4.3 Gaskets 

Gaskets have several important jobs in sealing systems [6]. 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. It 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 not take a “set” even though exposed for a long time to pressure applied with bolt torque and temperature changes. It 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 gasket material, or 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. 

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

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 °F 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 °F, 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.” 

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 °F 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. 

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

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. 

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 (described below), 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. (See published specifications for E-98 by manufacturer Dodge-Regupol Inc., Lancaster, PA.) 

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 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 installing. 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 °F before installation to prevent them from becoming hard. Gaskets should be cut and sealed (painted) indoors at temperatures above 70 °F for ease of handling and to reduce paint curing time. Installing neoprene-cork gaskets when temperatures are at or near freezing should be avoided because the gasket could be damaged and leak. Cork-neoprene gaskets must be evenly compressed 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 (minium 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” (Buna N) with 50 to 60 Duro (hardness) is generally the material that should be chosen for most transformer applications. 

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. 

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 nonmovable (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 or 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 °C (150 °F). 

Viton should be used only for gaskets and O-rings in temperatures higher than 65 °C or for applications requiring motion (shaft seals, etc.). Viton is very tough and wear resistant; however, it is very expensive ($1,000+ per sheet) and 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. Care should be taken to store Nitrile and Viton separately, or order them in different colors; the materials look alike and can be easily confused, and a much more expensive gasket can be installed unnecessarily. Compression and fill requirements for Viton are the as same as those for nitrile, outlined above and shown in table 1. 

Gasket sizing for standard groove depths. Nitrile is chosen as the example because it is the most commonly used material for transformer gasketing. As shown in table 1, nitrile compression should be 25 to 50%. Nitrile sheets are available in 1/16-inch-thick increments. 

Gasket thickness is determined by groove depth and standard gasket thickness. Choose the sheet thickness so that one-fourth to one-third of the gasket will protrude above the groove; this is the amount available to be compressed. (See table 2.) Gasket sheets come in standard thicknesses in 1/16-inch increments. Choose one that allows one-third of the gasket to stick out above the groove if you can, but never choose a Table 1.—Transformer Gasket Application Summary 

Best Percent Gasket Temperature Compres Compatible UV Best Material Range -sion Fluids Resist Applications 

Neoprene -54 to 60 °C 30 Askarels and Yes Use only with (use Nitrile except (-65 to 140 °F) to hydrocarbon fluids compression limits or where there is not good with 33 recesses and use only if ultraviolet [UV] temp. swings UV resistance is needed exposure) or use Viton

Cork-Neoprene 0 to 60 °C 40 Mineral oil No Use only for flat to flat  (Coroprene) R-Temp surface gaskets with no this material takes a set (32 to 140 °F) Alpha 1 grooves or compression easily limits 

Cork-Nitrile -5 to 60 °C 40 Mineral oil No Use only for flat to flat (best) does not take a R-Temp surface gaskets with no set as easily as cork- (23 to 140 °F) Alpha 1 grooves or compression neoprene limits 

Nitrile -5 to 65 °C 25 Mineral oil No O-rings, flat and extruded (Buna N) to gaskets; use with use this except in high (23 to 150 °F) 50 R-Temp, Alpha 1 compression limiters or temp., high wear, or UV Excellent for recess only Hydrocarbon fluids 

Viton -20 to 150 °C 30 Silicone, Yes High temp.; O-rings, flat use for high wear to Alpha 1 and extruded gaskets; use and high temp. (-4 to 302 °F) 33 Mineral oil with compression limiter applications groove or recess 

Note: Viton O-rings are best for wear resistance and tolerating temperature variations.  Nitrile (Buna N) can also be used in low wear applications and temperatures less than 65 °C. 

Table 2.—Vertical Groove Compression for Circular Nitrile Gaskets 

Standard Recommended Available to Available groove depth gasket thickness compress compression (in inches) (in inches) (in inches) (percent) 3/32 1/8 1/32 25 1/8 3/16 1/16 33 /16 1/4 1/16 25 1/4 3/8 1/8 33 3/8 1/2 1/8 25 thickness that allows less than one-fourth or as much as one-half to protrude above the groove. Do not try to remove old primer from the groove. 

