Gasket Material
The material must be compatible with oil, retain sealing capabilities through the expected range of operating temperatures, maintain flexibility over time and not
contaminate the oil insulation system.
Historically, grades of rubber and cork/rubber combinations have been the gasket material of choice for Transformer OEM's. Cork alone is not typically
used since over time, with exposure to oil and heat, cork loses sealing capability.
When cork/rubber combination gaskets are compressed there is little spreading of the material.
Compressed rubber gaskets bulge and tend to flow or move. (See Gaskets – Figure 1, Comparison of Compressibility) Cork/rubber combination gaskets are flat, being cut from sheets or rolls of
flat stock. Rubber gaskets material is also available in sheets or rolls of narrow stock but are also applied as circular "rope" or pre-made O-rings. Each material has an optimum
compression factor. Over compression or under compression of the material will compromise sealing capability. There are many "rubber" and "cork/rubber" materials available, most
of which are not suitable for use on oil filled transformers.
Gasket Systems (See Gaskets – Figure 1, Gasket Systems)
To ensure correct gasket
material compression and to control movement, gasket stop(s) or a gasket groove are often part of the gasket system. The thickness of the gasket stop or the depth
of the gasket groove is matched to the gasket material thickness and optimum compressibility for oil sealing.
Example 1 Some applications do not use stops. They tend
to be on older equipment, on smaller openings i.e. off-circuit tapchanger flange or low voltage bushings, and are always cork/rubber combination gaskets. The gaskets were applied by
tightening the flange bolts in turn until the gasket thickness was reduced to the optimum sealing thickness. In replacing this configuration, this approach can again be taken or if practical
a gasket stop can be added.
Examples 2 – 4
The use of gasket stops better ensures that the proper amount of gasket compression is achieved. The stop materials are
rectangular and circular steel rod, asbestos gasket (no longer used) and other hard, non-compressible fibre materials. An alternative stop method is to use a machined groove. The flange
bolting is tightened until there is a "metal to metal" stop and then the bolts are torqued to the normal value for the hardware being used. (Note: when torquing bolts on bushing flanges,
follow supplier's specific instructions)
Example 2 This option shows a stop on the outboard side of the gasket. This method may be used with cork/rubber and rubber gaskets,
typically on smaller openings. This method fell out of favour for rubber gaskets when it was discovered that in rare cases the rubber gaskets were sucked into the transformer tank when
applying vacuum.
Example 3 This option has a stop on the inboard and outboard side of the gasket. This offers better compression consistency particularly with larger
openings.
Example 4 With this option, rather than adding stops, a machined groove provides the retention and compression control required.
