High‑temperature flange connections represent one of the most challenging sealing scenarios in industrial applications. When operating temperatures rise significantly, flanges, bolts, and gaskets exhibit different thermal expansion behaviours, and the high‑temperature properties of materials change. These factors together make high‑temperature flange sealing far more complex than at ambient conditions.
The impact of elevated temperature on flange seal performance is mainly reflected in three aspects. First, bolt creep and stress relaxation. Bolts preloaded at room temperature, upon reaching operating temperature and after extended service, undergo creep—elastic strain gradually transforms into plastic strain, and the residual stress in the bolts progressively decreases. When the bolt tension decreases due to stress relaxation to a point where the gasket compression falls below the minimum required sealing pressure, leakage begins at the flange joint.
Second, high‑temperature degradation of the gasket. Gaskets subjected to high compression at elevated temperatures also undergo creep and stress relaxation, with the probability increasing as temperature rises. Common high‑temperature failure modes for gaskets include creep‑relaxation failure, high‑temperature resilience failure, and high‑temperature strength failure. Once the gasket loses sufficient resilience, it can no longer compensate for flange face separation caused by thermal cycling, and sealing failure is inevitable.
Third, differential thermal expansion. Flanges, bolts, and gaskets have different coefficients of thermal expansion. When temperature changes, each component expands differently, leading to redistribution of preload and changes in gasket stress.
To address these sealing challenges, the engineering practice has developed hot‑tightening techniques. Hot‑tightening refers to re‑tightening the flange bolts at elevated temperature after the unit has warmed up to operating conditions, to compensate for preload loss due to temperature increase. Traditionally, two hot‑tightening rounds are performed during the heat‑up process.
Hot‑tightening is not simply a matter of re‑tightening the bolts. Studies indicate that for flange connections using flexible graphite spiral‑wound gaskets at high temperatures, the hot‑tightening force should be about 1.25 times the initial preload. Each hot‑tightening operation is recommended to follow a two‑round sequential bolt‑tightening procedure to ensure adequate and uniform gasket stress. During hot‑tightening, attention must be paid to the following: if the connection is re‑tightened at high temperature and then cooled down, the bolts should be slightly loosened to prevent excessive compression and damage to the gasket upon cooling contraction.
In recent years, zero‑hot‑tightening technology has emerged as a significant innovation over traditional practices. This technique involves precise calibration of bolt preload and sealing‑face tightening for high‑temperature flanges during the cold stage before the unit is heated with oil, eliminating the need for hot‑tightening during subsequent heat‑up while achieving zero leakage. This technology has been successfully validated in some refinery units.
High‑temperature flange sealing is a complex system engineering project involving materials science, thermodynamics, and solid mechanics. Its success depends on precise control and deep understanding of every contributing factor.
All data are sourced from publicly available sources and are provided for learning, communication, and reference purposes only. If there are any errors, please contact for correction. Please make your own judgment, this website assumes no responsibility.