Air operated double diaphragm pumps make a comeback Air operated double diaphragm pumps provide an interesting sealless alternative to centrifugal and positive displacement pumps, and are invaluable for applications where it is impossible or dangerous to use electricity. First introduced more than 40 years ago, more recent innovations in pump design and the construction of the diaphragms are boosting demand for these pumps. Tom Rozee provides the details.

W hile air operated double diaphragm (AODD) pumps have never been away, their

popularity has been making something of a comeback over recent years. Indeed there are many manufacturers of AODD pumps throughout the world. Introduced over 40 years ago, the AODD pump has seen a number of innovations and offers an interesting sealless, displacement alternative to centrifugal pumps. AODD pumps are not always the first choice in pump selection, yet they have found applications in a wide range of industries. These include chemicals, food processing, paints, electronics, construction, industrial maintenance, utilities, processing, wastewater and sludge processing, and mining. With interesting variations on design principles and new diaphragm

designs being introduced, these pumps offer a number of benefits that set them apart from alternative pumping methods. They are self priming, will run dry without damage and need no electricity. Powered entirely from compressed air, they can be used in a number of appli- cations where electricity is not available or must not be used, such as hazardous or explosive environ- ments or at remote sites. AODD pumps come in a variety of sizes, shapes and materials of manufacture (Figure 1).

How the AODD pump works

The basic principle of operation

of the AODD pump is shown in

Figure 1. The Husky family of AODD pumps from Grace Inc.

Figure 2. The diaphragms A and B

are moved from left to right and back

again by directing compressed air

alternately to them via a central air

valve. The movement of diaphragm

A towards the centre of the pump

generates a partial vacuum in the

chamber. This causes ball valve 1 to

close while ball valve 2 opens as air

pressure forces liquid from the inlet I into the chamber. At the same time,

diaphragm B compresses the volume

in its chamber, which closes valve 3

and forces fluid past valve 4 and out

through the outlet port 0. When the

air valve reverses the movement of

the diaphragms, fluid is pumped via

the left hand side of the pump. Since

the diaphragms are clamped to

the walls of the pumping chamber,

there are no sliding seals in the

pump. The diagram shows the flow

of liquid from the bottom to the

top; however, some pump designs

feature flap valves and in these cases,

liquid flows from the top to the

bottom of the pump. This pumping

principle is used, in essence, in

AODD pumps manufactured by most

of the world’s leading manufactu-

rers, such as Wilden, Warren-Rupp,

Grace, Blagdon and Aro. However,

an alternative method was intro-

duced, originally by Flotronics and

more recently by Johnson Pump,

where the fluid flows through the

centre of the pump (Figure 3), which

can lead to smoother flows.

AODD pumps contain fewer moving

parts than many other types of

pumps and there is therefore less

maintenance. They also offer easy

access to those parts that may need

servicing. There are no shaft seals or

40 WORLD PUMPS May 2002 0262 1762/02/$ - see front matter 0 2002 Elsevler Science Ltd. All rights reserved

packing boxes involved. The diaphragm is a dynamic seal

that converts the pneumatic pressure on one side to a

fluid pressure on the other (the medium to be pumped).

The differential air pressure across the diaphragm and

corresponding internal stresses are low in comparison

with similar stresses in mechanically driven diaphragm

pumps that use the diaphragm to generate pressure.

Discharge flow rates are adjusted by controlling the air

inlet or the discharge flow, so no complex control sysrems

are required.

Versatile pumping

AODD pumps are particularly versatile in being capable

of pumping materials from fluidized dry powder to shear-

sensitive liquids, viscous liquids and abrasive slurries.

They offer significant benefits over pumps with close-

fitting sliding or rotating mechanisms, where the passage

of abrasive slurries can cause excessive wear to the moving

parts of the pump. This can lead to significant down-time

and refurbishment costs. These problems do not arise

with AODD pumps. In addition, their large internal

clearances and flow through prevent clogging, with the

size of solid particles that can be pumped related to the

pump size. There are many materials of construction for

the pump body, diaphragms and check valves, including

acetal, polypropylene, modified PTFE, Kynar@,

aluminium and stainless steel. This allows a wide variety

of abrasive fluids to be accommodated, including sludge,

slurries and solids in municipal and industrial waste

applications. By choosing the appropriate materials of

construction, AODD pumps can also be used in hygienic

applications in the food and beverage industries for fluids

such as beer, finings, milk, wine, tomatoes, strawberries

and cherries.

The AODD pump is also particularly suited to pumping

viscous fluids, because flow rate is directly proportional to

the speed of the pump. When viscous materials are

pumped there are friction losses within the pump itself

and between the fluid and the walls of the pipe supplying

the pump. The AODD pump automatically reduces the

pumping rate as viscosity increases, and at the point

where the pump can no longer move the liquid it simply

stops without damaging itself.

