Author: Lawrence Kirchner
If you do some research regarding what is most likely to be the death of your power transformer you will find that short-circuits and voltage surges are at the top of the list. There is no dispute that this is true. The fact is that the cellulose insulation in a transformer is not only responsible for electrically isolating the internal windings from each other and ground, but also mechanically supports the windings against movement during high current fault conditions. The paper insulation is strongest when the transformer is new and immediately begins to age and weaken as the transformer is loaded. The aging is a due to a combination of heat, moisture and time. To date, the industry has yet to figure out how to make transformers 100% efficient where there would be no loss and therefore no heat generated. Stopping time seems to be an issue also. We can, however, control the ingress of moisture from the environment into the transformer by maintaining a tight seal.
Unless otherwise specified during the purchasing process, the transformer will be provided with nitrile rubber (Buna-N) gaskets at all joints. This material has been the standard for many years and is well suited for the operating temperature range of an oil filled transformer. Depending on unit loading, load cycling and ambient conditions, the gaskets should perform well for many years. The general principle of how the gasket works is shown in the illustration below. The gasket thickness is greater than the depth of the provided containment (retainer) groove. As the two metal surfaces are pulled together, the gasket compresses. The compressed gasket pushes against the two mating surfaces to maintain a seal.
As the gasket ages, mostly due to heat, it becomes hard and loses its ability to push against the mating surfaces. It is not uncommon to develop leaks in transformers operating at higher temperatures after about 10 to 12 years. Leaking joints could potentially pull in humidity from the outside, depending on point in the load cycle.
Selecting gasket grade
Nitrile synthetic gasket material is rated for use in continuous operating conditions ranging from -40°C to approximately 110°C. For higher temperature locations, such as around high current bushings and turrets, Viton® synthetic rubber gasket material rated for temperatures as high as 200°C is recommended. Select durometer (shore A) of 70 to 75 for proper hardness.
The containment (retaining) gasket groove cross-sectional area is typically designed to be larger as compared to the gasket cross-sectional area. The gasket must have a space to expand into when compressed. A minimum compression of 15% is required to obtain a proper seal. Compressing beyond 33% will damage the gasket material.
Example: If a gasket groove is 8” ID* X 9” OD** X 0.15” W, the groove cross-sectional area is therefore 0.075 square inches. You should select a 3/16” thick material (gives 20% compression) with 8-1/8” ID X 8-7/8” OD. The cross-sectional area of the gasket is 0.0703 square inches.
The subject surfaces must be smooth and thoroughly cleaned. Any old adhesives or paint must be removed. Common practice is to scrub surfaces with a nonmetallic abrasive pad and then wipe the surfaces with alcohol and a lint-free rag to clean. It is acceptable to apply rated gasket adhesive to one mating surface to temporarily hold the gasket in place during assembly. Applying a light film of petroleum jelly to the other surface will help assure that the gasket remains in proper position during assembly.
Scarfing in this context is the joining or splicing of two gaskets. In cases where it is not possible, or it is uneconomical to disassemble a joint to a point that a continuous gasket can be used, scarfing is permitted. The splice joint should be made through the material in a dimension or angle so that the compression force is pushing the joint together. The general rule is that the splice length should be a minimum of three times the material thickness. Scarfed joints can be rejoined using a hot vulcanizing process or by using a proper adhesive.
General do's and don'ts
- Never stack multiple thicknesses of gasket material to achieve the desired gasket thickness.
- Both nitrile and Viton® materials flow when compressed and therefore require a containment groove or ring. Never use between two flat surfaces.
3. Only one of the two mating surfaces should have a retaining groove or ring.
4. Depending on the average operating temperature of your transformer, you should consider regasketing the entire unit about every 12 years.