Fuel tanks protection

[hssmiguel]

Hi all,

I'm trying to predesign a fire fighting system for a power plant with a huge amount of fuel to be stored. Providing that the minimum distances specified in NFPA 30 are fulfilled, I understand no cooling water would be needed (only foam according to NFPA 11 5.2.5). But my question is, what about if this minimum distances couldn't be possible?

Let's imagine we have two tanks, one in front of the other. The protection philosphy would be:

- Foam system for the two tanks (roof)
- Cooling water ring for exposure protection (only one half of each tank walls, since the only exposure possibility is the ignition in one of the tanks).
- Foam hydrants around the tanks basin.

Am I right? The aim for me would be to get the demand of water flow (according to NFPA 850, 2 hours with the most unfavourable system plus 500 gpm for hose reel). The key is the quantification of that most unfavourable demand, for what I'm assuming the scenario of the ignition in one of the tanks: cooling water for the non-burning tank + foam for the burning tank + foam for hydrants around the basin.

I'd really appreciate your opinion on this, since I'm quite a beginner about NFPA and fire fighting systems.

Thank you very much

 

[DavidCR]

When you mention that the min. dists. are not OK, there are many things to ask about the particular situation that you don´t mention. You are right in your basic approach for the water consume scenario but that is where other aspects are OK.

Remember the NFPA does not adress any fire protection as a substitution for the lack of security distances. When you are out of the code the fire protection goes to a field where you have no "legal" support for your protection equipment. Dealing with oil storage, only with very sophisticated design tools and strong enginnering resources, it is almost impossible to use NFPA codes to help you to design something particular or go to performance based desing. Sometimes if things cannot be changed you have to make the owner to understand that the fire protection equipment will help but not guarantee that all will be fine with authorities or permits.

You may give a glance to API 2001, API 2021, API 2030 to give you a more general point o view to help you. If you ony use NFPA11, NFPA 850, NFPA15, flows and water capacities to design your foam and water equipment, you may not consider some design important points (for example: aspects like extra capacity for starting up delay and periodic testing of foam equipment, extra firefighting time to use hydrants and monitors, type of foam systems, type of hand water systems, etc., etc.), that only experienced design criteria tell and that are not included in the code minimum requirements.

 

[hssmiguel]

Thank you David, I understand that the fire fighting design is not elementary at all, specially when there are several fuel tanks. I will take a look to API standards (if I'm not wrong, they involve some issues related to the tanks height and so on, but I'm not used to them). What I didn't know is what you tell about flows and water capacities using NFPA standards; I always assumed they were a good approach for this calculations.

On the other hand, what do you mean when saying "that is where other aspects are OK"? My point: when minimum distances (according to NFPA 30 tables) are fulfilled, in theory it is not necessary (or it is anyway??) cooling deluge systems, but only foam for each tank protection; when these minimum distances are not fulfilled, deluge water is more than recommenended and in addtion, its calculation might not be right using only NFPA. Is this OK?

 

[DavidCR]

API2030 is a standard from API with recommendadtions on water spray systems in oil industry. The other two are also for fire protection issues on oli industry and give some points related to fire water use.

Other aspects like: API650 tank design or others, dike geometry, rain water draining, spill oil-water separators, distances, tank farm layout, fire brigade operations considerations, insurance, tank contents, etc. If this aspects are OK then the NFPA 30´s protection requirement tables consodering diastances and diameters apply easily.

Yes cooling systems is a plus but depending on the tank layout and issues like distances and wind direction, etc., spray may be a strong recomendation.

 

[MJCronin]

What liquids do your tanks contain and can you give us a sketch ?

 

[hssmiguel]

Thank you both for answering;

MJCronin, let's imagine we have two tanks containing a liquid class III (diesel with flash point of 70ºC approx.), 30 meters diameter and 16 meters high each one, in a common dike. Let's also consider two general options: fixed roof or floating roof. I couldn't get API standards yet, but I'm asked to fulfill NFPA 30 reccomendations. This way, there's a table in chapter 22 where shell-to-sheel spacing is recommended providing the type of tank and the stored liquid. At the end of this chpater (I don't remember the specific point), a paragraph says something like "if tanks are installed according to all the recommendations given in this chapter, there's no need for protecting it". That is what disturbs me, since in my short experience, I've always seen protected fuel tanks in refineries and so on. Could an appropiated distance between tanks, in addition to remaining recommendations of NFPA 30, avoid protecting them (both cooling spraying and foam). If not, would it be possible following API 2030, as David explains?