Ethylene Storage Tank Failure during air drying

[sunnyjohn]

The above tank is dsigned to API 620 appendix Q , with an inner tank of ASTM A 353 material 31.2 diax 21.2 m height. The outer tank is of 33.2mx 24.2 m height. The inner tank is resting on hard wood block annular foundation , the bottom annular plate of the tank is 9.8 mm and the bottom center field is of 5.0mm, the bottom center field is insulated with 3 layers of cellular glas and the top layer of clean dry sand.The inner tank roof is of suspended aluminium deck from the outer tank roof.
The tank shell insulation consisits of the glass fiber insulation blanket of 150mm thickness and the loose fill expanded perlite. The perlite density assumed for the design is 65kg/m3.
The tank was dried with air and after a period of 3/4 weeks during the rotinr precommissioing checks , it was found that the bottom plates has buckled up and at one location has even buckeled the annular ring and the shell portion has been caved inside at which location the annular ring to the shell welding has also ruptured along the weld sfor aroundf 6inch. lentgh.The side walls has buckled in two locations on the inner tank. However as the inner tank is like an open vessel kept inside the outer tank and any partial vaccum created inside would have buckled in the weakest structure of the roof. There is no deformation noticed on the weakest structures. What other phenomena can cause such a failure. The materials comply to the API specifications and the properties and the PMI are established.
Can there be any influence of the air flow for drying operation for the failures such as a wind loading effct on the inner tank. Higher pressure is ruled out as the tank is equipped with 2 pilot operated safety valves and 2 mannual safety valves, and at no point in time the valves has propped.
The effect of the compressive stress by the insulation blanket and the perlite filling on calculation for the temperature differential of approximately 20 degrees during the drying operation due to the inner wall expansion does not seem to expalin the phenomenon.
Can a limited explosion under the tank bottom possibleas the failure seems to be due to a pressure wave or a very high compressive force on the external surface of thr inner tanks.
I can forward the failure photos if any one can assist in the analysis.
Can any one suggest the points to be checked and the corrective measures.

 

[sunnyjohn]

Additionally if we are to repair the tank which of the codes to be considered, API 620 only or API653. The replacement of a full plate shall need the removal of the HAZ as well as per the codes. How to address the same, ie. adjusting the plate size including the extra removed portion of the HAZ of the good plates on to which the new plates are to be welded.

 

[CHD01]

I am not sure I completely understand your description of the installation and failure. But it sounds like you may have installed an API 620 tank without consideration for addequate hold-down anchors for the tank. Either you have no hold down anchors at all, or the anchors provided were pulled from the base (you can check for that) or there is one other possiblity. You mentioned that the base consisted of hard wood blocks???? This is very strange. If so, you may have lifted the hard wood with the anchor. For all of this to be possible, you would have to pressure the tank above atmospheric pressure (it does not have to be greater than the MAWP of the tank to be a problem). This would bulge the bottom of the tank lifting the tank off its base, then when the pressure is removed it may or may not reverse itself which could cause the inward buckling you speak of. Note that this is possible even with a tank liquid full to the top tangent line since the weight of the liquid will NOT prevent the tank from lifting. Only the weight of the side wall and roof (when attahced to the wall) acts to hold the tank to its base.



The more you learn, the less you are certain of.

 

[JoeTank]

I would offer to review the photos and discuss it further with you. I am at braune@tankindustry.com.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[DSB123]

sunnyjohn,
See my reply on the other forum you posted this on re "It sounds to me that the thickness of the external resilient blanket is insufficient and has caused an external loading onto the inner tank during heating with the warm air. Perlite tends to settle and become like cement. If the thickness of the resilient blanket is insufficient then quite high pressures can be developed. The external insulation would have developed an external pressure on the tank during installation of the perlite and expansion due to the warm air will have increased this pressure depending on the resilience of the insulation blanket."

Regards

DSB123