closely spaced water tubes. Casing for HTW generators shall be double wall
construction. Reinforced, welded, gas-tight inner casing shall be
constructed of not lighter than 3.416 mm (10 gauge) 10 gauge black steel
sheets. Outer casing shall be constructed of not less than 1.897 mm (14
gauge) 14 gauge steel sheets. Outer casing may be either bolted or welded.
Inner casing shall be reinforced with structural steel to provide rigidity
and prevent buckling. Inner casing in furnace section shall abut furnace
tubes with no foreign sealer between the tube steel and the casing steel.
Casing shall not be attached to tubes. Each horizontal tube shall be
supported independently of casing at intervals not exceeding 1.8 m 6 feet.
The inner casing shall be applied so as to form expansion joints at the
point of tube support. Where refractory is installed at access doors, the
double casing shall be constructed to form a gas-tight seal and at no point
shall combustion gases be able to enter between inner and outer casing.
All welded joints and openings shall be checked by a pressure test. Any
casing leakage shall be repaired and made pressure-tight. The maximum
deflection of the reinforced panels shall not exceed 1/360 of the length of
the maximum span. Block insulation shall be applied between the inner and
outer casings and held securely with insulating pins. The casing tested
shall be capable of holding a pressure of 1.5 times the predicted maximum
furnace operating pressure.] [HTW generator walls shall be of welded-wall
construction. The width of the fins shall be limited to 19 mm 3/4 inch to
prevent overheating of the fins under all operating conditions. Designs
exceeding 19 mm 3/4 inch may only be used when provided with supporting
calculations and subject to the approval of the Contracting Officer. Tubes
shall be seamless type. The fin-to-tube weld shall be continuous and on
both the front (fireside) and back side of the fin. The fin shall not be
less than 6.4 mm 1/4 inch thick. The construction shall form a
pressure-tight structure capable of transferring a maximum amount of heat
to the tube. All welded joints and openings shall be checked by a pressure
test. Any casing leakage shall be repaired and made pressure tight. The
maximum deflection of the reinforced panels shall not exceed 1/360 of the
length of the maximum span. The structure tested shall be capable of
holding a pressure of 1.5 times the predicted maximum furnace operating
pressure.]
2.2.2.2
Walls
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NOTE: For personnel safety, the design temperature
of the casing surface should not exceed 65 degrees C
(150 degrees F). Should the designer wish to use a
design surface temperature between 55 and 65 degrees
C (130 and 150 degrees F), an economic evaluation
must be performed. The evaluation must determine if
the additional capital costs for insulation outweigh
the cost savings due to reduced boiler radiation
losses.
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[Refractory behind the waterwall tubes shall be high-duty refractory not
less than 65 mm 2-1/2 inches thick conforming to manufacturer's
requirements.] High temperature block and mineral wool blanket shall be
provided between the refractory backup and steel casing or between an inner
and outer casing. Thickness of insulation shall be such that an average
casing temperature in the furnace area will not exceed [55] [_____] degrees
C [130] [_____] degrees F with a surface air velocity of 508 mm/second 100
fpm, and an ambient air temperature of 25 degrees C 80 degrees F when
operating at full capacity.
SECTION 15555A
Page 23
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