Page 243 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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204 Chapter 6 Water Distribution Systems: Components, Design, and Operation
Equation 6.2 is the hydrant discharge equation using the SI units:
2
Q = 0.0668cd 1p (SI Units) (6.2)
where p is the pitot reading in kPa, c is the hydrant discharge coefficient, d is the outlet
diameter in mm, and Q is the hydrant discharge in L/min.

EXAMPLE 6.1 HYDRANT DISCHARGE
Determine the discharge rate of a hydrant using a smooth well rounded 2.5 in. (63.5 mm) outlet at
pressures of 13.2, 9.6, 16.8, and 14.5 psig (91.61, 66.62, 116.59, and 100.63 kPa).
Solution 1 (U.S. Customary System):
For smooth well rounded 2.5 in. outlets, c 0.9
Refer to Table 6.3:
2
Q = 30cd 1p
2
Q = 30 * 0.9 * 2.5 1p
Q = 1691p
For P = 13.2 psig Q = 169113.2 = 610 gpm
For P = 9.6 psig Q = 16919.6 = 520 gpm
For P = 16.8 psig Q = 169116.8 = 690 gpm
For P = 14.5 psig Q = 169114.5 = 640 gpm

Solution 2 (SI System):
For smooth well rounded 63.5 mm outlets, c 0.9
Refer to Table 6.3:
2
Q = 0.0668 cd 1p
2
Q = 0.0668 * 0.9 * 63.5 1p
Q = 242.421p
For p 91.61 kPa Q 2,320 L/min
For p 66.62 kPa Q 1,979 L/min
For p 116.59 kPa Q 2,618 L/min
For p 100.63 kPa Q 2,432 L/min

Pressure-discharge relations established in this test are illustrated in Fig. 6.6. If the true
static pressure is known, a more exact calculation is possible, although the additional labor
involved is seldom justified. In accordance with the common hydraulic analysis of Borda’s
mouthpiece, a pressure gage inserted in a hydrant in juxtaposition to the hydrant outlet to be
opened will also record the discharge pressure otherwise measured by hydrant pitots.
Hydrant tests are sometimes made to ascertain the capacity of individual hydrants and
advertise it to firefighters (particularly to engine companies summoned from neighboring
towns) by painting the bonnet a suitable color. The weakness of this practice is its restriction
of flow measurements to single hydrants. In firefighting, groups of hydrants are normally
brought into action. Tests of individual hydrants may be quite misleading.

6.6 OFFICE STUDIES OF PIPE NETWORKS
No matter how energetically distribution systems are field-tested, needed extensions and
reinforcements of old networks and the design of new ones can be adequately identified
only by office studies. Necessary analysis presupposes familiarity with processes of hy-
draulic computation, including high-speed computers. Even without computers, however,