The discharge flow rate from an orifice is calculated as follows:
Flow rate = Orifice discharge coefficient x Orifice cross-sectional area x square root (2gH)
Where g = acceleration due to gravity
Where H = head of fluid about outlet
As the tank empties the fluid head will reduce and the discharge flow from the orifice will reduce.
Fluid Surface Area
If the fluid surface area within the tank is not
constant, the rate of change in the fluid head will
not be constant. Initially the change in fluid head may be slow, but
as the tank empties the fluid head will change
more quickly.
The example shows that the time to reduce the
fluid level by 300 mm (12 inch) is initially 3m - 37s,
where as the final 300 mm of fluid will empty in 1m - 33 s.
Effect of Discharge Pipe
When a pipe is used to feed the tank discharge to
some other point the pipe may restrict the
discharge flow rate.
When a 4 metre (13 ft) length of pipe is used, in
this example, the time to empty is increased by
almost 8 minutes.
Flow Advisor for Channels and Tanks can help you save time and money.
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Discharge Flow Rate
Effect of Discharge Pipe