Controlling the load is essential to ensuring the gasketed joint will seal properly. Previous Sealing Sense articles have examined the types of gaskets to use, how flange finish affects gasket sealing and major pitfalls to avoid to properly assemble a gasketed joint. However, regardless of the type of gasket, controlling the load is probably the most important criteria for getting a gasketed joint to seal. A big problem is the load on the gasket cannot be measured directly and easily during installation.

However, applied torque on the flange bolts can be measured and controlled and is one of the most frequently used methods to control gasket load. This article explores bolt torque and the major considerations for converting measurable bolt torque into the gasket load necessary to seal a flanged connection.

##### Bolt Torque

Torque is the turning force measured in foot-pounds (ft-lb) or inch-pounds (in-lb) applied to tighten (turn) the nut on a bolt. Torque can be measured during flange assembly with a properly calibrated torque wrench. In a bolted flange, the applied torque generates the axial load in the bolt. The bolt acts like a spring. Tightening the nut stretches the bolt, which increases the load on the gasket. The relationship between torque, the turning force, axial bolt force and gasket load can be expressed by the simplified formula:

T= (k∙f∙d)/12

Where:

T=Torque in ft-lb

k=Dimensionless nut factor

f=axial force in pounds

d=Nominal bolt diameter in inches

The nut factor is a "modified" friction factor, but a nut factor involves more than just friction. It is more of a multiplier "in total," taking into account many other load losses. If the same torque is applied, a 0.1 nut factor would produce twice the axial force as a 0.2 nut factor. Small changes in the nut factor can result in large changes in the load experienced by the gasket. This illustrates the need for a well lubricated bolt, nut and washer.

##### Forces on a Bolted Connection

As described above, the bolts act like springs pulling the flanges together. They need to be stretched enough to keep the load on the gasket as the system is pressurized and as pressure and temperature cycle during normal usage. Additional loading above the minimum load required to seal will give the bolted joint flexibility to absorb these load changes and a safety margin to maintain the seal as these system forces fluctuate.