API-620 tanks Limits on Flat Bottomed design

[MJCronin]

To all,

I am aware of the 2.5 to 15 psig pressure design limits for tanks designed to API-620.

What are the practical pressure and diameter limits for flat bottomed tank design ?

At what combination of pressure and diameter make a flat bottomed tank unecononomical and make a tank with two dished ends a better choice ? (assume carbon steel at ambient temperature ?

For example, assume a 12 foot diameter tank, 33,000 gallons designed to 14 psig......is an API-620 design (flat bottom) or a "non-coded" ASME-VIII design (horizontal with dished heads) more econonomical ?

Is there some general criteria that could be used/shared here ?

Thank you for your input....

MJC

 

[JoeTank]

A large flat bottom AST per API-620 is often limited in size due to economic factors. If you have enough money, you could build a 300ft diameter tank for 15 psi. The foundation would be massive and so would the cost.

As a practical matter consider the large LNG tanks. They are often in the 200ft diameter range and are designed for 1 to 2 psi. They are designed to API-620, Appendix Q.

Also, consider the large LPG and Ammonia tanks designed to API-620, Appendix R. They are often built at around 100-120 ft in diameter and designed for a pressure of about 2 to 2.5 psi.

I would accept these sizes as indicative of the economocal size of a large-low pressure storage tank.

Historically, the large vertical tanks designed for higher pressures, say in the range of 6 to 10 psi, we not flat bottom tanks with anchors. They were built with dished bottoms of various configurations. I have an old Hammond Ironworks brochure around here somewhere that give a range of sizes vs pressure of tanks that they built. I could send you a copy of it to you if you wish. it is from the 1950s, so its quite outdated, but interesting nevertheless.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[CGArmesto]

In my not very long experience, i only have met large tanks designed in accordance with API 620, more than 200feet in diameter. I´ve seen smaller tanks to API 650, 30feet in diameter.

I don`t really think API 620 would be an economical option for your tank. Try the API 650 or any other option you can consider,

Best regards,

Armesto

 

[MJCronin]

To all,

I guess I should better phrase my question....

For a specific liquid with a specific vapor pressure ( say 10 psig) is there a "company standard", commonly accepted practice, position paper or guideline that would help me decide the economics between an API-620 flat-bottomed tank or a horizontal "dished end" tank ?

My search is for a general practice or guideline and I feel that someone out there has asked this question before....

I am aware that I can get quotes for each style and compare, but is there an easier, more general method ?

I suspect that some huge chemical company such as Dow, Monsanto or someone has evaluated this.....

MJC

 

[JoeTank]

Mr Cronin,

The answer is probably going to be driven by the tank volume too. If it's a small volume, say no more than about 30,000 gallons, I'd assume that a shop fabricated ASME design, w/o Code stamp, would be cheaper than an API-620 tank. The horizontal tank approach would be "self-contained", in that the foundation would not know the tank is pressurized.

A vertical flat bottom AST per API-620 would have a large foundation with a bunch of anchors to resist the pressure uplift on the tank.

Hope this helps.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[MJCronin]

Steve,

Thank you for your help.... I was fishing around for something that may not exist

I believe you are a real asset to this Forum...

 

[willisd1]

Steve,

One of the reasons for the upper limitation of 15 PSIG is that most jurisdictions exempt from regulation vessels that are adequately protected from over pressure at 15PSIG or less, and require ASME standard certification if pressure exceeds 15 PSIG. API 620 allows lapped bottom plates in some cases, where ASME Section VIII Div 1 would permit a flat bottom head, but not allow it to contain lap joints. Another practical limitation is that a bottom head that was flat would require a significantly greater thickness than a formed head (Torispherical, ellipsoidal, etc) of the same diameter.

Dan W