Piston seal

A reciprocating pump for pumping a cryogenic fluid, the pump comprising a cylinder body having a cylinder bore and piston mounted in said cylinder bore and supported by said cylinder body, said piston being operable to move within said cylinder bore in a reciprocating motion such that it can compress the cryogenic fluid in a compression chamber of said cylinder bore, said piston further comprising a ring seal installed in a piston groove associated with said piston such that it seals said piston's surface against a wall of said cylinder bore, said ring seal comprising a split ring with an outer substantially flat circumferential surface facing said cylinder bore when said ring seal is installed in said piston groove and an inner circumferential surface, opposite said outer circumferential surface and facing said piston, said split ring comprising overlapping first and second end segments defining a split substantially flat surface there between, said split surface being transverse to the direction in which said piston moves when reciprocating within said cylinder bore and extending from said inner circumferential surface to said outer circumferential surface, wherein said first end segment is associated with a leading lateral surface of said ring seal that faces said compression chamber, the first segment subjected to a larger portion of stress exerted by the pressurized cryogenic fluid within the compression chamber compared to the stress exerted by the pressurized cryogenic fluid in the compression chamber on said second end segment, and said second end segment is associated with a trailing lateral surface of said ring seal that is opposite said first leading lateral surface, and wherein said split surface is spaced further from said leading lateral surface than it is spaced from said trailing lateral surface.