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Reg. 585 a, Reg. 585 b,
Reg. 586
Stays for fire-boxes and circular furnaces
(a) Solid screwed stays
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For screw stays to combustion chambers and fire-boxes
and for longitudinal and cross stays, the maximum working
pressure for the stays is to be calculated from the
appropriate one of the following two formulae:-
| WP= |
C |
1.28
(D - ------------)²
N |
Equation
XII/19 |
| A |
| WP= |
C x D1² |
Equation XII/20 |
| A |
WP is the working pressure in pounds per square inch.
D is diameter of stays over threads in inches.
D1 is diameter of body of stay at its smallest part in
inches.
N is number of threads of stay per inch.
A is area in square inches supported by one stay for area
to be supported by stays near tubes in fire-box tube plates
of Loco-type boilers, [see Regulation
193(a)].
C=7,100 for steel or special wrought iron screw stays
to combustion chamber or fire-boxes,
C=8,640 for steel longitudinal or cross stays fitted with
nuts.
=4,700 for copper stays.
Where stays are made with enlarged ends and the body of
the stay is smaller in diameter than at the bottom of
the thread the working pressure shall be calculated from
the second formula.
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Reg. 585 b
Circumferential stays for circular furnaces and fire-boxes
The
diameter of the stay shall be not less than ¾ inch
or twice the thickness of the fire-box plate, whichever
is the greater. In the case of screwed threads the diameter
shall be measured over the threads.
The pitch of the stays at the fire-box shall not exceed
14 times the thickness of the fire-box plate.
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Reg. 586
Fire-box Crown stays for Loco-type Boilers
Where
fire-box crown stays are directly attached to a semi-cylindrical
outer casing, and the fire-box top is cambered and fitted
with stays arranged as shown in Figure
XII/78, the thickness
of the outer casing shall be sufficient to provide not
less than three engaging threads, of which not less than
one shall be full thread in stays not normal to the stayed
surface, but if the thickness of the plate is not sufficient
to give one engaging full thread the plate shall be reinforced.
Where vertical stays only are used to take part of the
load on the fire-box top, boilers with shells 4 feet and
6 inches diameter and over shall be fitted with transverse
stay (see Figure XII/79). Stays may be attached as shown
in Figures XII/39, XII/40 and XII/41.
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