TROUBLE Insufficient  capacity Does not develop enough discharge   pressure Works for a while and then   fails   to   deliver liquid Takes  too  much  power and  the  motor  overheats CAUSE Air leakage into the suction line Insufficient speed of the pump Excessive  suction  lift Clogged  impeller  passages Excessive  discharge  pressure Mechanical defects (such as worn wearing rings, im- pellers, stuffing box pack- ing, or sleeves) Insufficient speed of the pump Air or gas in the liquid being pumped Mechanical defects (such as worn wearing rings, im- pellers,  leaking  mechanical seals, and sleeves) Air leakage into the suction line Air leakage in the stuffing boxes Clogged  water  seal  passages Insufficient  liquid  on  the suction  side Excessive heat in the liquid being  pumped Operation of the pump at excess  capacity  and  insuffi- cient discharge pressure Misalignment Bent shaft Excessively  tight  stuffing box packing Worn Other wearing rings mechanical  defects TROUBLE CAUSE Vibration Misalignment Bent  shaft Clogged,  eroded,  or  other- wise unbalanced impeller Lack  of  rigidity  in  the foundation Insufficient   suction   pressure   may   also   cause vibration, as   well   as   noisy   operation   and fluctuating  discharge  pressure. Rotary Pumps Another  type  of  pump  you  find  aboard  ship is  the  rotary  pump.  A  number  of  types  are included  in  this  classification,  among  which  are the gear pump, the screw pump, and the moving vane pump. Unlike the centrifugal pump, which we have discussed, the rotary pump is a positive- displacement  pump.  This  means  that  for  each revolution  of  the  pump,  a  fixed  volume  of  fluid is moved regardless of the resistance against which the pump is pushing. As you can see, any blockage in the system could quickly cause damage to the pump or a rupture of the system. You, as a pump operator, must always be sure that the system is properly  aligned  so  a  complete  flow  path  exists for  fluid  flow.  Also,  because  of  their  positive displacement  feature,  rotary  pumps  require  a relief valve to protect the pump and piping system. The  relief  valve  lifts  at  a  preset  pressure  and returns the system liquid either to the suction side of the pump or back to the supply tank or sump. Rotary  pumps  are  also  different  from centrifugal  pumps  in  that  they  are  essentially self-priming.   As   we   saw   in   our   discussion   of centrifugal pumps, the pump is located below the liquid  being  pumped;  gravity  creates  a  static pressure head which keeps the pump primed. A rotary  pump  operates  within  limits  with  the  pump located  above  the  source  of  supply. A  good  example  of  the  principle  that  makes rotary pumps self-priming is the simple drinking straw.  As  you  suck  on  the  straw,  you  lower  the air   pressure   inside   the   straw.   Atmospheric pressure on the surface of the liquid surrounding the straw is therefore greater and forces the liquid up the straw. The same conditions basically exist for  the  gear  and  screw  pump  to  prime  itself. 9-7