Gasket Installation
From www.gasketwiki.com
Contents |
Flange Conditions
Sealed joints can be affected by a variety of reasons:
Non-Parallellism
The two pipes are parallel with each other, but the surfaces of the flanges are not. The result is an uneven load distribution on the gasket where one side will be understressed and not effective seal present, and the other side will be oversstressed where crushing can occur, losing the seal.
Axial or Radial Misalignment
The two flanges are parallel with each other but are not in line of each other (non-concentric). The result is an uneven load distribution on the gasket where one side will be understressed and not effective seal present, and the other side will be oversstressed where crushing can occur, losing the seal.
Excessive Gap between Flanges
The two flanges are not parallel with each other. The result is an uneven load distribution on the gasket where one side will be understressed and not effective seal present, and the other side will be oversstressed where crushing can occur, losing the seal.
Excessive Flange Gap
The flange faces are too far apart from each other which uses a majority of the bolt load to pull them together. This results in lower seating stress being applied to the gasket. If the effective seating stress is too low, the seal will be lost.
Flange Rotation
The flanges are rotated outward, crushing the outer edge of the gasket and unloading the inner edge. This is a result of a high internal pressure and high bolt load needed to seal the pressure. The effective seal area becomes very small and susceptible to blowout.
Surface Conditions on Flange Face
Three conditions exist for the surface of the flange faces
Warpage
Warped flanges will create high and low areas of bolt load, making the seal susceptible to a blowout.
Surface Finish
The surface should be machined finish, between 125 - 250µm.
Imperfections
Pitting caused by corrosion, tool marks from chisels etc., or chemical or steam damage on the flange can all create leak paths.
Fasteners and Lubrication
Fasteners are the only method of applying and maintaining a compressive load on a gasket. Force is applied through a wrench and torque that is developed through hand or impact wrench. This creates tension in the bolt body due to the interaction of the threads. Tension pulls the flanges together, placing force on the gasket surface area. This is called sealing stress.
There are four key factors to effective bolting of flanges
Fastener Quality
Bolts/studs must be chosen to suit the load required for an effective seal. For example, grade B7 bolts have a far higher tensile strength than grade 8 bolts, and are required for certain flanges. Nuts and washers should also be hardend to prevent galling and 'mushrooming'.
External Factors Affecting Performance
Service life of fasteners are to be considered as bolts will relax over time, losing their ability to 'stretch' and hold their 'torque'. Temperature also has a big effect on bolts and should be considered when choosing fasteners for the application.
Lubrication of Fastener Components
Bolt threads, nut faces that contact the hardened washer, and both faces of the hardened washer need to be lubricated to ensure effective 'torque to tension' transfer. If no lubrication is used during bolting, more of force is required to compensate for the high coefficient of fricition which means that less of the force applied is transferred to the gasket. Conversely, more of the force applied to the bolts is transferred to the gasket it every component is properly lubricated.
Fastener Torque and How to Apply it
Proper tightening sequences are highly recommended to prevent localized overloading of gasket sections.
Common Installation Misconceptions
NST Coating vs. Lubricants on Gasket FaceMisconception #1
"Cut gasket always stick, so put lube or anti-seize on the surfaces"
Historically compressed asbestos fibre gaskets were coated to help in the removal process, which was common practice. Asbestos sheet material has a low percent of rubber binder which meant that the gasket would soften but not disintegrate. Today however, non-asbestos compressed sythetic fibre sheet material has more that 60% rubber in it, so grease and anti-seize compounds will attack the gasket. The gasket typically loses its thickness and consequentially its sealing stress. Friction between the flange face and the gasket drops and the gasket then extrudes out of position.
A solution to the sticking gasket has been found in the form of a non-stick coating(NST). Non stick coating is a mica based coating that is inert with high temperature ratings and can be found on Klingersil CSF materials.
Misconception #2
"If an installed cut gasket leaks a bit, just torque it down more"
Historically compressed asbestos fibre gaskets were re-torqued if they started to leak which would temporarily solve the problem. This was possible because the asbestos sheet has a low percentage of rubber binder and a high percentage of asbestos fiber holding it together. Rubber binders harden and become brittle after exposure to high temperatures. So, today's non-asbestos compressed fiber sheets with more than 60% rubber are prone to cracking and creating a blowout failure during a retorquing after high temperature service.
Misconception #3
"I switched from a cut gasket to a spiral wound, and now the joint always leaks"
Spiral wound gaskets require a much higher minimum seating stress where the torque values used for cut gaskets are much lower. So, if the bolts are well used, they cannot transfer the torque to the gasket surface to create a proper seal.
