The  liquid  is  clean,  free  from  sediment  and chemical  precipitation,  and  compatible  with  seal materials. The temperature is above 32°F and below 160°F. The suction pressure is below 75 psig. The liquid has adequate lubricating qualities. The  liquid  is  nontoxic  and  nonvolatile. EXTERNAL   LIQUID   LUBRICATION.— When the liquid being pumped contains solids or is otherwise not compatible with packing materials, an outside supply of seal liquid should be furnished. In general, external-injection liquid (from an outside source) is required when the following conditions exist. Liquid being pumped contains dirt, grit, or other impurities. The temperature of the pumped liquid is below 32°F or above 160°F. The  liquid  being  pumped  has  nonlubricating properties. The liquid is toxic or volatile. The suction pressure is above 75 psig, vacuum, or high lift. Mechanical  seals  are  preferred  over  packing  on some applications because of better sealing qualities and longer serviceability. Leakage is eliminated when a seal is properly installed, and normal life is much greater than that of packing on similar applications.  A mechanical shaft seal is supplied in place of a packed stuffing box when specifically requested. The change from packing to an alternate arrangement may be made in the field. Conversion kits may be ordered from the manufacturer. SINGLE SEAL.—Pumps   containing   single mechanical  seals  normally  use  pumped  liquid  to lubricate the seal faces. This method is preferred when the pumped liquid is neither abrasive nor corrosive. If the liquid being pumped is not suitable, an external flush  should  be  provided.  (See  “External  Liquid Lubrication”  above.) DOUBLE SEAL.—A  double  mechanical  seal consists of two single seals mounted back to back and a suitable sealing liquid that is introduced into the seal chamber. The sealing liquid (preferably clear water) is injected into the box at a higher pressure than exists at the entrance to the seal cavity on the pump side. The pressure differential isolates the sealing faces from the pumped liquid. Double mechanical seals are normally preferred in pumps handling sewage, slurries, or any other solids suspended in the pumped liquid. Sealing liquid that is introduced through the tap in the  seal  cavity  provides  lubrication  for  the  double  seal. The sealing liquid pressure must always be higher than the pressure on the seal closest to the suction side. If sufficient  sealing  pressure  is  not  maintained,  the pressure within the pump can force open the lower seal and allow the pumped liquid to enter the box. This can damage the seals. Two methods are used to provide sealing liquid to the stuffing box. The first method uses a pressure line installed from a tap on the discharge nozzle to the tap in the stuffing box cartridge. A filter is installed in the line to trap the solid particles. The filter must be capable of screening out all particles above 25 microns in size.   Since the liquid is bypassed from the high- pressure (discharge) side of the pump and dead-ended in the stuffing box cartridge, there are no problems in maintaining a sufficient pressure differential, provided the filter is not clogged. The second method uses clear, clean water supplied from an external source. City water can be used if there is an air break between the water supply and the water being provided to the pump. Various municipal ordinances require this break  to  prevent  contamination  of  the  city  water supply. PUMP MAINTENANCE Operating  conditions  vary  so  widely  that  to recommend  one  schedule  of  preventive  maintenance for all centrifugal pumps is not possible. Yet some sort of regular inspection must be planned and followed. You  should  maintain  a  permanent  record  of  the periodic inspections and maintenance performed on a pump. This procedure will assist you in keeping the pump  in  good  working  condition  and  prevent  costly breakdowns. One of the best rules to follow in the proper maintenance of a centrifugal pump is to keep a record of actual operating hours. Then, after a predetermined period of operation has elapsed, the pump should be given  a  thorough  inspection.  The  length  of  this operating period varies with different applications and can  only  be  determined  from  experience.  New equipment, however, should be examined after a relatively   short   period   of   operation.   The   next 6-34