Why are there only FF and RJ as the sealing surface types for equipment flanges, compared to pipe flanges?

FF, used for flat flanges, especially with non-metallic pipes such as fiberglass-reinforced plastics and other fragile pipes with relatively low pressure resistance； RTJ: the pressure is relatively high; storage tanks also have such flanges, such as those in the API 6BX series with a pressure of 345 bar (5000 psi), used in crude oil storage tanks or on FPSOs for knockout drums/separation tanks.

Do you mean that the container flange can also use an RJ sealing surface? Is there any evidence? If that’s the case, how should I choose the gasket? Do I need to have it customized?

Perhaps due to size issues, the machining of the sealing surface and gasket is difficult and costly. The dimensions of flanges for general equipment (note: manholes, etc., do not belong to equipment flanges) are generally large. It is difficult to machine the levelness of the FF sealing surface ； The RJ sealing surface and steel ring gasket are difficult to manufacture and come at a high cost

The equipment flanges are large in size, and full-plane sealing FF is prone to leakage.

The views from the 5th and 6th floors seem correct; I agree with you guys.

The RJ sealing surface is available on the equipment flanges; it’s just not used very often, though it might be employed in high-temperature and high-pressure applications. I think the reason for its limited use is that the all-metal gasket used on the RJ sealing surface is too hard, requiring high bolt forces; this leads to a significant increase in the size of the flange and its cover, resulting in material waste. Furthermore, in the design of some high-pressure heat exchangers, it is common to opt for a structure in which the tube sheet is welded directly to the shells on both sides. This not only results in better stress distribution on the tube sheet and reduces material waste, but it also eliminates one sealing element, thereby significantly lowering the risk of leakage. As for full-plane sealing, in the pressure vessel industry, it is used very rarely even for pipe flanges, let alone equipment flanges. The reason is that full-flat seals are more prone to leakage than convex-seal types (some colleagues may not agree with the claim that they are more prone to leakage; in fact, from an engineering perspective, leakage issues with full-flat seals should be relatively rare). It’s similar to how, in most cases, MFM seals are chosen for equipment flanges rather than RF seals; but does that mean RF seals are necessarily inferior to MFM seals? Not necessarily. But in engineering, this approach is simply *the norm; it’s absolutely impossible to expect designers to give up MFM and switch to RF – no one would take such a risk. ), and the gasket tends to shift position.

FF face mounting is inconvenient for alignment; RJ is used in applications with high temperature and pressure, as well as in situations where the flange size is small.

For full-flat FF flanges, I use gaskets of the same specification as those for RF flanges, and the sealing effect is identical