How to Install a Flange

Proper flange installation is critical for achieving a leak-free joint in piping systems. A poorly installed flange is the leading cause of flange leaks, ahead of gasket defects or bolt issues. This guide covers the step-by-step procedure for installing a pipe flange in accordance with ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly).

Step-by-Step Flange Installation Procedure

Step	Action	Key Details
1	Inspect components	Check flange faces for damage, corrosion, scratches, or warping. Inspect bolt holes for thread damage. Verify gasket type and size match the flange rating.
2	Clean flange faces	Remove old gasket material, rust, and debris using a wire brush or plastic scraper. Never use a steel scraper on RTJ or raised face flanges; it damages the sealing surface.
3	Align the flanges	Bring the two flanges into parallel alignment. Maximum allowable flange face gap variation is 1.5 mm (1/16 in.) across the face. Use alignment pins if needed.
4	Check bolt hole alignment	Bolt holes must straddle the vertical and horizontal centerlines (not sit on them). Holes should align within 3 mm (1/8 in.) maximum offset.
5	Install the gasket	Center the gasket on the flange face. Never use gasket paste or sealant on spiral wound or RTJ gaskets unless specified by the manufacturer.
6	Insert bolts and nuts	Install stud bolts with equal thread engagement on each side. Apply anti-seize lubricant to bolt threads and nut bearing surfaces.
7	Hand-tighten all bolts	Tighten all nuts finger-tight to pull the joint together uniformly. Ensure the gasket remains centered.
8	Tighten in cross pattern	Tighten bolts in a star (cross) pattern using multiple passes: 30%, 60%, 100% of target torque.
9	Final torque pass	Complete a final clockwise (rotational) pass at 100% torque to verify uniform bolt load.
10	Verify and document	Record final torque values. Mark bolts to visually confirm completion.

Alignment Tolerances

Parameter	Maximum Allowable
Flange face parallelism (gap variation)	1.5 mm (1/16 in.)
Bolt hole offset (misalignment)	3 mm (1/8 in.)
Flange face separation (gap)	Gasket thickness + 3 mm
Rotational alignment (bolt hole clocking)	Within 1.5 mm
Pipe axial offset at flange	1.5 mm (1/16 in.)

Pro Tip

Never use flame heating or excessive force to align flanges. Forced alignment introduces residual stress in the piping, which causes the flange to spring back and leak after the bolts relax. If flanges cannot be aligned within tolerance, correct the pipe spool; do not force the joint.

Key Points

• Always use new gaskets. Never reuse gaskets, even if they appear undamaged.
• Lubricate all bolt threads and nut faces before tightening. Unlubricated bolts can require 30-50% more torque to achieve the same bolt load, leading to uneven gasket compression.
• For flanges rated Class 600 and above, consider using hydraulic bolt tensioning instead of torque wrenches for more uniform bolt load.
• ASME PCC-1 provides detailed guidance on flange joint assembly, including qualification of bolting specialists.

Refer to flange bolting torque charts for target torque values by flange size and pressure class.

Read the full guide to piping inspections