Ball valves are quarter - turn, straight through flow valves that have a round closure element with matching rounded seats that permit uniform sealing stress. Ball valves are used in situations where tight shut - off is required. The type of seat can vary with the valve pressure rating and material of construction. Some valve seats are composed of single molded forms, while other seats with higher - pressure rating often incorporate a trunnion design where each face of the ball is separately sealed. They are wide duty valves, able to transfer gases,liquids and liquids with suspended solids (slurries).