Pressure-Reducing Valves

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  (fig.  6-16,  view  B).  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. Mechanically Operated Sequence Valve The   mechanically   operated   sequence   valve (fig. 6-17) is operated by a plunger that extends through  the  body  of  the  valve.  The  valve  is mounted so that the plunger will be 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  (fig.  6-17)  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  depresses  the  plunger  as  it completes   its   operation.   The   plunger   unseats the  check  valve  and  allows  the  fluid  to  flow Figure 6-17.—Mechanically operated sequence valve. through  the  valve,  out  port  B,  and  to  the secondary  unit. This  type  of  sequence  valve  permits  flow  in the  opposite  direction.  Fluid  enters  port  B  and flows to the check valve. Although this is return flow from the actuating unit, the fluid overcomes spring  tension,  unseats  the  check  valve,  and  flows out  through  port  A. PRESSURE-REDUCING  VALVES Pressure-reducing   valves   provide   a   steady pressure into a system that operates at a lower pressure than the supply system. A reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the  valve  is  set,  the  reduced  pressure  will  be maintained  regardless  of  changes  in  supply pressure (as long as the supply pressure is at least as  high  as  the  reduced  pressure  desired)  and regardless of the system load, providing the load does not exceed the design capacity of the reducer. Figure 6-18.—Spring-loaded pressure-reducing valve. 6-12


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