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# Calculating capacity of vertical overflow pipe in box gutter sump (example attached)

## Calculating capacity of vertical overflow pipe in box gutter sump (example attached)

(OP)
Hello,

I have a situation where we are installing a new gutter on an existing roof which was previously leaking and had no overflow provision. I have attached a detail of our proposal.

My question is...how do I go about calculating the capacity of the vertical overflow pipe in this situation? We have a 150mm deep sump with the overflow rising 184mm above the bottom of the sump (so 34mm above the bottom of the gutter).

Australian Standard AS3500.3 Section 3.7.5.2 states that "A vertical pipe overflow that projects through the floor of the sump, is a possible alternative where a high capacity device is not required. No equations, graphs or examples are provided for vertical pipe overflow devices due to a lack of appropriate research data."

Anyone haev any ideas? I'm guessing this would this would either be solved with:

(a) Circular weir equation? If so, what formula should be used? Or,
(b) Orifice equation?

Also, how would you determine what H value to use?

Alex

### RE: Calculating capacity of vertical overflow pipe in box gutter sump (example attached)

I would use the orifice equation, with the head being the distance from the top of the overflow pipe to the WSE in the gutter.

For low-head conditions you could compare this to weir flow, using the sharp-crested weir equation based on the perimeter of the orifice.

Peter Smart

### RE: Calculating capacity of vertical overflow pipe in box gutter sump (example attached)

for self-venting (2-phase) flow in a vertical pipe, capacity may be less that that predicted by the orifice equation. And if there is any debris in the pipe, that will limit capacity also.

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