In the name of God

Pump diaghragms

hamedjonami

http://groupiranian.parsiblog.com

tranehkhafan

azad Islamic university of qazvin

NOV 2009

) صلوات ) عج زمان امام سالمتی برای

فرستادی؟

Catalog

Title

page

The Diaphragm Pump

3

The Structure of a Diaphragm

14

It’s the Diaphragm that Does It

20

Keaflex to make elastomer pump diaphragm for

Salamander

24

Sample system design and diaphragm pumps

27

Selecting the Proper Diaphragm Pressure Control Valve

34

2

The Logical Path - The Application of Ceramics to

Diaphragm Pumps

41

Getting Pumped Up on Diaphragms by Design

47

Source

51

The Diaphragm Pump

The unique property of the reciprocating diaphragm pump is that

the actuating piston does not come into direct contact with the

mobile phase and thus, the demands on the piston-cylinder seal are

not so great. The diaphragm has a relatively high surface area and

thus, the movement of the diaphragm is relatively small and

consequently the pump can be operated at a fairly high frequency.

High frequency pumping results in a very significant reduction in

pulse amplitude and, in addition, high frequency pulses are more

readily damped by the column system. Nevertheless, it must be

emphasized that diaphragm pumps are not pulseless. A diagram of

a diaphragm pump, showing its mode of action is depicted in figure

10

3

Figure 0. The Action of a Diaphragm Pump

A diaphragm pump is a positive displacement pump that uses a

combination of the reciprocating action of a rubber,thermoplastic

or teflon diaphragm and suitable non-return check valves to pump

a fluid. Sometimes this type of pump is also called a membrane

pump.

There are three main types of diaphragm pumps:

In the first type, the diaphragm is sealed with one side in the fluid

to be pumped, and the other in air or hydraulic fluid. The

diaphragm is flexed, causing the volume of the pump chamber to

increase and decrease. A pair of non-return check valves prevent

reverse flow of the fluid.

4

As described above, the second type of diaphragm pump works

with volumetric positive displacement, but differs in that the prime

mover of the diaphragm is neither oil nor air; but is electro-

mechanical, working through a crank or geared motor drive. This

method flexes the diaphragm through simple mechanical action,

and one side of the diaphragm is open to air.

The third type of diaphragm pump has one or more unsealed

diaphragms with the fluid to be pumped on both sides. The

diaphragm(s) again are flexed, causing the volume to change.

When the volume of a chamber of either type of pump is increased

(the diaphragm moving up), the pressure decreases, and fluid is

drawn into the chamber. When the chamber pressure later

increases from decreased volume (the diaphragm moving down),

the fluid previously drawn in is forced out. Finally, the diaphragm

moving up once again draws fluid into the chamber, completing the

cycle. This action is similar to that of the cylinder in an internal

combustion engine.

Diaphragm pumps can be used for the transfer and dosing of

liquids in many different applications.

This article concentrates on fast running diaphragm liquid pumps.

It describes the operating principle, construction and

characteristics of the fore-mentioned pumps and shows the

technical advantages. It concludes with an example of the use of

such pumps in laboratory equipment.

Introduction

The tasks of transferring and metering liquids are as old as

humanity. From history we know that 3500 years ago the

Babylonians used water-wheels for irrigating their fields; in the

same period, the Egyptians used metering techniques in the

5

manufacture of medicines. Even before the start of the Christian

calendar, Archimedes had invented his screw pump, and the

Greeks used piston pumps for handling water. With the alchemists,

metering liquids became increasingly important.Today the liquid

pump industry has developed to become a many-faceted branch of

mechanical engineering, and offers an extensive range of products

for applications ranging from analysis to water treatment.

Types of pump

For transferring and metering relatively small quantities of liquid,

oscillating or rotary positive-displacement pumps are generally

employed.

The following types are oscillating positive-displacement pumps:

1. diaphragm pumps

2. piston pumps

3. bellows pumps.

4. Rotary positive-displacement pumps include:

5. peristaltic pumps

6. gear pumps

7. rotary piston pumps

8. screw pumps.

This article concentrates on diaphragm pumps for transferring and

metering small quantities of liquids.

How diaphragm pumps work

6

Fig. 1. Construction and operation of a diaphragm liquid pump.

The operating principle and construction of a diaphragm liquid

pump are extremely simple (see Fig. 1).The diaphragm is clamped

at its circumference between the pump housing and the pump

head. An eccentric imparts movement to the connecting rod, which

in turn moves the diaphragm to and from. This produces a periodic

change in volume of the working chamber, similar to a piston

pump. In combination with automatic inlet and exhaust valves, this

change in volume produces a pumping action. To prevent excessive

stretching of the elastic diaphragm, a support is arranged below

the diaphragm.

Depending on the type of drive used, diaphragm liquid pumps can

be categorised as high-speed and low-speed pumps. Low-speed

diaphragm liquid pumps operate at speeds up to about 300 strokes

per minute.The flow rate Q of such pumps enjoys a substantially

linear relationship to the speed of rotation of the drive motor. This

makes such low-speed diaphragm pumps especially suitable for

metering in proportion to some process variable, or for execution of

a pre-determined number of strokes on receipt of a particular

signal. The flow rate can be varied, not only by changing the stroke

of the diaphragm, but also in response to change in an electrical

signal, such as frequency (ac motor) or voltage (dc motor).

7

Many requirements for transfer and metering can be met with

simpler means. These are potential applications for high-speed

diaphragm liquid pumps. They operate preferably in the range

2500 to 3000 strokes/min, that is to say 10 times as fast as their

slow-running counterparts. Their size, on the other hand, is in

inverse proportion to their speed, so that very compact dimensions

are one of the particular advantages of high-speed liquid

diaphragm pumps. In a period in which miniaturisation is one of

the main themes of technical development, this has time and again

been shown to be an outstanding feature, particularly in

comparison with other types of pump.

Fig. 2. Performance curves for high-speed liquid diaphragm pumps

NF60/61 DC and NF60 E.

The rest of this article concentrates mainly on high-speed liquid

diaphragm pumps for transferring liquids. Thanks to their

8

outstanding vacuum performance they are also often used for

suction of liquids or air/liquid mixtures. The pumps are driven by ac

or dc motors directly that is to say with no reduction gear between

the two. Figure 2 shows two typical performance curves for a

diaphragm liquid pump; it shows the flow rate in relation to the

suction head/pressure height for two different drives. So that the

same pump can produce different flow rates at given inlet and

outlet conditions, this manufacturer equips some of its products

with an adjustable diaphragm support. With this patented system

pumps can be set at the factory to a particular flow rate. It also

provides a convenient way of compensating for the effects of the

tolerances of components on performance.

Pumps with resonance chamber system and integral over-pressure

relief valve

Fig. 3. Mode of operation of resonance chamber.

