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Pressure Control Valves
Figure 6-13.Operation of compound relief valve,

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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Smaller  relief  valves,  similar  in  design  and operation to the main system relief valve, are often used in isolated parts of the system where a check valve or directional control valve prevents pressure from being relieved through the main system relief valve and where pressures must be relieved at a set  point  lower  than  that  provided  by  the  main system  relief.  These  small  relief  valves  are  also used   to   relieve   pressures   caused   by   thermal expansion  (see  glossary)  of  the  fluids. Figure 6-11 shows a typical relief valve. System pressure simply acts under the valve disk at the inlet  to  the  valve.  When  the  system  pressure exceeds the force exerted by the valve spring, the valve  disk  lifts  off  of  its  seat,  allowing  some  of the system fluid to escape through the valve outlet until the system pressure is reduced to just below the  relief  set  point  of  the  valve. All  relief  valves  have  an  adjustment  for increasing  or  decreasing  the  set  relief  pressure. Some  relief  valves  are  equipped  with  an  adjusting screw  for  this  purpose.  This  adjusting  screw  is usually  covered  with  a  cap,  which  must  be removed  before  an  adjustment  can  be  made.  Some type  of  locking  device,  such  as  a  lock  nut,  is usually provided to prevent the adjustment from changing through vibration. Other types of relief valves  are  equipped  with  a  handwheel  for  making adjustments  to  the  valve.  Either  the  adjusting screw  or  the  handwheel  is  turned  clockwise  to increase the pressure at which the valve will open. In addition, most relief valves are also provided Figure  6-11.—Relief  valve. with an operating lever or some type of device to allow  manual  cycling  or  gagging  the  valve  open for  certain  tasks. Various  modifications  of  the  relief  valve shown in figure 6-11 are used to efficiently serve the  requirements  of  some  fluid  power  systems; however,  this  relief  valve  is  unsatisfactory  for some  applications.  To  give  you  a  better  under- standing of the operation of relief valves, we will discuss some of the undesirable characteristics of this valve. A  simple  relief  valve,  such  as  the  one illustrated in figure 6-11, with a suitable spring adjustment can be set so that it will open when the system pressure reaches a certain level, 500 psi  for  example.  When  the  valve  does  open,  the volume of flow to be handled may be greater than the  capacity  of  the  valve;  therefore,  pressure  in the system may increase to several hundred psi above  the  set  pressure  before  the  valve  brings  the pressure under control. A simple relief valve will be effective under these conditions only if it is very large. In this case, it would operate stiffly and the valve  element  would  chatter  back  and  forth.  In addition, the valve will not close until the system pressure  decreases  to  a  point  somewhat  below  the opening  pressure. The  surface  area  of  the  valve  element  must  be larger  than  that  of  the  pressure  opening  if  the valve is to seat satisfactorily as shown in figure 6-12. The pressure in the system acts on the valve element  open  to  it.  In  each  case  in  figure  6-12, the   force   exerted   directly   upward   by   system pressure when the valve is closed depends on the area  (A)  across  the  valve  element  where  the element  seats  against  the  pressure  tube.  The moment  the  valve  opens,  however,  the  upward force exerted depends on the horizontal area (B) of the entire valve element, which is greater than area A. This causes an upward jump of the valve element immediately after it opens, because the Figure 6-12.—Pressure acting on different areas. 6-7







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