come from grease cups which are used on some water pumps to lubricate bearings. If the cups are turned  down  too  much  or  too  often,  grease  is forced into the circulating water. A hole in the element of an oil cooler permits oil to flow into the cooling system. Any source of oil or grease should be located and repairs made as soon as possible. Corrosion or erosion of the element in a heat exchanger,  as  well  as  operation  at  excessive pressure,  may  cause  LEAKS.  These  leaks  can develop either in the element or in the casing. Leakage from the cooler casing can usually be detected  by  inspection.  Element  leaks,  however, are more difficult to detect. Any noticeable decline or  rise  in  the  freshwater  tank  level,  with  the temperature  remaining  normal,  usually  indicates leakage. A hole made by corrosion in a cooler element indicates   that   corrosion   probably   exists throughout the element, and a thorough inspec- tion should be made. Corrosion can be prevented to a large extent by using the prescribed freshwater treatment, inspecting as necessary and venting the cooler to remove entrapped air. Holes due to erosion are usually caused by particles   of   grit   (sand,   dirt,   etc.,   resulting usually from operation in shallow water) striking an element at high velocity. Grit is for the most part  so  fine  that  it  passes  easily  through  the strainer. If the strainer is defective, even the larger particles of grit may enter the cooler. Erosion by water at high velocity may also result in holes in a cooler element. This occurs when water flow has to be increased above the rated  capacity  in  order  to  maintain  a  desired freshwater  temperature.  Whenever  it  is  found necessary to greatly increase the water flow, the cooler should be cleaned. If  the  designed  maximum  operating  pressure (indicated on the exchanger name plate) is ex- ceeded, leaks are apt to result. Excessive pressure is likely to occur in conjunction with clogging, because  additional  pressure  is  necessary  to  force a  given  quantity  of  water  through  a  clogged element. MAINTENANCE  AND  REPAIR Because of the difference in their construction, methods of cleaning both types of heat exchangers (radiator  and  tubular)  differ  in  some  respects. Radiator-type  heat  exchangers  are  cleaned  by chemical  means  because  mechanical  cleaning  is not  satisfactory  for  this  type  heat  exchanger. Chemical   cleaning   of   radiator-type   units   is discussed  in  Engineman  3  &  2,  NAVEDTRA 10541  (current  edition).  Tubular  heat  exchangers, on  the  other  hand,  are  cleaned  by  mechanical means. In both types of heat exchangers, loose foreign matter such as seaweed, sand, and dirt may be removed  by  blowing  steam  through  the  element in  a  direction  opposite  to  the  normal  flow  of water. When an element is badly clogged, care must be exercised not to admit steam at a pressure exceeding the maximum specified for the element. If a film of oil or grease is evident, the element should be cleaned like an oil cooler element. Leakage from the CASING of a radiator-type heat  exchanger  may  be  caused  by  a  damaged gasket. If so, the heat exchanger should be re- moved from the piping in order that flange faces may be tightened evenly after a new gasket is installed. If there is any reason to suspect that there are leaks in a heat exchanger element, the best method for locating them is by an air test. This test may be accomplished as follows: 1. Remove the element from the casing. 2.   Block off the discharge side of the element. 3. Attach a pressure gage to the inlet line of the  element. 4. Supply low-pressure air to the inlet side of the element. Remember: Air pressure must NEVER  exceed  design  pressure  for  the element. 5. Immerse the element in a tank of water. 6. Check for bubbles. An element of a heat exchanger may also be tested hydrostatically by filling the element with water under pressure and checking for leaks. Emergency repair of leaks in the element of a radiator-type heat exchanger can be made as shown in figure 3-13. When emergency repairs to the radiator-type heat exchanger are necessary, they may be made with the use of soft solder and a small torch or soldering iron. Extreme care must be taken to prevent the surrounding area from being overheated, thus causing the existing solder to  melt.  Small  radiator-type  heat  exchangers ENGINEMAN  1  &  C 3-14