Minimum Submergence and Froude Number

[Mech85]

Hi,

The HI standard 9.8 "Intake Design for Rotodynamic Pumps" states that the Minimum Submergence for a horizontal bottom outlet from a tank can be given by S=D (1+2.3Fr) where D is the nozzle diameter and Fr is the Froude number
The Froude No. Fr = V / SQRT(g*D)
where
V = nozzle velocity
g = gravity constant.

My query is for a rectangular outlet nozzle, should I use an equivalent hydraulic diameter for D such as
dh = 2 a b / (a + b)
where
dh = hydraulic diameter
a = width of the duct (mm, in)
b = height of the duct (mm, in)

or do I just use the height ‘b’ of the rectangular nozzle as the value for D?

 

[BigInch]

Not sure, but I'd tend to use the equivalent diameter, since velocity is a principle argument in these types of problems. If you used the height of a square, velocity could actually be considerably slower than what you calculated due to all the excess area in the corners. However a rectangle may be somewhat different and results could be expected to vary considerably with great width to height ratios, in which case I might be tempted to use height for distance to floor and width for distance to edge of tank. I think if you're working with typical rectangles, not of extreme width to height ratios, go ahead and use the equivalent diameter.

Of course you could calculate it both ways and see how much the difference in velocity is then base your method on how conservative you want the result to be in case it doesn't work out like you expect (ie how expensive will it be to fix it).

 

[georgeverghese]

In addition, a vortex breaker of some sort at the inlet to this tank bottom exit would help.