The present invention relates to a method of producing an electrically conductive connection between metallic components which have a non-conductive coating. In particular, the invention relates to a method of producing an electrically conductive coating between the metallic components which are coated with an enamel, glass or similar coating that is resistant to corrosive media.
In the chemical and pharmaceutical industries, it is common for agitators to be used in corrosive environments. In such cases, the agitator blades and the agitator shaft to which the blades are connected are usually coated with materials such as enamel or glass, which are stable in such environments and can withstand attack by such media. It is normal for both the agitator shaft and the agitator blades to be completely coated by the stable medium so that they only contact one another by way of the medium, which typically is not electrically conductive.
EP 0 189 992 describes an agitator assembly wherein the exterior surfaces of agitator blades as well as the exterior surface of a drive shaft for the agitator blades are coated with glass and a hub of the agitator blade assembly is interference fitted to the drive shaft in glass-to-glass surface contact sufficient to withstand torque imparted to the blades by the drive shaft. The shrink-fitting of agitator blades to a drive shaft in this way has been shown to be impermeable to liquids and is therefore liquid-tight, it having been verified that liquid particles penetrate the joint only to a small extent in a region at the periphery of blade hub/drive shaft connection area.
However, it will be appreciated that in such an assembly there is no electrical connection between the agitator blades and the drive shaft. The lack of any electrical connection between the agitator blades and the drive shaft means that the agitator cannot be electrically grounded. Regulations now require that within certain vessels used in chemical and pharmaceutical processes all components must be grounded to prevent electrostatic charges building up.
Also, the lack of any electrical connection between the agitator blades and the drive shaft means that known methods of monitoring the state of the enamel coating the blades cannot be used. In such a method, electrical means for detecting damage would be connected between an electrode extending into, for example, a conductive liquid contained in the vessel and an external conductor connected to the drive shaft. When enamel damage occurs, the conductive liquid would come into direct contact with the metal of the agitator blades, thus closing the electrical circuit to actuate an alarm. If an electrical connection is required, it is currently necessary to provide metallic rings around the blade hub which can contact a metallic area of the agitator shaft, both of which metallic areas must be made from chemically stable material. These rings are typically made from corrosion-resistant steel and are welded in the interior of a blade hub and the shaft of an agitator assembly. It is critical, however, that the rings are sealed with respect to the adjoining enamel coating to prevent corrosive attack on the underlying metal. This is a potential source of damage to the enamel coating. As a result of these requirements and the fact that only chemically stable metals can be used, this method is very costly. Also, it is not possible to upgrade an existing agitator assembly to apply it. In an alternative approach, chemically stable screws, wires and cables can be used to conjoin components together but this in itself can be a cause of considerable damage to the enamel or other non-conductive coating. Also, both of these methods can: lead to a high contact resistance existing between the two components which is not always desirable.
The known insulating connection method does have one significant advantage. When products are being agitated they tend to charge electrostatically; thus there is always a risk that electrostatic discharges can lead to a puncture of the enamel with consequent damage to the metal of the agitator. The tendency for electrostatically caused punctures to occur is greatest in those places where the relative speed between the product and the agitator is at its greatest, which means that such punctures usually occur on the tips of the agitator blades. The risk of such punctures occurring is directly related to the high electrical charging potential difference between an electrostatically charged product and the agitator. If mixing is carried out using a non-grounded agitator, then the agitator itself builds up an opposite charge to that of the product, which then reduces the potential difference between it and the product. As a result electrostatically caused punctures are less likely to occur.
The object of the present invention is to provide a method of producing an electrically conductive connection between the metallic components that have a non-conductive coating, which overcomes the aforementioned disadvantages.