Self priming and dry running

AODD pumps are inherently dry self-priming, that is,

capable of drawing up liquid even when installed above

the source liquid level, whereas centrifugal pumps require

an initial charge of liquid. This self-priming characteristic

greatly helps with physical placement of the pump. It also

means that by attaching a hose to the inlet, the pump can

be used as a ‘scavenger’, to empty drums, barrels, sumps

etc. This can offer significant cost savings to the user

since the scavenging action completely empties the

container. Most positive displacement and centrifugal

pumps fail if they run dry, frequently because their shaft

seals or stuffing boxes may require lubrication or cooling

Figure 2. Principles of operation of AODD pumps. A and B are the diaphragms; 14 the ball valves; and I and 0 the inlet and

outlet ports for the pumped fluid.

from the pumped fluid. AODD pumps, however, simply

run faster when running dry without damage. Air shut-off

valves can be added to reduce the amount of wasted

compressed air during dry running.

Critical components

AODD pumps benefit from their simplicity of operation.

The key components in the pump are the air valve (which

is usually a shuttle valve to route the compressed air

alternately to each diaphragm), the valves, and of course,

the diaphragms themselves. The position of the various

components of an AODD pump is shown in the cutaway

Figure 3. Central product flow through Optiflo AODD pump from Johnson Pump.

www.worldpumps.com WORLD PUMPS May 2002 41

Figure 4. Cutaway showing component parts in Husky 1040 pumps (Grace Inc).

diagram (Figure 4). The air valve is

an important part of the pump

and in the early days of AODD

pumps these were prone to stalling,

causing the pump to stop. This was

due to icing of the valve, caused by

sudden expansion of air to the

atmosphere and consequent rapid

cooling, which instantly freezes any

moisture contained in the air. The

ice formed on the moving parts

Figure 5. One-Step and One-Step Plus PTFE-bonded rubber diaphragms from Tri-Ark Ltd.

of the air valve caused them to

stick. Naturally manufacturers have

addressed this problem in different

ways and each has its own particular

designs to eliminate the problem.

In general, therefore, stalling is

no longer a significant problem for

this type of pump. Moreover, many

pumps are designed to have the

air valve serviceable without the

need to dismantle the rest of

the pump. The diaphragms are

another key component for AODD

pumps.

Diaphragm selection

The key factors that determine

the suitability of the diaphragm

for a particular application are:

flex life, the chemical composition

of the fluids to be pumped and

their effect on the diaphragm; the

temperature range that the pump

must operate in, including extremes;

and the abrasive nature of the

media being pumped, including the

size of any particulates. Most pump

manufacturers can supply chemical

resistance guides to assist in the

selection of suitable diaphragms.

However, since the diaphragm is

continually being stressed, stress

corrosion can result. Information on

the previous practical performance

of a particular material used for

the diaphragm is therefore also

important.

Diaphragm materials

Various rubber components are

used in the manufacture of dia-

phragms, with a nylon fabric mesh

to provide improved dimensional

stability and additional strength.

Elastomers such as Neoprene, Buna-

N, EPDM (Nordel) and Viton are the

most commonly used. Their chief

characteristic is flexibility, which

allows more volume to be displaced.

Characteristics of the different

rubber materials are summarized in

Table 1.

Thermoplastic materials are also av-

ailable for manufacturing diaphragms.

These are produced by an injection

moulding process and require no

fabric reinforcement for dimensional

stability or tensile strength. The most

popular are polyurethane, Hytrel@,

Santoprene@ and Teflon@. Their

characteristics are summarized in

Table 2.

Polyurethane gives excellent general-

purpose diaphragms with extremely

good flex life, and is the most

economical material of all. Teflon@

(PTFE) is one of the most chemically

inert man-made compounds known,

but has very little elasticity, which

can reduce flow rates by up to 20%.

A rubber back-up diaphragm is

required to provide flexibility and

memory. Many other proprietary

42 WORLD PUMPS May 2002 www.worldpumps.com

materials, usually derivatives of the

above, have been developed by in-

dividual pump companies for use in

their own pumps.

One-piece PTFE/rubber diaphragms

The pump industry is well aware of

the virtue of Teflon@ for chemical

resistance but also of its limitations

as regards operational life and cost,

and so modified PTFE diaphragms

have been developed that show

improved flexibility and memory and

also have improved flow rates.

