Valves can be normally-seated if the direction of stem movement is towards the valve to shut it. Reverse-seated hydraulic valves are those where the stem movement is away from the valve to shut it. Stems transfer motion from the handle to the disc. Gaskets are mechanical seals that prevent spilling of gas or liquids. A valve ball can be made of plastics like PVC, PVDF or metals like nickel, steel, brass or titanium. Valve balls are used in thermal power plants or atomic reactors. Springs help shift the disc position. Inconel X750 springs are used in heating systems. The trim contains all the internal elements. Visit tridenthydraulics.co.uk, they supply hydraulic and electronic solutions.