Fire Case - Heat Input Rate

Fire can cause overpressure of storage or vessel. Either liquid vaporization of wetted vessel or vapor expansion of unwetted vessel due to heat input will increase the pressure. Heat input rate of fire exposure is not calculable from standard of heat transfer fundamental. Fortunately, OSHA regulations specify standards which are to be followed for particular material in storage vessel and API also provides formulas for calculating heat input rate which are to be followed for particular condition of process vessel.

The following picture presents OSHA Venting Requirement of Fire Exposure to Storage Vessel.

For flammable and combustible liquid, heat input refers to NFPA 30 as follow.

Note :

1. Information in table applies to liquified components ; see original standard for nonliquified gases

2. CGA = Compressed Gas Association
NFPA = National Fire Protection Association
ANSI = American National Standard Association

3.Heat Input expressed in BTU/hr, with area in square feet.

4.See source documents for details of area calculations

My friend, if you want to check the correctness of value in above table, please refers to original reference, Guidelines for Pressure Relief and Effluent Handling System, AMERICAN INSTITUTE OF CHEMICAL ENGINEERS (AICHE), Table 3.3-1 (page 131 of 538). You can find that book in our library.

API STD 521 provides formulas for calculating heat input rate to process vessel containing liquid.
Q = 21.000 FA^0.82

Where adequate drainage and firefighting equipment do not exist, equation below should be used,
Q = 34,500 FA^0.82

Regarding a drainage, as long as the amount of liquid which is spilled out around the vessel is possibly decrease, use the first formula ( = heat flux 21000).

F is environment factor. The value F = 1 for bare vessel. Refer to API STD 521 Table 5 (page 17) for other than bare vessel.
A is wetted area of the vessel.

The value of A ^0.82 reflect that not 100% vessel area exposed by fire. It is true for large vessel, for small vessel, we can use assumption that 100 % vessel area is totally exposed by fire.

For example, for small bare vessel, the equation to be Q = 21.000 A.

My friend, please note, based on the formula, when 82% fraction area is applied the heat flux is 21.000 BTU/ft^1.64/hr or 34500 BTU/ft^1.64/hr. And when 100% fraction area is applied, the unit will be BTU/ft^2/hr. This understanding will useful in perform depressuring simulation using HYSYS.

The above formula is for pool fire case which has heat flux of 21.000 BTU/ft1.64/hr. Whereas, based on API STD 521, the heat flux for jet fire is 95.500 BTU/ft2/hr (average). From the unit of heat flux for jet fire, its show that 100% fraction area is used (for jet fire is localized heat flux).

Hope this drawing helps you get better understanding of pool fire and jet fire

And below is jet fire

My friend, in this posting we discuss about heat input rate of fire exposure for process vessel. The following are the important point,

1. For adequate drainage and fire fighting exist, use formula Q = 21.000 FA^0.82

2. For small vessel, use 100% fraction area is exposed by fire, therefore Q =21.000 FA. In my understanding, vessel with wetted area less than 200 ft2 can be considered small vessel (see NFPA 30)

3. For wetted vessel, overpressure caused by liquid evaporation, the relieving load capacity can be calculated as heat input divided by latent heat of vaporization. (W = Q/Hv).

4. Heat flux of jet fire is very high (95.500 BTU/ft2/hr average), and in localized area.

5. I suggest you to sizing PSV on fire case for pool fire case only.

6. The relieving temperature can be higher than the vessel collapse temperature. Vessel will collapse before increasing pressure reach PSV’s set pressure. Therefore, actually, PSV doesn’t provide sufficient protection. Depressuring is one of the additional protections against fire. (We will discuss about fire case depressuring both for pool and jet fire in other posting)

My friend, thank you for reading, please correct me if I'am Wrong.