9

Because of the high speed (up to 3000 strokes /min) special

attention must be paid to avoiding cavitation. For this reason this

manufacturer fits most of its products with a patented resonance

chamber system. The chamber is connected to the suction side of

the pump and works as a pulsation damper; it restricts acceleration

peaks in the liquid on the suction side of the pump, and thus

restricts cavitation. Operation (see Fig. 3): in the resonance

chamber (9), an additional diaphragm oscillates at the same

frequency as the working diaphragm (1). During the exhaust stroke

of the pump, the column of liquid on the suction side is not forced

to stop abruptly, but can flow into the resonance chamber. 'On the

next suction stroke, the column of liquid is not required to

accelerate from a stationary condition, instead the pump can use

the speed remaining in the liquid. The effect is that the resonance

chamber system produces significantly better pump operation.

Fig. 4. Mode of operation of over-pressure relief valve.

10

There is a general danger with pumps that, because a filter is

blocked or a valve accidentally closed, they may have to operate

against a closed system and hence undesirable operating

conditions. In such cases the pressure can exceed the permissible

operating range of the pump.To prevent this happening, KNF has

integrated an over-pressure relief valve into the pump head. A

bypass, which connects the pressure port of the pump to the inlet

side, is closed by a spring-loaded diaphragm (see Fig. 4). If the

pressure on the pressure side of the pump reaches the setting of

the over-pressure relief valve, this opens and circulates liquid

through the internal bypass from the pressure side to the inlet side,

and back to the pressure side. An adjusting screw enables the

pressure to be easily and precisely set. Generally the overpressure

relief valve should be set about 0.5 bar above the normal system

pressure.

High-speed liquid diaphragm pumps can be used for metering as

well as transferring liquids. Because of the large number of strokes

per unit time, the relationship between number of strokes and flow

rate is not truly linear. Metering is therefore not, as for,slow-

running pumps, achieved by varying the motor speed, but, is

dependent on running time or system volume; the pump runs for a

precisely defined time, or until a level sensor is triggered. The

precision of metering of a high-speed liquid diaphragm pump can

be significantly increased if a solenoid valve is installed either

before or after the pump, or if a pressure control valve is fitted to

the outlet of the -pump. In this way, high-speed liquid diaphragm

pumps can be successfully used for metering even very small

quantities.

In some applications the pulsation of a diaphragm pump is a

disadvantage compared with rotary pumps. These problems can be

reduced by using pumps with mcre than one head. The heads are

11

then connected in parallel, and their eccentrics arranged so that

they operate sequentially.

Characteristics

High-speed liquid diaphragm pumps have several important

characteristics. They:

1. are self-priming because they can also handle gases and

gas/liquid mixtures,

2. can run dry,

3. are maintenance-free,

4. have long working lives,

5. are reliable, thanks to their simple and sturdy construction,

6. are chemically resistant; all medium-contact parts can be

made of chemically resistant materials such as PTFE, FFPM

or PVDF,

7. can operate in any position,

8. are very compact,

are quiet.

Typical applications

An example of the use of high-speed liquid diaphragm pumps in

laboratory equipment will be described briefly. In medical

diagnosis, blood analysis has a very important role.Today, using

photo spectrometry equipment, this analysis is almost entirely

automatic. Blood samples are fed to the analyser in sample tubes.

The analyser prepares the sample by treating it with certain

reagents, and then examines its spectral properties. A computer

processes the results, assigns them to the sample, and prints them.

12

Fig. 5. Photo-spectrometric blood analyser flow diagram for the

liquid system.

The sample is then passed to a disposal container, and the sample

tube washed.

In the apparatus described, four high-speed liquid diaphragm

pumps are fitted. Figure 6 shows the flow diagram for the liquids

system. The first pump feeds the reagents to the sample tube; it is

controlled by a timer for metering purposes. The second pump

empties the sample tube it operates as a transfer pump. Two

further transfer pumps look after cleaning the sample tube. One of

them feeds a detergent solution into the soiled tube, and the

second sucks this solution out again. Because of the combination of

properties described above, a highspeed liquid diaphragm pump

has proved entirely suitable for all these functions.

13

Fig 6. High-speed liquid diaphragm pumps from KNF.

Numerous further applications for high-speed liquid diaphragm

pumps are to be found in the following fields:

1. analysers

2. repro equipment

3. the cleaning industry

4. laboratory equipment

5. water treatment

Certainly the Babylonians, the ancient Egyptians, Archimedes, or

the alchemists would be astonished by some of these technical

solutions. But technical progress does not stop here. The trend to

ever smaller products and smaller flow rates continues unabated.

KNF Flodos announced a new, product of small size which will

precisely meter micro-litre quantities.

Applications

Diaphragm pumps have good suction lift characteristics, some are

low pressure pumps with low flow rates; others are capable of

higher flows rates, dependent on the effective working diameter of

the diaphragm and its stroke length. They can handle sludges and

slurries with a good amount of grit and solid content.

1. Diaphragm pumps have good dry running characteristics.

2. Diaphragm pumps are low-shear pumps.

3. Diaphragm pumps can be used to make artificial hearts.

4. Diaphragm pumps can be up to 97% efficient.

14

5. Diaphragm pumps have good self priming capabilities.

6. Diaphragm Pumps can handle highly viscous liquids.

7. Diaphragm Pumps are available for industrial, chemical and

hygienic applications

The Structure of a Diaphragm

 

The type of diaphragm determines to a great extent the

performance of the pump.

15

The patented structured diaphragm makes it possible to

significantly reduce the size for a given performance. Diaphragm

pumps for compressing and evaluating gases can be classified by

the type of diaphragm they employ. It is the diaphragm that

determines the performance of the pump. KNF Neuberger’s

development of the patented structured diaphragm can only be

described as precision design.

 With a diaphragm separating the compression chamber from the

mechanical parts and operating practically without friction,

diaphragm pumps are completely oil-free. This makes them

suitable for applications for which oil-lubricated pumps could not

even be considered, for example:

Inhalers for medical purposes

Analyzers and gas-samplers in the analysis field

Vacuum evaporator in process engineering

Stack gas analysis in environmental technology

Diaphragm Pump Advantages

Apart from the wide range of potential uses, the diaphragm pump

offers further advantages over oil-lubricated pumps. First of all

there is the low maintenance requirement. Not only are oil

changes not required, but also the simple design and small

number of parts, reduce the need for maintenance. At the same

time the simple design ensures high reliability. In addition the

hermetically sealed compression chamber prevents losses of the

gases or liquids being handled, and makes compression and

evacuation of expensive, toxic inflammable, radio-active, or

otherwise dangerous gases possible. Currently diaphragms can be

divided into three groups:

Flat diaphragms

Molded diaphragms

Structured diaphragms

16

The Flat Diaphragm

The simplest type of diaphragm – the flat diaphragm - consists of

a flat rubber plate. It is clamped art its edge between the

crankcase and the pump head, and at its center to a rigid metal

disc.

Top Surface of a KNF Flat Diaphragm PTFE-Coated with

Clamping Disk

A connecting rod imparts movement to the diaphragm. Depending

on the expected duty, the diaphragm hay have reinforcing fabric

vulcanized into it to carry the forces arising from pressure

created in the gas. The advantages of this type of diaphragm in a

pump are:

Simple and economical design

Low wear

Suitable for high pressure, if necessary this can be increased

by appropriate choice of reinforcing fabric

Higher flow-rate

The disadvantages of the flat diaphragm lie in the metal clamping

disc and screw head, which are of necessity situated in the

compression chamber. Applications with corrosive or aggressive

gases demand a better solution.

