Every flange material combination has a “temperature window”—within this temperature range, the flange, gasket, and bolts can work in coordination and maintain a reliable seal; beyond this window, the sealing performance of the system deteriorates sharply due to differences in the temperature sensitivities of the components. Identifying and respecting this temperature window is one of the most technically demanding judgements in flange selection.
The temperature limits of gasket materials are important determinants of both the lower and upper boundaries of the temperature window. Non‑metallic gasket materials differ significantly in temperature resistance: PTFE gaskets soften or even decompose at elevated temperatures; flexible graphite gaskets offer a wider high‑temperature range but become more brittle at very low temperatures. Metallic gaskets (such as the metal winding in spiral wound gaskets) have greater temperature resistance, but sealing performance depends on the temperature stability of the non‑metallic filler material. During gasket selection, it must be ensured that the temperature application range covers the actual working temperature range of the flange with adequate margin.
High‑temperature relaxation of bolt material is the primary constraint on the upper boundary of the temperature window. At elevated temperatures, bolt material undergoes creep and stress relaxation, and preload decays over time. The decay rate is exponentially related to temperature—for every certain increase in temperature, the relaxation rate accelerates significantly. Sealing failure occurs when bolt preload drops below the level required to maintain the gasket sealing pressure. Therefore, the upper boundary of the temperature window depends not only on the instantaneous strength of the bolt but also on its long‑term relaxation behaviour at the target temperature.
Thermal expansion matching between flange and bolts affects sealing stability within the temperature window. If the thermal expansion coefficient of the flange material is significantly greater than that of the bolt material, the flange expansion during heating exceeds the bolt elongation, the gap between flange faces increases, and the gasket compression force actually decreases. Conversely, if bolt expansion is greater, the gasket may be over‑compressed. Ideally, the thermal expansion coefficients of flange and bolt materials should be as close as possible to minimise preload fluctuations caused by temperature changes.
The “erosion” of the temperature window by thermal cycling is a dynamic process. Each temperature cycle leaves a small amount of accumulated plastic deformation in the gasket and a small amount of residual relaxation in the bolts. As the number of cycles increases, the system’s ability to maintain sealing gradually declines. This means that a flange connection fully within the temperature window at initial start‑up may gradually approach the boundary of sealing failure after multiple thermal cycles.
The engineering application of the temperature window lies in the fact that during flange selection, not only should the instantaneous sealing performance at the design temperature be verified, but the long‑term sealing capability across the entire expected service temperature spectrum should also be assessed. For applications with significant temperature fluctuations, material combinations with lower temperature sensitivity should be selected, and design margins should be appropriately increased. For applications approaching the boundary of the temperature window, stricter temperature monitoring and more frequent maintenance inspection regimes should be established.
The temperature window of flange sealing is a boundary defined jointly by materials science and thermodynamics. Respecting this boundary means acknowledging that the sealing system has inherent capability limits; designing carefully within this boundary is the prerequisite for achieving long‑term reliable sealing.
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