24 July 2003
including Change 1, Jan 2004 and Change 2, March 2005
All openings leading to external attachments-such as water column connections, low
water fuel cut-off devices, openings in drypipes, and openings to safety valves-will be
examined to ensure they are free from obstruction.
The inspector will also check fire surfaces for bulging or blistering. Bulges often result
from overheating of the entire thickness of the metal, lowering the strength of the metal
and allowing it to be deformed by the pressure in the boiler. Bulges may also be
caused by creep or temperature gradients.
Blisters may be caused by metal defects, such as lamination in which the side exposed
to the fire overheats but the opposite side retains its strength due to the cooling effect of
the boiler water. Overheating can cause serious boiler deterioration. Metal parts can
oxidize, and pressure parts can deform and even rupture. Tubes can also be damaged
by poor circulation, steam binding, or deposition of scale.
The inspector will pay particular attention to the plate or tube surfaces exposed to the
fire, looking for any deformation such as bulging or blistering. If a bulge or blister shows
evidence of leakage or is large enough to weaken the plate or tubes seriously, the
boiler will be put out of service for repair. The blister area must be removed, the
remaining thickness determined, and repairs made as required. Although a bulge on a
water tube must always be repaired, a bulge on a plate, if not extensive, can be driven
back into place. Otherwise the affected area must be patched.
Another type of flaw noted by the inspector is cracking. Cracks can result from flaws
originating in the material from which the boiler was made, the boiler's basic design and
operating conditions, or metal fatigue. They can be accelerated by corrosion. Fire
cracks are caused by the thermal differential when the cooling effect of the water is not
adequate to transfer the heat from the metal surfaces exposed to the fire. Cracks can
result from a combination of these causes. Cracks noted in shell plates usually are
The inspector will examine areas where cracks are most likely to appear, such as the
ligaments between the tube holes on watertube boiler drums, between the tube holes
on the tube sheet of firetube boilers, areas of stay bolts, at any flange where there may
be repeated flexing of the plate during operation, and around welded pipe and tube
If cracks are suspected, a hydrostatic test to determine their location may be necessary.
A suitable nondestructive examination method may also locate such cracks.
The inspector will also look for corrosion, which causes metal surface deterioration.
Corrosion can affect large areas or be localized as pitting. Isolated, shallow pitting is
not considered serious if it is not active.
Boiler corrosion is usually caused by free oxygen and dissolved salts. If the inspector
finds active corrosion, he will advise the owner or user to obtain competent remedial
action. To estimate what effect severe corrosion over large areas has on the safe
working pressure, the thickness of the remaining sound metal will be determined by