Especially in the vacuum field a flat diaphragm is not the optimum

solution. At   top dead center, the clamp disc and its fastening

17

give rise to gaps and recesses that increase the dead volume, so

reducing the volumetric efficiency of the pump. These limitations

of the flat diaphragm led to the development of the molded

diaphragm.

Molded Diaphragm

In contrast to the flat diaphragm, which is stamped from a flat

sheet of rubber, the molded diaphragm is manufactured by

pressing the finished part in an axially symmetrical mold. To

provide good elastic deformability, the thickness of the diaphragm

is reduced towards its circumference. The threaded stud that

projects from the lower side of the diaphragm secures it to the

connecting rod. The diaphragm presents an unbroken surface to

the compression chamber. The contour of the pump here can be

made to conform very precisely to the form of the diaphragm at full

stroke .

Top Surface of a

PTFE-Coated KNF Molded Diaphragm

This means that the dead volume can be reduced to a minimum,

so that compared with the flat diaphragm a lower ultimate

vacuum is achieved. The unbroken surface of the molded

diaphragm increases the gas-tightness. Only the joint formed by

clamping the diaphragm between the crankcase and the pump

head, and the permeability of the elastomer from which it is

made, set a limit to the possible gas-tightness. Because the metal

parts of the diaphragm are protected by a layer of rubber, the

18

molded diaphragm is particularly important in applications with

corrosive or chemically aggressive media. The smooth surface of

the diaphragm makes it practicable to coat it, for example, with

PTFE, to provide protection against highly aggressive substances.

Now, if the components of the pump head are made from PTFE,

the resulting pump is reliable, and resistant to chemicals. The

advantages of the molded diaphragm are:

Simple and economical design

Low wear

Makes a low ultimate vacuum possible

High gas-tightness

Reliable, chemically resistant pump possible

Compared to the flat diaphragm, pump, a molded diaphragm

pump of the same size and the same design life has a lower flow

rate. Whereas, the central region of the flat diaphragm is rigid the

pressure in the compression chamber deforms the molded

diaphragm much more; this can result in up to 20% reduction in

performance. If the stiff central part of the diaphragm were

extended too far without reducing the size of the elastic part, the

permissible stresses for the diaphragm material would be

exceeded.

The KNF Structured Diaphragm

In the development of the structured diaphragm, the primary

objective was to combine the advantages of the flat and molded

diaphragms, and at the same time to eliminate the disadvantages.

This was achieved by analyzing the stresses and led to the

development of a structure for the underside of the diaphragm. By

stiffening the diaphragm in the center it was possible to reduce

the size of the vulcanized-in metal part significantly, compared

with the molded diaphragm. These measures produce an average

reduction of 15% in mechanical loading in the diaphragm, and

reduce the effort needed to turn the pump.

19

Bottom Surface of aKNF Sturctured Diaphragm

The result is a diaphragm with:

Simple and economical design

Low wear

Comparatively high pressure capability

High flow rate

A low ultimate vacuum

Chemical resistance

High Efficiency

For many applications the miniaturization of the pump that the

structured diaphragm makes possible is particularly important.

The optimized diaphragm structure makes it possible to design a

smaller diaphragm, so that the successor model of a pump with a

flow rate of 5 standard litres of air per minute could be smaller in

all the main dimensions. This new pump occupies only 63% of the

volume of its predecessor. The user can reduce the space for the

pump in his product by more than two-thirds. Naturally, when the

volume is smaller the weight is reduced as well.

By using the structured diaphragm the desire for increased

performance without making the pump bigger can also be

satisfied. Compared with a molded diaphragm pump, one fitted

with a structured diaphragm can accommodate a longer stroke,

with the same elastic deformation of the diaphragm material, and

without compromising the mechanical data. At the same time less

20

power is required to drive the pump. Combining as it does the

advantages of flat and molded diaphragms, the new structured

diaphragm represents a milestone in diaphragm pump

technology. Pumps with structured diaphragms cannot only met

the demands of existing fields of application better than was

previously possible, but also fulfill the needs of completely new

applications.

In Summary

The flat diaphragm is, because of its simple construction,

inexpensive.

Metal parts in the compression chamber can be a

disadvantage.

The KNF-patented structured diaphragm has a decisive

influence on the size of a diaphragm pump.

KNF Neuberger has managed to achieve the same

performance from a volume only 1/2 that of its predecessor.

Employing the molded diaphragm reduces wear and

considerable improves gas-tightness.

Coating the diaphragm with PRFE, and making the pump

head from PTFE provide for the highest chemical resistance.

The patented structured diaphragm combines the advantages

of molded and flat diaphragms.

Large-scale FEM analysis and stress calculations resulted in

the structured diaphragm.

21

It’s the Diaphragm that Does It

Characteristics determine areas of application for gas and vapor

diaphragm pumps

Diaphragm pumps are an attractive solution today for a wide range

of applications, irrespective of whether corrosive or toxic media are

being pumped. A particular role is played by the application-

oriented design of the diaphragm and its careful manufacture.

Diaphragm pumps for gas and vapor have a place in the analytical,

medical, and process engineering markets, as well as in the

laboratory. They can deliver media uncontaminated, have high gas

tightness, and can be designed so that the parts coming in contact

with the media are chemically resistant. Depending on the

diaphragm type used, the properties and applications of these

pumps can vary in considerable detail. There are three basic

22

designs of diaphragm in use today: the flat diaphragm, the molded

diaphragm, and the structured diaphragm.

Flat Diaphragm

The flat diaphragm is the classic diaphragm type, and consists of a

rubber disk. The connection between the diaphragm and the

connecting rod, which provides the up and down motion, is

provided by a clamping disk, normally made of metal, and a screw

which is guided through a hole in the center of the diaphragm (Fig.

1). Pumps with flat diaphragms provide high compression strength,

because the connecting rod and the diaphragm support disk

actually support the diaphragm. For vacuum applications, on the

other hand, flat diaphragms are not the best choice, because they

cannot achieve optimal vacuum. The geometric design of

diaphragm, clamping disk, and compression chamber - in

conjunction with the slight tilting motion of the diaphragm actuated

23

by the connecting rod - results in a high clearance volume at the

upper turning point of the diaphragm motion; the result of this is a

dead volume on evacuation. In addition, the gas tightness of such

diaphragm pumps has been shown to be unsatisfactory for many

applications because, among other things, the gas can creep along

at the connecting rod fastening screw into the pump chamber, and

be admitted into the chamber. The leakage rate which can usually

be achieved, normally 1 mbar 1/s, restricts the use of these pumps

in the areas of analysis, chemistry, and medical technology.

