Wind and Overturning in API 650 2

[Elrae]

I've been out of API for some time and things have changed. I need some help.

Used to be you looked at overturning like this;
Pressure * Area (height * Diameter) * (height /2) (for shell)
plus
(Pressure * Area ((diameter * height of roof) /2) * (Centroid of roof + height of tank)

We then converted the moment found above to uplift to evaluate anchorage.

Now the wind pressure on the roof is in uplift but the overturning calculations are still in moment.

Can I just disregard the term uplift and calculate overturning moment like I used to? If not, what is expected now?

Any help is appreciated.

 

[JStephen]

There are some inconsistencies in the current API approach.

You calculate moment about one edge of the tank, including the uplift force on the roof, and neglecting lateral load on the roof. So you have horizontal pressure x projected shell area x half the shell height plus vertical pressure x cross-sectional area x radius. Then use the indicated equations to check for overturning.

For the overturning check, this works okay. However, when you use this wind moment moment to figure anchorage, it is overly conservative. The practical difference is not usually that large, though.

 

[LeonEarle]

So, if I understand, although we calculated uplift on the roof, we use the result as the moment in the calculations for overturning. Is this correct? If so, then I presume that this value, Mw, would also be used in the evaluation of anchor bolt tension, 3.11.3, and anchorage, 3.12.2. Is this also correct?

 

[JStephen]

That is the way I understand it.

 

[kvbalu]

What requirements other than Process conditions dictate the requirement of PWHT of Plates for Storage Tanks as per API 650 ?

 

[LeonEarle]

kybalu,

You should have started another thread with your question. However, since I was interested in this thread, I have read your question and will respond.

In API 650, tanks built with group 1, 2, or 3 materials that exceed 1" in thickness are required to have nozzles 12" in diameter or greater installed into insert plates and stress relieved prior to installation into the tank. Additionally, if the tank us made using group 4 or higher materials and the thickness exceeds 1/2", all nozzles that require reinforcement are required to be installed into an insert and stress relieved prior to installation into the tank. In additiona to these nozzles, all clean-out doors, regardless of materials used are required to be stress relieved prior to installation into the tank. There may also be a requirement to stress relieve flush-type nozzles but I would have to look that one up to confirm.

Hope this helps
LeonEarle

 

[Elrae]

JStephen,

In your reply, you indicated that the uplift was to be multiplied by the cross-sectional area. In API, they indicate that it is to be multiplied by the "horizontal projected area". I have searched and searched and can not find how to calculate "horizontal projected area". The projected area of a cone is diameter * height of cone / 2. Is this the same as "horizontal projected area"?

Thanks

 

[IFRs]

Horizontal projection area is simply the area of the tank when looking straight down on the tank from above ( diameter squared timex pi divided by 4). Your calculation is for vertical projection area which might be used for overturning but not uplift.

 

[rab1944]

For an API 650 F and S tank with a design temperature in excess of the API 620 250 degree limit, what is the best approach in choosing the allowable stress for an umbrella roof design?