It’s the Diaphragm that Does It

by Werner Trares and Erwin Hauser

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 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 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 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 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.

Werner Trares is employed by Freudenberg Spezialdichtungsprodukte KG

Erwin Hauser is employed by KNF Neuberger GmbH