Page 98 - Introduction to Marine Engineering

Boilers 85

Exhaust gas boilers
Auxiliary boilers on diesel main propulsion ships, other than tankers,
are usually of composite form, enabling steam generation using oil firing
or the exhaust gases from the diesel engine. With this arrangement the
boiler acts as the heat exchanger and raises steam in its own drum.

Boiler mountings

Certain fittings are necessary on a boiler to ensure its safe operation.
They are usually referred to as boiler mountings. The mountings
usually found on a boiler are:
Safety valves. These are mounted in pairs to protect the boiler against
overpressure. Once the valve lifting pressure is set in the presence of a
Surveyor it is locked and cannot be changed. The valve is arranged to
open automatically at the pre-set blow-off pressure.
Mam steftm stop valve. This valve is fitted in the main steam supply line
and is usually of the non-return type.
Auxiliary steam stop valve. This is a smaller valve fitted in the auxiliary
steam supply line, and is usually of the non-return type.
Feed check or control valve. A pair of valves are fitted: one is the main
valve, the other the auxiliary or standby. They are non-return valves and
must give an indication of their open and closed position.
Water level gauge. Water level gauges or 'gauge glasses' are fitted in
pairs, at opposite ends of the boiler. The construction of the level gauge
depends upon the boiler pressure.
Pressure gauge connection. Where necessary on the boiler drum,
superheater, etc., pressure gauges are fitted to provide pressure
readings.
Air release cock. These are fitted in the headers, boiler drum, etc., to
release air when filling the boiler or initially raising steam.
Sampling connection. A water outlet cock and cooling arrangement is
provided for the sampling and analysis of feed water. A provision may
also be made for injecting water treatment chemicals.
Blow down valve. This valve enables water to be blown down or emptied
from the boiler. It may be used when partially or completely emptying
the boiler.
Scum valve. A shallow dish positioned at the normal water level is
connected to the scum valve. This enables the blowing down or removal
of scum and impurities from the water surface.
Whistle stop valve. This is a small bore non-return valve which supplies
the whistle with steam straight from the boiler drum.