A substrate processing apparatus, includes a sealed pressure vessel, such as an Atomic Layer Deposition, ALD, apparatus, a fluid inlet assembly attached to a wall of the sealed pressure vessel, the fluid inlet assembly having a fluid inlet pipe passing through the wall, and a resilient element in the fluid inlet assembly around the fluid inlet pipe coupling the inlet pipe to the wall, where one of an interior surface and an exterior surface of the resilient element sees pressure prevailing within the pressure vessel and the other sees ambient pressure, and where the fluid inlet pipe prevents fluid carried inside from being in contact with the resilient element, and a relating method.