Pcl

. i__i. .. i i i i

-20 -10 0 10 20 30 40 50 60 Angle o! inclination (degrees)

FIGURE 9 Effect of angle of inclination on Durand deposition parameter (after Wilson and Tse, 1984)

EXAMPLE 5 Consider an inclined suction pipe located on a dredge ladder. Pipe length = 59 ft (18 m), D = 25.6 in. (0.65 m), angle to the horizontal = 30°. Determine the deposition velocity and the head losses at the proposed operating velocity of 21.3 ft/s (6.5 m/s). Assume Ss = 2.65 and Cv = 0.20. For a horizontal pipe, it is calculated that Vsm = 15.7 ft/s (4.8 m/s) and at the proposed operating velocity iw = 0.0373, (im — iw) = 0.0239.

Solution From Figure 9, at 30°, AD = 0.33, equivalent to an increase in Vsm of

0.33 [(64.4) • 1.65(25.6/12)]1/2 = 5.0 ft/s in USCS units, or

0.33 [(19.62)(1.65 )(0.65)]1/2 = 1.5 m/s in SI units.

The resulting limit velocity in the inclined pipe is 15.7 + 5.0 = 20.7 ft/s, 4.8 + 1.5 = 6.3 m/s, which is slightly less than the proposed operating velocity. At the operating velocity Eq. 31 gives the excess gradient, Ai(30°), as

where 0.0239 represents the solids effect in the horizontal pipe. Thus Ai(30°) equals 0.0207 + 0.1650 or 0.1857. The clear water gradient iw was 0.0373, giving a total value of 0.223, and on multiplying this by the suction-pipe length of 59 ft (18 m), the drop is found to be 13.2 ft (4.0 m) of water.

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