NFPA 15 Tank/Vessel Run Down

[DTR1968]

Dear All,

We have a situation where our customer has specified the area of operation for a Storage Tank Deluge System for exposure protection without including the area for rundown i.e. the area of coverage to which the design density is applied is calculated by the area covered by the Spray Pattern of the Nozzles.

My questions are...has anyone else come across this interpretation of the code? and is this a correct interpretation?

We have always applied the design density to the entire surface area of the Tank.

Comments/thoughts would be appreciated.

Thanks

Dave

 

[DavidCR]

I understand that the intention of your client is to locate the nozzles with nozzles everywhere to cover the protected surface and not considering that the rundown water would reduce the quantity of nozzles required.

Spray cooling is more effective cooling than water "courtain" running downcooling, so the idea is to have spray instead of "courtains". The geometry of the protected surface rules. If you are forced to locate nozzles everywhere your client should be aware about the cost of the system could be lower if a reasonable run down effect is taken on acount. In theory, the total flow does not change, but you have to be sure that the sprayed water is delivered uniformely, or you have to considered an extra flow due to run down, wind, etc, and also overlap the nozzle patterns.

For example, for a cilindrical tank, spray is applied over the upper ring plates, and some run down effect is taken on account to define if you will need an extra ring of nozzles on a lower part. If you are strictly followind a "no rundown rule" you would have to locate nozzles rings all over the walls, that is unnecesarily expensive.

 

[DTR1968]

Thanks for your reply. However I think, my original post was not clear.The issue is not the request for complete coverage with overlapping spray patterns from the nozzles. The issue is that our customer does not want to include the run down area in the hydraulic calculation ...this does not appear to be code compliant based on my experience and interpretation of the code.

My experience is that the density is applied over the entire tank surface area (Unless IP Codes are applicable) with the flow rate required to achieve the density, even when applying run down.

Has anyone ever come across this before?

Thanks

 

[chicopee]

What type of a tank is it (horizontal, vertical), how is the tank supported and what is the commodity stored in the tank? The reason that I am asking is because the NFPA has certain requirements on fire protection of tanks containing flammable and combustible liquids.

 

[DavidCR]

As Chicopee sugest more data is needed to understand client intention. As I see it the client intention is the same way as I see it, he wants all area to be multiplicated by the gpm/ft2 needed, all area is thoeretically sprayed and the run down is a waste does not diminish or increase the gpm needed.

The code does not mention a difference in gpm/ft2 for area subect to direct spray and area with run down. All exposed or protected area is included.

 

[DTR1968]

Thanks...to clarify, the Tanks are Fixed Cone Roof, product stored is diesel Fuel Oil used as a stand by fuel for a GE Turbine on a Power Plant in North Africa.

DavidCR the fact is the opposite, the customer does not want the run down area to be calculated in the water demand, only the area that is being directly sprayed by the Nozzles, and to be clear, they have identified the number of rings on the tank and the area covered by the rings, this is how they have calculated the area of operation and minimum flow rate by applying the density by area method.This area is less than the total surface area of the tank.

I believe DavidCR has already confirmed my thoughts/experience in his comment "The code does not mention a difference in gpm/ft2 for area subject to direct spray and area with run down. All exposed or protected area is included."

The reason for the post was to find out if anyone has come across this scenario before,sometimes these odd situations crop up.

Thanks again.

 

[RaffO81]

Dear DTR1968,

I'm facing a very similar problem: same tanks, same product, same continent.

I submitted an official query to NFPA since as a result of comparison between various standards was noted that the application rate suggested by NFPA is much more higher:

NFPA 15: 10,2 l/min/m2
API 2030: 4,1 l/min/m2
IP19: 2 l/min/m2

The paragraph of NFPA 15 related to "vessels" seems to be more relevant to pressure vessels and in general not fully applicable to large storage tanks.

Considering the relatively long time that this large tanks need in order to increase their temperature up to hazardous level under the heat radiation of external fire I think that the application rate of 10,2 is excessive, unless the interpretation of your client is correct.

It shall be also noted that if you are binded to NFPA standards, NFPA 30 does not require for protection systems in case of fuel oil tanks (fluid group II or III) if the installation is according to distances given by the standard itself.

If NFPA will respond to my query I'll give you an update.

For sake of clarity, notwithstanding the possible explanation above, I believe that they will respond that all the protected surface shall be considered and so your interpretation is correct, even if the application rate is so high in my opinion.

Kind regards