Simple-type in-line check valve (ball-type) Orifice-type in-line check valve (ball-type) Flow direction marking on simple-type in-line check valve Flow direction marking on orifice-type in-line check valve Outlet port Outlet port Inlet port Inlet port Drilled passage A B C D Inlet Inlet Outlet Outlet Figure 12-38.  An in-line check valve and orifice type in-line check valve. obtained by adjusting the tension of the spring that normally  holds the piston in the closed position. (Note that the top part  of the piston has a larger diameter than the lower part.) Fluid  enters the valve through the inlet port, flows around the lower  part of the piston and exits the outlet port, where it flows to  the primary (first) unit to be operated.  [Figure 12-39A]  This  fluid pressure also acts against the lower surface of the piston. The mechanically-operated sequence valve is operated  by a plunger that extends through the body of the valve.  [Figure 12-40]  The valve is mounted so that the plunger is  operated by the primary unit. A check valve, either a ball or  a poppet, is installed between the fluid ports in the body. It  can be unseated by either the plunger or fluid pressure. Port  A and the actuator of the primary unit are connected by a  common line. Port B is connected by a line to the actuator of  the secondary unit. When fluid under pressure flows to the  primary unit, it also flows into the sequence valve through  port A to the seated check valve in the sequence valve. In  order to operate the secondary unit, the fluid must flow  through the sequence valve. The valve is located so that the  primary unit moves the plunger as it completes its operation.  The plunger unseats the check valve and allows the fluid to  flow through the valve, out port B, and to the secondary unit. When the primary actuating unit completes its operation,  pressure in the line to the actuating unit increases sufficiently  to overcome the force of the spring, and the piston rises.  The valve is then in the open position.  [Figure 12-39B]  The  fluid entering the valve takes the path of least resistance and  flows to the secondary unit. A drain passage is provided to  allow any fluid leaking past the piston to flow from the top  of the valve. In hydraulic systems, this drain line is usually  connected to the main return line. 12-26 Adjusting screw Outlet to secondary unit Spring Drain Piston Inlet Outlet to primary unit A B Figure 12-39.  A pressure-controlled sequence valve. Mechanically-Operated Sequence Valve