Transferring and Metering Liquids with High-Speed Diaphragm Pumps

by Erwin Bolt, KNF Flodos AG

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 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:

diaphragm pumps
piston pumps
bellows pumps.

Rotary positive-displacement pumps include:

peristaltic pumps
gear pumps
rotary piston pumps
screw pumps.

This article concentrates on diaphragm pumps for transferring and metering small quantities of liquids.

How diaphragm pumps work

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).

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 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.

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.

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 then connected in parallel, and their eccentrics arranged so that they operate sequentially.

Characteristics

High-speed liquid diaphragm pumps have several important characteristics. They:

are self-priming because they can also handle gases and gas/liquid mixtures,
can run dry,
are maintenance-free,
have long working lives,
are reliable, thanks to their simple and sturdy construction,
are chemically resistant; all medium-contact parts can be made of chemically resistant materials such as PTFE, FFPM or PVDF,
can operate in any position,
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.

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.

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:

analysers
repro equipment
the cleaning industry
laboratory equipment
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.

Author

Dipl.-Ing. Erwin Bolt studied mechanical engineering and production at the Technical High School in Konstanz. After 15 years international industrial experience, he joined the KNF Neuberger Group in FreiburgMunzingen, specialists in the field of diaphragm pumps. Since 1988 he has managed the Group's product centre for liquid pumps, KNF Flodos AG in Switzerland.