Gaskets

Gaskets have several important jobs in sealing systems. A gasket must create a seal and hold it over a long period of time. It must be impervious and not contaminate the insulating fluid or gas above the fluid. It should be easily removed and replaced. A gasket must be elastic enough to flow into imperfections on the sealing surfaces. It must withstand high and low temperatures and remain resilient enough to hold the seal, even with joint movement from expansion, contraction, and vibration. It must be resilient enough to avoid taking a “set,” even when exposed for a long time to pressure applied with bolt torque and temperature changes. The gasket must have sufficient strength to resist crushing under applied load and resist blowout under system pressure or vacuum. It must maintain its integrity while being handled or installed. If a gasket fails to meet any of these criteria, a leak will result. Gasket leaks result from improper torque, choosing the wrong type of gasket material, or choosing the wrong size gasket. Improper sealing surface preparation or the gasket taking a “set” (becoming hard and losing its resilience and elasticity) will also cause a leak. Usually, gaskets take a set as a result of temperature extremes and age.

Sealing (Mating) Surface Preparation

Clean the metal surface thoroughly. Remove all moisture, oil and grease, rust, etc. A wire brush and/or solvent may be required. After rust and scale have been removed, metal surfaces should be coated with Loctite Master gasket No. 518. This material will cure after you bolt up the gasket, so additional glue is not necessary. If the temperature is 50 ºF or more, you can bolt up the gasket immediately. This material comes in a kit (part No. 22424) with primer, a tube of material, and instructions. If these instructions are followed, the seal will last many years, and the gasket will be easy to remove later, if necessary. If the temperature is under 50 ºF, wait about ½ to 1 hour after applying the material to surfaces before bolting. If you are using cork-nitrile or cork-neoprene, you can also seal gasket surfaces (including the edge of the gasket) with this same material. Loctite makes other sealers that can be used to seal gaskets, such as “Hi-tack.”

CAUTION: Take extra care that rust and dirt particles never fall into the transformer. The results could be catastrophic when the transformer is energized.

GE glyptol No. 1201B-red can also be used to paint gasket and metal surfaces; but it takes more time, and you must be more cautious about temperature. If possible, this work should be done in temperatures above 70 ºF to speed paint curing. Allow the paint to completely dry before applying glue or the new gasket. It is not necessary to remove old glyptol, or other primer, or old glue if the surface is fairly smooth and uniform. Choose the correct replacement gasket. The main influences on gasket material selection are design of the gasket joint, maximum and minimum operating temperature, type of fluid contained, and internal pressure of the transformer.

CAUTION: Most synthetic rubber compounds, including nitrile (Buna N), contain some carbon, which makes it semiconductive. Take extra care to never drop a gasket or pieces of gasket into a transformer tank. The results could be catastrophic when the transformer is energized.

Cork-Nitrile

Cork-nitrile should be used if the joint does not have grooves or limits. This material performs better than cork-neoprene because it does not take a set as easily and conforms better to mating surfaces. It also performs better at higher temperatures. Be extra careful when you store this material because it looks like cork-neoprene, and they easily are mistaken for each other. Compression is the same as for cork-neoprene, about 45%. Cork-nitrile should recover 80% of its thickness with compression of 400 psi in accordance with ASTM F36. Hardness should be 60 to 75 durometer in accordance with ASTM D2240.

CAUTION: Cork nitrile has a shelf life of only about 2 years, so do not order and stock more than can be used during this time.

Cork-Neoprene

Cork-neoprene mixture (called coroprene) can also be used; however, it does not perform as well as cork-nitrile. This material takes a set when it is compressed and should only be used when there are no expansion limiting grooves. Using cork-neoprene in grooves can result in leaks from expansion and contraction of mating surfaces. The material is very porous and should be sealed on both sides and edges with a thin coat of Glyptol No. 1201B red or similar sealer before installation. Glyptol No. 1201B is a slow drying paint used to seal metal flanges and gaskets, and the paint should be allowed to dry totally before installation. Once compressed, this gasket should never be reused. These gaskets should be kept above 35 ºF before installation to prevent them from becoming hard. Gaskets should be cut and sealed (painted) indoors at temperatures above 70 ºF for ease of handling and to reduce paint curing time. Avoid installing corkneoprene gaskets when temperatures are at or near freezing because the gasket could be damaged and leak. Cork-neoprene gaskets must be evenly compressed at about 43 to 45%. For example, if the gasket is ¼-inch thick, 0.43 x 0.25 = 0.10. When the gasket is torqued down, it should be compressed about 0.10 inch. Or you may subtract 0.1 from ¼ inch to calculate the thickness of the gasket after it is compressed. In this case, ¼ = 0.25 so 0.25 minus 0.10 = 0.15 inch would be the final distance between the mating surfaces after the gasket is compressed. In an emergency, if compression limits are required on this gasket, split lock washers may be used. Bend the washers until they are flat and install enough of them (minimum of three), evenly spaced, in the center of the gasket cross section to prevent excessive compression. The thickness of the washers should be such that the gasket compression is limited to approximately 43%, as explained above.

Nitrile “NBR”

Nitrile “NBR” (buna N) with 50 to 60 duro (hardness) is generally the material that should be chosen for most transformer applications. Replace all cork neoprene gaskets with nitrile if the joint has recesses or expansion limiting grooves. Be careful to protect nitrile from sunlight. It is not sunlight resistant and will deteriorate, even if only the edges are exposed. It should not be greased when it is used in a non-movable (static) seal. When joints have to slide during installation or are used as a moveable seal (such as bushing caps, oil cooler isolation valves, and tap changer drive shafts), the gasket or O-ring should be lubricated with a thin coating of DOW No. 111, No. 714, or equivalent grease. These are very thin and provide a good seal. Nitrile performs better than cork-neoprene, when exposed to higher temperatures, it will perform well up to 65 ºC (150 ºF).

CAUTION: Do not confuse this material with butyl rubber. Butyl is not a satisfactory material for transformer gaskets. The terms butyl and buna are easily confused, and care must be taken to make sure nitrile (buna N) is always used and never butyl.

Viton

Viton should be used only for gaskets and O-rings in temperatures higher than 65 ºC or for applications requiring motion (shaft seals, etc.). Viton is very tough and wear resistant; however, it is very expensive ($1,000+ per sheet). Therefore, it should not be used unless it is needed for high wear or high temperature applications. Viton should only be used with compression limiter grooves and recesses. Store nitrile and viton separately, or order them in different colors. The materials look alike and can be easily confused, causing a much more expensive gasket to be installed unnecessarily. Compression and fill requirements for Viton are the same as those for nitrile, as outlined above and shown in Table on Picture 1.

Picture 1: Transformer Gasket Application Summary Table