Another disadvantage of flat diaphragms is the poor resistance of

the metal parts (clamping disk and screw head) to corrosive or

aggressive gases. As a result, for chemically-resistant pump

requirements, the diaphragm, the clamping disk and the screw

head must be coated with chemically resistant materials such as

PTFE. Care must be taken to avoid nicks in the coating when using

tools to install or remove the metal clamping disk.

Molded diaphragm

With the molded diaphragm, developed at KNF Neuberger, the

metal stud required to actuate the diaphragm is vulcanized into the

center of the diaphragm and forms a rigid zone at that point (Fig.

2). This means that the side of the diaphragm located in the

pumping chamber is fully enclosed. The pumping chamber can be

well adapted to the contour of the actuated diaphragm, and the

clearance volume of the pump is reduced, without the risk of the

diaphragm striking the pump head at the upper turning point of the

movement. This results in a good vacuum. At the same time the

closed surface of the diaphragm achieves very good gas tightness

for the pump.

In addition, with the molded diaphragm, reliable chemically-

resistant pump designs for applications with corrosive or

aggressive gases or vapors can be readily created. The metal stud

24

of the pump is vulcanized within; in other words, covered by

elastomer. The PTFE coating of metal parts, unlike flat diaphragm

designs, is not necessary. The diaphragm itself can be provided

with a protective coating against aggressive media, with the

protective coating being permanently cross-linked to the elastomer

diaphragm by vulcanization.

These properties described elevate the molded diaphragm to a

preferred status over the flat diaphragm. And, because the molded

diaphragm does not have the rigid clamping disk and diaphragm

support of the flat diaphragm, it suffers from restricted

compression strength, something which is particularly relevant for

compressors.

Structured diaphragm

The patented, structured diaphragm combines the advantages of

both the flat and molded diaphragm, and at the same time largely

eliminates the disadvantages from which both these diaphragm

types suffer. As with the molded diaphragm, the structured

diaphragm has the metal stud, required to actuate the diaphragm,

vulcanized centrally into the diaphragm, where it forms a rigid

zone. The side of the diaphragm located in the pumping chamber

is, therefore, entirely enclosed. The difference is that the underside

of the diaphragm is ribbed to accommodate the particular load to

be imposed, and the diaphragm is stiffened in the center (Fig. 3).

The advantages are reduced mechanical loading (and therefore less

wear), smaller size, comparably high compression strength, and

good delivery capacity.

As with the molded diaphragm, the closed surface of the structured

diaphragm allows for reliably chemically-resistant diaphragm

designs to be created with no problems. Last but not least, very

high gas tightness can also be achieved, particularly as compared

25

with the flat diaphragm. In contrast with the conventional molded

diaphragm, a special design is available to improve the gas

tightness of the structured diaphragm even more. In this case, the

periphery of the structured diaphragm features a lip, which serves

as a positive-fit seal at the clamping point between the pump

housing and head. As a result of all these features, the leakage rate

of a pump with a structured diaphragm is, as a rule, lower by a

factor of 100 than a pump with a flat diaphragm.

Development and manufacture

It is only the development evolution, from the flat diaphragm,

through the molded diaphragm, to the structured diaphragm which

has made the versatile application of the diaphragm pump possible.

For this reason, the demands on the diaphragms with regard to

mechanical, chemical, and thermal loading have risen sharply. The

simple rubber-component diaphragm has developed into a device

which is designed by finite-element calculations, and undergoes

very complex manufacturing processes. The challenge has been to

create a rubber/metal connection between the diaphragm and the

metal stud of the eccentric connection, which, despite the differing

elasticity behavior of the materials, and despite the dynamic load

during pump operation, allows a component with long service life

to be produced. In addition to the search for suitable materials, the

focus of attention for the diaphragm manufacturers, such as

Freudenberg, was on optimizing the manufacturing process.

A key feature in the performance of the diaphragms is a

permanent, firm chemical bond between the PTFE film and the

metal part on the one hand, and the elastomer on the other. In this

case, it has proved possible, by matching suitable bonding agent

systems to the materials available, to produce compounds in which

the strength is greater than that of the elastomer.

26

Keaflex to make elastomer pump

diaphragm for Salamander

UK-based Salamander Pumps has chosen James Walker Keaflex to

manufacture elastomer diaphragms for its domestic water-pressure

booster pumps.

The ethylene propylene diaphragm is a component of the 1 litre

pressure vessel that is used in the company's ESP CPV models

which can boost the pressure of single showers or entire house-

water systems.

Tooling for the moulded part was designed by James Walker

Keaflex and manufactured at its in-house tool shop. The multi-

cavity design is manufactured to precise tolerances, and, after

moulding, the diaphragms are post-cured to remove any volatile

contaminants that could otherwise leach into the water, the

company says.

The high specification EP62/65 ethylene propylene elastomer used

for the diaphragm was developed within the James Walker Group.

Approved by the Water Regulations Advisory Scheme for use in hot

and cold potable water at temperatures up to 85°C, the material

meets the requirements of British Standard BS6920.

EP62/65 is said to be suitable for other uses in the water industry,

such as gaskets and O-rings. EP62 is also available with increased

hardness, and can be used in applications that require greater

mechanical strength, such as tap washers and ball valve discs.

27

Ethylene propylene diaphragms manufactured by James Walker

Keaflex for Salamander Pumps for use in its domestic water-

pressure booster pumps.

28

Sample system design and diaphragm

pumps

The design of sampling systems can be influenced by many factors,

including cost, size, maintenance needs, source conditions and

analyser requirements. Dan Martin, of diaphragm pump

manufacturer Air Dimensions, discusses several common aspects of

sample system design, taking into account the operating

characteristics of a diaphragm pump, and choices that will improve

system performance, lessen measurement errors and reduce pump

maintenance.

Sample system design can take many forms, influenced by factors

including cost, size, maintenance, source conditions and analyser

requirements – to mention a few. Illustrated here are a few

techniques that are commonly used in many sampling systems; this

list is not complete, but serves as an introduction to the topic of

system design.

The first section looks at sample line sizing and the location of the

pump relative to the sample source connection tap. Next, the effect

of pump pulsation and its amelioration are discussed, together with

the use of filters. The final section reviews the various types of flow

and pressure control methods.

Pump location

A sample pump is required if the process pressure is too low to

provide adequate pressure for measurement. The location of the

pump and the selection of line size are the most important factors

when dealing with low-pressure samples. This may be of little

concern for short line lengths, but when long distances are

29

involved, this can be a serious and expensive mistake if not

evaluated correctly.

Piping design is best illustrated by the example in Figure 1. For the

illustrated system, the analyser requires a flow rate of 10 standard

litres per minute (SLPM) at an absolute pressure of 14.7 psia (=

0.101 MPa), i.e. atmospheric pressure. The process pressure is

atmospheric, that is 14.7 psia, and the sample line length is 300 ft

(c. 91.5 m). The filter pressure drop is assumed to be negligible.

Full-size image (15K)

Figure 1. The location of the pump and the sample line size are the

most important factors when designing piping to deal with low-

pressure samples.

There are two main choices to be made: the sample line size, and

the location and selection of the pump.

Table 1 illustrates how sample-line pressure drop is affected by

pump location and sample line size.

