In the structure of a flange gasket, the inner and outer reinforcing rings are not located at the core sealing position and are therefore often regarded as optional “accessories.” However, these two small components play irreplaceable roles in installation quality, sealing reliability, and system safety.
The functions of the outer reinforcing ring operate on three levels. First, it helps centre the sealing element during installation—semi‑metallic gaskets such as spiral wound gaskets are prone to displacement during bolt tightening if not positioned by an outer ring, causing the sealing element to deviate from the effective sealing face. Second, the outer ring prevents over‑compression and damage to the sealing element. When bolt preload is excessive, the metallic outer ring shares part of the load, protecting the internal non‑metallic sealing material from being crushed. Third, the outer ring serves to prevent gasket blow‑out and reduce flange rotation. Under pressure fluctuation, a gasket without outer ring restraint may undergo radial displacement or even be “blown out” through the flange gap by media pressure.
Outer ring material selection is relatively flexible because it does not contact the sealing medium and is not required to resist media corrosion; it is therefore often made of carbon steel. This design logic reduces cost while ensuring functionality. The outer ring can also be made integral with the sealing element, as in metal serrated gaskets and corrugated composite gaskets.
The function of the inner reinforcing ring is entirely different from that of the outer ring. The inner ring contacts the fluid, and its material must be capable of resisting corrosion by the sealing medium. The core function of the inner ring is to prevent the gap between the sealing element and the vessel or pipe flange from interfering with fluid flow. Without an inner ring filling the annular gap between the gasket inner edge and the pipe bore, fluid passing through the flange connection forms eddies in this gap; such eddies not only increase pressure drop but may also erode the gasket inner edge, accelerating damage to the sealing element.
Another less obvious function of the inner ring is to limit radial contraction of the gasket. Under temperature changes or pressure fluctuations, certain gasket materials may undergo radial dimensional changes; the presence of the inner ring constrains such changes and maintains the positional stability of the sealing element.
In installation practice, the presence or absence of inner and outer rings directly affects the complexity of installation operations. Gaskets with outer rings require ensuring alignment between the outer ring and the flange bolt holes during installation—this is particularly important for large‑diameter flanges, as the centring function of the outer ring can only take effect when properly installed. Gaskets with inner rings require confirming that the inner ring does not obstruct the effective flow cross‑section of the pipe bore.
The inner and outer rings of a gasket, though small, embody the concept of “functional redundancy” in sealing system design. Their value lies not in performing the primary sealing function, but in providing the conditions that enable the sealing function to be effectively realised. In gasket selection and procurement, attention to the material, dimensions, and fit precision of inner and outer rings should be elevated to the same level of importance as the sealing element itself.
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