Quantcast MAINTENANCE - 14104_148

 
  
 
this color scheme provides uniformity among all naval   surface   ships   and   shore-based   training facilities. MAINTENANCE Preventive  maintenance  is  the  best  way  to extend  the  life  of  valves  and  fittings.  Always  refer to  the  applicable  portion  of  the  Standard   Navy Valve  Technical  Manual,   NAVSEA   0948-LP- 012-5000,  if  possible.  When  making  repairs  on more sophisticated valve types, use the available manufacturer’s  technical  manuals.  As  soon  as  you observe  a  leak,  determine  the  cause,  and  then apply  the  proper  corrective  maintenance.  Mainte- nance may be as simple as tightening a packing nut or gland. A leaking flange joint may need only to have the bolts tightened or to have a new gasket or  O-ring  inserted.  Dirt  and  scale,  if  allowed  to collect, will cause leakage. Loose hangers permit sections  of  a  line  to  sag,  and  the  weight  of  the pipe and the fluid in these sagging sections may strain  joints  to  the  point  of  leakage. Whenever you are going to install a valve, be sure you know the function the valve is going to perform—that  is,  whether  it  must  start  flow, stop  flow,  regulate  flow,  regulate  pressure,  or prevent backflow. Inspect the valve body for the information  that  is  stamped  upon  it  by  the manufacturer:  type  of  system  (oil,  water,  gas), operating  pressure,  direction  of  flow,  and  other information. You  should  also  know  the  operating  character- istics of the valve, the metal from which it is made, and  the  type  of  end  connection  with  which  it  is fitted. Operating characteristics and the material are  factors  that  affect  the  length  and  kind  of service  that  a  valve  will  give;  end  connections indicate whether or not a particular valve is suited to the installation. When   you   install   valves,   ensure   they   are readily  accessible  and  allow  enough  headroom  for full operation. Install valves with stems pointing upward   if   possible.   A   stem   position   between straight up and horizontal is acceptable, but avoid the inverted position (stem pointing downward). If  the  valve  is  installed  with  the  stem  pointing downward,  sediment  will  collect  in  the  bonnet  and score the stem. Also, in a line that is subject to freezing temperatures, liquid that is trapped in the valve  bonnet  may  freeze  and  rupture  it. Since  you  can  install  a  globe  valve  with pressure either above the disk or below the disk (depending on which method will be best for the operation, protection, maintenance, and repair of the machinery served by the system), you should use caution. The question of what would happen if  the  disk  became  detached  from  the  stem  is  a major  consideration  in  determining  whether pressure should be above the disk or below it. If you are required to install a globe valve, be SURE to check the blueprints for the system to see which way  the  valve  must  be  installed.  Very  serious casualties  can  result  if  a  valve  is  installed  with pressure above the disk when it should be below the  disk,  or  below  the  disk  when  it  should  be above. Valves  that  have  been  in  constant  service for  a  long  time  will  eventually  require  gland tightening, repacking, or a complete overhaul of all  parts.  If  you  know  that  a  valve  is  not  doing the job for which it was intended, dismantle the valve  and  inspect  all  parts.  You  must  repair  or replace all defective parts. The repair of globe valves (other than routine renewal of packing) is limited to refinishing the seat and/or disk surface. When doing this work, you  should  observe  the  following  precautions: .    When  refinishing  the  valve  seat,  do  not remove more material than is necessary. You  can  finish  valves  that  do  not  have replaceable   valve   seats   only   a   limited number  of  times. l    Before  doing  any  repair  to  the  seat  and disk of a globe valve, check the valve disk to make certain it is secured rigidly to and is  square  on  the  valve  stem.  Also,  check to be sure that the stem is straight. If the stem is not straight, the valve disk cannot seat properly, l   Carefully inspect the valve seat and valve disk for evidence of wear, for cuts on the seating  area,  and  for  improper  fit  of  the disk to the seat. Even if the disk and seat appear to be in good condition, you should perform  a  spot-in  check  to  find  out whether  they  actually  are  in  good  condition. Figure  9-32  shows  a  standard  checkoff diagram for performing a routine inspection and minor  maintenance  of  a  valve. Spotting-In  Valves The  method  used  to  visually  determine  whether the  seat  and  the  disk  of  a  valve  make  good contact with each other is called spotting-in. To 9-22

 


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