Check Valves are used to prevent the oil flowing in the opposite direction. The check valves contain two kinds: straight-through and right-angle style. The straight-through check valves are fixed on the pipeline by the screw connection. The right-angle check valves contain three types: high performance butterfly valves and flanged butterfly valves
. The liquid-control check valves are also called as locker valves or pressure-keeping valves. The same as check valves, they are used to avoid back-stream of oil. However, the liquid back-stream may need oil flowing in the opposite direction to control the oil pressure. When opening the check valves, the oil flowing could go through both directions. Liquid-controlled check valves apply conical spool to ensure sound sealing feature. If the shut-down of oil supply is needed, check valves can be used to lock in the single direction and keep the pressure. There are two types of oil leakage ways for liquid-controlled check valves: inner leakage and outer leakage.

Check Valves are the directional controlling valves to manage the gas flowing in single direction. Its working theory is the same with liquid-controlled check valves. The pressed air enters from the gate P and resists the spring force and friction to open the check valves. Then, the pressed air would flow from P to A. If there is no pressed air on the gate P, the valve would shut down under the function of spring force with rest air at the gate A. That can ensure the still air from gate A to P. The check valves are applied in the places which prohibit the air flowing backward. For instance, when filling the tank via inflatable air compressor, a check valve would be set between them. If the inflatable air compressor stops working, the valve can avoid the pressed air in tank to flow backward. Check valves are frequently used with throttle, sequential into one-way valve throttle, or one-way valve for single direction.