Flanged Joint Bolt Load Relaxation in Pipe Fitting Assemblies: Creep, Thermal Cycling, and Re-Torquing
A flanged joint between a pipe fitting and connecting pipe relies on bolt preload to compress the gasket and maintain a seal. The bolt load applied during assembly is not static — it decreases over time and through operating cycles due to several mechanisms collectively called bolt load relaxation. If bolt load falls below the minimum seating stress required for the gasket, the joint leaks. Understanding and managing bolt load relaxation is particularly important for high-temperature pipe fitting flanges in refinery and power plant service.
Mechanisms of Bolt Load Loss
Bolt load relaxation in flanged pipe fitting joints occurs through four distinct mechanisms. Gasket creep: soft gaskets (spiral wound, PTFE, compressed fibre) consolidate and thin under the initial bolt load — a spiral wound gasket can lose 10–20% of its thickness in the first 24 hours after bolt-up as the windings seat against the flange face irregularities. This thinning reduces the gasket thickness and allows the bolt to shorten, reducing bolt strain and hence bolt load. Embedding: microscopic surface asperities on the bolt threads, under the nut face, and on the flange washer face deform plastically under the initial bolt load — this embedding loss is irreversible and typically accounts for 5–15% of initial bolt load. Thermal expansion mismatch: when the flanged joint is heated to operating temperature, the flange and bolt expand at different rates. If the flange body expands more than the bolt (e.g., a cast iron flange with a stainless steel bolt), the bolt elongates further and bolt load increases; if the bolt expands more than the flange gap (e.g., a long bolt through a thin flange), bolt load decreases. Creep relaxation of the bolt itself: at temperatures above approximately 400°C for carbon steel bolts or 550°C for alloy steel bolting (B7/B16), the bolt material creeps under the sustained tensile stress of the preload — the bolt shortens at constant length by creep strain, reducing elastic strain and hence bolt load. This is a dominant loss mechanism in high-temperature flanges.
Re-Torquing: The ASME PCC-1 Requirement
ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) recommends re-torquing flanged joints after initial heat-up to operating temperature, to compensate for gasket creep and embedding losses that occur during first thermal loading. The re-torquing procedure: after the joint reaches operating temperature and pressure for the first time, the plant is shut down (or the joint is accessible and depressurised); each bolt is re-torqued to the original target torque value in the same cross-bolt sequence used during initial assembly. This restores bolt load to the target value after the initial relaxation. For spiral wound gaskets in steam service, re-torquing after first heat-up typically restores 15–25% of the bolt load that was lost during the first thermal cycle. ASME PCC-1 also requires that re-torquing is performed on a hot joint where possible (within the joint's safe temperature for hand work) rather than after cooling — re-torquing a cold joint that will then be reheated may over-stress the gasket.
High-Temperature Flange Design: B16 Alloy Bolting
For pipe fitting flanges operating above approximately 400°C, carbon steel bolting (ASTM A307, Grade B) is inadequate — the creep relaxation rate of carbon steel above 400°C is too high, and bolt load can fall to near zero within months of operation. ASTM A193 Grade B7 (Cr-Mo alloy steel, 4140 equivalent) is the standard bolting for CrMo alloy steel flanges up to approximately 450–480°C. Above 480°C, ASTM A193 Grade B16 (Cr-Mo-V alloy steel) provides better creep resistance and is used in high-temperature power plant flanges with P91/P92 fittings up to approximately 540–565°C. For steam service above 565°C (ultra-supercritical power plant), austenitic stainless bolting (A193 Grade B8M, 316 stainless) or nickel alloy bolting (A453 Grade 660, Alloy 718) may be required. The key criterion is that the bolt material must have a creep relaxation rate low enough to maintain adequate gasket seating stress over the intended service life — typically demonstrated by long-term creep relaxation testing of the bolt material at the design temperature.
Gasket Selection to Minimise Relaxation
The gasket material selection significantly affects bolt load relaxation. Soft gaskets (full-face rubber, compressed fibre sheet, PTFE) have higher creep rates than semi-metallic or metallic gaskets — they offer low seating stress requirements but lose a higher fraction of bolt load to gasket creep. Spiral wound gaskets (stainless winding with graphite filler) are the standard for most process piping flanges — they have moderate seating stress requirements and moderate creep rates. Kammprofile (grooved metal gasket with graphite overlay) and ring-type joint (RTJ) metallic gaskets have very low creep rates and are preferred for high-temperature, high-pressure flanges or where bolt load relaxation must be minimised. RTJ metallic ring gaskets (octagonal or oval cross-section per ASME B16.20) rely on plastic deformation of the ring into the flange groove rather than gasket creep for seating — once seated, they are very resistant to further load relaxation. The tradeoff is that RTJ flanges require higher bolt loads for initial seating and are not re-usable without replacing the ring.