Page 99 - Introduction to Marine Engineering

86 Boilers

Boiler mountings (water-tube boilers)
Watertube boilers, because of their smaller water content in relation to
their steam raising capacity, require certain additional mountings:
Automatic feed water regulator. Fitted in the feed line prior to the main
check valve, this device is essential to ensure the correct water level in.the
boiler during all load conditions. Boilers with a high evaporation rate
will use a multiple-element feed water control system (see Chapter 15).
Low level alarm. A device to provide audible warning of low water level
conditions.
Superheater circulating valves. Acting also as air vents, these fittings
ensure a flow of steam when initially warming through and raising steam
in the boiler.
Sootblowers, Operated by steam or compressed air, they act to blow
away soot and the products of combustion from the tube surfaces.
Several are fitted in strategic places. The sootbiower lance is inserted,
soot is blown and the lance is withdrawn.

Water level gauges
The water level gauge provides a visible indication of the water level in
the boiler in the region of the correct working level. If the water level
were too high then water might pass out of the boiler and do serious
damage to any equipment designed to accept steam. If the water level
were too low then the heat transfer surfaces might become exposed to
excessive temperatures and fail. Constant attention to the boiler water
level is therefore essential. Due to the motion of the ship it is necessary to
have a water level gauge at each end of the boiler to correctly observe the
level.
Depending upon the boiler operating pressure, one of two basically
different types of water level gauge will be fitted.
For boiler pressures up to a maximum of 17 bar a round glass tube
type of water level gauge is used. The glass tube is connected to the
boiler shell by cocks and pipes, as shown in Figure 4.11. Packing rings
are positioned at the tube ends to give a tight seal and prevent leaks. A
guard is usually placed around the tube to protect it from accidental
damage and to avoid injury to any personnel in the vicinity if the tube
shatters. The water level gauge is usually connected directly to the
boiler. Isolating cocks are fitted in the steam and water passages, and a
drain cock is also present. A ball valve is fitted below the tube to shut off
the water should the tube break and water attempt to rush out.
For boiler pressures above 17 bar a plate-glass-type water level gauge
is used. The glass tube is replaced by an assembly made up of glass plates
within a metal housing, as shown in Figure 4.12. The assembly is made