Flashover Calculator

About This Tool

Flashover is the transition from a fire in one item to a room in which every exposed combustible surface is burning. A hot gas layer about 500 °C above ambient, or roughly 20 kW/m² of radiation at the floor, is the usual sign. Three hand correlations estimate the heat release rate a room needs to get there. All of them depend on the ventilation factor A_o√H_o, the opening area times the square root of its height, because the air an opening admits sets how much fire the room can hold.

MQH. Solving the McCaffrey, Quintiere and Harkleroad temperature correlation for a 500 K rise gives Q_fo = 610 (h_k A_T A_o√H_o)^1/2 in kW, where A_T is the area of walls, floor and ceiling less the opening and h_k is the wall heat transfer coefficient: k/δ once the lining has heated through (after t_p = (ρc/k)(δ/2)²) and √(kρc/t) before that. Babrauskas. An energy balance with radiation to the walls and test data put flashover at about half the stoichiometric burning rate: Q_fo = 750 A_o√H_o. Thomas. An average of measured losses gives Q_fo = 7.8 A_T + 378 A_o√H_o. The opening itself can feed at most about 1,500 A_o√H_o kW of burning inside the room.

Worked example

A 14 by 12 ft room, 8 ft high, lined with 5/8 in. gypsum board, has one 3 by 7 ft door. The ventilation factor is 55.6 ft^5/2 (2.85 m^5/2) and A_T is 731 ft² (67.9 m²). The board heats through in about 6.5 minutes, so for a 30 minute exposure h_k = k/δ = 10.7 W/m²·K. MQH gives 878 kW, Thomas 1,607 kW and Babrauskas 2,137 kW. A 500 kW design fire is 57 percent of the lowest value, so flashover is not expected, while a fire over about 900 kW would reach the MQH limit. Early in the fire, at 2 minutes, the lining is still cold and absorbs more heat, so MQH rises to 1,669 kW and Thomas becomes the lowest.

Assumptions and limits

The methods treat the room as one well-mixed upper layer with a single opening and a steady fire. They scatter by a factor of about two, so the lowest value is the prudent screening limit. The test data behind them cover A_T/(A_o√H_o) of roughly 9 to 65 m^−1/2 and ventilation factors of 0.03 to 7.5 m^5/2, and the MQH data came from rooms no more than 2.7 m high. Very large or tall rooms, rooms with combustible linings, several openings at different heights and mechanical ventilation are outside these methods. For several openings, one opening with the total area and an area-weighted height is a common approximation.

When to bring in an engineer

Flashover potential drives decisions on sprinkler omissions, interior finish, fire-rated separations and tenability in performance-based designs. DGA provides fire and smoke modeling and code consulting when the hand estimate is close to the design fire or the room is outside the methods.

References

1. NFPA 555, Guide on Methods for Evaluating Potential for Room Flashover, National Fire Protection Association, 2021 edition.
2. McCaffrey, B.J., Quintiere, J.G., and Harkleroad, M.F., “Estimating Room Temperatures and the Likelihood of Flashover Using Fire Test Data Correlations,” Fire Technology, Vol. 17, No. 2, 1981, pp. 98 to 119.
3. Babrauskas, V., “Estimating Room Flashover Potential,” Fire Technology, Vol. 16, No. 2, 1980, pp. 94 to 103.
4. Thomas, P.H., “Testing Products and Materials for Their Contribution to Flashover in Rooms,” Fire and Materials, Vol. 5, No. 3, 1981, pp. 103 to 111.
5. Iqbal, N., and Salley, M.H., Fire Dynamics Tools (FDTs), NUREG-1805, U.S. Nuclear Regulatory Commission, 2004, Chapter 13, and Supplement 1, 2013: flashover methods and lining material properties.