263), the lengths of the stay bars, and the weights of the various sized rivets (p. 254), ready for ordering. All the dimensions should be carefully checked, and if the boiler is to be built under the inspection of any society, the working pressure for each part should be calculated from the invoiced dimensions and with the help of the latest edition of their rules, and preferably without the use of any tables. If required, a tracing should also be submitted for approval, so as to obviate all risks of having to make alterations.

The two following tables, which contain the effective diameters and the effective sectional areas of screwed stays, may be of use when other stresses than those of the tables, pp. 354, 392, are to be adopted.

Effective Sectional Areas

Sq. Ins. Sq. Ins. Sq. Ins. Sq. Ins. Sq. Ins. Sq. Ins. Sq. Ins. Sq. Ins.

.44

10

•60

⚫30

⚫42

⚫79

•99

1.23

1.48

8776

8

333

CHAPTER X

LLOYD'S REGISTER BOILER RULES

EXTRACTS FROM RULES PUBLISHED IN JUNE 1899

Rules for Determining the Working Pressure to be allowed in New Boilers

Cylindrical Shells of Iron Boilers.-The strength of circular shells of iron boilers to be calculated from the strength of the longitudinal joints by the following formula :

where C coefficient as per following table, T thickness of plate in inches,

D = mean diameter of shell in inches,

B = percentage of strength of joint found as follows-the least percentage to be taken.

(In case of rivets being in double shear, 1.75a is to be used instead

MEM. In any case where the strength of the longitudinal joint is satisfactorily shown by experiment to be greater than given by this formula, the actual strength may be taken in the calculation.

NOTE. The inside butt strap to be at least the thickness of the plate.

Cylindrical Shells of Steel Boilers.-The strength of cylindrical shells of steel boilers is to be calculated from the following formula :

CX (T-2) x B

where D

D

= working pressure in lbs. per square inch,

= mean diameter of shell in inches,

= thickness of plate in sixteenths of an inch,

= 21 when the longitudinal seams are fitted with double butt straps of equal width,

C = 20-25 when they are fitted with double butt straps of unequal width, only covering on one side the reduced section of plate at the outer lines of rivets (fig. 438, p. 318),

C = 195 when the longitudinal seams are lap joints. If the minimum tensile strength of shell plates is 28 or 29 tons per square inch, instead of 27 tons per square inch, these values of C may be correspondingly increased.

B= the least percentage of strength of longitudinal joint, found as follows:

In case of rivets in double shear 1·75a is to be used instead of a,

where p pitch of rivets in inches,

=

t = thickness of plate in inches,

d =

diameter of rivet holes in inches,

n = number of rivets used per pitch in the longitudinal joint,

a = sectional area of rivet in square inches.

NOTE. The inside butt strap to be at least the thickness of the plate.

NOTE. For the shell plates of superheaters or steam chests enclosed in the uptakes, or exposed to the direct action of the flame, the coefficients should be of those given above.

Proper deductions are to be made for openings in shell. All manholes in circular shells to be stiffened with compensating rings.

The shell plates under domes in boilers so fitted to be stayed from the top of the dome or otherwise stiffened.

Stays. The strength of stays supporting flat surfaces is to be calculated from the smallest part of the stay or fastening, and the strain upon them is not to exceed the following limits, namely:

Iron Stays. For stays not exceeding 13 in. smallest diameter, and for all stays which are welded, 6,000 lbs. per sq. in.; for unwelded stays above 1 in. smallest diameter, 7,500 lbs. per sq. in.

Steel Stays. For screw stays not exceeding 1 in. smallest diameter, 8,000 lbs. per sq. in.; for screw stays above 11⁄2 in. smallest diameter, 9,000 lbs. per sq. inch. For other stays not exceeding 1 in. smallest diameter, 9,000 lbs. per sq. in., and for stays exceeding 1 in. smallest diameter, 10,000 lbs. per sq. in. No steel stays are to be welded.

Stay Tubes. The stress is not to exceed 7,500 lbs per sq. in. Flat Plates. The strength of flat plates supported by stays to be taken from the following formula :—

Cx T2
P2

=

working pressure in lbs. per square inch,

where T = thickness of plate in sixteenths of an inch,

P2=square of pitch in inches; if the pitch in the rows is not equal to that between the rows, then the mean of the squares of the two pitches is to be taken,

6

C90 for iron or steel plates thick and under, fitted with screw stays with riveted heads,

16

C=100 for iron or steel plates above thick, fitted with screw stays with riveted heads,

C110 for iron or steel plates thick and under, fitted with stays and nuts,

C = 120 for iron plates above thick, and for steel plates above and under thick, fitted with screw stays

and nuts,

C = 135 for steel plates
stays and nuts,

thick and above, fitted with screw

= 140 for iron plates fitted with stays with double nuts, = 150 for iron plates fitted with stays, with double nuts and washers outside the plates, of at least of the pitch in diameter and the thickness of the plates, C = 160 for iron plates fitted with stays, with double nuts and washers riveted to the outside of the plates, of at least of the pitch in diameter and the thick

ness of the plates,

C = 175 for iron plates fitted with stays, with double nuts and washers riveted to the outside of the plates, when the washers are at least of the pitch in diameter and of the same thickness as the plates.

For iron plates fitted with stays, with double nuts and doubling