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6.5 Field Performance of Existing Systems 203

Reservoir
Static pressure
A
Normal operating pressure B

Test pressure
C
Engine-pressure
D

E

Figure 6.9 Pressure and Discharge Relations Established by Hydrant-Flow Test (See Fig. 6.8 and
Table 6.3)
A: Static water table
B: No hydrant discharge. Pressure 74 psig (514 kPa gauge); pressure drop P 0 due to coincident draft Q
C: Hydrant discharge. Pressure 46 psig (319 kPa gauge); pressure drop P 1 (74 46) 28 psi
(194 kPa) accompanies discharge of Q 1 2,980 gpm (908 L/min)
D: Engine streams. Pressure 20 psig (140 kPa gauge); pressure drop P 2 (74 20) 54 psi (375 kPa)
accompanies discharge Q 2 4,200 gpm (1,280 L/min)
E: Hydrant 1, recording residual pressure of hydrant groups shown in Fig. 6.5

Necessary hydrant-flow calculations for the flow test may be worked out using
Equations 6.1 and 6.2 and are recorded as shown in Table 6.3.
For outlets of diameter d in. the discharge Q in gpm is
2
Q = 30cd 1p (U.S. Customary Units) (6.1)
where p is the pitot reading in psig and c is the coefficient of hydrant discharge.

Table 6.3 Record of a Typical Hydrant-Flow Test
Observed Discharge Calculated
Pressure at Velocity Flow (Q),
Conditions of Test Hydrant 1 psig Head psig gpm Remarks
All hydrants closed 74 … … All hydrant outlets are
2 ⁄2 in. in diameter.
1
Hydrant 2 opened,
1 outlet — 13.2 610 Total Q 2,980 gpm
Hydrant 3 opened, Calculated engine
2 outlets … 9.6 2 520 streams 4,200 gpm
Hydrant 4 opened,
1 outlet … 16.8 690
Hydrant 5 opened,
1 outlet 46 14.5 640
All hydrants closed 74 … …
Conversion factors:1 psig gauge pressure 6.94 kPa; 1 gpm 3.785 L/min; 1 in. 25.4 mm