A side port nozzle for a reverse osmosis pressure vessel, the nozzle 60 includes a sealing flange 62 for seating the nozzle in a pressure vessel wall port seat 57, in which the seat-contacting sealing surface of the flange is curved. The nozzle may be pushed through a nozzle port 55 that extends through a pressure vessel wall 55 to be held by a retaining clip 66. The nozzle may be used in pressure vessels for use in reverse osmosis systems that purify water. Further inventions directed to i) a reverse osmosis pressure vessel characterised by one or more ports for receiving nozzles, the or each port comprising a port hole including a nozzle seat, in which the nozzle seat has a radial curve following the circumference of the port hole, ii) a pressure vessel characterised by one or more side ports for receiving nozzles, the or each port comprising a nozzle seat for receiving the nozzle flange, in which the seat is generally curved, and iii) a sealing gasket for a pressure vessel nozzle, the gasket being formed f
