Facts At Your Fingertips
Bolted Flange Joint Assemblies
Department Editor: Scott Jenkins
F
lange-bolted joints enable connections of singular segments
of pipes into more complex sections, as well as joining measuring and
process devices, such as flowmeters,
pumps, fans and pressure vessels.
These joints are potential sources of
leakage, which can introduce safety
and environmental hazards, as well as
process inefficiencies. This one-page
reference reviews concepts associated with preventing leaks in flangebolted joints.
Design considerations
The design of flange-bolted joints depends on the selection of a gasket, as
well as the proper assembly. Required
design data include the nominal diameter of the pipeline, the temperature and pressure of the process fluid
and the chemical and physical properties of the transported medium.
Additional data are external loads on
joint (and their variations in time), required tightness and durability of the
joint. Over the past several decades,
computational algorithms have been
developed to help achieve the desired
bolt tightness. In most cases, the algorithms apply to flange-bolted joints
with circular gaskets, and the results
of the calculations provide the torque
at which the nuts should be tightened
to ensure sufficient tightness [1].
Forces on joints
Three main forces act upon a bolted
flange joint assembly (BFJA; Figure 1).
The flange/bolt load acts to compress
the gasket enough to fill any serrations or imperfections on the sealing
surface. This helps prevent potential
leak paths. The hydrostatic end load,
caused by the internal pressure of the
fluid in the system, acts to push the
two flanges apart. The internal blowout pressure acts upon the gasket
and tries to push it out through the
gap between the flanges.
A main concern surrounding the
design and installation of the BFJA
is determining the gasket stress or
load that will be applied to the gasket (the flange/bolt load minus the
hydrostatic end load). This remaining
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gasket load must then
be greater than the internal blowout pressure
to ensure the integrity of
the seal. If it is not, a leak
or gasket blowout can
occur [2].
Leakage rate
Bolt load
Hydrostatic
end force
Gasket
Blowout/
internal
The leakage rate refers
pressure
to the quantity of fluid
that passes through the
body and over the faces
of a gasket per unit periphery of the gasket over FIGURE 1. The three main forces at play in a bolted flange joint asa specified time [3]. The sembly (shown here) act to prevent leaks and gasket blowouts
leakage rate is highly influenced by process media, pressure,
calibrated torque-control device
temperature, surface pressure and
to ensure proper torque values
other factors. It is usually measured
are applied
under a specific gasket load and at a • Keep assembly records to verify that
specific fluid pressure. For given set
proper procedures were followed
of conditions, the lower the leakage
rate, the better the fluid is retained by Gasket materials
Common gasket materials include
the gasket material.
As a general guideline, increas- elastomers, polytetrafluoroethylene
ing internal pressure leads to higher (PTFE), flexible graphite, natural fileakage rates. Increasing surface bers or mineral-based materials.
pressure results in lower leakage Gasket manufacturers provide presrates. Increasing molecular size gen- sure-versus-temperature charts to
erally means lower leakage rates. help determine whether or not the
Increasing temperature gives lower material is safe to use for a particuleakage rates for some gasket ma- lar application.
terials, although toward the end of
service life, the leakage rate is likely Flange compressibility
Flange compressibility is considto increase [3].
ered to be the percentage reducInstallation tips
tion in thickness of a flange under
The following are tips to consider compressive pressure (applied at a
when installing BFJAs [2].
constant rate, at ambient tempera• Make sure the flange sealing sur- ture). This value provides a means to
faces are clean and free of dings, measure the deformation properties
marks or indentations
of a gasket, and is an indication of
• Flanges should be aligned to maxi- the ability of the material to conform
mize sealing contact and to pro- to the flange surface irregularities [3].
vide a uniform gasket load
Gasket materials with higher com• Working surfaces of bolts, nuts and pressibilities will more effectively fill
washers should be lubricated to surface roughness.
n
ensure uniform friction
• Verify the material, grade and con- References
dition of the bolts. Nuts should 1. Jaszak, P. and Adamek, K., Design and analysis of the flangebolted join with respect to required tightness and strength, Open
spin freely onto the bolt thread
Engineering, 2019. https://doi.org/10.1515/eng-2019-0031
without binding
2. Norton, C., Back to Basics: Soft Gaskets, Fluid Sealing Association (FSA), Sealing Sense, 2016.
• Number and tighten bolts using a
proven tightening sequence or 3. European Sealing Association (ESA) and the Fluid Sealing Association (FSA). Flange Gaskets: Glossary of Terms, ESA/FSA,
assembly pattern, and use a
Publication 018/09. 2009.
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OCTOBER 2023
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