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

3-86 API RECOMMENDED PRACTICE 581
5.8.2 Pool Fires
5.8.2.1 Overview
When a flammable liquid is released from a piece of equipment or pipeline, a liquid pool may form. As the pool
forms, some of the liquid will evaporate and, if the vaporizing flammable materials find an ignition source while
it is above its LFL, a pool fire can occur. Pool fires are considered to occur as a result of immediate ignition of
a flammable liquid from a pressurized process vessel or pipe that develops a hole or ruptures.
Important characteristics of pool fires include its burning velocity, rate of heat release, flame height, flame
plume deflection, and radiative heat flux. To model a pool fire correctly, necessary data for the calculations
include the extent of the pool surface, the physical properties of the burning fluid, the physical and thermal
properties of the substrate, and the ambient conditions.
A method for calculating the consequences associated with a pool fire is provided by CCPS [17] . This method
entails calculating the burning rate off the surface of the pool that is a function of the pool area and the heat of
combustion, the latent heat of vaporization, and the specific heat of the flammable liquid. The maximum size
of the pool is determined at that point where the burning rate off the surface of the pool is equal to the release
rate calculated through the hole or rupture from the protected piece of equipment (see Section 5.8.2.3).
The consequence area is estimated by considering the potential for personnel injury and component damage
due to the effects of exposure to thermal radiation in the vicinity of the fire.
5.8.2.2 Pool Fire Burning Rate
The burning rate off of the surface of a pool fire is the rate at which the flammable material is evaporated during
the fire is given in TNO [18] and can be determined using the following equations.
For non-boiling pools:
C
⋅ HC
(3.128)
17 l
mb
= Cp
l ( T
b
− T
atm ) +∆ H
v
For boiling pools, such as cryogenic liquids or LPGs:
C
⋅ HC
17 l
b
=
(3.129)
∆Hv
m
Note that for liquid mixtures (such as gasoline), the burning rate can be approximated by calculating the burning
rate for each component in the mixture, m , and summing as follows:
bi ,
N
∑
m = frac ⋅m (3.130)
b mole, i b,
i
i=
1
5.8.2.3 Pool Fire Size
The ultimate size of the pool fire is then determined to be the size where the liquid portion (rainout) of the release
rate from the pressurized system, W , is equal to the burning rate off the surface of the pool, m
b
, or:
pool
n