As stated previously, the purpose of a gasket is to create a static seal between two stationary ﬂanges. The seal itself is effected by achieving the proper compression on the gasket causing it to ﬂ ow into the imperfections on the surface of the ﬂange. This results in a tight, unbroken barrier, impervious to the ﬂuid being contained.
In many instances, a good seal is obtained through the limited “swell” caused by the reaction of the inside edge of the gasket material with the ﬂuid being contained. A certain amount of swell is desirable, as long as it reaches an equilibrium and does not reach a condition of degradation where the gasket begins to breakdown. In many instances, the ﬂuid being contained may “cauterize” the inside edge of the gasket and “seal off” the gasket from further ﬂuid penetration.
Bolting should be of sufﬁcient strength to achieve proper compression of the gasket, to not only seal the joint, but to maintain the seal without exceeding the yield strength of the bolts being used. The torque values in our torque tables are based on using ASTM A193 Grade B7 studs and 2H heavy hex nuts lubricated with never seize.
Since sheet gasket materials have micropores, they must be sufﬁciently compressed to reduce porosity. Without adequate compression the system pressure can force the contained ﬂuid into the gasket and degrade it.
Therefore, when installing the gasket it is important that good technique be followed including cleaning the ﬂanges, inspecting the ﬂange face and the bolts and bringing the ﬂanges together parallel and in stages. Many ﬁeld problems arise from improperly installed gaskets. Refer to the section on gasket installation.
Minimizing Torque Loss
Proper gasket selection and installation should be based on minimizing torque loss. Torque loss can be caused by the tendency of the gasket to relax or remold after it has been compressed and/or by elongation of the bolts. This loss can be minimized several ways:
Use of a thinner gasket: The surface of the gasket is actually the sealing surface. The internal portion of the gasket is used primarily to insure that the imperfections in the sealing surface are filled. Since it is this internal portion that is primarily affected by creep relaxation, the thinner the gasket, the more effective the seal. However, if the surface to be sealed is pitted or marred or is somewhat distorted, it may not be feasible to switch to a thinner gasket.
Use of a denser gasket: In general, the denser the gasket material, the less creep relaxation will occur. With materials of similar composition, greater density will require greater seating stresses to seal. Therefore, some lighter flanges may not be strong enough to use with a denser material.
Use of conical washer: The elastic effect of a conical washer helps to compensate for some of the loss in gasket resilience. The washer also lengthens the bolt to a slight degree, lessening the effect of bolt elongation.
Greater bolt load: The use of stronger bolts or more bolts can also help in the reduction of torque loss. Care should be taken to insure that the maximum loads on the bolts are not exceeded.