The application of ceramics to diaphragm pumps

The Logical Path - The Application of Ceramics to Diaphragm Pumps

by Erwin Hauser, KNF Neuberger GmbH

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 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.

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KNF Model N1200.3CT Ceramic/PTFE pump

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:

Chemical resistance,
Elastic de-formability,
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 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 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 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.