API 2000 Emergency Venting for Water Storage

[pbc825]

I'm designing an API 650 tank and the governing venting requirements are from API 2000. There are a number of venting requirements from API 2000 (liquid from pumps, thermal, and emergency), but the one I have a question about it emergency venting. If the contents of the tank are not hydrocarbons (i.e. water for example), would the emergency venting still apply? I'm a mech (BE) and structural (ME), so my chemistry classes are a distant memory, but my gut feeling is telling me a tank full of water subject to fire would expire vapor at a lesser rate than a tank full of gasoline subject to the same fire. Am I correct in that assumption? If so, how do I "factor down" the volumetric flow rate to account for the less volatile contents?

Many thanks for any responses.
PC

 

[bimr]

Water tanks do not have emergency venting scenarios. The vent size should be based on the maximum exiting flow volume.

 

[don1980]

All API tanks need an in-breathing and out-breathing vent device for normal breathing (thermal, pump-in/out) but it's up to the user to determine whether an emergency relief vent (ERV) is needed. For tank located in the US, an ERV is legally required (OSHA 1910.106) for tanks containing liquids with a flash point of < 200 degF. To my knowledge, there are no other countries that have a similar legal requirement - this decision is left to the discretion of the user.

In petro-chem plants it's common, although not legally required, to protect tanks containing non-flammables from fire if that tank is located within the fire zone of another tank that does contain flammables. For example, if a water tank is near a tank containing gasoline (or within the same dike area), then that water tank is clearly at risk of fire exposure. OSHA doesn't require one to protect that water tank from fire, but many companies chose to do so for obvious reasons.

In response to your question about the chemistry difference between water and gasoline, it's true that a given amount of heat will vaporize more gasoline than water. That's simply due to the fact that water has such a high heat of vaporization, as compared to gasoline and other hydrocarbons. However, that physical property difference is irrelevant to the question of whether or not a water tank should be protected from fire exposure. Once the liquid in an API tank starts to boil, the tank is going to fail if it is not equipped with a sufficiently sized vent device.

 

[bimr]

don1980 brings up some additional issues. A water tank may require an emergency vent if located in a dike area with flammable storage. However, this would appear to be unusual scenario as a) water storage tanks are not typically installed within dikes; and b) since water storage tanks may be used for fire water storage supply, the water storage would not typically be located in the vicinity of areas at risk from fires.

Some additional comments:

If the API-650 tank has a frangible roof, then emergency venting is not required by the code.

API-650 is a frequently used code for non-specific water storage tanks.

NFPA 30 section 22.7.1 also has an emergency venting exception:

Emergency venting for fire exposure is not required if the tank is larger than 12,000 gallons US and flash point is not less than 200 Deg F; and
tank is not exposed to other liquids with flash point <140 Deg F.

If your question has not been addressed, please respond with some further details on your application. The Water Storage Tank forum is also a good source for information.

 

[pbc825]

Thanks to all for your replies. I’m better informed now on tank venting than ever. One follow up question.

Let’s say there’s a scenario where a water storage tank is completely engulfed in flames from an external fuel source. Water was evaporating from the interior and expiring from a vent. Would the water storage tank be expiring gas at the same rate as a tank filled with gasoline in the same scenario? If not, what is the reduction factor for volumetric flow rate?

 

[bimr]

Evaporation of water requires a substantial amount of energy due to the high heat of vaporization of water.

Water -- Latent heat of Vaporization 2259.197 KJ/kg . K = 539.6 BTU/lb . F

Gasoline -- Latent heat of Vaporization About 586-628 KJ/kg . K = 140 -150 BTU/lb . F depending on fuel blend

Based on the heat of vaporization and for the same amount of applied heat, approximately 3.7 times more gasoline would vaporize.

Gasoline has an initial boiling point at atmospheric pressure of about 35 °C (95 °F) and a final boiling point of about 200 °C (395 °F). Water boils at 100 °C (212 °F).

 

[MJCronin]

pbc825....

You state:

a water storage tank is completely engulfed in flames

I do not believe that this is even remotely credible....

Even when detailed fire protection studies are performed, there has to be a source for the fuel and the flames are only postulated to go so high.

How big is your water tank ? Is it larger than the 12,000 gallon limitation, as stated above ????

MJCronin
Sr. Process Engineer

 

[pbc825]

Fair point MJCronin.

That was more a hypothetical situation for other tanks that I work with. This specific situation is a baby tank. It’s about 5m tall and 3m diameter so about 9,000 US gallons. Sometimes the tanks I work on are not purely water but a non-hydrocarbon storage tank.

You’re correct on the flame height. It’s between 9 and 10m (I think it’s 30’) in API 2000 for defining the wetted area. Taller tanks don’t consider wetted area above this height.