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Diaphragm control valve, downward-seating type.
VALVE   MANIFOLDS

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Figure  9-29.—Arrangement  of  control  pilot  and  diaphragm control valve for supplying reduced-steam pressure. Figure 9-29 shows one arrangement that you might use. Assume that the service requirements indicate  the  need  for  a  direct-acting,  upward- seating diaphragm control valve. Can you figure out  which  kind  of  a  control  pilot—direct  acting or   reverse   acting—should   be   used   in   this installation? Try it first with a direct-acting control pilot, As  the  controlled  pressure  (discharge  pressure from   the   diaphragm   control   valve)   increases, increased pressure is applied to the diaphragm of the direct-acting control pilot. The valve stem is pushed  downward  and  the  valve  in  the  control pilot is opened. This increases the operating air pressure from the control pilot to the top of the diaphragm  control  valve.  The  increased  operating air pressure acting on the diaphragm of the valve pushes the stem downward, and since this is an upward-seating  valve,  this  action  OPENS  the diaphragm  control  valve  still  wider.  Obviously, this   won’t   work   for   this   application.   An   IN- CREASE in controlled pressure must result in a DECREASE  in  operating  air  pressure.  Therefore, we made a mistake in choosing the direct-acting control  pilot,  For  this  particular  pressure-reducing application,   you   should   choose   a   REVERSE- ACTING   control   pilot. It  is  not  likely  that  you  will  be  required  to decide which type of control pilot and diaphragm control valve is needed in any particular installa- tion. But you must know how and why they are selected  so  you  do  not  make  mistakes  in  repairing or replacing these units. Figure 9-30.—Priority PRIORITY VALVES.— In valve. systems  with  two or  more  circuits,  it  is  sometimes  necessary  to  have some  means  of  supplying  all  available  fluid  to  one particular circuit in case of a pressure drop in the system. A priority valve is often incorporated in the  system  to  ensure  a  supply  of  fluid  to  the critical/vital   circuit.   The   components   of   the system are arranged so the fluid to operate each circuit, except the one critical/vital circuit, must flow through the priority valve. A priority valve may also be used within a subsystem containing two  or  more  actuating  units  to  ensure  a  supply of fluid to one of the actuating units. In this case, the priority valve is incorporated in the subsystem in such a location that the fluid to each actuating unit,   except   the   critical/vital   unit,   must   flow through  the  valve. Figure 9-30 shows one type of priority valve. View  A  of  figure  9-30  shows  the  valve  in  the priority-flow position; that is, the fluid must flow through the valve in the direction shown by the arrows  to  get  to  the  noncritical/vital  circuits  or actuating  units.  With  no  fluid  pressure  in  the valve,  spring  tension  forces  the  piston  against  the stop and the poppet seats against the hole in the center of the piston. As fluid pressure increases, the spring compresses and the piston moves to the right. The poppet follows the piston, sealing the hole in the center of the piston until the preset pressure  is  reached.  (The  preset  pressure  depends upon the requirements of the system and is set by  the  manufacturer.)  Assume  that  the  critical/ vital circuit or actuating unit requires 1500 psi. 9-20






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