Table 1.

Sample tubing characteristics for flow rate of 10 SLPM

Size

(inches)

ID

(in)

Line pressure drop

(psi) – position A

Line pressure drop

(psi) – position B

1/4 × 0.180 ΔP = (20.0 – 14.7) = ΔP = (14.7– 7.3) = 7.4

30

Size

(inches)

ID

(in)

Line pressure drop

(psi) – position A

Line pressure drop

(psi) – position B

0.035 wall 5.3

3/8 ×

0.035 wall0.305

ΔP = (15.0 – 14.7) =

0.3

ΔP = (14.7 – 14.3) =

0.4

Full-size table

Note: Line pressures are expressed as absolute values measured in

psia.

The selection of pump position A (Figure 1) is dependent on flow-

rate capacity for inlet pressure equal to the ambient pressure. The

outlet pressure is the sample-line back pressure.

For position B, the selection is dependent on the reduced pressure

at the pump inlet (due to sample line loss) and the required flow

rate with the pump outlet at atmospheric pressure.

In summary

• Comparing tubing pressure drops for pressurized and vacuum

sample-line conditions illustrates the importance of line size. In

addition, a larger displacement pump may be required for smaller

line sizes because of vacuum conditions and decreased density.

• Vapour condensation may be a problem if the pump is located a

long distance from the process inlet and sample pressure is

reduced below gas vapour pressure.

• Final selection of line size and pump location is affected by many

factors, including local codes, environmental considerations,

system design and installation requirements.

Pump pulsations

31

The effect of pump pulsation is a subject not often considered in

sampling system design. The connection at the diaphragm pump

inlet will see a suction vacuum for one-half of the motor shaft

rotation followed by ‘no flow’ for the remaining half of the rotation

– the exhaust pulse (flow) portion of the pump cycle. As a result,

both the inlet and exhaust strokes cause peaked flows several times

greater that the average flow rate specified for the pump. This

raises several concerns for the selection of components (line size,

filter, flow indicators, etc.) for optimum design. The inlet and outlet

conditions are depicted in Figure 2.

Full-size image (4K)

Figure 2. Typical pump inlet and outlet pulsation pattern.

Inlet pulsation

Filters are used as protective devices in most sampling systems. A

filter is typically installed before the pump to remove

contamination from the sampling system. Placing the filter close to

the pump inlet will have the effect of a pulsation dampener, where

the inlet suction vacuum is stored in the filter body (Figure 3). This

will reduce the vacuum and flow-rate peaks and increase the

average pump flow rate. Overall system performance is thus

improved at little or no additional cost.

32

Full-size image (8K)

Figure 3. Filters used as pulsation dampeners.

Outlet pulsation

In addition, placing a filter at the outlet will reduce the amplitude

of the pump outlet pressure pulse. This is important when

flowmeters or other pressure-sensitive instruments are used, as

pulsation will give false readings.

Balanced stroke design

A two-head opposed-stroke pump will greatly reduce the effects of

pump pulsation. This pump design utilizes two heads oriented 180°

out of phase; as one head begins the suction stroke, the other head

is starting its exhaust stroke (Figure 4).

Full-size image (4K)

Figure 4. Ideal dual-head pulsation pattern.

When the inlet and outlet ports of the two heads are connected in

parallel, there will be continuous action at the interconnect

junctions and no dead time between inlet or outlet pulses. The

result of this design is smoother gas flow with greatly reduced

pulsation effects at the inlet and outlet connections; in addition,

sample-line pressure losses are correspondingly reduced because

of the reduced peaked and nearly-constant flow rate.

33

Use of a two-head pump with balanced pump stroke design will be

of particular interest in those systems where pump selection is

marginal, or system design is extremely sensitive to pulsating

flows. In some cases, a smaller two-head, balanced-stroke pump

will give better performance than a more-expensive, single-head

pump.

Flow & pressure control

Following are several basic control systems that are used to

regulate flow and pressure.

Control valve

The simplest form of flow control is a manual throttle valve (TV) in

series with the pump. This may be located before the pump –

configuration A – or after the pump – configuration B (Figure 5).

Full-size image (11K)

Figure 5. Flow control valve placement. (FI = flow instrument or

flowmeter.)

In configuration A, the flow rate is reduced as a result of the

increased restriction introduced by the throttle valve, resulting in

pressure reduction at the pump inlet. With the TV closed, the

gauge pressure, Pa, at position A can decrease to less than 28 in

Hg vacuum (depending on pump capability).

In configuration B, flow rate is again reduced as a result of the

increased restriction by TV, but this has the effect of increasing

pressure at the pump outlet – up to a maximum of the pump

34

capacity. Depending on the pump characteristics, the increase in

pressure Pb may range from 30 psig to 100 psig or more.

Excessive pressure on the pump diaphragm (configuration B) will

result in increased diaphragm and bearing stress, reducing pump

life and increasing service requirements. However, locating the

valve before the pump inlet avoids the condition of high diaphragm

pressure.

Relief valve flow control

Use of a relief valve (RV) reduces the high-pressure pulses on the

pump diaphragm, as in the above case where a throttle valve is

located at the pump outlet (Figure 6). Flow pulses will therefore be

reduced. The return line can either be connected back to the

process or returned to the pump inlet. Connecting the return line

back to the pump inlet is generally the lower cost alternative

because the distance to the process can be much greater.

Full-size image (16K)

Figure 6. Flow control via use of a relief valve.

Constant outlet pressure

Using a back-pressure regulator (BPR) will provide better pressure

regulation than a relief valve. A BPR located before the analyser

will provide the best pressure regulation (Figure 7). Adding one or

35

more filters is optional, but will give additional pulsation reduction

and therefore also help system performance.

Full-size image (19K)

Figure 7. Pressure control using a back-pressure regulator (BPR).

Constant system flow rate

Using a downstream pressure regulator (DPR) with a throttling

valve configured as shown in Figure 8 provides a constant flow rate

independent of the pressure upstream of the pump. Downstream

pressure is assumed to be constant. This design may be used for

systems that have constant time delay or constant response-time

requirements.

Full-size image (9K)

Figure 8. Flow control using a downstream pressure regulator

(DPR).

Conclusion

36

The above discussion only touches on a few basic methods and is

by no means complete, but is intended to provide a starting point

for system design using diaphragm pumps. There are many

approaches to sampling system design, some better than others,

but, in general, ‘the simplest ways are the best’

37

Selecting the Proper Diaphragm

Pressure Control Valve

FDV Valves in PVDF materials

1. How It Works

The possible uses of the pressure control valve are widely varied

and well established. It can be used in two possible ways

depending on the application:

Pressure Control Valve

Maintain constant back pressure for exact flow rates under free

flow conditions, with positive pressure on the inlet side or with

varying back pressure, or with the operation under vacuum etc.

Bypass Valve

Serves as a safety device for protection of the pump, motor,

pipework, vessels and other accessories. Installed as a bypass

valve, it prevents excessive pressure build up in the system

caused by dirt, misuse or other problems.

The pressure control valve FDV30/1.30 are able to handle air, gas

and liquids.

