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.

 

[Goahead]

If an unexplained explosion took place somewhere near the bottom of the inner tank, it should have left its traces also on the inside surfaces of the external tank. If no signs can be seen in the last one, could it be that the inner tank fell down during assembly and the accident was not discovered or reported?
Also the fractographic examination of the welding rupture could possibly determine if it was sudden brittle failure due to an explosion or to overload. Minimum temperature at the presumed time of occurrence should be guessed or found.

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[JStephen]

It sounds to me like maybe the tank floor was much cooler than the shell, resulting in compressive hoop stress at the shell-floor junction. Possibly putting cold product on the floor, or blowing warm air on the shell while floor is maintained colder due to material below it.

 

[DSB123]

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.

 

[chicopee]

what you are not telling is if there was ethylene vapor in the tanks to cause this speculative explosion. Also how was the warm air generated and introduced.

 

[sunnyjohn]

Dear Chicopee,
The chance of having any ethylene vapor is remote, as the plant was yet to be commissioned, though trial of the ethylene compressor was done with the associated systems and purging and inerting using nitrogen gas was done. Also as the pressure required for any ethylkene gas is very high there is absolutely no probability of any ethylene liquid transfer. The probability of any ethylene gas vapor also is remote, though the chances of any gas pockets from the system is not ruled out.Crack gas compressor was used for the generation of air at 4.5 bart for the drying operation of the tank, at a wet bulb temp of around -40degree celsius.
The chance of an explosive mixture is assumed from any of the binding materials used under the tank or any chemicals used for the cleaning etc.
The probability of having a very high temperatue gradient between the tank floor and walls as suggested by Jstephen donot seem to be likely as the air was introduced through a tank bottom ring line and top ring line inside the inner tank.
The probability as pointed out by DSB123 seems to be a real one which we will have to evaluate during the repair activity.
With warm regards to all of you,
Sunny John