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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-107
tox
i) STEP 9.9—Calculate the probability weighted or final toxic personnel injury consequence area, CA ,
f , inj
using Equation (3.191).
5.10 Determine Nonflammable Nontoxic Consequences
5.10.1 General
Many nonflammable nontoxic fluids will still result in a consequence area caused by loss of containment. These
include steam, acids, and other fluids where the concern is for personnel being sprayed or splashed. Other
nonflammable gases such as air and nitrogen, although not flammable, can have significant consequences if
the equipment ruptures under excessive pressure.
5.10.2 Physical Explosions
5.10.2.1 General
A physical explosion occurs when a pressurized piece of equipment containing a vapor or two-phase fluid
ruptures. Since a physical explosion can only occur after rupture, the consequence area for physical explosions
is equal to zero for all hole sizes except the rupture case. An explosion or blast wave occurs as the contained
energy is released into the atmosphere. A physical explosion can result with ruptures of equipment containing
flammable or nonflammable materials. If the contained fluid is flammable, the pressure wave can be followed
by other events, such as fireballs, pool fires, flash fires, or VCEs, depending on whether or not the release
ignites and whether or not there is an immediate or delayed ignition.
5.10.2.2 TNT Equivalency Method
As with a VCE, a conservative method for calculating the effects of physical explosions is to use the TNT
equivalency method. The energy associated with the rupture of a gas-filled vessel can be estimated using
Equation (3.192), which is provided by Brode [31] and modified here to convert to an equivalent TNT.
⎛P
− P
⎝ k −1
s atm
TNT
=
29
⋅
s
⋅ ⎜ ⎟
W C V
⎞
⎠
(3.192)
5.10.2.3 Physical Explosion Safe Distance and Consequence Area
At this point, the calculation of the consequence area as a result of the release of energy from a gas-filled
vessel rupture is identical to that described earlier for VCEs. The calculation of the blast overpressure uses
blast curves as described in Section 5.8.5.5 c). The calculation of the consequence area is identical to Section
5.8.5.5.
pexp
In general, the procedure results in a safe distance for both component damage, xs
cmd
, and personnel injury,
pexp
xs , from which a consequence area can be calculated per Equation (3.193) and Equation (3.194).
inj
pexp
f , cmd
pexp
( ) 2
cmd
CA = π ⋅ xs for rupture case only
(3.193)
pexp
f , inj
pexp
( ) 2
inj
CA = π ⋅ xs for rupture case only
(3.194)
5.10.3 BLEVEs
5.10.3.1 General