Horizontal groove fill is determined by how wide the groove is. The groove width is equal to the outer diameter (OD) minus the inner diameter (ID) divided by two: OD− ID . Or just measure the groove width with an accurate caliper. 

The width of the groove minus the width of the gasket is the room left for the gasket to expand while being compressed. For nitrile, the amount of horizontal room needed is about 15 to 25%. Therefore, you need to cut the gasket cross section so that it fills about 75 to 85% of the width of the groove. −

For example, an 8-inch OD groove with a 6-inch ID, OD− ID is 86 = 1 inch. Therefore, 2 2 the width of the groove is 1 inch. Because we have to leave 25% expansion space, the width of the gasket is 75% of 1 inch, or ¾ inch. So that the gasket can expand equally toward the center and toward the outside, you should leave one-half the expansion space at the inner diameter of the groove and one-half at the outer. In this example, there should be a total space of 25% of 1 inch or (¼ inch) for expansion after the gasket is inserted, so you should leave �-inch space at the OD and �-inch space at CROSS SECTION OF CIRCULAR GASKET IN GROOVE the ID. See figure 14. Figure 14.—Cross Section of Circular Gasket in Groove. 

Always cut the outer diameter first. In this example, the outer diameter would be 8 inches minus ¼ inch, or 7¾ inches. 

Note: Since �-inch space is required all around the gasket, ¼ inch must be subtracted to allow� inch on both sides. The inner diameter would be 6 inches plus ¼ inch or 6¼ inches. Note that ¼ inch is subtracted from the OD but added to the ID. 

To check yourself, subtract the inner radius from the outer to make sure you get the same gasket width calculated above. In this example, 3�-inches (outer radius, ½ of 7¾), minus 3� inches (inner radius, ½ of 6¼), is ¾ inch, which is the correct gasket width. 

Rectangular Nitrile Gaskets larger than sheet stock on hand can be fabricated by cutting strips and corners with a table saw or a utility knife with razor blade. Cutting is easier if a little transformer oil or WD-40 oil is applied. Nitrile is also available in spools in standard ribbon sizes. The ends may be joined using a cyanoacrylate adhesive (super glue). There are many types of this glue; only a few of them work well with nitrile, and they all have a very limited shelf life. Remember to always keep them refrigerated to extend shelf life. The one proven to stand up best to temperature changes and compression is Lawson Rubber Bonder No. 92081. The Lawson part number is 90286, and it is available from Lawson Products Co. in Reno, Nevada, (702-856-1381). Loctite 404 is commonly available at NAPA auto parts stores and works also but does not survive temperature variations as well. Shelf life is critical. A new supply should always be obtained when a gasketing job is started; never use an old bottle that has been on the shelf since the last job. 

When bonding the ends of ribbon together, ends should be cut at an angle (scarfed) at about 15 degrees. The best bond occurs when the length of the angle cut is about four times the thickness of the gasket. With practice, a craftsperson can cut 15-degree scarfs with a utility knife. A jig can also be made from wood to hold the gasket at a 15-degree angle for cutting and sanding. The ends may be further fine-sanded or ground on a fine bench grinder wheel to match perfectly before applying glue. A jig can be fabricated to hold the gasket at 15 degrees while cutting, sanding, or grinding. 