The pressure control valve can used with KNF products as well as

other pump systems. The FDV30/1.30 series are recommended for

use with the following pumps.

38

Metering pumps - all FM Products

Transfer pumps NF10, NF30, NF1.30

Vacuum pumps. Up to a max flowrate of: CALL KNF

Note! Pressure control valves are not absolutely tight shut-off

valves. They should always be installed on the pressure side of the

pump.

2. Construction

The FDV pressure control valve is based on the principle of the

diaphragm valve. The essential components are the lower

housing, the upper housing, the spindle and the diaphragm.

The required pressure is achieved by adjusting the tension in the

spring. The spring tension exerts pressure on the diaphragm with

is then transferred to the fluid system. By turning the spindle

clockwise the opening pressure increases at a given flow rate and

by turning it counter-clockwise the opening pressure decreases. A

locking nut prevents adjustment from the set position.

In the normal position the diaphragm rests on both of the ports and the system is

then closed. When the pressure produced by the pump exceeds the pre-set opening

pressure the diaphragm is pushed open and the medium can flow. The pressure

control valve is now in the working mode and is opened.

Index

10 Lower housing

20 Upper housing

30 Protective surface of diaphragm

40 Lip diaphragm

50 Flexi-washer

60 Support plate

70 Washer

80 Spring

90 Washer

100 Shaft spindle

110 Locking nut

39

120 Screw

 

The parts in contact with the media are the diaphragm and the

lower housing. They can be produced in a variety of materials

which can be selected according to the liquid or gas to be

transferred.

The following material combinations are available:

Base Model Code

Head components

Material (liquid contacting parts)

FDV30KP, FDV1.30KP

lower hosing PP

diaphragm EPDM

 

FDV30KV, FDV1.30KV

lower housing PP

diaphragm Viton

FDV30KT, FDV1.30KT

lower housing PP

diaphragm Viton/FFKM

FDV30TV, FDV1.30TV

lower housing PVDF

diaphragm Viton

FDV30TT, FDV1.30TT

lower housing PVDF

diaphragm Viton/FFKM

The upper housing which is not in contact with the media is

produced in Ryton for all types.

3. Technical Data

40

The correct control valve is selected according to the following

criteria:

1. Pressure

2. Flow rate

3. Aggressive nature of the media

4. Size

Note that the pre-set opening pressure should not exceed the

maximum pressure of the pump. Other important factors include

connecting sizes, overall geometry, temperature etc. The

following table assists in making the correct selection.

Following base models are available:

Parameter  FDV30

Z

FDV1.30

Z

Adj. pressure range bar g 0.2 0 2.5 2.0 - 6.5

Standard pressure (factory

set)bar g 0.5 3.0

Max. flow with liquids ml/min 600 600

Max. flow with air/gases l/min    

Max. environment temp °C 80 80

Max. media Temp °C 80 80

Threads for hose

connectorin. G 1/8" G 1/8"

Weight (depends on

model) grams 50 - 60 50 - 60

The above flow rates should not be exceeded. If required the

factory pre-set opening pressure can be adjusted to other values.

The adjusted opening pressure will be noted on the identification

plate.

Dimension Drawing

41

4. Options

The pressure control valve is also available with other options:

turning knob instead of locking nut

other materials / combination

with hose connectors

5. Applications

Pressure control valves are used in diverse operations and can

therefore undertake several functions:

1. As pressure control valve for precise dosing in systems with

fluctuating pressure, for operating in a vacuum and for

operating with back pressure on the suction side.

2. As bypass valve to prevent the build up of excessive pressure

on the operating side of the system, for protection of the

pump, pipework, vessels, glassware, etc.

3. Anti-injection function to avoid unintended injection of liquid

when metering into pipework at high flow rates.

Examples of applications

1. Pressure control valve

The precision of diaphragm pumps can be influenced by other

factors such as system pressures. The following illustrations

demonstrates the use of the control valve in achieving precise

metering.

a. Operation in a system with fluctuating back pressure

42

Varying back pressure can significantly influence the performance

and thus the precision of the flow rate. The use of the pressure

control valve promotes more stable system pressure. The pressure

variation is reduced and so precise dosing is guaranteed.

1a. Flow diagram

 

b. Operation with vacuum on the outlet of the pump

When it is required to meter into a vacuum on the outlet side of

the pump, use of the pressure control valve restricts the free flow

of the media through the pump. Without a pressure control valve,

the vacuum on the outlet side of the pump would cancel the effect

of the pump and the media would flow unaided through the pump.

This applies whether the pump is being used or not.

1b. Flow diagram

c. Operation with positive pressure on the suction side of

the pump

43

If positive pressure exists on the suction side of the pump,

accurate dosing is not possible. Even when the pump is not being

used, it is still possible for the pressure head to force the media

through the pump. The pressure control valve will withstand the

higher pressure therefore guaranteeing optimum performance.

1c. Flow diagram

2. Bypass function

The bypass valve serves to restrict the build up of excessive

pressure on the pump outlet side of the system. Therefore the

pump, pipework and motor are adequately protected against

malfunction or failures as a result of high pressure build up. In

the case of excessive back pressure on the outlet side of the

pump, the bypass valve opens and the media is recirculated. The

media recirculates and the pump is protected until the restriction

is removed.

A bypass valve should also be used if the pump has to operate

against a closed system on the pressure side.

Pressurized measuring systems, pipework, receivers etc. can also

be protection against excessive pressure build up by using the

bypass valve. If excessive pressure build up occurs, the media will

flow back through the valve and into a storage vessel.

44

2. Flow diagram

3. Anti-injection function

When pumping into a tube that contains a continuously high

speed of flow of media, “venturi action” can occur. This means

that when the pump is stationary, suction produced by the fast

flow of liquid in the tube overcomes the resistance of the valves

and pulls the dosing medium through the pump. This venturi

action has a negative affect on the dosing accuracy.

With the built in diaphragm pressure control valve, the venturi

action can be stopped. Furthermore the pressure control valve

function assists the dosing accuracy even though the pipework

pressure may vary.

3. Flow diagram

4. Other functions

Further uses are:

Improved sealing against back flow, for example in analyzers

45

Over-pressure valves in liquid, air/gas systems

Charging pipework with constant pressure without the flow being

drawn off through the bypass valve

The Logical Path - The Application of

Ceramics to Diaphragm Pumps

To achieve the highest possible safety in service, pumps employed

in the chemical industry must be gas-tight, chemically resistant and

maintenance-free.

To avoid unwelcome chemical reactions and to maintain the purity

of the gases, contamination by the pumping process must be

prevented. Clean vacuum is indispensable for many applications.

For these extreme conditions a special type of positive

displacement pump, the diaphragm pump, has become an

46

important asset for many users. The principles of design of the

diaphragm pump make it gas-tight and absolutely oil-free. The use

of PTFE and ceramic ensure excellent chemical resistance.

