the  outer  hub  of  the  impeller  and  that  part  of the  pump  casing  in  which  the  hub  rotates.  The purpose of this is to minimize leakage from the discharge side of the pump casing to the suction side. Because  of  the  high  rotational  speed  of  the impeller and the necessarily close clearance, the rubbing  surfaces  of  both  the  impeller  hub  and  the casing at that point are subject to stress, causing rapid  wear.  To  eliminate  the  need  for  replacing an  entire  impeller  and  pump  casing  solely  because of wear in this location, most centrifugal pumps are  designed  with  replaceable  casing  wearing  rings. In most centrifugal pumps, the shaft is fitted with  a  replaceable  sleeve.  The  advantage  of using  a  sleeve  is  that  it  can  be  replaced  more economically  than  the  entire  shaft. Mechanical seals and stuffing boxes are used to  seal  between  the  shaft  and  the  casing.  Most pumps are now furnished with mechanical seals; mechanical  seals  do  not  result  in  better  pump operation; but, they do provide a better environ- ment,  keep  bilges  dry,  and  preserve  the  liquid being  pumped. Seal  piping  (liquid  seal)  is  installed  to  cool  the mechanical seal. Most pumps in saltwater service with total head of 30 psi or more are also fitted with  cyclone  separators.  These  separators  use centrifugal  force  to  prevent  abrasive  material (such  as  sand  in  the  seawater)  from  passing between  the  sealing  surfaces  of  the  mechanical seal.  There  is  an  opening  at  each  end  of  the separator.  The  opening  at  the  top  is  for  “clean” water,  which  is  directed  though  tubing  to  the mechanical seals in the pump. The high-velocity “dirty”  water  is  directed  through  the  bottom  of the  separator,  back  to  the  inlet  piping  for  the pump. Figure  9-3.—Centrifugal  pump  flow. Bearings  support  the  weight  of  the  impeller and  shaft  and  maintain  the  position  of  the impeller—both radially and axially. Some bearings are  grease-lubricated  with  grease  cups  to  allow  for periodic   relubrication. The  power  end  of  the  centrifugal  pump  you are  to  work  with  has  an  electric  motor  that  is maintained  by  your  ship’s  Electrician’s  Mate. OPERATION.—  Liquid  enters  the  rotating impeller  on  the  suction  side  of  the  casing  and enters  the  eye  of  the  impeller  (fig.  9-3).  Liquid is  thrown  out  through  the  opening  around  the edge of the impeller and against the side of the casing  by  centrifugal  force.  This  is  where  the pump  got  its  name.  When  liquid  is  thrown  out to the edge of the casing, a region of low pressure (below atmospheric) is created around the center of  the  impeller;  more  liquid  moves  into  the  eye to replace the liquid that was thrown out. Liquid moves into the center of the impeller with a high velocity  (speed).  Therefore,  liquid  in  the  center of  the  impeller  has  a  low  pressure,  but  it  is moving  at  a  high  velocity. Liquid   moving   between   the   blades   of   the impeller spreads out, which causes the liquid to slow down. (Its velocity decreases.) At the same time, as the liquid moves closer to the edge of the casing, the pressure of the liquid increases. This change  (from  low  pressure  and  high  velocity  at the center to high pressure and low velocity at the edge)  is  caused  by  the  shape  of  the  opening between the impeller blades. This space has the shape  of  a  diffuser,  a  device  that  causes  the velocity-pressure  relationship  of  any  fluid  that moves  through  it  to  change. A  centrifugal  pump  is  considered  to  be  a nonpositive-displacement   pump   because   the volume  of  liquid  discharged  from  the  pump changes  whenever  the  pressure  head  changes.  The pressure  head  is  the  combined  effect  of  liquid weight, fluid friction, and obstruction to flow. In a  centrifugal  pump,  the  force  of  the  discharge pressure of the pump must be able to overcome the  force  of  the  pressure  head;  otherwise,  the pump  could  not  deliver  any  liquid  to  a  piping system.  The  pressure  head  and  the  discharge pressure  of  a  centrifugal  pump  oppose  each  other. When the pressure head increases, the discharge pressure  of  the  pump  must  also  increase.  Since no energy can be lost, when the discharge pressure of the pump increases, the velocity of flow must decrease. On the other hand, when the pressure head decreases, the volume of liquid discharged from  the  pump  increases.  As  a  general  rule,  a 9-3