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TABLE 19.-Of the STRENGTH of DRAWN LEAD PIPES of the ordinary
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 is } of 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 = 1, or with factor 3, 1 = 3 = Ig of the ultimate dead load. Taking these values (} and 1s) for the sake of round numbers, at ly and to 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 :
10 In which d = diameter of the pipe in inches, and t = the thickness when the pressure is practically nothing; thus for a
✓9 9-inch pipet
+ 15 = .45 inch; col. 3 of Table 20 has
10 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 :
Hw xd (99.)
+ .15 10
x 25000 = d.
TABLE 20.-Of the THICKNESS and WEIGHT of CAST-IRON SOCKET-PIPE TO BEAR SAFELY DIFFERENT
PRESSURES of WATER.
's s' +
thick. cwt. qrs. lbs. thick. cwt. grs. Ibs. thick, cwt. qrs. lbs. thick. cwt. qrs. Ibs. thick. cwt. qrs. Ibs. thick. cwt. qrs. Ibs.
300 1 7.31 0 1 8 .33 0 1 10
0 1 23 • 35 0 1 26 37
0 2 11 •37 0 2 14 •40
1 0 19 • 44 1 1 0
1 3 1 1 3 18
4 1 4
In which d
diameter of the pipe in inches, t = the thickness in inches, and H, = safe working head of water, in feet. Thus, the thickness for an 8-inch pipe for 250 feet of water
250 x 8 becomes by the rule (99) t =
· 15 10
25000 ·513 inch. Again, to find the safe head of water for a 9-inch pipe .63 inch thick, the rule (100) gives Hw =
x 25000 = 9 = 500 feet head. 10 (101.) Table 20, taken from the author's Practical Hydraulics,' has been calculated by the rule (99), and gives the thickness and approximate weights of cast-iron socket-pipes for pressures ranging from gas-pipes up to 1000 feet of water. The usual practice of engineers is to specify the weights of pipes, rather than the thickness, leaving the founder to determine that for himself, which long practice enables him to do with considerable precision; of course absoluto accuracy cannot be attained and should not be expected: a margin for unavoidable variations should be allowed, say 1 lb. to the inch either way, so that, for instance, a 7-inch pipe for 250 feet head, specified to weigh 3 cwt. O qr. 9 lbs. as per Table 20, should not be rejected if its real weight is between 3 cwt. 0 qr. 2 lbs. and 3 cwt. O qr. 16 lbs., &c., 14 lbs. being thus allowed for variation in a 7-inch pipe. Founders consider this to be a fair allowance.
STRENGTH OF CHAIN, ROPES, ETO.
(102.) The strength of chain is not equal to that of a straight bar of the same material ; experiments at Woolwich have shown that a straight bar being 1.0, that of a chain = 1.822 instead of 2.0. Table 21 gives the result of direct experiments on the strength of chain, which shows that wrought iron is the best material. Steel seems entirely to lose its superior tensile