I've got a task to re-check an existing tank's overflow protection since the old pump supplying the tank has been replaced by two new ones.
The tank receives oily water and is used as a settle tank where the water flows to the water treatment facilities and the oil is skimmed off. The tank is an atmospheric tank with floating roof. The overflow line has a syphon break at its top and the size of the line is 14" (I.D=0,3398 m) both upstream and downstream of syphon break. The maximum inflow from the pumps is 700 m3/h.
I came across an old post (http://www.cheresour...-overflow-line/) which had a lot of good information but I am still not totally clear if I have understood the problem correctly.
Based on equation 3 in P.D Hills article about gravity flow, h is calculated as follows:
h > 0.811 * (QL)2 / (g * d4) = 0,811 * (700/3600)2 / (9,81 * 0,33984) = 0,234 m (2,29 kPa)
I also calculated the pressure drop from the inlet of the overflow pipe to the top based on darcy weisbach equation (just for comparison) and got a result of 5.21 kPa which equals 0,532 m.
Based on the calculated heights I draw the following conclusions.
1. If the maximum level of the floating roof is above the height of the U-bend + the height calculated above there will be no problem.
2. If the maximum height is not high enough, then the floating roof must be able to handle at least 2,29 kPag (or 5.21 kPag based on darcy weisbach) caused by liquid pushing upwards on the floating roof. At this pressure the full inflow can be accomodated in the overflow line.
Next thing, is to check the froude number. As defined by P.D Hills it is calculated like this:
JL < (2 * h / d)0.5 = (2 * 0,234 / 0,3398)0.5 = 1,175
From this I draw the conclusion that the flow is not self-venting and air will be sucked in through the syphon break which in my application does not matter.
Any comments? Have I understood the problem and the theory correctly?