The procedure for cleaning saltwater heaters is as follows: 1. Remove the waterheads. 2. Insert the special cleaning tool in the tube and drive it with a 250 to 300 rpm motor. The motor should be of the reversible type. 3.  Feed  a  light  stream  of  water  into  the opposite end of the tube to wash the scale from the cutting tool and out of the tube. A light stream of compressed air may be substituted in place of the water. Care should be taken not to drive the tool  too  fast  and  to  be  certain  that  the  tool  is straight when it is inserted into the tube. An 8 to 10 psig hydrostatic test should be per- formed on the shell of the saltwater heater before replacing the heads. If a greater test pressure is used, the relief valve will have to be plugged or removed. Cleaning  Feed  Boxes If feed flow is below normal and the distiller feed  pump  discharge  is  normal,  the  first-stage flash  orifices  may  be  plugged.  Fouling  of  the second-stage orifices may be evidenced by water backing  up  into  the  first  stage;  however,  the second-stage orifices are larger and will not be as readily plugged. Water backing into the first stage may also be caused by insufficient pressure dif- ference between the stages. Since the temperatures that exist in the feed boxes are well below the range in which saltwater scale forms, the only plugging or fouling expected at the orifices would come from the introduction of  foreign  matter  into  the  system.  Should  the orifices in either stage become plugged, it will be necessary to remove the access plate at the front of the unit, remove the perforated plates from the feed box, and remove the obstructing material from the orifices. The feed boxes are constructed so  that  the  front  can  be  readily  removed  for access to the orifices. AUTOMATIC PRESSURE CONTROL  DEVICES Most shipboard systems and machinery are protected  by  pressure  or  temperature  control devices. Their maintenance and operation are the responsibility of the EN1 or ENC on duty. Of the various types of pressure and temperature con- trol devices, the temperature control regulator, the relief valve, and the reducing valve are the types that you will encounter more often. Since temperature regulators were discussed in an earlier chapter, we will not cover them in this chapter. We will discuss only the relief valve and the reduc- ing valve. Remember, the information given in this chapter is general. More detailed information can be obtained from the manufacturer’s technical manual. All  reducing  valves  should  be  inspected, cleaned, and repaired semiannually, or whenever they do not operate properly. RELIEF  VALVES Relief valves are designed to open automatic- ally  when  the  pressure  in  the  line  or  the  unit becomes  too  high.  They  are  commonly  installed in steam, water, and oil lines, and on various units of machinery where pressure must not exceed a certain  limit.  Relief  valves  prevent  the  building- up of an excessive pressure which may be caused by such conditions as the sudden closing of an outlet valve or the failure of a reducing valve. There are many different types of relief valves, but most of them consist of a valve body contain- ing  a  disk  or  ball.  Under  normal  pressure  condi- tions, the compression of a coil spring holds the disk or ball on its seat. When the pressure in a valve exceeds the resistance of the spring, the disk or ball lifts off its seat and the pressure is reduced until it falls below the pressure for which the valve is set. Relief  valves  should  be  set  at  the  lifting pressure  specified  by  the  manufacturer.  The  ten- sion on the valve spring can be adjusted by means of an adjusting nut. The nut should be locked when the desired setting is attained. Since the set- ting for a specific valve will depend on the design of the valve and its use, the instructions in the applicable  manufacturer’s  technical  manual should be followed when any relief valve is being set. Continual lifting (popping) of a relief valve indicates either excessive pressure or malfunc- tioning of the valve. Either condition should be corrected immediately. A relief valve which is not Chapter  7—AUXILIARY  MACHINERY 7-33