Table 3.—Vertical Groove Compression for Rectangular Nitrile Gaskets 

Standard groove Standard ribbon Recommended Available to Available depth width gasket thickness compress compression (in inches) (in inches) (in inches) (in inches) (in inches) 3/32 1/4 1/8 1/32 25 

1/8 5/16 3/16 1/16 33 3/16 3/8 1/4 1/16 25 1/4 3/4 3/8 1/8 33 3/8 3/4 1/2 1/8 25 

Note: Maximum horizontal fill of the groove should be 75 to 85% as explained above in the circular gasket section. However, it is not necessary to fill the groove fully to 75% to obtain a good seal. Choose the width of ribbon that comes close to, but does not exceed, 75 to 80%. If one standard ribbon width fills only 70% of the groove and the next size standard width fills 90%, choose the size that fills 70%. As in the circular groove explained above, place the gasket so that expansion space is equal on both sides. The key point is that the cross sectional area of the gasket remains the same as the cover is tightened; the thickness decreases, but the width increases. See below and figure 15. 

Caution: Nitrile (Buna N) is a synthetic rubber compound and, as cover bolts are tightened, the gasket is compressed. Thickness of the gasket is decreased and the width is increased. If a gasket is too large, rubber will be pressed into the void between the cover and the sealing surface. This will prevent a metal-to-metal seal, and a leak will result. It is best if the cross sectional area of the gasket is a little smaller than the groove cross sectional area. As cover bolts are tightened, the thickness of the gasket decreases but the width increases so that cross sectional area (thickness times the width) remains the same. Care must be taken to ensure that the gasket cross sectional area is equal to or slightly smaller (never larger) than the groove cross sectional area. This will provide space for the rubber to expand in the groove so that it will not be forced out into the metal-to-metal contact area. (See figure 15.) If it is forced out into the “metal-to-metal” seal area, a leak generally will be the result. When this happens, our first response is to tighten the bolts, which bends the cover around the gasket material in the metal-to-metal contact area. The leak may stop (or more often not); but the Figure 15.—Cross Section of Gasket next time the cover is removed, getting a Remains Constant Before Tightening proper seal is almost impossible because and After. w x d = gw x gt the cover is bent. Take extra care sizing the gasket, and these problems won’t occur. 

Caution: On some older bushings used on voltages 15 kV and above, it is necessary to install a semiconductive gasket. This type bushing (such as GE type L) has no ground connection between the bottom porcelain skirt flange and the ground ring. The bottom of the skirt is normally painted with a conductive paint, and then a semiconductive gasket is installed. This allows static electric charges to bleed off to ground. The gaskets are typically a semiconductive neoprene material. Sometimes, the gasket will have conductive metal staples near the center to bleed off these charges. When replacing this type gasket, always replace with like material. If like gasket material is not available, use cork-neoprene. 

Thin metal conductive shim stock may be folded over the outer perimeter around approximately one-half the circumference. These pieces of shim stock should be evenly spaced around the circumference and stick far enough in toward the center so that they will be held when the bolts are tightened. As an example, if the gasket is 8 inches in diameter, the circumference would be �D or 3.1416 times 8 inches = 25.13 inches in circumference. Fifty percent of 25.13 is about 12½ inches. Cut 12 strips 1-inch wide and long enough to be clamped by the flange top and bottom when tightened. Fold them over the outside edge of the gasket leaving a little more than 1-inch space between, so that the shim stock pieces will be more or less evenly spaced around the circumference. 

Note: Failure to provide a path for static electric charges to get to ground will result in corona discharges between the ground sleeve and the bushing flange. The gasket will be rapidly destroyed, and a leak will be the result. 

Bolting sequences to avoid sealing problems: If proper bolt tightening sequences are not followed or improper torque applied to the bolts, sealing problems will result. The resulting problem is illustrated in figure 16. A slight bow in the flange or lid top (exaggerated for illustration) occurs, which

applies uneven pressure to the gasket. This bow compromises the seal, 

Proper bolting sequences are illustrated for various type flanges/covers Bolt numbers show the correct tightening sequences. 

The numbers do not have to be followed exactly; however, the diagonal tightening patterns should be followed. By using proper torque and the illustrated sequence patterns, sealing problems from improper tightening and uneven pressure on the gasket can be avoided. Use a torque wrench and torque bolts according to the head stamp on the bolt. Check manufacturers instruction book for pancake gasket torque values.