Change of Product in an Existing Tank

[sajk14]

Hello all,

I have a client who wishes to change the product in their existing tank to a new one with a much higher specific gravity.

I have ran the relevant shell thickness calculations using the newer/higher SG value and the existing shell thicknesses for the lower 3 tiers of the tank are not thick enough to take the internal loads from the new product.

However I have advised the client that if he reduces his fill height within the tank to a specified max level, he can in fact use the tank for the higher SG as long as he does not exceed the new max fill height.

My client has gone away and come back stating his engineers have done some calculations to show that if they were to add 3 sets of stiffening rings to each of the lower 3 tiers, then the tank will be fine to fill to the original fill height ( i.e. top of the tank shell ) with the new product!? The 9 new stiffening rings will be made from rolled channel and welded to the tank shell using intermittent welds.

They have told me that they have carried out an Finite Element Anaysis to back this up.

What is everyones thoughts/comments on this? My opinion is that this should not be done as I believe this type of modification to the tank is not in accordance with any of the tank codes irrespective of what the F.E Analysis shows? My understanding is that stiffening rings will help the shell from a stability point of view against buckling and external pressures. I do not see how the stiffening rings will benefit the tank shell from a thickness point of view against the internal product and pressure loads?

Also what other affects will adding numerous stiffening rings to each tier have on the tank in future that may not have been taken into account?! i.e corrosion etc..

Your comments and feedback are much appreciated.

 

[fegenbush]

Horizontal stiffeners will be of negligible benefit alone in resisting internal pressure. Used in conjunction with vertical stiffeners, there is a chance of success, although with significant cost. Replacing the shell courses, even a single sheet at a time, would appear to be more economical.

 

[bimr]

Stiffening rings are not typically used for internal pressure. API 650 does not recognize shell stiffeners to resist hoop tension. The elastic analysis method was not intended to be used for stiffening rings.

What code is listed on the tank nameplate?

Stiffening rings have been used to repair or reinforce existing tanks due to changes in service. However, there is no recognized code or standard to use. It means that the tank will be none code. There is a business risk to the owner to pursue this approach. If something went wrong with the work, they will be legally responsible for the damages.

 

[sajk14]

Thanks for your comments and confirming my view on the matter.

Bimr...The tanks have been designed to EN 14015:2004.

I have advised the client that this proposed modification to the tank is not compliant with any tank codes and therefore is not recommended!

 

[JStephen]

The theory is fine, and stiffeners can in fact be used to strengthen a tank for hoop stress, without vertical stiffeners. Nobody does it that way due to cost. Adding a little bit of extra thickness (in a new tank) is a whole lot cheaper than doing an extra mile of overhead welding. The result will be a bastard tank that doesn't fit any particular design code. Weld distortion may be an issue. Working out details around manways, etc., could be an issue.
I would say, research applicable regulations and ordinances and find out if there is a specific requirement to conform to any particular specifications or rules, and go from there.

 

[sajk14]

Thanks JSTEPHEN. You make a good point about clashes with manways, fittings etc...