drum  and  header  are  used  to  remove  scale  and  other foreign matter that have settled in the lowest part of the water  spaces. Boilers are also blown down to control concentration ofdissolved and suspended solids in boiler water.  The  water  column  blowdown  permits  removal  of scale   and   sediments   from   the   water   column. Additionally, some boilers have what is called a surface blowdown.  The  surface  blowdown  is  located  at  the approximate water level so as to discharge partial steam and water. The surface blowdown removes foaming on the top of the water surface and any impurities that are on the surface of the water. FUSIBLE PLUGS FUSIBLE  PLUGS  are  used  on  some  boilers  to provide added protection against low water. They are constructed of bronze or brass with a tapered hole drilled lengthwise through the plug. They have an even taper from end to end. This  tapered  hole  is  filled  with  a low-melting  alloy.  consisting  mostly  of  tin.  There  are  two types  of  fusible  plugs—fire  actuated  and  steam  actuated. The FIRE-ACTUATED plug is filled with an alloy of tin, copper. and lead with a melting point of 445°F to 450°F.  It  is  screwed  into  the  shell  at  the  lowest permissible  water  level.  One  side  of  the  plug  is  in  contact with the tire or hot gases, and the other side is in contact with the water (see item 9). As long as the plug is covered with water, the tin does not melt. When the water level drops below the plug, the tin melts and blows out. Once the core is blown out, a whistling noise will warn the operator. The boiler then must be taken out of service to replace the plug. The STEAM-ACTUATED plug is installed on the end of a pipe outside the drum. The other end of the pipe. which is open, is at the lowest permissible water level in the steam drum. A valve is usually installed between the plug and the drum. The metal in the plug melts at a temperature below that of the steam in the boiler. The pipe is small enough to prevent water from circulating in it. The water around the plug is much cooler than the water in the boiler as long as the end of the pipe is below the water level. However, when the water level drops below the open end of the pipe, the cool water runs out of the pipe and steam heats the plug. The hot steam melts and blows the tin out, allowing steam to escape from the boiler warning the operator. This type of plug can be replaced  by  closing  the  valve  in  the  piping.  It  is  not necessary to take the boiler out of service to replace the plug. Fusible plugs should be renewed regularly once a year. Do NOT refill old casings with new tin alloy and use again. ALWAYS USE A NEW PLUG. WATER COLUMN A WATER COLUMN (fig. 1-7) is a hollow vessel having  two  connections  to  the  boiler. Water  columns come in many more designs than the two shown in figure 1-7; however, they all operate to accomplish the same principle. The top connection enters the steam drum of the boiler through the top of the shell or drum. The water connection enters the shell or head at least 6 inches below the lowest permissible water level. The purpose of the water column is to steady the water level in the gauge glass through the reservoir capacity of the column. Also, the  column  may  eliminate  the  obstruction  on  small diameter,   gauge-glass   connections   by   serving   as   a sediment chamber. The water columns shown are equipped with high- and low-water alarms that sound a whistle to warn the operator. The whistle is operated by either of the two floats or the solid weights shown in figure 1-7. Water Level Control The  water  level  control  not  only  automatically operates the boiler feed pump but also safeguards the boiler against low water by stopping the burner. Various types  of  water  level  controls  are  used  on  boilers.  At Seabee activities, boilers frequently are equipped with a float-operated type, a combination float and mercury switch  type,  or  an  electrode  probe  type  ofautomatic  water level control. Each of these types is described below. The   FLOAT-OPERATED   TYPE   of   feedwater control, similar in design to the feedwater control shown in  figure  1-8,  is  attached  to  the  water  column.  This control  uses  a  float,  an  arm,  and  a  set  of  electrical contacts. As a low-water cutoff, the float rises or lowers with  the  water  level  in  an  enclosed  chamber.  The chamber is connected to the boiler by two lines, which allow the water and steam to have the same level in the float  chamber  as  in  the  boiler.  An  arm  and  linkage connects  the  float  to  a  set  of  electrical  contacts  that operate the feedwater pump when the water lowers the float. When the water supply fails or the pump becomes inoperative and allows the water level to continue to drop,  another  set  of  contacts  operates  an  alarm  bell, buzzer, or whistle, and secures the burners. The  COMBINATION  FLOAT  AND  MERCURY SW ITCH TYPE of water level control shown in figure 1-8 1-9