7 replies to this topic

[Seemsata]

Hi,

 

I am new to this comunity and this is my first post on this forum.

 

On my project, there are few atmospheric storage tanks used to store wastewater. There is a possibility of hydrocarbon carryover and the tank is fitted with oil skimming facility. There is no bund provided around the tanks.

 

My question is whether the fire case (internal or external) overpressure scenario should be considered while calculating emegency venting?

 

There is an electric substation located at ~ 30m from the tank. Other couple of tanks located @ 20m separation distance also contain waste water.

 

Regards,

[aroon]

Hi Seemsata,

 

Welcome to this forum!

 

First and foremost check is that you should check whether this area is coming under potential fire zone or not. In many of the projects, these type of zones are already defined. Generally, some waste-water, utility units, non-explosive storage facility, etc are not coming under potential fire zone. However, it is not always true, because sometimes there may be chance of pool fire due to presence of some source for the spillage of explosive liquid. No need to think about source for the ignition, there are plenty of sources available in addition to the electrical spark such as hammering, chipping, some maintenance activities in surrounding areas.

 

If fire scenario is applicable as per above criteria, then you must protect your tank from possible fire. Goose-neck vents, normal open vents, or Hatches should be provided to protect the tank.

 

Note that the frangible type fixed roof tank may not require protection against fire as generally this roof are for emergency venting only and this roof will blow-off prior to over-pressure.

[ankur2061]

Note that the frangible type fixed roof tank may not require protection against fire as generally this roof are for emergency venting only and this roof will blow-off prior to over-pressure.

 

aroon,

 

Irrespective of the theoretical connotations provided in API 2000 or any other document, practically any tank which can be exposed to a pool fire needs to be provided a emergency venting device, be it a relief valve or a blow-off hatch. You do not depend on the frangible roof to blow-off to protect the tank and surroundings from catastrophic damage.

 

There is definitely one aspect of providing emergency venting that is often overlooked which is the maximum emergency venting rate. Annex B of API 2000 provides a very informative description of emergency venting basis. One important point that it mentions is that the maximum emergency venting rate required regardless of the size of the tank was 17,400 m3/h based on their field tests.This makes a lot of sense. For the contents of a very large tank exposed to a fire to reach boiling point would take a very very long time. By logic, this would provide ample time to either extinguish the fire completely or partially and thus reduce the heat input to the tank exposed to fire. So practically, it is possible to define a maximum emergency venting rate irrespective of the size of the tank.

 

I would love to hear views of other members on topic of maximum emergency venting rate.

 

Regards,

Ankur.

[aroon]

Thanks Ankur for correcting on my above quote. My above quote is with respect to service of the tank and since it is in waste water service, which will not cause any catastrophic damage when roof is blown-off.

 

However, generally, the selection of frangible roof tank itself is wrong if the tank type is frangible for the service which can cause catastrophic damage. please correct me if I am wrong.

 

PS: Above quote is with reference to API 2000 Section-4.3.3.2.

[Seemsata]

Dear all,

 

Thanks for your prompt responses.

 

I am adopting following approach to conclude that the fire scenario is not applicable:

1) This is a fixed roof tank provided with insulation and used for wastewater storage;

2) Only a trace level of hydrocarbon carryover is expected hence fire would not sustain for longer duration;

3) The tank is not dyked, so no accumulation of tank content in case of any accidental damange;

4) The wastewater treatment facility is located away from the main plant facility, hence this area is not under fire zone as such

5) The tanks are not located in the low point area where other spillages can get accumulated

6) Although there are potential sources such as skimmed oil storage or switch gear room in the vicinity, they are located at least 30m away from the wastewater tank

 

Hope I am not missing anything here!

[aroon]

 

2) Only a trace level of hydrocarbon carryover is expected hence fire would not sustain for longer duration;

​Trace level or any composition present inside the vessel has nothing to do with fire sustainability. Fire sustainability is based on spillage and accumulation out side the vessel. This just for your information only.

4) The wastewater treatment facility is located away from the main plant facility, hence this area is not under fire zone as such

​Based on this, obviously you can get rid of fire scenario.

Please refer my reply in quote for better clarification.

[fallah]

Seemsata,

 

Your justification appears to be right to ignore the fire scenario for the tank, but there are few concerns as follows to be clarified:

 

-Being oil skimmer inside the tank and skimmed oil storage facility indicates, contrary to your statement, HC carryover would be higher than a trace level.

-If the vapor pressure of the HC would be high enough, the possibility of the HC vapor ignition by the ignition source even at 30 m away from the tank should be investigated.

-The possibility of the explosion inside the tank by static electricity due to falling/splashing the incoming liquid from top of the tank, if any, as an ignition source should be investigated.

[Seemsata]

The points raised by Fallah are are important I suppose and not very easy to rule out

1) Probably you are right to point out the fact that provision of oil skimming and storage means higher than traces carryover can be envisaged;

2) The inlet nozzle is at the bottom of the tank hence static may not a big concern I suppose.

 

In fact I should apologise as I forgot to mention a very important point that the tank is provided with open vent fitted with "end of line" flame arrestor (FA).

Unfortunately due to sever winter conditions (-40 Deg c), FA tends to get blocked becasue of ice formation (in act one tank was lost due to vacuum pulled in when FA was blocked due to ice!). Hence PVSV are being added as an additional safety measure.

However my thinking is that even with blockage FA will protectstank from external source as flame can not travel in to the tank. Hence concern vide point no. 2 can be mitigated. 

 

Regards