GasketsPresented by Sunil Dhuri

P. M. Cell

Synopsis

IntroductionSealing Gaskets positioningGasket selectionClassificationCommon gasket types in useHeat exchanger gasket selectionReliability Performance analysis

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. 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.

What are gaskets?• Terms joint, jointing and gaskets are used

synonymously to describe static seals cut or fabricated from any compressible flat sheet material

• Primarily to prevent leaks at joints and to withstand harsh operating conditions of temperature, pressure loading, relative physical movement of equipment

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..

Positioning gaskets..

Gaskets’ selection• Gasket thickness• Sealing and surface stress• Surface finish• Gasket stress• Temperature effect• Other compressive and thermal properties..

Classification of gaskets

Ring type gaskets

Full faced gaskets

Cork in Electrical Industry Special cork-and-rubber have been developed to prevent leakage of the very searching cooling fluids used in modern electrical transformer, similar material are also widely used in switch gear circuit breakers, lightning arresters, and other transmission equipment, as well as in conduit fittings, gear cases and cover plates in washing machines. Rubbers cork gasket materials specification as per is -4253 part -II 1988 ( ASTM - F-104 ).

RUBBERIZED CORK GASKET

RUBBERIZED CORK SHEETCORK in combination with RUBBER Provides unique properties. By itself Cork is amazing, it is natural, light, imputrescible, impervious to liquids and gases, flexible, resistant, it has great dimensional stability and is a thermal and acoustic insulator.

Remarkably when combined with rubber we get properties of the polymer plus the property of true compressibility. This unique combination has a never-ending potential of applications.

Rubberized Cork has major superiority over other gasket material, which makes it ideal for wide range of other gasket material where application of oil is involved and only choice for flanges, which are not fully flat, or having limited surface area. Other material under pressure crush, distort or extrude. Cork is endowed with a natural property allowing it to recover its initial shape after being compressed. Cork is the only material that can compress without lateral spread, and can recover up to 80% of its original thickness. This property ensures there is a Relaxation stress (outward pressure) on the flanges to maintain the seal.

APPLICATION :Synthetic Rubber bonded cork sheets have wide application where gas, oil or any other liquid is used at low bolt loads. Thus it has wide application in Transformers & switch gears, Automobiles and General electrical industry. Many applications ranging from gearboxes, inspection covers to water piping gaskets use rubber - cork. In high voltage transformers rubberized cork sheet is used as oil sealants. Even at elevated temperature, Rubberized Cork Sheets have a deformity of less than 0.1% and the physical stability of the products is excellent. They do not swell, have high tensile strengths and are resistance to the temperature as high as 110 degree Celsius. In automobile industry, rubberized cork gaskets are used as a sealant for engine heads, sumps or packing to prevent leakage of oil.

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

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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)/2. 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.

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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 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.

23Always cut the outer diameter first. In this example, the outer diameter would be 8 inches minus ¼ inch, or 7 ¾ inches.

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Proper bolting sequences are illustrated for various type flanges/covers in figure 17. Bolt numbers show the correct tightening sequences.

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Proper bolting sequences are illustrated for various

type flanges/covers in figure 17. Bolt numbers show

the correct tightening sequences.

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Many larger transformers have 8 bolt butterfly valves to control the oil flow between the radiator and the main tank. Often there are 8 bolt flanges on the pump assemblies. Gasket deterioration invariably leads to leaks. (Figure 1) Like 4-bolt flapper valve failure, to repair the 8 bolt flange gasket normally the radiator bank must be drained and the bank removed and all gaskets replaced on all connecting flanges and valves. Since 8 bolt butterfly valves are associated with large service transformers,

down time for a traditional repair is very expensive.

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