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Flange installation is a critical process in piping systems, ensuring that
pipes, valves, and equipment can be connected securely while maintaining a
pressure-tight seal. The first consideration is alignment. Before inserting the
bolts, the two flanges must be perfectly parallel and concentric. If the piping
is forced into place to align mismatched flanges, it creates "stress risers"
that can lead to cracks in the flange or gasket failure under thermal expansion.
Using pry bars to force alignment is strictly prohibited; the piping should be
supported independently so the flanges mate naturally.
The second consideration is the gasket selection. The gasket material must
be compatible with the fluid being transported and the operating temperature.
For high-pressure steam, spiral wound gaskets (stainless steel windings with
graphite filler) are standard. For chemical applications, PTFE (Teflon) or EPDM
rubber gaskets prevent corrosion. The gasket must be centered on the bolt
circle; if it protrudes into the pipe bore, it can cause turbulence and erosion.
For ring-type joint (RTJ) flanges, the metal ring gasket must sit perfectly in
the groove, and the flange faces must be machined to a specific smoothness
(usually 125-250 RMS) to ensure the metal-to-metal seal works.
Torque application is the third vital step. Bolts must be tightened in a
star or crisscross pattern to ensure even compression of the gasket. Tightening
one bolt fully before moving to the next can tilt the flange and squeeze the
gasket out unevenly, creating a leak path. A calibrated torque wrench is
essential; under-torquing leads to leaks, while over-torquing can stretch the
bolts (yielding) or crush the gasket, causing it to "take a set" and lose
resilience. For critical services, hydraulic tensioning is used to stretch the
bolt slightly beyond its yield point for a precise clamp load.
Lubrication is the fourth consideration. The threads and nut faces of the
bolts should be coated with a high-temperature anti-seize compound (usually
graphite or molybdenum disulfide). This serves two purposes: it prevents the
threads from galling (cold welding) during tightening, and it ensures that the
applied torque translates accurately to clamp load. Without lubrication, up to
50% of the turning force is lost to friction, meaning the gasket may not be
compressed enough even if the wrench clicks. However, care must be taken not to
get lubricant on the gasket surface, as this can cause slippage.
Finally, leak testing and re-torquing are mandatory. After initial
installation and pressurization, thermal cycling (heating up and cooling down)
will cause the metal to expand and contract, potentially loosening the bolts. A
"hot torque" check is often required 24 hours after startup to re-tighten the
bolts to the specified value. For hazardous materials, a non-destructive test
(NDT) like ultrasonic testing may be used to check for micro-leaks. Proper
documentation of torque values and bolt grades is essential for maintenance
tracking and safety compliance.
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Flange Dimensions and Tolerance Specifications
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Orifice Flange Systems: Flow Measurement and Differential Pressure Applications
