Cone Roof vs. Domed Roof - API 650 tanks !! 1

[MJCronin]

To all,

With regard to API-650, flat bottomed storage tanks....

I am aware that API-650 covers tanks with internal pressures to 2.5 psig...and

I am aware that a domed roof configuration is somewhat more expensive than a cone roof and is frequently used for liquids with slightly higher vapor pressure than water or fuel oil....

By what objective criteria should I recommend use of a "domed roof" storage tank over a "cone-roof" configuration ?

Is there a recommended vapor pressure based criteria ?

I have asked this question on several enginerring bulletin boards and have only recieved vague feeling and recollections. I have also reviewed Phillip Myers "Storage Tank Handbook" looking for an answer to this question

Is there any in-house company based criteria developed that can be shared ?

Thank You.....


MJC

 

[pablo02]

all of our (fuel oil) tanks had the somewhat flat cone-roof (40 ft and larger dia.) -- we had to climb on top to take monthly temperature and level measurements... [not very appealing during windy and icy conditions -- laying on our bellies at the edge trying to measure the circumference at 18" below the roof line for volume (construction) audit was even worse -- but we were young and foolish back then]

 

[MJCronin]

pablo02,

Thank you for sharing your feelings and recollections.....I hope that your belly has warmed up since those halcion "youthful times"

But you have submitted exactly the kind of answer I DO NOT WANT !!!!!

Let me try again.....

By what objective criteria should I recommend use of a "domed roof" storage tank over a "cone-roof" configuration ? Is there a recommended vapor pressure based criteria ?

Anyone out there ???

 

[EGT01]

MJCronin,

I don't have a lot of experience with atmospheric storage but was involved with a project or two some years back. I looked through some old papers I had and really didn't see much about vapor pressure as criteria for roof type.

Here's a couple notes I saw:

* Dome and umbrella roofs are self-supporting and are generally limited to tanks 60 feet or less in diameter. Except for small sizes, cone-roof tanks are supported by columns and other structural members and may reach 300 ft diameter.

* Self-supporting dome or umbrella roof designs are normally used in tanks with small, positive vapor pressures (less than 2.5 psig).

I found the last note interesting since it was written as "less than". Guess I was expecting to see a "greater than" value as a breakpoint between cone roof and dome roof but maybe the self-supporting aspect is more of a criteria than vapor pressure.

It will be interesting to see if anyone else has criteria about vapor pressure. Initial cost and ease of maintenance might also factor in there.

I would add that there was a note that with self-supporting roofs it is difficult to provide a frangible roof to top angle joint so dome roofs were required to have other means for emergency venting per API 2000.

 

[MJCronin]

EGT01

Thank you for your response.....

The 60 foot maximum diameter limit (for self-supporting dome roofs) may the the parameter that most influences the selection.

Cone roofs are only used to about 32 feet in diameter ( This was only told to me by a fabricator, as his best relollection)

Perhaps vapor pressure is not the criteria for selection....perhaps it is a desire for a self supporting roof over 30 feet in diameter..

Anybody else out there ?

MJC

 

[dave2109]

This may not be applicable to your case, however, I've generally specified cone or dome type roofs depending on the corrosiveness of the fluid contained in the tank.

For example, a demineralized water tank would have a dome type, self supporting roof with no internal bracing. Thus there is no worry about trying to coat all of the roof channels and center plate normally associated with an internally supported cone type roof.

For plain water storage, an internally supported cone type roof would be acceptable and is generally less expensive to install than a dome type roof.

Hope this helps.

 

[johnnymist2003]

Hi all,

further to all the above, I think that the main consideration for the specifying of a cone vs dome roof will be cost. For a given diameter, the self-supporting dome roof will work out more expensive to fabricate, due to the shape & forming costs, although this cost will be offset by the (normally) thinner plate section vs the cone roof required to comply with the code requirements. The cone roof will be thicker, but will be easier to manufacture. You would probably have to go out for fabrication prices to compare which option would be cheaper. In "Process Equipment Design", following a discussion wrt the shape of roofs, they say "Tank diameters for self-supporting (cone) roofs generally do not exceed 60 ft and are usually less than 40 ft. Any greater spans require such heavy rafters that it is simpler to use one or more supporting columns and thereby reduce the span". Looks to like at over 60 ft diameter, a column supported cone roof would be the way to go. I don't know the background source of this statement, and I also cannot comment on it's accuracy. Sorry I can't be of more assistance.
John

 

[EGT01]

MJCronin,

I happened to find another guideline for selection of storage tanks. For what it is worth,

Cone roof tanks were indicated for vapor space pressure/vacuum rating of 1.5"wc/1.5"wc respectively.

Dome, umbrella roofs were indicated for vapor space pressure/vacuum rating of 6"wc/1.5"wc respectively.

 

[KennyRay]

If you are preparing a design/build specification for a tank you must consider what is going on inside the tank when making this decision. If vapor space corrosion is a problem you may want to minimize what may corrode. In that case a dome roof is the way to go.

If that is not a consideration you should specify the design conditions, that is pressure, internal and or external, corrosion allowance and design temperature and leave the configuration decision to the designer/contractor assuming you are using API 650 as the design code.

Unless there are unusual conditions in the operation of the tank the configuration choice is best left to the individual responsible for the proper operation of the tank.

 

[tesarik]

I believe that one of the cone roof usages on low pressure storage tanks has got to do with a desirable weak connection between the tank shell and the roof. In other words, one would want the roof to fail first, and at as low pressure as possible. The cone roof to shell filet weld offers this better than the domed, or dished, head connection. Think of the cone roof as a giant rupture disk.

 

[DSB123]

tesarik,
You are talking about "Frangible Roofs". As such a domed or coned roof can be frangible since both can be connected to the shell via a fillet weld so your argument does not hold.