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Storage tank defect spray distance

Storage tank defect spray distance

(OP)
Dear,

I don't know if I should ask this question here, or in the proccess engineering forum, but let me start by asking it here.

I have to calculate the spray distance resulting from a small defect (say 10 or 20 mm) in a storage tank shell.

My first assumption was to calculate the outlet velocity using the hydrostatic pressure:
where Head = the liquid height above the defect

Then calculate the time it takes for gravitation to get the liquid down:
droptime = (2*height of defect / g)^0.5

Then using droptime * velocity to get the horizontal distance of the spray.

However if I compare my results to the example results I have from someone elses calculation my distances are much larger (about 1.6 times).
I know I calculated the distance with constant horizontal speed of the flow which may not be accurate.

Can any of you help me to get a more accurate calculation method for this problem?

Thanks in advance for the help.

RE: Storage tank defect spray distance

What you've posted is how I would calculate it. Of course, the following items could change the calculation:
1. Pressure (if it's a pressurized tank, the velocity would increase, v = (2*g*Head + pressure/density)^0.5
2. Viscosity, if the fluid is more viscous than water then you may have more losses due to friction.
3. The calculation is assuming no losses occurring at the defect.

All in all, if you are wanting to find the worst case scenario, then your calculation (unless the tank is pressurized) are correct.

Note: I noticed you double posted this in the Chemical Process Engineering forum as well, which is against the forum rules. I would suggest you request one of them to be deleted.

RE: Storage tank defect spray distance

The velocity, and the flowrate, are affected by the shape of the 'orifice', so you apply a coefficient.
It's around 0.7 for a round orifice without burrs or edge radii.
It's around 1.0 for a round orifice without burrs and with a generous radius on the inlet.
Of course you can't assume a round orifice, and you don't know anything about the defect's shape in any direction, so 1.0 is conservative. I'm guessing your friend used a less conservative value.

Mike Halloran
Pembroke Pines, FL, USA

RE: Storage tank defect spray distance

(OP)
Thanks for the replies.

@MikeHalloran: Where to use this coefficient? Is it in velocity = (2*g*Head)^0.5, becoming velocity = (2*g*Head*coefficient)^0.5?

RE: Storage tank defect spray distance

It's the 'flow coefficient' Fc in the orifice equation here:
http://www.efunda.com/formulae/fluids/calc_orifice...
used as a factor to estimate the orifice's effective area relative to its measured area.

... which is unnecessary for your method. Sorry.

Mike Halloran
Pembroke Pines, FL, USA

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