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Other experiments were made on lighter pipes of the same kind 1 inch external and inch internal diameter; most of these failed at once under the pressure of 3.82 tons per square inch. In this case, R = inch, r = Finch, and rule (82)

gives S =

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52

16

= 8.7 tons per square inch of

metal, if the pressure had been borne, which it was not. We may take the ultimate or breaking tensile strain of wrought iron in drawn tubes at 7 tons per square inch, which is 7÷25·7 =27 or 27 per cent. only of the strength of ordinary bar iron.

(93.) "Wrought-iron Gas-pipe."-Ordinary drawn wroughtiron gas tubing is also extensively used for steam-pipes, watermains, &c., where the pressure is considerable and a knowledge of its strength becomes important. Taking 2-inch pipe as an example, the thickness would be about inch: hence R = 1% r = 1 inch, and taking S = 7 tons or 15,680 lbs. per square 15680 x (1,3 - 1')

inch of metal, rule (81) gives p

=

=

2

1362 +12 pressure for constant

2667 lbs. per square inch bursting steady load equivalent by (909) to 2667 x = 1780 lbs. for intermittent load.

The proper value of the "factor of safety" will depend on circumstances, and must be fixed with judgment; for a waterpipe where the pressure is not only intermittent but where the sudden closing of a cock may create heavy shocks, whose effect cannot be calculated, but must be provided for by the use of a high factor, we may take it at 15, giving in our case 2667 ÷ 15 = 178 lbs. per square inch, or 178 × 2·3 = 410 feet of water, safe working pressure.

(94.) "Lead Pipe."-Mr. Jardine made two experiments on ordinary drawn lead pipes, the pressure being obtained by a force pump. One pipe 1 inch internal diameter, inch thick, bore without apparent alteration a head of 1000 feet of water; with 1200 feet it began to swell, and with 1400 feet, or 140023606 lbs. pressure per square inch, it burst. We may find from this the maximum breaking strain on the metal

by the rule (82), which with R = ·95, r = 75, becomes 606 x(·952 + ·752)

S =

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= 2611 lbs. per square inch.

=

In the other experiment, the pipe 2 inches internal diameter, inch thick, bore without alteration 800 feet of water, and burst with 1000 feet, or 10002·3 435 lbs. pressure per square inch; then R being 1.2 and r = = 1, we obtain 435 x (1•2 +1) = 2412 lbs. per square inch of metal.

S=

1.29 12

The mean of the two, or 2510 lbs., may be taken as a basis for the strength of lead pipes of all sizes. Table 19 gives the thickness, weights, and safe pressures for standard sizes of lead pipes; they are commonly rated by the weight per 15-foot length up to 1 inch diameter, and per 12-foot length for larger sizes. The weight per foot being given, the thickness may be calculated by finding the external diameter by the rule :

(D2 — d2) = W÷3·86.

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(95.) In which D = the external, and d the internal diameter in inches, W = the weight per foot run in lbs. Thus, say we take 3-inch pipe, 32 lbs. per 15 feet, therefore 2 133 lbs. per foot: then 2.133÷3.86 = 5503, which is D2 - d2; then ·5503 +32 or 5503 + ·5625 1.1128, the square root of which or 1.055 = d; hence the thickness = 1525 inch, as per col. 2 of Table 19. Having thus found the thickness, and thereby R, the bursting pressure will be given by the rule:

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=

=

(1.055 •75) ÷ 2

5800 × (R2 - r2)

R2 + p2

In which pw = the bursting pressure in feet of water, and the rest as in (82): thus with the 3-inch pipe, 1525 inch thick as in (95) r = ·375, R = 375·1525 5275, and the rule gives 5800 (52752-3752) × 5800 x (2782-1406)

Pw =

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=

or, 527523752

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27821406

= 1906 feet

of water, as in col. 5 of Table 19, &c.

TABLE 19.-Of the STRENGTH of DRAWN LEAD PIPES of the ordinary Standard Weights.

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To find the safe working pressure we have to determine the value of the factor of safety (880), which requires care and judgment: Table 137 shows that for lead with a perfectly dead pressure, the safe load may be of the bursting, or the factor

=

3, but this is seldom the case in practice; by the sudden closing

of a cock, &c., heavy shocks are common, producing the wellknown knocking sound, moreover the pressure is frequently intermittent as in all the lead service-pipes of a town supplied on the ordinary system. When a ball-cock is used, which shuts off the water gradually, the strain may be taken as an intermittent dead load, for which by col. 2 of Table 141, the ratio isof the equivalent constant dead load. Taking the factor of safety at 3, we obtain÷3 of the dead pressure, as the intermittent working pressure. But where the pipe is of considerable

=

=

length, and is subjected to heavy shocks from the sudden closing of a cock, the strain becomes an intermittent dynamic one, for which col. 3 gives the ratio , or with factor 3, ÷ 3 = of the ultimate dead load. Taking these values (and) for the sake of round numbers, at and respectively and applying them to col. 5 of Table 19, we obtain cols. 6 and 7, &c.

ORDINARY WATER AND GAS PIPES: CAST IRON.

(97.) With very low pressures, such as for gas and low-service water-pipes, the rules we have given will not apply without correction. The first question is to determine the minimum thickness with which it is practicable to cast them; here we have nothing but experience to guide us, and from that we obtain the empirical rule:—

(98.)

In which d

=

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√ d
10

+15.

the diameter of the pipe in inches, and t = thickness when the pressure is practically nothing; thus for a

9-inch pipe t =

√9
10

+15= 45 inch; col. 3 of Table 20 has

been calculated by this rule, and will apply for gas and low pressures in water say up to 50 feet head.

For higher pressures such as occur in ordinary water-mains the rules become :

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TABLE 20. Of the THICKNESS and WEIGHT of CAST-IRON SOCKET-PIPE to BEAR SAFELY DIFFERENT PRESSURES of WATER.

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