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Reg. 585 a, Reg. 585 b, Reg. 586

Stays for fire-boxes and circular furnaces

(a) Solid screwed stays

 

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.



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.



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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