The packing should not block off the liquid seal line connection to the lantern ring after the gland has been tightened. Figure 9-6 shows how the packing is arranged. Notice  how  the  lantern  ring  lines  up  with  the liquid seal connection when the gland is tightened. Renewing Shaft Sleeves.— In some pumps the shaft sleeve is pressed onto the shaft tightly by a hydraulic press. In this case, the old sleeve must be  machined  off  with  a  lathe  before  a  new  one can be installed. On others, the shaft sleeve may have  a  snug  slip-on  fit,  butted  up  against  a shoulder on the shaft and held securely in place with a nut. On smaller pumps, new sleeves can be  installed  by  removing  the  water  end  casing, impeller, and old shaft sleeves. New sleeves are carried as repair parts; they can also be made in the  machine  shop.  On  a  large  pump,  the  sleeve is  usually  pressed  on;  the  old  sleeve  must  be machined off before a new one can be pressed on. You  must  disassemble  the  pump  and  take  the sleeve to a machine shop, a repair shop, or a naval shipyard  to  have  this  done. To prevent water leakage between the shaft and the sleeve, some sleeves are packed, others have   an   O-ring   between   the   shaft   and   the abutting  shoulder.  For  detailed  information, consult the appropriate manufacturer’s technical manual  or  applicable  blueprint. Renewing Wearing Rings.— The clearance between  the  impeller  and  the  casing  wearing  ring (fig.  9-7)  must  be  maintained  as  directed  by the  manufacturer.  When  clearances  exceed  the specified  amount,  the  casing  wearing  ring  must be replaced. On most ships, this job can be done by  the  ship’s  force,  but  it  requires  the  complete disassembly of the pump. All necessary informa- tion on disassembly of the unit, dimensions of the wearing  rings,  and  reassembly  of  the  pump  is specified by PMS or can be found in the manufac- turer’s  technical  manual.  Failure  to  replace  the casing  wearing  ring  when  the  allowable  clearance is  exceeded  results  in  a  decrease  of  pump  capacity and efficiency. If a pump has to be disassembled because  of  some  internal  trouble,  the  wearing  ring should  be  checked  for  clearance.  Measure  the outside  diameter  of  the  impeller  hub  with  an outside  micrometer  and  the  inside  diameter  of  the casing wearing ring with an inside micrometer; the difference   between   the   two   diameters   is   the actual  wearing  ring  diametric  clearance.  By checking the actual wearing ring clearance with the  maximum  allowable  clearance,  you  can  decide whether to renew the ring before reassembling the pump. The applicable MRCs area readily available source  of  information  on  proper  clearances. Wearing  rings  for  most  small  pumps  are carried  aboard  ship  as  part  of  the  ship’s  repair parts  allowance.  These  may  need  only  a  slight amount  of  machining  before  they  can  be  installed. For some pumps, spare rotors are carried aboard ship. The new rotor can be installed and the old rotor  sent  to  a  repair  activity  for  overhaul. Overhauling   a   rotor   includes   renewing   the wearing  rings,  bearings,  and  shaft  sleeve. Operating Troubles.—  You will be responsible for  the  maintenance  of  centrifugal  pumps.  The following  table  is  a  description  of  some  of  the problems  you  will  have  to  deal  with  together  with the  probable  causes: TROUBLE CAUSE Does  not  deliver  any      Insufficient  priming liquid Insufficient speed of the pump Excessive discharge pressure (such as a partially closed valve or some other obstruc- tion in the discharge line) Excessive suction lift Clogged impeller passages Wrong direction of rotation Clogged suction screen (if used) Figure 9-7.—Impeller, impeller wearing ring, and casing wearing ring for a centrifugal pump. Ruptured  suction  line Loss of suction pressure 9-6