API 650 Roof Nozzle Allowable Load

[AwesomeAndy3]

Hey everyone,

Just wanted to see if anyone has ever done calculations or has experiences on allowable load for a roof nozzle on an API 650.

Any help is greatly appreciated. I'm just unsure where to start.

Thanks,

 

[JStephen]

Somewhere online, I remember seeing an analysis of a scaffold cable hitch via finite element.
Roof nozzles are not often analyzed.

 

[LittleInch]

Start from the basis that no load or torsion is allowable and you then won't be disappointed.

It usually needs some serious FEA type work or use of flexible connectors / spring hangers / structural support of the joining pipes.

Not many roof nozzles do anything other than hold up their own weight and occasionally some sort of vent valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AwesomeAndy3]

Thanks guys,

What do you guys think of my approach as follows:

Section 5.10.4.4 of API 650 provides calculation on rafter spacing. Is it acceptable for me to work backwards if I know the maximum roof plate span (6.5' in my case), Fy being 30,000psi, and the corroded roof thickness, and try to calculate the uniform pressure p from the equation.

From this p, I can work on the load combinations from section 5.2.2, whichever case with the maximum psf, I subtract that maximum number by the p I got from the rafter equation, and the remaining difference is the psf that the roof plate can still handle (I can convert the nozzle load into psf based on the area of the nozzle). Then I will calculate the load acting on the area of the nozzle with the self weight of the nozzle, as well as the additional appurtenance that is being attached to that nozzle. Compare the two numbers at the end.

Does this approach seem reasonable by any means?

 

[JStephen]

If it helps any, the equation in 5.10.4.4 is just based on a fixed-fixed beam with uniform loading, with allowable bending stress of 0.75Fy.
Note that the specified load for this check is the load combinations in API-650, not the total of all possible loads.
First step, decide if you really need to do this check, or if this is just another roof nozzle.
I would be inclined to check bending on a strip several feet wide spanning between rafters with the nozzle in the center.
Another potential approach is to treat the plate as a catenary (zero stiffness, supported by tension in the plate only).
In either case, you may show adequate strength, but the fitting will still be fairly flexible.

 

[AwesomeAndy3]

Hey JStephen,

So wouldn't using 5.10.4.4 be a conservative approach? Since it assumes uniform loading throughout the entire length.

I'll only be looking at the area of the nozzle and its repad.