121.33 Figure 3-13.—Emergency repair of a tube leak in a radiator type heat exchanger. should be replaced as soon as a leak develops, if a replacement is available. The presence of one leak,  unless  caused  by  dropping  or  accidental puncture, indicates that other areas in the heat exchanger may be eroded. In shell-and-tube heat exchangers, a leaking tube must be replaced as soon as possible. In an emergency,  a  faulty  tube  may  be  blocked  off  by inserting a special plug at each end, until the tube can  be  replaced.  An  air  lance  or  water  lance should be used to clean the tubes of a shell-and- tube heat exchanger. If the scale has hardened in the  tubes,  a  round  bristle  brush  or  soft  rubber plugs may be used to clean the tubes. When clean- ing  the  tubes  by  mechanical  means,  avoid damaging the protective coating inside the tubes. These tubes should never be polished, as the tar- nish on the tubes acts as insulation to prevent Chapter  3—ENGINE  MAINTENANCE further corrosion. Removing the tarnish will also reduce the tube wall thickness and over a period of time and a number of cleanings, could suffi- ciently  reduce  tube  strength,  resulting  in  tube failure. For the proper procedures for cleaning shell  and  tube  type  heat  exchangers  and  the safety  precautions,  use  the  PMS  maintenance requirements  cards,  the  manufacturer’s  technical manual  and  Naval  Ships’s  Technical  Manual, chapter  254. LUBRICATING  SYSTEM To ensure that all the parts of an engine receive adequate lubrication, it is essential that all parts of the lubricating oil system be properly main- tained at all times. Some parts which may be a source of trouble are considered in this section. For other information on lubricating systems, see Engineman  3  &  2, NAVEDTRA 10541 (current edition). LUBE OIL PUMPS Pumps used in engine lubricating systems are of the positive displacement type. In some pumps pressure  control  is  maintained  by  pressure regulating  or  pressure  relief  valves  built  directly into the pump; in other pumps, valves exterior to  the  pump  are  used  for  this  purpose.  Most regulating devices recirculate excess lube oil back to the suction side of the pump, but some pumps discharge excess oil directly into the engine sump. Pump casualties, as well as many other lube systems failures, are indicated by the loss of lube oil  pressure.  The  loss  of  oil  pressure  can  be recognized  by  checking  the  pressure  gages  at prescribed intervals, or by means of an electrical alarm system. Most lube oil pump failures are generally due to wear, and develop gradually. Failures may also occur abruptly if a drive shaft breaks, or some parts suffer physical deforma- tion. Such failures are usually indicated by ab- normal noise in the pump and by sounding of the low-pressure lube oil alarm. The   warning   system   should   be   tested   at specified  intervals,  usually  when  an  engine  is being started or secured. Warning systems do not excuse  personnel  from  their  responsibility  for keeping  a  vigilant  and  accurate  watch  on  engine 3-15