FT pumps with flat diaphragms

The first step on the path to diaphragm pumps with almost

universal chemical resistance was taken several years ago. The FT

pumps were designed for small and medium flow-rates and

represented a significant innovation at the time. FT stands for Full-

Teflon® and thus for the best possible chemical resistance. The

idea of making all the gas-contact parts of a diaphragm pump from

PTFE was a challenge to the design and development engineers.

The disadvantages of this material were common knowledge but its

disadvantages just as well know.

It is a property of PTFE that it deforms or creeps under constant

tensile or compressive loads. In manufacture it is a disadvantage

that PTFE cannot be injection molded because it has practically no

melting point.

A primary goal was to devise a diaphragm which fulfilled the

following criteria:

1. Chemical resistance,

47

2. Elastic de-formability,

3. Low permeability to gases.

These conditions can only be met by a PTFE/elastomer

combination, the first attempt employed the customary flat

diaphragm abut coated with a layer of PTFE (Fig.1a). The

diaphragm retainer-plate was made of steel and coated with a

fluorinated polymer to provide the necessary chemical resistance.

This design, however, has intrinsic problems.

The PTFE layer experiences severe strain because it is rigidly

restrained by clamping with the retainer plate. This problem can be

relieved by reducing the stroke, but this in turn means that the

pump must be larger for a given flow rate.

To prevent damage to the PTFE layer the diaphragm retainer plate

must be carefully radiused and the radii must be very well blended

into their neighboring surfaces. This makes it relatively expensive

to produce. Even with the best possible design this concept

involves a certain dead volume which has an unfavorable effect on

the attainable ultimate vacuum.

High gas-tightness demands that there is a minimum number of

gas seals between the compression space and the world outside.

Leaks, when they occur, do so generally at such points, and so a

diaphragm with a central hole was not considered an optimum

solution.

When operating under extreme conditions a diaphragm may

deteriorate or be damaged. In this case it must be changed and

ease of servicing becomes important to reduce downtime. There

was no satisfactory solution to this problem with the retainer plate

design. If the retainer plate was plastic-coated the holes in its

upper surface could not be used to tighten or loosen it because this

would immediately result in damage to the plastic coating by the

pegs of the spanner employed and loss of protection against

chemical attack. To overcome this problem it is necessary to devise

some means of clamping which can be tightened from the

48

crankcase side of the diaphragm, this is possible but decidedly

cumbersome.

FT pumps with molded diaphragms

Finally all these considerations led to another type of diaphragm. In

the new range of FT Pumps the flat diaphragm was replaced by the

molded diaphragm. The molded diaphragm consists of a neoprene

body, which is vulcanized under precisely defined pressure and

temperature conditions simultaneously bonded to a chemically-

treated PTFE film and the steel carrier component, to ensure

permanent and reliable bonds. This compact diaphragm element

can, after removal of the head, be removed and refitted by simply

screwing it into or out of the con-rod. The curved form of the upper

surface of the diaphragm in designed to conform to the shape of

the head with the least possible dead volume so as to provide

excellent ultimate vacuum. Since it has no central perforation, this

diaphragm provides practically a hermetic seal between

compression space and crankcase.

The head, the valve discs and the valve bodies of the FT range are

made from PTFE . As already mentioned, the major disadvantage of

PTFE is its tendency to creep. Over a period of time the PTFE

molecules reposition themselves to relieve internal stresses so that

clamping forces between two components reduce with time. The

design compensates for this effect by clamping the PRGE in a

"sandwich" between a metal plate and the crankcase and

employing disc springs to maintain clamping force when the PTFE

relaxes. The diaphragm pumps of the FT range have flow-rates

between 10 and 60 NI-min-1 are used mainly in the laboratory

where, because of their versatility, they have found wide

acceptance.

For chemical plant and pre-production trials these flow-rates are

often not sufficient. For removal and circulation of aggressive

49

gases, volume flows in the region of 100 to 250 NI-min-1 are

required. To reduce the flow losses in the connecting pipe work

and to keep the number of connections (and hence potential leaks)

as small as possible , diaphragm pumps with high flow-rates should

not have more than two heads.

Ceramic Materials

The successful path which employs PTFE as a practically universal

chemically resistant material for the head parts of diaphragm

pumps has been extended by the use of ceramics. The excellent

chemical resistance allied to hardness, strength and wear

resistance make this material particularly attractive for larger

diaphragm heads. Due to the limitations that the diaphragm

imposes on the stroke, diaphragm compressors and vacuum pumps

require a large effective diaphragm diameter, which means that the

diaphragm head must fulfill particularly exacting conditions with

regard to strength and stability. Tight tolerances characterize this

component which has an important influence on the ultimate

vacuum pump . Only tight tolerances can ensure a consistently

small dead volume and thus consistent performance in series

production. For this PTFE cannot be used for larger diaphragm

heads.

Up to now its brittleness together with the difficulty of manufacture

and machining have discouraged designers from using ceramic

parts. In recent years there has been much progress in the

technology of ceramic manufacture and aluminum oxide ceramic

manufacture and aluminum oxide ceramic has become particularly

significant. By optimizing the time and temperature of sintering as

well as the purity and particle size of raw material, quality has

been improved and costs reduced. Precision parts, and the

diaphragm head is one of them, must be machined with diamond

tools after sintering. Since ceramics have great compressive

50

strength but are very sensitive to tensile loads the designer must

take care that the component is practically only subjected to

compressive loads.

The molded diaphragm used with the FT head is also used for the

ceramic head pump. Even in this much larger version it can be

made to conform to the head shape. The valve discs and valve

bodies are again of PTFE. Ceramic combined with PTFE has made

possible the development of a pump with a high flow-rate and first-

class resistance to chemicals.

This new development of single and twin-headed pumps with

ceramic diaphragm heads has extended the performance range of

chemical resistant pumps by a factor of 3, to 130 NI-min-1 for

single or 230 NI-min-1 for twin heads respectively, and has thus

opened these products to applications which were closed to the FT

range.

If service trials are successful, ceramics could start a materials-led

technological revolution in engine design. Practically every motor

manufacturer is today testing ceramics for his future products. In

the field of diaphragm pumps the future has already begun.

51

Getting Pumped Up on Diaphragms by

Design

All diaphragm process pumps for use with gases and vapors share

certain fundamental characteristics, including relatively simple

construction, oil-free operation without maintenance, high gas

tightness, and uncontaminated delivery of the gas. Beyond the

basics, though, specialized performance can be realized from

diaphragm pumps due to their design versatility and application

adaptability.

Diaphragm pumps transfer, compress, recirculate, or evacuate

gases or vapors in industry and research applications for medical

technology, analytical instrumentation, control engineering,

chemical and process engineering, or in the laboratory, among

others. The proper design and selection of a pump’s diaphragm and

how effectively a pump can be customized to handle the demands

ultimately will govern success in an application.

Diaphragms by design

52

The diaphragm primarily functions to displace the working gases

from the pump’s compression chamber. Integration into a pump is

relatively simple: The elastic diaphragm is clamped pressure-tight

between the pump head and the housing to separate the transfer

compartment from the housing’s interior. The diaphragm then is

connected pressure-tight to a connecting rod.

