Page 101 - Introduction to Marine Engineering

88 Boilers

Upper union
piece

Lower union
piece

Cover
Section through gauge «aemt»ty

Drain Code handle
pipe led. operated by
to bilge wires from Drain cock
firing platform
body
Detail of ball valve
Figure 4.12 Plate-type gauge giass

passages are dear a 'blowing through' procedure should be followed.
Referring to Figure 4.11, close the water cock B and open drain cock C.
The boiler pressure should produce a strong jet of steam from the drain.
Cock A is now closed and Cock B opened. A jet of water should now pass
through the drain. The absence of a flow through the drain will indicate
that the passage to the open cock is blocked.

Safety valves
Safety valves are fitted in pairs, usually on a single valve chest. Each valve
must be able to release all the steam the boiler can produce without the
pressure rising by more than 10% over a set period.
Spring-loaded valves are always fitted on board ship because of their
positive action at any inclination. They are positioned on the boiler
drum in the steam space. The ordinary spring loaded safety valve is
shown in Figure 4.13. The valve is held closed by the helical spring