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Overflow pipe

In the mining industry, we frequently have tanks that holds slurry or other type of liquid. These tanks are usually design as atmospheric as per API 610.
1-tank-overlow-pipe
Normally, overflow pipe is shown on PID, however there will times when sizing of overflow pipes can fall on the piping designer shoulders. So, how do we determine the size of overflow pipe? As you know, atmospheric tank is not design for internal pressure and even small internal pressure can cause the tank to fall apart. Overflow pipe must have enough capacity to take all intake flow. Another question that rises is velocity in gravity driven flow. There are no investigation conducted on this subject and there is no suitable equation that will describe flow in vertical gravity pipe. However, experience shows that for gravity driven flow, 1 ft/sec could be a good assumption. By knowing velocity in overflow pipe (1 ft/sec) and required flow (intake flow),  we can now calculate inner pipe diameter.