API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

3-90 API RECOMMENDED PRACTICE 581
5.8.3 Jet Fires
5.8.3.1 General
Jet fires occur as a result of immediate ignition of a flammable fluid from a pressurized process vessel or pipe
that develops a hole. Jet fires do not occur as a result of an immediate ignition from a loss of containment due
to a rupture. See Figure 5.4. Similar to pool fires, the main deleterious effect is the heat flux produced by the
jet fire.
One method for calculating the consequences from a jet fire is provided in CCPS [17] . The method involves
calculating the flame length of the jet fire and the radiative heat flux at distances away from the jet source. The
distance at which the calculated thermal radiation from the jet fire equals the thermal radiation limit specified
by the risk analyst [limit is 12.6 kW/m 2 (4,000 Btu/hr-ft 2 ) for personnel and 37.8 kW/m 2
(12,000 Btu/hr-ft 2 ) for equipment] provides the radius for the consequence area.
A conservative assumption is made that the jet fire arises vertically at a point located at grade since this will
provide the largest effected area that exceeds the thermal radiation limit.
5.8.3.2 Jet Fire Radiated Energy
The amount of energy radiated by the jet (often referred to as surface emitted heat flux) is a fraction of the total
combustion power of the flame. The fraction of the total combustion power that is radiated, β , is often quoted
in the range of 0.15 to 0.35. A conservative value of 0.35 can be chosen. Therefore:
Qrad = C ⋅β
⋅W ⋅ HC
(3.152)
jet
jet
n 14 n v
For mixtures, the heat of combustion can be evaluated using a mole weighted average of the individual
component heats of combustion.
5.8.3.3 Jet Fire Safe Distance and Consequence Area
The amount of the radiated energy that actually reaches a target at some location away from the jet fire is a
function of the atmospheric conditions as well as the radiation view factor between the source and the target.
The received thermal flux can be calculated as follows:
Ith = τ ⋅Qrad ⋅ Fp
(3.153)
jet
jet
n atm,
n n n
If a point source model is used, then the radiation view factor between the source flame and the target can be
approximated as follows:
Fp
n
1
=
4π
⋅ xs
2
n
(3.154)
The point source view factor provides a reasonable estimate of received flux at distances far from the flame.
More rigorous formulas that are based on specific flame shapes (e.g. cylinders; see Equation (3.142)] or that
assume a solid plume radiation model may be used as alternatives to the simplified calculation shown above.
Note that the atmospheric transmissivity and the point source view factor are functions of the distance from
the flame source to the target. An iterative approach is required to determine the acceptable distance away
from the jet fire and the resultant consequence area.