Today the majority of the industry have gone away from RTJ flanges in favor of RF flanges.
It used to be common for refiners to specify RTJ’s for elevated pressure services. Sometimes this philosophy was also applied to elevated temperature services and/or hydrogen services over a certain partial pressure. Various combinations were also used, i.e. hydrogen service requiring Class 1500 or greater flanges. The benefit of RTJ’s is that the gasket is confined in a groove and has two sealing surfaces. It also tends to be self-actuating (i.e., the sealing pressure, at least on one side, tends to increase with increased service pressure).
Over time, most refiners modified their criteria, reducing the instances in which RTJ flanges were required. For example, it might initially have applied to all Class 900 and greater flanges, then to Class 900 flanges over 12 inch, Class 1500 over 6 inch and then to Class 2500.
current philosophy is to call for raised face flanges with spiral wound gaskets for all services. We have done this for more than 20 years without any problems. Prior to making this change, we had used raised face flanges for all Class 1500 and lower services for many years, also without issues. Recently some refiners have preferred to use Kammprofile gaskets instead of spiral wound.
The change to all raised face flanges was made after observing them perform without incident in Class 2500 service. Many refiners had recommended this change (based upon their experiences) prior to our making it.
Another reason for the change is the potential for crack formation at the intersection of the sides and bottom of the groove in RTJ flanges. These cracks can propagate into the base metal. They can be prevented by properly contouring the groove intersections, but avoiding the “problem” geometry all together was preferred.
Ring type joint grooves may also accumulate dirt, moisture, etc when the flange is open, i.e., they are more difficult to keep clean. A small amount of damage (including corrosion) on the narrow sealing surface(s) may lead to a leak. Also, as the use of RTJ’s declined, warehousing (and finding) the proper gasket(s) became a more important concern. Another factor is that the grooves and gasket must align properly in order to assemble the flange and obtain a seal. They must stay aligned throughout the operating cycle. The latter point becomes an issue when mating flanges are made of different materials and not providing different hardness materials for flange and ring and also thermal expansion is a concern. The first point is an issue at the initial assembly but can be more of a concern at reassembly after service.
Furthermore in those cases where dissimilar joint materials are in contact and where the piping is 10” nominal size or larger the use of RTJ Flanges is strongly not reccomended. In fact, due to the different nature of the two materials, ring joint flanges would experience local stresses due to differential expansion that potentially could lead to ring groove cracking. There are known cases of ring joint flange failures that have led to loss of reactor containment.
