11
2.5 Hazard Detection Definition
T-Squared Estimation
According to Section 2.3 - the possible combustible material types; Section 2.4 -
Detection time definition; we can calculate what’s the smallest or minimum scale of fire
hazard that VFDS™ is able to detect. In the NPFA 92B, it tells us the heat release rate
in Ignition Period is rather low, though the heat release rate increases by time. VFDS™
designer could use parabolic curve equation (Q =
α
(t−t
0
)
n
, Heskestad, 1984) as an ideal
growth model of fire hazard. It is to say, “n=2” in the equation. The different fire
ignition will have different fire hazard growth time presenting as Q =
α
t
g
2
. This
equation means when the fire ignition growth and reach a critical point, the heat release
rate will be the direct proportion of the square of time.
Q =
α
t
g
2
Q
：
Heat release rate of fire ignition
（
kW
）
α
：
Growth rate of fire ignition
（
kW/s
2
）
-> Refer to Table 2.5-1
t
g
：
Effective ignition time
（
s
）
; the alarm trigger time
Table 2.5-1 T-Squared Fires Growth Rate Coefficient
Item description
Growth rate
α
(kW/s
2
)
(Slow)
0.002931
(Medium)
0.01127
(Fast)
0.04689
(Ultra Fast)
0.1878
The other calculation reference on hazard definition
Beside T-Squared method, VFDS™ designer could refer to the report by NFPA 2008, Video
Image Detection System Installation Performance Criteria Research Project in VID
(Video
Image Detection). It recommends the detection definition on the suitable and the smallest fire
ignition. The reports defines different environments such as warehouse, large factory scale
(ex: chemical and electrical factory), and high-ceiling courtyard. Please see below: Table
2.5-2 NFPA VID research on definition of fire ignition size
Item description
Scale of fire hazard
under flame detection
Scale of fire hazard
under smoke detection
Warehouse
300kW
20~100kW
Large Scale Factor
(chemical & electrical
factories)
100kW
25~100kW
high-ceiling courtyard
100kW
100kW