Another important development in

PTFE diaphragm technology is the

singleapiece diaphragm where the

PTFE diaphragm is bonded directly

to a rubber diaphragm. The One-

Up@ diaphragm from W.L. Gore 61

Associates (UK) Ltd is one example,

together with the new One-Step

and One-Step Plus diaphragms from

Tri-Ark (Figure 5). These PTFE

protected diaphragms combine the

chemical resistance associated with

PTFE with the flex characteristics of

an elastomer. Tri-Ark’s One-Step

uses virgin PTFE, whilst One-Step

Plus features a modified PTFE that

allows a much thinner outer layer to

be used. This maintains the same

chemical resistance properties but

increases flex life even further. The

PTFE is chemically bound to the

underlying rubber. Since it is

manufactured as one piece, the risk

of separation normally associated

with two-piece overlaid diaphragms

is eliminated, increasing time on-

line. One-piece construction avoids

the possibility of leakage and also

prevents abrasive particulate mat-

erial getting between the PTFE and

rubber, which could cause damage

and lead to reduced diaphragm

lifetime.

Installation and maintenance

AODD pumps are very reliable since

they have very few moving parts or

components that can wear. They are

easily dismantled for maintenance or

diaphragm replacement. They can be

mounted wherever there is physical

space for them, even submerged if the

www.worldpumps.com

Material Flex life Temp limits PC) Applications

Material Flex life Temp limits (“C) Applications I

Efficient sludge transfer with: ODS@ diaphragm pump

n Heavy duty construction

n Air ooerated

Service:

w Automatic Flow & Pressure-

Control adjustment package 1

n High viscosity sludges

w Concentrations: 4%, 6%, 8

n Self-priming

n Run dry ability

w Abrasive sludges

n Corrosive sludges

570-455 2051 X 298 Fax: 570-459 2761 E-maikdorr-oliver@gIv.com

tj@ki$W / ~ORRMOL~VER

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WORLD PUMPS May 2002 k3

material of the pump is compatible

with the fluid and the exhaust is

ported out of the liquid. Their simple

mode of operation means that there is

no need for bypass piping or pressure

relief valves. If vibrations and surges

cause problems in the piping systems,

pulsation dampeners can be fitted.

The AODD pump uses compressed air

as its power source and therefore can

be used in areas either where

electricity simply isn’t available, or in

hazardous areas where electrical

supplies are forbidden. Compressed

air is explosion-proof and there

are no electrical or heat-generating

components to spark and cause

product flashing. This gives AODD

pumps greater versatility than

centrifugal or positive displacement

pumps and offers pollution-free

operation. Most AODD pumps use

oil-free compressed air, which has

environmental benefits since no oil

mist is vented into the atmosphere,

and also there can be no oil

contamination of the pumped fluid.

In addition, there are no costs

associated with oil consumption or

lubrication.

Life cycle costs

Assessing the cost of ownership of a

pump is actually a function of the

overall installation as much as the

costs associated with purchasing and

running the pump itself. If the plant

is not appropriately designed for

the application, the pump may well

cost more to run than it should. In

terms of the pump itself, costs can

be broken down into purchase and

installation costs, maintenance costs

(labour and spares), energy costs

and removal/disposal costs. The main

consumable items in an AODD

pump are valves, valve seats and

diaphragms. In general, spares are

low cost, but it is important to use

the most appropriate components.

For example, neoprene is excellent

for general-purpose, low cost dia-

phragms giving the highest flex life.

Buna-N@ (also known as nitrile) is

approximately 70% more expensive

than neoprene, while Nordel@ (also

known as EPDM) is approximately

40% more expensive than neoprene.

Viton@ is highly resistant but is

approximately nine times more

expensive than neoprene. Teflon

diaphragms are typically ten times

more expensive than polyurethane.

Thus choice of the most suitable

diaphragm for the application can

significantly affect maintenance

costs. Compressed air is a relatively

costly utility in many plants, requir-

ing compressors and air distribution

systems and higher energy costs, so

engineers must weigh up the cost

benefits.

However, the sheer versatility of the

AODD pump has a value that is

almost impossible to calculate. The

AODD pump offers a real alternative

to centrifugal and positive dis-

placement pumps and should not be

overlooked in the pump selection

process. n

CONTACT Tom Rozee, Managing Director Tri-Ark Ltd. The Sail Loft, Burnham Business Park, Springfield Road, Burnham on Crouch, CM10 8TE. UK. Tel: +44-1621-781144 Fax: +44-1621-781155 E-mail: salesatri-ark.com www.tri-ark.com

I

SO_

design, manufacturing and after sales Modular dosing

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Dosing and mixing of liquids 10, Grande Rue - 27360 Pont-Saint-Pierre - France Tel : +33 (0)2 32 68 30 12 l Fax : +33 (0)2 32 68 30 91 www.dosapro.com

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