In operation, the drive in the interior of the housing reciprocates

the connecting rod, which causes the diaphragm to move up and

down. In the downward stroke, the suction created in the pump

chamber causes the inlet valve to open, allowing flow into the

chamber. In the upward stroke, the pressure caused by the rising

diaphragm causes the outlet valve to open, allowing flow out of the

chamber.

The most common standard diaphragm designs include flat,

molded, and structured.

The simples and least expensive is the flat diaphragm, which is

essentially an elastomer disk. The connection between the

diaphragm and the connecting rod is provided by a clamping disk

(usually metal) and a screw guided through a hole in the center of

the diaphragm. Pumps with flat diaphragms provide high

compression strength, because the connecting rod and the

diaphragm-support disk contribute support.

Among tradeoffs, however, pumps with flat diaphragms will not

typically achieve optimal vacuums; uncoated metal parts will be

prone to corrosive or aggressive gases; and relatively poor gas

tightness can be expected, leading to higher leakage rates

(normally 1 mbar 1/s) and restricting their application potential in

the areas of analysis, chemistry, and medical technology.

Molded diaphragms represent a significant improvement by fully

enclosing the side of the diaphragm located in the pumping

chamber. This is accomplished by vulcanizing the metal stud

(required to actuate the diaphragm) into the center of the

53

diaphragm, which forms a rigid zone at that point and eliminates

any need for a rigid clamping disk (and possible leakage path).

With this design the pumping chamber adapts to the contour of the

actuated diaphragm and the clearance volume of the pump is

reduced without the risk of the diaphragm striking the pump head

at the upper turning point of the movement. An ideal vacuum is

created and the closed surface of the diaphragm promotes superior

gas tightness for the pump.

In addition, this design inherently allows for the development of

chemically resistant versions without a need to coat metal parts;

the vulcanized metal stud of the pump is already covered by

elastomer. For applications that will experience corrosive or

aggressive gases or vapors, added protection for the diaphragm

can be provided with an appropriately enabling coating.

One noteworthy tradeoff: The lack of the flat diaphragm’s rigid

clamping disk and other support can restrict the compression

strength critical for compressor applications.

Structured diaphragms combine the advantages of both the flat and

molded types and dispatch many of the drawbacks. As with the

molded diaphragm, the metal stud required to actuate the

diaphragm is vulcanized centrally into the diaphragm, where it

forms a rigid zone, and the side of the diaphragm located in the

pumping chamber becomes entirely closed.

The difference with the patented structured diaphragm is that its

underside is ribbed to accommodate the particular load to be

imposed and the diaphragm is stiffened in the center. The outcome:

Reduced mechanical loading (and less wear), smaller size (for more

compact designs), comparably higher compression strength, and

good flow capacity. Standard structured diaphragms exhibit

extremely high gas tightness (which can be improved upon with

special designs) and will demonstrate significantly lower leakage

rates (reduced by a factor of 100 compared with flat diaphragm

pumps).

54

Variations on a theme

Regardless of industry, evolving applications for process pumps

have imposed increasing demands on diaphragms and their

capability to satisfy ever-burgeoning mechanical, chemical, and

thermal loading requirements. In response, special versions can

offer properties tailored to application parameters. Among them:

“Gas tight” diaphragm pumps seal exposed areas with O-rings to

achieve dramatically lower leak rates (5 x 10-3 mbar l/s to 5 x 10-6

mbar/s). These can prove especially useful in applications involving

poisonous or radioactive gases, whose traces in the surrounding air

could endanger workers and the environment.

Corrosion-Resistant Considerations

Corrosion-resistant diaphragm pumps benefit from combining high-

grade steels and solid PTFE (or other inert materials for the wetted

head portion) with a laminated layer of corrosion-resistant material

over the diaphragm. Such a combination imparts mechanical and

thermal resistance, resistance to corrosion, and high tensile

strength and resistance to pressure. By laminating the PTFE,

pumps can become more flexible and exhibit longer service life.

The high-grade steels will equip the suction channel and the output

channel of the head parts with robust threads. With secure and

reliable screw joint connections, pumps can greatly resist pressure

and significantly lower the potential for leaks.

Heated-Head Sampling Pumps

Heated diaphragm pumps will be specified where a small cooling

down of the working gas leads to “condensing out” of parts of the

gases, which can distort measurement results if the gases are

55

transferred as samples. In order to prevent such condensation, the

sample gas must be guided via a heated pipeline and the pump

head, too, must be heated. An electric heating element installed in

the pump head does the job. (The current supply to the heating

element can be switched off using a thermal switch attached to the

head or, as a recent innovation, temperature sensors can be

mounted on the head to regulate electronically.) The heated head

offers a corollary advantage by keeping the gas dry and preventing

the formation of corrosive compounds in the pump chamber.

Diaphragm pumps with explosion-proof AC motors offer solutions in

potentially hazardous locations, such as for applications in the

chemical, mining, hydrocarbon processing, plastics, and petroleum

industries. Specialty pumps for compliance with Class 1, Division 1,

Groups C & D and ATEX hazardous locations have been engineered

to deliver high performance and long service life during

continuous, heavy-duty operation.

Double Diaphragm Safety Pumps

Double diaphragm pumps pair a “safety” diaphragm with a

“working” diaphragm to safeguard applications where hazardous,

toxic, or otherwise harmful (or valuable) gases must be transferred.

Such applications raise the bar on the demands for gas tightness

and leakage prevention and have been commonly engaged for

decades to monitor emissions at nuclear power plants.

Together with additional sealing rings, a double diaphragm

system’s arrangement enhances gas tightness with leak rates as

low as < 6 x 10-6 mbar l/s. (In very rare cases, should the working

diaphragm become damaged, the pumped medium will not escape

but will be captured in the intermediate space between the two

diaphragms.) The “safety” diaphragm is subject only to low

mechanical and thermal loads during pump operation; the

56

“working” diaphragm is elastically distorted and warmed by the

compression process.

A rupture of the working diaphragm will easily be detected through

a sudden and dramatic drop in the pump’s pumping or compression

capacity. If the pump is generating only low pressure, a sensor can

be fitted to monitor the intermediate area between the working

diaphragm and safety diaphragm to detect any damage to the

working diaphragm. Suitable pressure and gas sensors can be

specified for this task.

Footnote: As all these designs suggest, the simple rubber-

component diaphragm has evolved into a device supported by

finite-element calculations and complex manufacturing processes.

With all the choices and capabilities to customize, users can benefit

from partnering at the outset with an experienced manufacturer to

develop the best-suited diaphragm pump for the application.

57

Source

1. http://www.absoluteastronomy.com

2. http://www.chromatography-online.org

3. http://www.knf.com

4. http://www.wisegeek.com

5. article by Werner Trares and Erwin Hauser

6. article by Elsevier Ltd

7. article by KNF Flodos R&D Center,

Switzerland

8. article by Erwin Hauser, KNF Neuberger

GmbH

58

9. article by Richard J. Aerts, Process Products

Engineer for KNF Neuberger, Inc

10. article by Erwin Bolt, KNF Flodos AG

59