Tank roof explosion 1

[dipak2]

Dear Freinds/Experts Hello..Every body!!

30 M3 Tank which is used as Catch/Safety Tank in the Plant.The diffret Safety Valve Pop Up lines come in this tanks.Diffrent lines handle Hydrogen Gas.This tank vent around 8" in Diameter is open to atmosphere and Steam Line provided in the Vent.This tank is continously in positive Pressure with Low Pressure Nitrogen.But its roof got exploded.All Safety Valves checked found no pop up condition.No fire observed.What could be probable reason ?

Can you pl. enlighten about this matter or any possible explosive mixture with Hydrogen ?

Thanks in advance.

 

[JoeTank]

You have not provide much info to grasp the problem. Could you elaboarate a bit more on the situation.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[dipak2]

It is the Hydrogenation Reaction.Various Vents from the tanks,From the Reactor Safety valve,Other Equipement in the plant(Downstream of Reactor) which may have Unreacted hydrogen gas its vent lines goes in to this tank.Some of the Equipment in the plant which overflow line directly goes to this catch tank and has maximum possibility to have hydrogen gas is also Nitrogen blanketed i mean having 5-10 Nm3/hr Nitrogen flow.
The catch tank itself is also having Positive Low pressure Nitrogen pressure which pressure remains around 0.5 to 0.9 barg.
The catch tank vent open to Atmosphere and its outlet line Steam injection is provided to nulify Hydrogen in atmosphere(Cold flaring)if at all Safety valve pop ups from Reactor or any other tanks.

My question is how Hydrogen gas explosion may occur even with Nitrogen Blanketed Tanks ?Catch tank roof deteched with heavy noise.

 

[JStephen]

It sounds like rather a complex situation, with which we won't be able to offer much help without a lot of firsthand knowledge and examination.

Are you sure that anything actually exploded? That would be the first question. What I mean is, is it possible that the tank was overpressured, and ruptured the roof, without actually having a chemical reaction take place in the tank?

I have seen a case where a hot-water storage tank ruptured the roof. As best we could tell, the water going into the tank was near boiling. It got a little too hot, and generated large quantities of flash steam, which exceeded the vent capacity. The operators were quite sure that this could not have happened, for what that's worth. Anyway, in that case, it didn't blow the roof off, just buckled the top angle and opened up a hole in the joint.

If something did actually burn in the tank, it will take some detective work to figure out what. If none of the substances in the tank can react with each other, and yet, there's combustion, then either your assumptions about what is going into that tank are faulty, or your assumptions about what can react with what are faulty. Check if any of the products going into that tank can ever lose pressure and suck in air. Check if any can ever get contaminated for one reason or the other. Check if air can diffuse back through that vent even with positive flow out- doesn't seem possible, but that may be a faulty assumption.

 

[dipak2]

Dear Freinds/Experts..!!

Yes,Upper Tank Roof is completely deteched with the Tank Body with Explosive Sound --This is seen by One of our operator.This tank is design with atmospheric +150 mmwc.Tank Shell Body is insulated(Material is CS).At the time of Explosion the upper part of shell insulation is also thrown away.
Inside Inspection done---No shell thickness reduction found.
The Tank has dead level matieral Containing Organic+water+Tary Material as usual since last 8-10 years we operate this tank.No even single incidence happened if Hydrogen Gas enteres in this tank with more flow rate so many times in past also and this time no any major Hydrogen gas entered in this tank from Process Equipments even though Incident happened.Inside Tank has dead level mixture of Materials --material has 9.6 PH,Conducitivity around 1250 microohm/cm.Approx. 50 % water,50 % Organic & Spent Solids contain Pd ,Pt based Catalst slurry.This type of material in this tank remains during Normal Plant Operatoion since last 8-10 years.Reactor Material also drain during shut down but no single incident happened in past 10 years.This time it is doubt that whether in Nitrogen some unforseen material came or not ? otherwise any explosive reaction happened in the tank or not ?Air ingressed from 8" dia vent of the tank even tank is N2 blaneted with Positive N2 flow in N2 rotameter line & 0.5 to 0.94 Nitrogen pressure in the line as per DCS Trend(But since last 10 years we operate the same way).Oxygen PPm in N2 is continously montiored there was no deviation.
Or what could be possible reason ? it is still mistory.

Pl. be enlighten.

Thanks.

 

[EdStainless]

You may have carried some oxygen into the tank from another source. Maybe a chemical with a more stable, lower oxidation state, would provide enough oxygen for an explosion.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[epoisses]

dipak2,

Please... it's absolutely impossible to analyse this situation and even more improbable that anyone here could come up with advice that would make your plant safer. This is not something you can address in a text based forum. Please carry out a rigorous Hazop study based on up-to-date P&ID's, check N2 blanketing failure contingency and check incompatiblities of streams sent to the tank/possible tank contents. Why are you sending H2 to a tank in the first place?

 

[sshep]

Sending a small amount of dissolved H2 to a storage tank has proved literally fatal in a company I worked for via a similar situation to what you described, except that workers were on the roof when the tank "popped". The stream came from a hydrogenation unit operation and was degassed in a low pressure drum, but not totally, and the hazard went unrecognized for years until the fatal day.

It is best that you put your efforts to a well designed degassing of your streams before the tank. Solubility of H2 increases with temp in many fluids and any partial pressure of H2 will cause a pick-up of dissolved gas into the liquid. On the otherhand even a small N2 sparge can bring the H2 down to zilch quickly.

best wishes,
sshep

 

[dipak2]

Dear Freinds/Experts,

The Technology is designed in such a way that there is no any degasifiying equipement prior to gas goes to Catch /Safety tank.But yes,you are right Regorouse Case study & Hazop will be required.

Thanks to all of you for your valuable advises.

1.The tank vent which goes to Atmospheric Vent has not Breather valve/NRV.is it necessary to avoid Air in to come back to Nitrogen Blanketed Tank.

Once Again thank to all of you.

Thanking You,

 

[dipak2]

This tank has one additional Tapping at top of Roof that is for Flame arrestor.Is there any chance to air go back to this tank.

Dear SShep --Can you elarborate your case study ?

Thanking You,
Dipak Shukla

 

[dipak2]

Dear Mr.SSHep I would like to know more about the case study that has happened in your previous indsutry ?

Thanks in Advance !!!

 

[sshep]

dipak2,

I think I can probably get a copy of the findings and refresh myself on what happened. I was not working at the site where the accident happened, but the findings impacted our site because we had to analyze our similar systems and implement recommended changes.

At this time I hate to give out detailed info using my memory of events of approx 5 years ago. Because there was loss of life it could be a sensitive topic for discussion in an open forum. I have temporarily updated my profile with an email address, please use that email to make some off-line contact.

best wishes, Sean