

alkalies. PE is very similar to polypropylene and retains good physical

properties at low temperatures and also up to 200°F. Its hard smooth

surface makes PE useful for abrasive solutions.

 

FIBERGLASS/GLASS REINFORCED POLYESTER (FRP/GRP). These

thermosetting materials are polyesters reinforced with glass or other

fibers to provide additional strength. They are closer to metals in their

structural properties than the thermoplastic materials, but are not as

chemically resistant. Since FRP/GRP is a composite material rather than

a homogeneous polymer, there is potential for forming capillary

passageways that result in wicking. This limits the application of these

materials, particularly if the same pump is to be used for different

chemicals. It also limits their use in abrasive service. Maximum service

temperatures for pumps made of these materials is 230°F.

 

ETHYLENE CHLOROTRIFLUORETHYLENE (ECTFE). This material resists

an extremely broad range of acids including oxidizing types, the

alkalies, and most other corrosive and abrasive fluids. ECTFE is very

similar to PVDF. It offers excellent barrier properties suitable for

ultrapure water and similar fluids where contamination could be a

problem. ECTFE has extremely high tensile strength as well as impact

resistance. It is recommended for temperatures to 300°F.

 

POLYTETRAFLUORETHYLENE (PTFE). This material is most commonly

referred to as Teflon. It is one of, if not the most inert of the

thermoplastics available. PTFE retains useful mechanical properties at

temperatures as high as 500°F. It offers excellent impact and abrasion

resistance. Compared with the other fluoropolymers, however, its

tensile strength and creep resistance are low.

 

The above covers the rigid materials generally used for pump

construction. Other materials mentioned by respondents refer to

elastomeric components, primarily gaskets, O-rings, flexible liners and

other parts that require corrosion and impact resistance combined with

flexibility.

 

Why plastic pumps are selected

 

Plastics represent a large family of synthetic materials with a number of

common characteristics. They share the same basic molecular

structure, and being manmade, they can be readily modified and

compounded to achieve a wide variety of physical properties and

behavior patterns. The term "polymers" frequently used in relation to

plastics refers to their molecular structure. They consist of very large

chain-like molecules, made from various organic substances called

monomers. The chemical reaction used to link and form the monomers

into polymers is polymerization.

 

Although thousands of plastic compounds exist, the reasons for

selecting specific ones for pump construction are summed up in Table

3.

 

 

Corrosion resistance, not surprisingly, was listed as the primary reason

for selecting plastics for pump construction. We were somewhat

surprised, however, to see the relatively high position of "price" since

pumps made of engineered plastic are similar in cost to pumps made of

type 316 stainless steel.

 

Engineered plastic pumps, built to the standards required for resisting

corrosives or for use in rugged service offer advantages over metal

pumps in terms of longer service life and lower maintenance, but not

necessarily in terms of original price. They are, however, less expensive

than pumps of exotic materials such as Alloy 20, Hastelloy, Nickel or

Monel.

 

Other reasons for using plastic pumps mentioned by respondents were

purity, environmental protection, flexibility and non-sparking in an

explosion-proof environment. Another reason for specifying engineered

plastics is low maintenance. Plastic parts do not gall or seize the way

metal parts do. Nuts and bolts and other hardware are easy to remove

and threaded plastic components can be unscrewed readily and reused.

Plastic pumps are easy to keep clean and do not need to be painted

since they don't rust.

 

Non-metallic pump applications

 

Approximately 100 specific applications for nonmetallic pumps in

wastewater and pollution control service were listed by the respondents

(Table 4). These applications fall into several basic groups:

 

• Transfer chemicals from bulk storage to day tanks.

• Metering chemicals and polymers.

• Chemical feed to scrubbers and condensate pumping.

• Odor control systems.

• Groundwater remediation and landfill leachate treatment.

• Lift station collection and wastewater treatment.

 

Bad experiences with non-metallic pumps

 

The majority of respondents said they had not had a bad experience

with non-metallic pumps. Of those who had, most offered their own

carelessness, negligence or lack of knowledge as the primary cause.

The bad experiences of respondents fell into four basic categories:

 

• Buying low quality because of low price.

• Improper applications, selecting the wrong materials, the wrong pump

or improper design.

• Poor installation, often due to carelessness

• Improper maintenance, often due to unfamiliarity with recommended

maintenance procedures for plastic pumps.

 

The most common material-related selection problem appears to be

improper use of thermosetting FRP/GRP pumps in severe

corrosive/abrasive applications. Switching from thermoset to

thermoplastic pumps helped solve the problem in many cases. In some

abrasive applications where polypropylene pumps were originally

selected, impeller wear was overcome by switching from polypropylene

impellers to PVDF.

 

A number of respondents referred to difficulties under cold or freezing

temperatures, particularly when PVC was the basic material. One

solution is to specify PVDF instead of PVC because it maintains its

strength at temperatures as low as -40°F. Other solutions involve heat

tracing, proper insulation and special handling techniques.

 

Operating problems due to seal and bearing failure are the result of poor

maintenance practices. Analysis of the bad experience mentioned in the

responses points out the importance of working closely with the pump

manufacturers before specifying materials or design for difficult

applications.

 

Pump headaches

 

The final question asked for information on pumping applications that

still cause the biggest headaches. The answers fit into two basic

groups: difficult-to-pump products and demanding operating conditions

(Table 5). Responses cut across all pump materials — metals and

plastics.

The biggest headaches are associated with pumping sludges, heavy

slurries and high head, low flow conditions — things that have nothing to

do with the basic material selection and can cause problems for metal

as well as plastic pumps. With the exception of thick sludges and

sand/oil slurries, all of the difficult products listed by respondents can

be pumped with properly engineered non-metallic pumps. None of the

difficult operating conditions are beyond the scope of non-metallic

pumping systems.

 

It also is obvious from the responses that satisfactory service life is a

relative term. Downtime is expensive. Extending it by days or even

hours is frequently of significant value. The larger the operation the

greater the significance in most cases. But in the environmental

industry, where equipment often operates in remote locations,

continuous operation is unrelated to the size of the operation.

This again highlights the importance of discussing pump problems with

the manufacturer and seeking out manufacturers who will modify

existing designs when off-the-shelf products can lead to expensive

problems.

 

Engineered plastic pumps have made strong inroads in water and

wastewater treatment, pollution control and related environmental

services. For maximum performance and efficient service life, the

emphasis should be on quality products engineered for the specific

corrosive or abrasive applications.

 

In addition to selecting the right materials of construction, the right

pump type and the necessary design features, it is imperative that care

be taken during installation and that sound maintenance programs be

followed.

 

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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mail@vantonpump.com

(+44) 01260 277040

Vanton Pumps (Europe) Ltd.

Unit 4, Royle Park

Royle Street

Congleton CW12 1JJ

UNITED KINGDOM

www.vantonpump.com