4 replies to this topic

[keverne]

Dear all,

Storage tank: 2600 metric tan
PV relief valve: Varec 2010B (1 unit) on tank roof

Initially the storage tank is used to store biodiesel cargo for shipment at terminal located nearby a harbour. The cargo was hauled off and one week later; without clean-up on the tank; it was re-used as the client wanted to store the same product.

Prior to bulking, it was subjected to nitrogen purging process. Normally N2 purging requires approximately 11 hours to reach the required oxygen level (in percentage) within the tank. On top of the tank only the bleeder and the pressure vacuum relief valve were opened. There are 2 bulking lines; 4 inch & 8 inch. The former is used to purge N2 while the latter was closed.

About 1 hour and 30 minutes later, the roof of the tank rupture and the tank shell on the top section buckled on one side. It was understood that this was a case of overpressure but under what circumstances can the PR relief valve fail?

Please advice.

[Qalander (Chem)]

Dear keverne Hello/good afternoon;

the pictures depict buit-up of higher than design pressure internally.
That induced the Roof-Shell sigle fillet weld to fail if this storage tank was designed as per API 650 code guidance.
You mentioned explosion,was this this seam shearing or explosion?More explaination would be useful.

Additionally the tank's venting system(the bleeder plus PVRV) not clear to me if it could cater the inflow quantum,
this indeed could creat a residual pressure multipied to roof surface area
which creates huge force resulting with what is apparent (even with few inches of water column in excess to design)
Moreover I assume that all functional elements of venting system were checked perfectly OK!
Hope this is helpful
Regards
Qalander

[keverne]

Dear Qalander,

First of all, thank you for the input. In my opinion, I think this was a case of mechanical explosion which rendered such deformation on the storage tank. The operation side has been practising the same manual methods to conduct the tank operation for more than 30 years. As for this tank, it was about 1-2 years in age and this was the first incident which broke down the tank. The PVRV was selected and installed as recommended by the manufacturer in tandem with the tank design as well as requirements; also compliant to API 2000.

So my concern is as to whether this incident occur due to errorneous operation; failed to function safety devices or the tank failure itself.

Kindly advice and thanks again.

[Andrei]

Looking at your pictures seams more like an implosion to me. That vessel failed because of external pressure not internal pressure. I do not think overpressure is involved here, more like the vessel was subject to vacuum. Was your vessel protected against vacuum? Can you track the vessel temperature before the incident? temperature variations can trigger these incidents.
These kind of incidents are much more frequent in the storage facilities than actual explosions. And another thing, explosion means a violent combustion process, that involves a fuel (seams you had it) air/oxygen (plenty of it outside), and an ignition initiator (did you have it?), and of course all of these have to be in close contact.

[Qalander (Chem)]

Dear Qalander,

First of all, thank you for the input. In my opinion, I think this was a case of mechanical explosion which rendered such deformation on the storage tank. The operation side has been practising the same manual methods to conduct the tank operation for more than 30 years. As for this tank, it was about 1-2 years in age and this was the first incident which broke down the tank. The PVRV was selected and installed as recommended by the manufacturer in tandem with the tank design as well as requirements; also compliant to API 2000.

So my concern is as to whether this incident occur due to errorneous operation; failed to function safety devices or the tank failure itself.

Kindly advice and thanks again.

Dear,
May I add that we saw almost identical 100ft dia tank top seem rupture failure; although with lesser severity; During various blending streams inflow.
Material stored was blended fuel oil and equipped with bed coil(150psi steam)
However the malfunction of PVRV could not be ruled out; especially we also had two nos. inverted J-shaped vents along with flame arrestors. Which were probably clogged with particulates during service, thus malfunctioned and contributed in failure..

In your case the pictures show one Inverted J shaped item(not sure what is this exactly)could have failed on similar account.
Best regards
Qalander