Flange Bolt Tightening Sequence

The bolt tightening sequence defines the order in which flange bolts are tightened to achieve uniform gasket compression. A cross (star) pattern is the standard method per ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly). Tightening bolts in a circular sequence or random order causes uneven gasket load, flange distortion, and leaks.

When the Cross Pattern Is Used

The cross-pattern tightening sequence is mandatory for all flanged joints in pressure piping per ASME PCC-1. It applies to:

• Raised face (RF) flanges with spiral wound or sheet gaskets
• Ring-type joint (RTJ) flanges
• Large-diameter flanges (ASME B16.47 Series A and B)
• Heat exchanger flanges and pressure vessel nozzle flanges
• Any bolted joint requiring controlled gasket compression

Cross-Pattern Tightening Sequences

Number of Bolts	Tightening Order (bolt positions)
4 bolts	1-3-2-4
8 bolts	1-5-3-7-2-6-4-8
12 bolts	1-7-4-10-2-8-5-11-3-9-6-12
16 bolts	1-9-5-13-3-11-7-15-2-10-6-14-4-12-8-16
20 bolts	1-11-6-16-3-13-8-18-5-15-10-20-2-12-7-17-4-14-9-19
24 bolts	1-13-7-19-4-16-10-22-2-14-8-20-5-17-11-23-3-15-9-21-6-18-12-24

The general rule: tighten the bolt directly opposite (180 degrees) the last bolt tightened, then move approximately 90 degrees for the next pair. Continue alternating across the flange until all bolts are addressed.

Number of Tightening Passes

Pass	Target Torque	Purpose
Pass 1	20-30% of target	Initial snug to seat gasket and pull flanges together
Pass 2	50-70% of target	Intermediate load to compress gasket uniformly
Pass 3	100% of target (cross pattern)	Full target torque applied in cross-pattern sequence
Pass 4	100% of target (clockwise rotation)	Final verification pass in clockwise (circular) order

Pro Tip

The final pass (Pass 4) should always be a rotational (clockwise) pass, not a cross-pattern pass. This compensates for elastic interaction; the phenomenon where tightening one bolt partially unloads adjacent bolts. ASME PCC-1 recommends at least two additional rotational passes until no nut movement is detected.

Key Points

• Bolt numbering starts at the 12 o’clock position and increases clockwise. All personnel working on the joint must use the same numbering system.
• For critical joints (Class 900 and above, high temperature, toxic/lethal service), ASME PCC-1 recommends using qualified bolting specialists and documenting each pass with torque values.
• Stud bolt dimensions and grades must match the flange class. Never substitute lower-grade bolts.
• Anti-seize lubricant on threads and nut faces is required for achieving accurate torque-to-load conversion. The nut factor (K-factor) changes significantly with lubrication condition.

Refer to the flange bolting torque charts for target torque values by flange size, class, and bolt grade.

Read the full guide to piping inspections