When the primary actuating unit completes itsoperation, pressure in the line to the actuating unitincreases sufficiently to overcome the force of thespring, and the piston rises. The valve is then inthe open position (fig. 6-16, view B). The fluidentering the valve takes the path of least resistanceand flows to the secondary unit.A drain passage is provided to allow any fluidleaking past the piston to flow from the top ofthe valve. In hydraulic systems, this drain line isusually connected to the main return line.Mechanically Operated Sequence ValveThe mechanically operated sequence valve(fig. 6-17) is operated by a plunger that extendsthrough the body of the valve. The valve ismounted so that the plunger will be operated bythe primary unit.A check valve, either a ball or a poppet, isinstalled between the fluid ports in the body. Itcan be unseated by either the plunger or fluidpressure.Port A (fig. 6-17) and the actuator of theprimary unit are connected by a common line.Port B is connected by a line to the actuator ofthe secondary unit. When fluid under pressureflows to the primary unit, it also flows into thesequence valve through port A to the seated checkvalve in the sequence valve. In order to operatethe secondary unit, the fluid must flow throughthe sequence valve. The valve is located so thatthe primary unit depresses the plunger as itcompletes its operation. The plunger unseatsthe check valve and allows the fluid to flowFigure 6-17.—Mechanically operated sequence valve.through the valve, out port B, and to thesecondary unit.This type of sequence valve permits flow inthe opposite direction. Fluid enters port B andflows to the check valve. Although this is returnflow from the actuating unit, the fluid overcomesspring tension, unseats the check valve, and flowsout through port A.PRESSURE-REDUCING VALVESPressure-reducing valves provide a steadypressure into a system that operates at a lowerpressure than the supply system. A reducing valvecan normally be set for any desired downstreampressure within the design limits of the valve. Oncethe valve is set, the reduced pressure will bemaintained regardless of changes in supplypressure (as long as the supply pressure is at leastas high as the reduced pressure desired) andregardless of the system load, providing the loaddoes not exceed the design capacity of the reducer.Figure 6-18.—Spring-loaded pressure-reducing valve.6-12
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