Page 110 - Introduction to Marine Engineering

Boilers 97

Boiler operation

The procedure adopted for raising steam will vary from boiler to boiler
and the manufacturers' instructions should always be followed. A
number of aspects are common to all boilers and a general procedure
might be as follows.

Preparations
The uptakes should be checked to ensure a clear path for the exhaust
gases through the boiler; any dampers should be operated and then
correctly positioned. All vents, alarm, water and pressure gauge
connections should be opened. The superheater circulating valves or
drains should be opened to ensure a flow of steam through the
superheater. All the other boiler drains and blow-down valves should be
checked to ensure that they are closed. The boiler should then be filled
to slightly below the working level with hot de-aerated water. The
various header vents should be closed as water is seen to flow from them.
The economiser should be checked to ensure that it is full of water and
all air vented off.
The operation of the forced draught fan should be checked and
where exhaust gas air heaters are fitted they should be bypassed. The
fuel oil system should be checked for the correct positioning of valves,
etc. The fuel oil should then be circulated and heated.

Raising steam
The forced draught fan should be started and air passed through the
furnace for several minutes to 'purge' it of any exhaust gas or oil
vapours. The air slides (checks) at every register, except the lighting up'
burner, should then be closed. The operating burner can now be lit and
adjusted to provide a low firing rate with good combustion. The fuel oil
pressure and forced draught pressure should be matched to ensure
good combustion with a full steady flame.
The superheater header vents may be closed once steam issues from
them. When a drum pressure of about 210kPa (2.1 bar) has been
reached the drum air vent may be closed. The boiler must be brought
slowly up to working pressure in order to ensure gradual expansion and
to avoid overheating the superheater elements and damaging any
refractory material. Boiler manufacturers usually provide a steam-
raising diagram in the form of a graph of drum pressure against hours
after flashing up.
The main and auxiliary steam lines should now be warmed through
and then the drains closed. In addition the water level gauges should be