Carbon graphite seal rings are used in rotary joints to provide a mechanical seal between a rotating rotor and a fixed wear plate or body housing. During normal operation, carbon graphite seals transfer a stable film to a counterface (for example, a seal plate or nipple face) and lose material at a slow rate. The transfer film is critical to self-lubrication. A couple of conditions are required for this film to be established: (1) the presence of moisture and (2) thermal equilibrium whereby frictional heat generation is equal to heat loss from the rubbing parts. What is more, carbon graphite is not self-lubricating in the absence of water vapor.
Superheated steam adversely affects the life of the carbon graphite seal because it removes the required moisture from the seal environment. If the level of superheat is high, a stable transfer film isn’t established and the seal rubs against the counterface with higher friction and accelerated wear. Secondly, the frictional heat generation and operating temperature are increased exacerbating the condition.
Kadant Johnson recommends limiting the superheat to less than 50° F to avoid accelerated seal wear in rotary pressure joint applications.