Z-Flex Inc. is a major manufacturer of category 3 stainless steel venting material (AL29-4C). All of the stainless steel venting is a sealed vent pipe. The stainless steel vent pipe has a seal on one end inside the female end of the pipe. The vent pipe has a built in clamp on the female end also.
The seal on the inside of the 3" & 4" stainless venting keeps the vent pieces sealed when connecting them together. If there is ever a need to take the venting pieces apart then the gaskets would have to be replaced as they would be damaged by the prior heat and condensation.
Globaltowne has extra 3" & 4" replacement silicon seals in stock in case the venting has to be taken apart. These seals are required for all the stainless venting and prevent any gasses from getting out into the home.