Gasket Installation Factors

Surface Condition -  Bolting the Flanges -  Selection of Gasket Material -  Common Misconceptions -



Sealed joints can be affected by:

  • Non-parallelism
  • Axial or radial misalignment
  • Excessive gap between flanges
  • Flange rotation
  • Surface conditions of flange face


Surface conditions of flange face

  • Warpage:
    • Warped flanges will create high and low areas of bolt load, making seal susceptible to blowout.
    • Surface finish:
    • Surface should be machined finish, between 125 – 250 mm.


  • Imperfections – the following will affect a seal:
    • Pitting caused by corrosion
    • Tool marks from chisels etc. Will create leak path
    • Chemical or steam damage on flange


Fasteners are the only method to apply and maintain a compressive load on a gasket

1-     Force is applied through wrench

2-     Torque developed (through hand or impact wrench)

3-     Interaction of threads creates tension in stud / bolt body.

4-     Tension pulls flanges together, placing force on gasket surface area; this is called sealing stress




Key factors to effective bolting of flanges: 

  • Fastener quality
  • External factors affecting performance
  • Lubrication of fastener components
  • Fastener torque and how to apply it


 Fastener quality

  • Bolt/studs must be chosen to suit the load required for an effective seal, (I.E. Grade b7 bolts have far higher tensile strength than grade 8, and are required for certain flanges)
  • Nuts and washers should be hardened to prevent galling and ‘mushrooming’



External factors affecting performance:

  • Service life of fasteners is to be considered; bolts/studs will relax over time, losing their ability to ‘stretch’ and hold their ‘torque’.
  • Temperature has big effect on bolts / studs, and should be considered when choosing fasteners.



Lubrication of fastener components:

  • To ensure effective ‘torque to tension’ transfer, one must lubricate:
  • Bolt / stud threads
  • Face of nut that contacts hardened washer
  • Both faces of hardened washer


Lubrication of fastener components:

  • Torque applied to bolts without lubrication galls fastener surfaces, and can cause ‘cold-welding’.
  • Stress used to deform fastener material, not sealing gasket.



Fastener torque and how to apply it

  • Torque accuracy is critical to effective seal:


Fastener torque and how to apply it

  • Fluid sealing association guidelines should be followed when torquing fasteners.
  • Proper tightening sequences are recommended to prevent localized overloading of gasket section.




Factors that affect the selection of a gasket material

  • Chemical compatibility: media sealed must not chemically attack gasket material
  • Pressure and operating temperature: must fall within gasket material range.
  • Flange surface:  condition of flanges must be considered when selecting a gasket material
  • Gasket sealing stresses: gasket should be selected to match available bolt load.


  • Gasket materials are generally grouped as:

Elastomer (rubber) sheet materials

Compressed asbestos fiber sheet

Compressed synthetic fiber sheet

P.T.F.E materials (filled & expanded)

Semi-metallic gaskets

Metallic gaskets



Elastomer gasketing

  • Each rubber has favorable physical characteristics and chemical resistance
  • Temperature limits depend on rubber used.
  • Pressure capability is low; considered 150 psi max.
  • Can be used in standard same flanges, frt. Flanges (fiberglass reinforced plastic), and custom made sheet metal.
  • Low seating stress needed to seal low pressures, so can be used with low-grade bolts.


Compressed synthetic fiber gasketing

  • Chemical compatibility depends on elastomer binder within material
  • Combination of numerous synthetic fibers to make up physical ‘strength’
  • Very application-specific; requires matching of rubber binder to specific chemical.
  • High seating stress required; so not recommended for non-std flanges, or with low strength bolts
  • High rubber content makes it sensitive to chemical attack


P.T.F.E  gasketing

  • Ptfe gasketing has virtually universal chemical resistance
  • Temperature is limited to 500 deg f
  • Available as filled ptfe, pure ptfe, or expanded ptfe.
  • Filled ptfe: styles can be used in ASME, FR.P. Or custom flanges.  Types available with FDA compliance, oxygen certification, or a.P.I. 6fa fire-test approval.
  • Expanded ptfe: available in sheet or tape form, with oxygen approval.  High pressure capabilities, works well with FR.P. And seals corroded or pitted flanges well 


Semi metallic gasketing

  • Uses both metal and non-metallic components to create and maintain seal.
  • Very wide range of gasket types to choose from, each with advantages:
  • Spiral wound gaskets
  • Maxiprofile gaskets
  • Double/single jacketed gaskets
  • Corrugated metal gaskets


Metallic gasketing

  • Reserved for ultra-high pressure or ultra-volatile applications where a leak can be catastrophic.
  • Ring joint gaskets are used in specially designed asme flanges, and are ‘crushed’ into flange to make a seal.


 Common Misconceptions


Misconception #1:

            “Cut gaskets always stick, so put lube or anti-seize on the surfaces”


  • Historically: C.A.F. Gaskets were coated to help removal;  this was common practice to aid in removal
  • C.A.F. Sheet has a low % of rubber binder;  the gasket would soften, but would not disintegrate
  • Today:  non-asbestos (C.S.F.) sheets are 60+% rubber, so grease and anit-seize attack the gasket:
  • Gasket loses its thickness, and sealing stress
  • Friction between flange face and gasket drops, gasket extrudes out of position
  • Klingersil C.S.F sheets are ‘nst’ treated to prevent sticking.
  • 'nst’: mica based coating: inert with high temp rating


Misconception #2:

            “if an installed cut gasket leaks a bit, just torque it down more……”


  • Historically: C.A.F. Gaskets were re-torqued if they were starting to leak; this would temporarily solve the problem.
  • Rubber binders harden and become brittle after exposure to high temperatures.
  • C.A.F. Sheet have a low % of rubber binder; the gasket would not disintegrate due to the high % of asbestos fiber holding it together.
  • Today:  non-asbestos (C.S.F.) sheets are 60+% rubber, so retorquing a gasket after high temp service may crack the gasket and create a blowout failure.


Misconception #3:

            “I switched from a cut gasket to a spiral wound, and now the joint always leaks….”


  • Spiral wound gaskets have much higher minimum seating stresses, the torque values used for cut gaskets are most likely too low.
  • Bolts are well used, and cannot transfer the torque to the gasket surface to create a seal on the new spiral wound gasket.