Automatic check valves contain a seat on which a movable body (ball, cone, or poppet) seats by means of spring tension. See figure 8-12. The valve opens when pressure in the direction of flow (indicated by an arrow on the body of the valve) is strong enough to unseat the movable body. Flow in the reverse direction, along with spring tension, tends to seal the movable body against the valve seat.

When the pressure on the downstream side of the valve exceeds that on the upstream side, the resultant unbalanced force seals the valve closed, as shown in view A of figure 8-12. When the pressure is reversed, the valve is forced open against the tension of the spring, and the fluid flows freely through the valve, as shown in view B of figure 8-12. The tension of the spring is relatively weak, and is intended to be barely sufficient to support the ball in its proper position.

Bypass check valves serve the same purpose as automatic check valves, but are so constructed that they may be opened manually to allow the flow of fluid in both directions. An example of the possible use of a bypass check valve is in the line between the hand pump and the accumulator. Installation of a

Figure 8-12.Typical check valve.

bypass check valve in this line would allow hand pump pressure to be directed to either the accumulator or the selector valve.