The body of the mechanically-operated (fig. 8-13) sequence valve is usually aluminum, and contains all the working parts. As for the number and location of the fluid ports, there are many variations, depending upon how the valve is to be used. At least two ports are needed. Some models have four ports, and those not needed are plugged. The valve shown in figure 8-13 has two ports.

A contact plunger extends from the body. The plunger is held in the extended position by a plunger spring. The valve is mounted so that the plunger will be depressed by the first unit operated.

A check valve, either a poppet or ball, is installed between the fluid ports of the body, and is held against a seat by the check valve spring. The seated check valve spring prevents fluid flow through the valve. The plunger, driven into the valve by the first unit, unseats the check.

The balanced sequence valve will not permit fluid flow in either direction unless the plunger is depressed. This check valve, with equal working areas (balanced), cannot be unseated by fluid pressure in either direction. Thermal relief valves are needed in this system.

The unbalanced valve can be unseated by fluid pressure below it without having the plunger depressed. This movement allows thermal expansion to be relieved. Thermal relief valves are NOT needed in this system.

Pressure from the selector valve goes directly to the first unit. To operate the second unit, fluid must pass through the sequence valve, which it can do only when the check valve is unseated. On completing its operation, the first unit depresses the plunger on the sequence valve, which unseats the check valve and allows fluid to flow through the valve to second unit. Thus, the second unit cannot operate until the first unit operation is complete. In reverse, when contact force is removed from the plunger, the spring extends it and the check valve reseats.

Improper adjustment of plungers on the mechanical-type sequence valve is the most common cause of trouble. If the adjustment is off, it could cause the second unit to operate too soon or not at all. The adjustment is made either on the plunger of the sequence valve or the striker that depresses the plunger.

Adjustment should be checked at every periodic inspection. If a valve leaks internally, disassemble, clean, and inspect the check valve and its sealing surface. Replace faulty O-rings. Internal leakage could cause the second unit to operate before it should.

Figure 8-13.-Typical sequence valve.