Are overflow pipes required by any code?

[PotashMechy]

Hello,

I have been working on teh design of a holding tank in our mill and I am trying to find whether overflow pipes are required by any code. From what I could tell, API 650 does not specify that overflow pipes are needed, yet the code does mention them alot. Any tips on this would be great.

Thanks

 

[JStephen]

On oil tanks, if you don't have an overflow, you can damage your tank, which is big money. If you do have an overflow, you can create your own little environmental catastrophe, which is also big money, and the trend is away from overflows, and to increased use of level sensing and controls. The overflows would normally be slots, rather than piping, for an oil tank.

AWWA D100 requires overflow pipes on municipal water tanks, where flow rates are usually higher and the environmental issue isn't there.

For general industrial tanks built to API-650, some have overflow nozzles, some don't. The code allows them but does not require them.

 

[LittleInch]

Over flow pipes are only useful when the flow into a tank is relatively low and the overflow can be taken somewhere that makes it safe. Sizing the overflow is often difficult as the tank has very low strength at the top and the only driving force into the hole is a few inches of liquid. It all depends on what your contents are and what the impact is of a curtain of it falling off the tank is. Cone roof tanks with large slots cut in the roof for an overflow can result in a cascade of liquid.

One reason given for the huge buncefield tank farm explosion was that when the petrol (gasoline) ran off the roof, it then hit a wind girder and sprayed into the air, vapourising very effectively and creating, in part, a huge vapour cloud. If this instead had gone down a pipe and even if simply filled the bund, the explosion may not have occurred.

As noted above, primary safety should be your control and esd system to prevent overflow in the first place, but as a means of limiting impact of an overflow situation, pipes are a bit better than the alternatives...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

Overflows for smaller API-650 tanks (non-petrol)tend to be pipes, not slots which can be internal or external.

Sizing these "pipe type" overflows has been discussed on other threads on this forum.



MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com