Common Causes of Flange Leakage
By Vincent A. Carucci
A previous article highlighted the primary causes of flange leakage.
This article provides more information about these.
- Uneven Bolt Stress. An incorrect boltup procedure or cramped
working conditions near the flange can leave some bolts loose while
others are overtightened and crush the gasket. This can cause
in-service leaks, especially in high temperature services when the
heavily loaded bolts relax.
- Improper Flange Alignment. Improper flange alignment,
especially flange face parallelism, causes uneven gasket compression,
local crushing, and can cause subsequent leakage. Improper
flange centerline alignment can also cause uneven gasket compression
and flange leaks.
- Improper Gasket Centering. If a gasket is installed off
center compared to the flange faces, the gasket will be unevenly
compressed and make the joint prone to leakage. Spiral wound and
double jacketed gaskets usually have a centering ring that extends to
the inner edge of the bolts. A sheet gasket can be cut so that
its outside diameter matches the inner edge of the bolts.
- Dirty or Damaged Flange Faces. Dirt, scale, scratches,
protrusions, weld spatter on gasket seating surfaces, and warped
seating surfaces provide leakage paths or can cause uneven gasket
compression that can result in flange leakage.
- Excessive Piping System Loads at Flange Locations. Excessive
forces and bending moments can loosen the bolting or distort the
flanges and lead to leaks. Common causes are inadequate piping
flexibility, using cold spring to align flanges, and improper location
of supports or restraints.
- Thermal Shock. Rapid temperature fluctuations can cause
flanges to deform temporarily. This is typically a greater
potential problem in high temperature applications. Process
variations cannot always be avoided. A related problem is
temperature variation around the flange circumference (e.g., cooling
on top due to rain, or cool liquid at the bottom and hot gas at the
top). Where this is a problem, sheet metal shields can be
installed to protect against rain or snow impingement that could cause
thermal gradients across the flange and cause leakage. Such
shields also serve to keep the flanges and bolts at a more uniform
- Improper Gasket Size or Material. Sometimes, the wrong
gasket size or material is installed. The wrong size should be
fairly obvious during installation, and something that a trained
boltup crew will immediately identify. The wrong material may
not be apparent until corrosion or blowout damages the gasket.
- Improper Flange Facing. Deeper serrations than specified
will prevent the seating of double jacketed or spiral wound gaskets
and provide a leakage path. Normal raised face flange finishes
have grooves that are 0.002 to 0.005 in. (0.05 to 0.13 mm) deep.
- High Vibration Levels. Excessive vibration can loosen flange
bolts and ultimately cause flange leakage.