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Measuring Free-Spilling Pipe Flow with the California Pipe Method
Free-spilling discharge offers challenges in how to best measure the flow. Unlike piped or channeled flow, free-spilling discharge means that many of the flumes and weirs that could otherwise don’t adapt well. However, the California Pipe Method is one approach that requires little in the way of instrumentation and provides reasonably accurate (+/-10%) flow numbers.
California Pipe Method
In applications where there is not space to install a primary device at the end of the pipe, the pipe itself can act as the device, so long as it is level.
The California Pipe Method can be used to determine the flow rate from the open end of a partially full pipe freely discharging into the air. It is particularly adapted to measuring flow in comparatively small pipes and can also be used to measure flows in small channels if the liquid can be directed into a pipe (so that it does not run full).
The pipe must be level for at least 6D (D=pipe diameter) and the discharge must freely spill off the end. The pipe cannot run full and the discharge must be non-jetting (overly energetic).
If the pipe runs nearly full a vent should be placed back from the outlet of the pipe to ensure the free circulation of air in the unfilled portion of the pipe.
California Pipe Method Flow Equation
The flow equation for the California Pipe Method have been empirically derived and is as follows:
Q = K [1 - a / d]1.88 d2.48
Q = flow rate
a = distance from the inside top of the pipe to the water's surface (measured at the end of the pipe)
d = pipe diameter
K = constant depending upon the uniits
Applying the relevant K constant for various units of flow rates yields:
CFS (ft) = 8.69 [1 - a / d]1.88 d2.48
GPM (ft) = 3,900 [1 - a / d]1.88 d2.48
MGD (ft) = 5.62 [1 - a / d]1.88 d2.48
L/S (m) = 4,680 [1 - a / d]1.88 d2.48
M3/HR (m) = 16,900 [1 - a / d]1.88 d2.48
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