Needle Valves

manner.  Another  reason  for  not  leaving  globe valves  in  the  fully  open  position  is  that  it  is sometimes  difficult  to  determine  if  the  valve  is open or closed. If the valve is jammed in the open position, the stem may be damaged or broken by someone  who  thinks  the  valve  is  closed,  and attempts  to  open  it. It is important that globe valves be installed with the pressure against the face of the disk to keep  the  system  pressure  away  from  the  stem packing when the valve is shut. NEEDLE  VALVES Needle  valves  are  similar  in  design  and operation  to  the  globe  valve.  Instead  of  a  disk, a needle valve has a long tapered point at the end of  the  valve  stem.  A  cross-sectional  view  of  a needle valve is illustrated in figure 6-8. The  long  taper  of  the  valve  element  permits a much smaller seating surface area than that of the globe valve; therefore, the needle valve is more suitable as a throttle valve. Needle valves are used to   control   flow   into   delicate   gauges,   which might be damaged by sudden surges of fluid under pressure. Needle valves are also used to control the end of a work cycle, where it is desirable for motion to be brought slowly to a halt, and at other points  where  precise  adjustments  of  flow  are necessary  and  where  a  small  rate  of  flow  is desired. Although  many  of  the  needle  valves  used  in fluid  power  systems  are  the  manually  operated type (fig. 6-8), modifications of this type of valve are often used as variable restrictors. This valve is constructed without a handwheel and is adjusted to provide a specific rate of flow. This rate of flow will  provide  a  desired  time  of  operation  for  a particular subsystem. Since this type of valve can be adjusted to conform to the requirements of a particular system, it can be used in a variety of systems.  Figure  6-9  illustrates  a  needle  valve  that was  modified  as  a  variable  restrictor. HYDRAULIC   AND   PNEUMATIC GLOBE  VALVES The valve consists of a valve body and a stem cartridge assembly. The stem cartridge assembly includes the bonnet, gland nut, packing, packing retainer, handle, stem, and seat. On small valves (1/8 and 1/4 inch) the stem is made in one piece, but  on  larger  sizes  it  is  made  of  a  stem,  guide, and stem retainer. The valve disk is made of nylon and is swaged into either the stem, for 1/8- and 1/4-inch  valves,  or  the  guide,  for  larger  valves. The   bonnet   screws   into   the   valve   body   with left-hand  threads  and  is  sealed  by  an  O-ring (including  a  back-up  ring). Figure 6-8.—Cross-sectional view of a needle valve. Figure 6-9.—Variable  restrictor. 6-5


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