A flange connection is a system composed of three core components—flange, gasket, and bolts—together with the installation process. The sealing reliability of this system depends not on the strongest link but on the weakest—this is the “cask effect” in flange connections. Understanding this principle is crucial for avoiding common pitfalls in selection.
Strength matching among flange, gasket, and bolts is the first principle of system design. When one of a pair of flanges is cast iron, bronze, or another cast flange, the fasteners shall be of a material compatible with the lower‑strength flange. The engineering logic is that if fastener strength far exceeds the load‑bearing capacity of a cast flange, the flange body may be crushed during tightening rather than achieving an effective seal. Similarly, when flanges of different ratings are bolted together, the tightening torque shall comply with the requirements of the lower‑rated flange.
Strength matching between gasket and fasteners also requires systematic consideration. For flange connections with spiral wound gaskets sealing low‑pressure highly toxic media, although the pipeline operating pressure and temperature are not high, the relatively high specific pressure required to form the initial seal with a spiral wound gasket demands greater load‑bearing capacity from the fasteners. In such cases, high‑strength alloy steel fasteners are required. The lesson from this example is that the sealing characteristics of the gasket may impose requirements on the fasteners that exceed those arising from the pipeline service conditions themselves.
Matching between flange stiffness and bolt load is a more subtle system issue. If flange stiffness is insufficient to resist the combined action of bolt preload and media pressure, the flange will deform excessively, causing separation of the sealing faces. Under high‑temperature conditions, the creep resistance of the flange material should be superior to that of the bolt material—if the flange creep rate exceeds that of the bolts, the overall flange stiffness will decline more rapidly, and sealing failure will occur earlier.
Matching between gasket compression deformation and flange face roughness is the “cask effect” at the microscopic level. The gasket must undergo sufficient compression deformation to fill the micro‑irregularities of the sealing face. If the flange face roughness is too great while the gasket’s compression deformation capacity is insufficient, sealing will fail; conversely, if the gasket is too soft while the flange face is too smooth, the gasket may be extruded under high pressure. The matching relationship between the two must be verified for the specific service conditions.
The “short‑board effect” of installation practice should not be overlooked. Even if all components are correctly selected, non‑uniform bolt preload distribution, gasket offset, or sealing face damage during installation may cause sealing failure at the weakest link in the system. Installation quality is the only link in the flange connection system entirely controlled by human factors, and it is the most common trigger of the “cask effect.”
The design philosophy for flange connection systems should shift from “pursuing the strongest” to “pursuing the most balanced.” Identifying weak links in the system and taking measures to reinforce or protect them at the design, selection, and installation stages is more effective than simply enhancing the performance of a single component.
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