The Importance of Joint Compression

Outside of material selection, joint compression is the most important variable in ensuring that the BFJ performs to its design standard.

Bolting in a BFJ is subject to axial loads. Hence, installation instructions for joint compression must ensure that joint is preloaded to equal, at least, the hydrostatic forces it will experience. The preload value may also include a safety factor.

Too much or not enough joint compression can result in:

  • Joint member fracture
  • Fluid or gas leaks passed the gasket
  • Fastener loosening due to load cycles
  • Reduced fastener life due to fatigue

Bolting Strength

Bolting must be sufficiently strong to form an initial gasket seat when hydrostatic forces are absent and have reserve strength to maintain joint compression after adding hydrostatic forces.

Preload Value

Generally, the correct bolt preload value is one that brings the joint members together at
a sufficient level of compression, but does not yield the bolting, crush the gasket or fracture the flange.

When field personnel apply either insufficient or excessive preload joint failure can occur. In addition to completing engineering calculations prior to design, specifying installation protocols prior to installation can prevent BFJ failure. One of the most important variables in field installation is controlling bolt tension.

Controlling Bolt Tension

Below the proportional limit, bolt elongation is directly proportional to axial stress, that is, elongation increases in the same proportion as stresses increase. Because of flange joint design, the resulting modes of failure—documented in the field—include gasket crush, excessive bolt yield, and flange fracture. With this linear relationship, there are two basic methods for controlling initial bolt tension. These are direct tension and torque-tension.

Direct tension achieves a direct bolt stress through elongation measurement while torque-tension achieves a desired bolt elongation through torque measurement. Choosing a method, some of which are manual, is a function of anticipated preload accuracies
and relative cost.

Regardless of tensioning method, tension may decrease with time if the:

  • Bolt/nut/washer seating surfaces deform under compressive load
  • Bolt stretches or creeps under tensile load
  • Cyclic loading causes relative motion between the joint members

Independent testing has found this can be as high as ±50%

Torque Values

Torque values for genuine Saint Ferrer® Products are available by contacting technical support at technical@saintferrer.com.