Fuel Storage Tank Alteration

[Ibeam]

I'm hoping someone may have some insight as to the code ramifications on altering an existing fuel storage tank (steel plate construction).

My question is: If the tank is altered, such as installing anchors to a previously unanchored tank, does that open the door to new code requirements for the entire tank and nozzles.?

The client suspects that the tank (41' ht. x 40.5' dia.) was not originally designed to the applicable code for seismic overturning (Zone 3), drawings are dated 1980. I'm in the middle of determining that, and if it proves true then will look to see if it's feasible to retrofit an anchoring system. The ringwall foundation is 12" wide x 4' deep. The tank bottom rests on oil saturated 6" sand base, with compacted granular fill below that, reaching down to bottom of ringwall.

 

[Tankman650]

Most state regulations and industry requirements require that owners maintain their tanks per API 653. API 653 refers back to the requirements of API 650.

I recommend that you determine the manufacturer and have them review the design and determine the need for anchors. If needed, they can determine the modifications. Tanks are not like building structures. Adding anchors may actually cause a failure of the tank rather than prevent one.

 

[Ibeam]

Tankman; Thank you for your suggestion. When you say "Manufacturer", I imagine an entity that produces shop fabricated tanks. This tank was built on-site by an independant contractor, and designed by an owner retained engineer. Please excuse me if I'm stating the obvious. I don't know the background as to why the original designer isn't reviewing the code requirements, but I believe the tank does go through an inspection process according to API 653, and as a result, the client has been given some sort of indication that the tank does not stand up to the Appendix E requirements of API 650.

If this is the case, then aside from limiting the fill height, the tank can either be anchored or be modified by increasing the bottom shell, or bottom plate thickness.

This leads me to another question: In regards to the option of increasing the bottom plate thickness, how can this be acceptable if API 650 (E.4.2) restricts the thickness value to 1/4" max. (which is a common bottom plate size I believe?) for calculating wL, which is corollated to the bottom shell forces?

 

[Tankman650]

By manufacturer I was refering to companies like CBI, Pasidena Tank, etc... which will not help in your case since it is a owner fabricated tank since the owner hired all the parties.

An annular base plate can be located under the shell as noted in E.4. The maximum thickness of the base plate is limited to the thickness of the lower shell course (if greater than 1/4&quot. The radial dimension of the annular base plate is given in E.4.2. Annular base plates are difficult to install compaired to normal bottom plates. They must meet the requirements of 3.1.5.6, 3.5, and figure 3-3C (if greater than 1/2&quot.

 

[Ibeam]

Tankman,

Do you have any thoughts on "Manos" method of evaluating existing tanks? Can this be used instead of API650? The results are so different.

The way it looks to me, API650 gives me no choice but to install anchors..my wL value is held to a maximum by 1.25GHD, so increasing my bottom course and annular plate doesn't seem to help the M/(D2(wt+wL)) ratio of 2.84.

Also, it looks like it's a zone 4 area, not zone 3 as I originally mentioned.

 

[Tankman650]

I am not familiar with "Manos".

Have you determined if the uplift on the anchor bolts exceeds the dead weight of the ring wall??

I would start there and if the uplift exceeds the dead weight , then try reducing the level of the tank to reduce the uplift. I think that normal factor of safty for uplift is 1.5.

I would present the option to the owner of either reduced level of operation or replace the tank. The owner may have to replace the tank from the foundation up to meet the requirments of the current API 650 standard and have full capacity of the tank.