Potential causes of flange leakage include uneven bolt stress, poor alignment, off-center gaskets, dirty or damaged flange faces, excessive bending moments, thermal shock, incorrect gasket size or material, incorrect flange facing, and high vibration levels. Commonly used flange bolt tightening techniques include manual wrenches, impact wrenches, hammer wrenches, hydraulic torque wrenches, and hydraulic bolt tensioners, each with pros and cons. The proper bolt tightening technique requires experience and engineering judgment, and qualification of both the tools and crew.

1. KNOWLEDGE CAPSULE
Flange Leakage - Will It Ever Go Away?
Following are the primary potential causes of flange leakage:
Uneven bolt stress
Poor flange alignment
Off-center gasket installation
Dirty or damaged flange faces
Excessive bending moments imposed by the connected piping
Thermal shock during system operation
Incorrect gasket size or material
Incorrect flange facing
High vibration levels
Flange Bolt Tightening Methods
The following table summarizes the most commonly used techniques.
Method Pros Cons
Manual Wrench • Readily available, easy
to use
• Wide variation in stud pre-stress
• Limited to relatively small Dia.
studs
Impact Wrench • Fast easy to use
• May overstress small studs
• Final Torque level not adjustable
and may vary
Hammer Wrench • Can tighten larger
studs than manual
wrenches
• Readily available
• Pre-stress depends on
accessibility of studs and
experience of mechanic
Hydraulic Torque
Wrench
• Cost effective
• Uniform preload
achievable
• Higher initial purchase cost than
conventional wrenches
Hydraulic Bolt
Tensioners
• Most accurate and
achieves uniform
preload
• Eliminates galling
problems
• Different set of tensioner heads
are required for each stud size.
• Sometimes not enough
clearance around nuts to install
tensioner heads.
• Must remember to order longer
studs to permit attachment of
tensioner heads.
• Not useful for hot bolting since
hydraulic seals can rapidly
overheat.

2. KNOWLEDGE CAPSULE
The selection of the proper bolt tightening technique requires experience and good
engineering judgment. The successful application of any technique also requires
qualification of both the tools that will be used and the crew who will do the work.
In concept, a hydraulic torque wrench is simply a conventional wrench which
has been modified such that a hydraulic cylinder pushes on the end of the
wrench handle.
Hydraulic bolt tensioners employ a high-pressure hydraulic cylinder that
attaches to the stud that is to be tensioned and then stretches the stud
directly. The nut is then tightened by hand using a short bar or bevel gear
arrangement. When the hydraulic pressure is released, the load is then
transferred to the nut, which then maintains load on the stud and
compression of the gasket.
When a torque wrench is used for flange boltup, it is necessary to have an
approximate torque value that must be applied to achieve the required
preload stress for standard size bolts. The only reliable way to determine bolt
stress is to measure bolt elongation during procedure qualification, and then
to adjust the torque values as necessary to achieve the desired bolt preload
stress.
The majority of flange joints in process plants are assembled based on
achieving average preload stress using ASTM A193/A193M Grade B7 or B16
bolts. Lower bolt stresses are appropriate for some combinations of flange
type, rating, and bolting material. This is necessary to avoid damage to the
flange, gaskets, or bolts (e.g., ring joint flanges, flange rating Classes 900
and higher, austenitic stainless steel bolts, etc.).
Selecting the appropriate boltup method and having trained crews using
documented boltup procedures will help achieve leak-free flanged joints.