Bolted Joint Glossary

Terms

Axial backlash

The axial play between a nut and stud thread, measured before double nutting.

Bolting assembler

The person who assembles and tightens a bolted joint, trained to the skill level the owner requires.

BLLF (bolt load loss factor)

In tensioning with less than 100% tool coverage, the share of load the first set of bolts loses when the second set is tensioned.

Check pass

A final circular pass at full target torque, repeated until no nut turns.

Circular pattern

Tightening bolt to bolt around the flange in one direction.

Cold relaxation pass

Another name for the pre-start-up retorque: an extra pass at ambient temperature after a waiting period.

Double nutting

Using two nuts on a stud end, a thin jam nut inside and a full nut outside, locked against each other to resist loosening.

Effective length (L_eff)

The length of stud that stretches under load, taken as the grip plus one bolt diameter.

Gasket seating surface

The part of the flange face the gasket bears on.

Grouped bolting

Treating a small arc of adjacent bolts as one position in the pattern, used on flanges with many bolts.

Half-bolting

Running a joint with only some of its bolts installed, for example during a turnaround. Needs engineering review.

Hot bolting

Removing and replacing bolts on a joint that is in service. See single stud replacement.

Jam-nutting

Locking a thin jam nut against a full nut. See double nutting.

Joint assembly record

The record made at assembly: who, what, when, method and target, tools and problems found.

Joint historical record

The long-term file for a joint: leak history, root cause findings, inspections, drawings and changes.

Live tightening

Tightening bolts while the joint is under pressure or at temperature.

NLLF (nut load loss factor)

The share of tensioner load kept after the nut is turned down and the tool pressure is released.

Nut factor (K)

The factor relating torque to bolt load, T = K D F / 12. It lumps together thread and nut-face friction and thread geometry.

Pre-SSR tightening

A tightening pass, by torque or turn-of-nut, done before single stud replacement.

Pre-start-up retorque

An extra circular pass at ambient temperature before the joint goes into service.

Relaxation pass

An extra pass to make up load lost to gasket and bolt relaxation after assembly.

Root area

Cross-section at the thread root diameter. PCC-1 App O bases bolt stress and torque on it.

Single stud replacement (SSR)

Replacing studs one at a time on a pressurized joint. Covered by PCC-1-2026 App G.

Slack

Looseness in the joint or bolting taken up before load builds.

Snug

The light initial tightening after hand tightening: 10–20 ft-lb, no more than 10% of target.

Star pattern

The main PCC-1-2026 tightening sequence, crossing the flange so load builds evenly.

Start-up retorque

A pass done as the joint warms up, at 300–450°F or within 24 hours of start-up.

Target torque

The torque chosen to give the target bolt load for that joint.

Tensile stress area

The effective area used for thread strength. Larger than the root area.

Thermal cycle

A heat-up and cool-down of the joint in service.

Torquing

Tightening a nut by applying a controlled torque.

Turn-of-nut

Tightening by turning the nut a set angle past snug, rather than to a torque.