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Monday, July 21, 2008

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A free surface vortex potentially form when :

i) liquid exit via a nozzle or hole below it liquid surface (bottom or side) and
ii) liquid height (between liquid surface and nozzle/hole) is below a minimum submergence height (S).

When free surface vortex is formed, gas entrainment will occur and pull vapor / gas above liquid surface exit together with liquid via the nozzle / hole. The following image shows series of events taken place when a free surface vortex is formed.





Similarly, gas entrainment occurs at pump suction intake point causes vapor/gas enter pump reduce pumping capacity and vibration (due to pump impeller imbalance).

Submergence (S) is defined as the height between liquid surface and the exit hole. Following images show submergence (S) for different intake location and nozzle type.



Submergence subjects to intake velocity with the following relationship :



where

d = intake diameter (m)
Q = intake flowrate (m3/s)



Example :
A pump transferring liquid from a suction tank to a reactor at a rate of 227.1m3/h. The pump suction inlet nozzle with a internal diameter of 304.9mm is located horizontally. What is the minimum submergence to avoid gas entrainment ?

d = intake diamter = 304.9 /1000 = 0.3049 m
Q = intake flowrate = 227.1 / 3600 m3/h = 0.063 m3/s

S = 0.3049 + 2.3 (4 / SQRT(9.81) x PI) (0.063 / 0.3049)
S = 0.66 m

Thus, minimum submergence (S) is 0.66 m above intake nozzle.

* SQRT = Square-root
** PI = 3.141592654


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posted by Webworm, 11:25 PM

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