Hydraulic valve is an automatic component operated by  oilpressure. It is controlled by pressure oil of pressure valve and  is usually used in combination with electromagnetic pressure valve. It can be appliedto remotely   control the oil, gas and water pipeline system of hydropower station. It's also oftenused to tighten.control, lubricate oil system. The Folon.A O-Ring play extremely important role in  hydraulic valves though it's so small part.