Time to Empty or Drain a Tank, Pond, Reservoir Containing Water or other Liquid 
Compute time to empty (or lower) water or other liquid. Also can solve for discharge coefficient, orifice diameter (or area), tank diameter (or area), initial liquid depth, final liquid depth. 

Register to enable "Calculate" button. Units: cm=centimeter, ft=foot, hr=hour, m=meter, min=minute, sec=second. Tank (or pond or reservoir) is open to the atmosphere. The tank can be cylindrical or other crosssection but must have the same crosssection for its entire height. The orifice can be circular or noncircular. If tank or orifice is noncircular, enter their area into our calculation. H_{i} , H_{f} , and h are measured vertically from the centerline of the orifice. Introduction Equations Note: The above equation is valid if both the tank and orifice are at the same pressure, even if the pressure is not atmospheric For a tank with a constant crosssectional geometry A in the plan view (i.e. as you look down on it), substitute: Integrate h from H_{i} to H_{f} and integrate t from 0 to t, then solve for time t, which is the time required for the liquid to fall from H_{i} to H_{f}: If the tank is circular in plan view (i.e. looking down on it): If the orifice is circular: Our calculation allows you to solve for any of the variables: C, d, a, D, A, H_{i}, H_{f} , or t. The orifice and tank can be either circular or noncircular. If noncircular, then the diameter dimension is not used in the calculation. Validity The tank and orifice discharge are assumed to be at the same pressure. The tank is assumed to be of sufficiently larger area than the orifice so that friction effects from the tank wall are negligible; that is, A>10a as a rule of thumb suggested by LMNO Engineering. Orifice Discharge Coefficient Type: Rounded Sharpedged
Short tube Borda The short tube value is valid for L ~ 2.5d. Borda is also known as "reentrant" since it juts into the tank. Variables Error Messages given by calculation "Need H_{i} > H_{f}". Initial depth must be larger than final depth. "Best if A ≥ 10a". To ensure that tank friction is negligible, should have tank area much larger than orifice area. "Need 0 < C ≤ 1". Discharge coefficient must be greater than 0.0 and less than or equal to 1.0. Typical value is 0.6 for water flowing through squareedged orifice. "Infeasible input". This message can occur if the final depth is being computed and H_{i}^{0.5} < [t a C (g/2)^{0.5}] / A References Daugherty, R. L., J. B. Franzini, and E. J. Finnemore. 1985. Fluid Mechanics with Engineering Applications. McGrawHill, Inc. 8ed. Streeter, V. L., E. B. Wylie, and K. W. Bedford. 1998. Fluid Mechanics. McGrawHill, Inc. 9ed.
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To: LMNO Engineering home page (more calculations) Discharge from a tank (steady state) Orifice flow meter in pipe (D<5cm) Orifice flow meter in pipe (D>5cm)
