drum and header are used to remove scale and other
foreign matter that have settled in the lowest part of the
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 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 plugsfire 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
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.
A WATER COLUMN (fig. 1-7) is a hollow vessel
having two connections to the boiler.
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
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