Flange Joint Assembly and Bolt-up Procedures
By Vincent A. Carucci
This article summarizes good flange assembly practices that may be used
to help develop detailed flange boltup procedures. Controlled-torque
boltup procedures are the methods of choice for achieving reliable boltup
of most flanged joints in typical process plant services. However, the
specific flange boltup procedure used (e.g., hammer and wrench,
controlled-torque, stud tensioner, etc.) depends on flange service and
- Check studs, nuts, and flange/nut contact surfaces for cleanliness
and burrs. Clean them using a wire brush.
- Check flange nut bearing surfaces. Clean the flange nut contact
surfaces around the entire bolt circle using a wire brush. Use a brush
with stainless steel bristles on alloy surfaces. Ensure that these
contact faces are free of scratches, dirt, scale, burrs, and other
protrusions. Remove defects by grinding.
- Uniformly lubricate the stud and nut threads on all contact
surfaces, including the nut bearing surface that contacts the flange.
- Check condition of flange faces. Clean gasket seating surface on
flange face using a wire brush. Ensure that the surface is free from
scratches, dirt, scale, remnants of old gaskets, and other
protrusions. For flanges that are used in typical and critical
services, ensure that the flange face at the OD is parallel to the
gasket contact face by measuring the gap between them using a steel
ruler. The gap should not exceed 0.010 in. (0.25 mm) at any point.
- Check flange-to-flange alignment to verify that it is within the
specified tolerances. Excessive flange misalignment (especially lack
of parallelism) increases the likelihood of in-service leakage.
- Check flange faces for proper gasket insertion gap. The gap
between flanges should be just sufficient to allow for gasket
insertion. Excessive gap will result in needing to force the flanges
together, which will increase the likelihood of in-service leakage.
- Install studs in the lower half of the flange to support the
gasket when it is inserted.
- Inspect the gasket to ensure that it is in accordance with the
specification and free from defects.
- Insert the gasket between the flanges and ensure its proper
placement (i.e., centering in the joint), taking care not to damage
the gasket. If necessary to use something to hold the gasket in place,
a light spray of adhesive can be used. Alternatively, thin cellophane
or masking tape may be used on the outside edge of the gasket with
enough material protruding to allow removal during the initial
tightening process. Tape should be located to avoid contacting the
flange face/gasket seating surfaces since this could provide a leak
path during operation.
- Install remaining studs and nuts and ensure that there is complete
thread engagement in both nuts. Use hardened steel washers if the
studs are 1-1/2 in. (38 mm) diameter and above if the studs are to be
torqued. If a bolt tensioner will be used, the stud should protrude
beyond the nut by at least one (1) bolt diameter on one side only in
order to permit attachment of the tensioner head.
- Mark bolting sequence numbers and reference bolt locations on the
flange OD. Use a criss-cross bolt tightening sequence (like tightening
the wheel on a car).
- In all cases, tightening should proceed in stages (i.e., not to
the maximum stud stress at one time), and proceed in a criss-cross
pattern. This helps ensure uniform stud load and gasket compression
around the flange circumference.
The use of appropriate flange assembly and boltup procedures will
eliminate many flange leakage problems. Go “back to the basics” first when
dealing with such problems.