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Model a Connection to an Existing System
When possible, it is always best to model a water system back to the source. But this is not always possible. In such a case, it is possible to model a connection to an existing system using a reservoir and a pump with a three-point pump curve may be used instead.
NOTE: This method is only an approximation, so the results will not be as accurate as if you modeled the system back the actual source. It is also important to note that you cannot model multiple connections to an existing system. The results in such a case could be skewed and will not be viable.
The reservoir simulates the supply of water from the system. The Elevation of the reservoir should be equal to the elevation at the connection point. The pump and the pump curve will simulate the pressure drops and the available flow from the existing water system. The points for the pump curve are generated using a mathematical formula (given below), and data from a fire flow test. The pipe should be smooth, short and wide. For example, a Roughness of 140, length of 1 foot, and diameter of 48 inches are appropriate numbers. Please note that it is ALWAYS best to model the entire system back to the source. This method is only an approximation, and may not represent the water system under all flow conditions.
Qr = Qf * [(Hr/Hf)^.54] where:
Qr = Flow available at the desired fire flow residual pressureQf = Flow during testHr = Pressure drop to desired residual pressure (Static Pressure minus Chosen Design Pressure)Hf = Pressure drop during fire flow test (Static Pressure minus Residual Pressure)
EXAMPLE: DETERMINING THE THREE-POINT PUMP CURVE
1. The first point is generated by measuring the static pressure at the hydrant when the flow (Q) is equal to zero.
Q = 0 gpmH = 90psi or 207.9 feet of head (90 * 2.31)
(2.31 is the conversion factor used to convert psi to feet of head).
2. The engineer chooses a pressure for the second point, and the flow is calculated using the Formula below. The value for Q should lie somewhere between the data collected from the test.
Q = ?H = 55 psi or 127.05 feet (55 * 2.31) (chosen value)
Qr = Qf * (Hr/Hf)^.54
Qr = 800 * [((90 - 55) / (90 - 22))^.54]
Qr = 800 * [(35 / 68)^.54]
Qr = 800 * [.514^.54]
Qr = 800 * .69
Qr = 558
Therefore, Q = 558 gpm
3. The third point is generated by measuring the flow (Q) at the residual pressure of the hydrant.
Q = 800 gpmH = 22 psi or 50.82 ft. of head (22 * 2.31)
Pump curve values for this example:
Head (ft.) Discharge (gpm)
207.9 0127.05 55850.82 800