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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-95
c) Use of Blast Curves—To determine the blast effect, the side-on blast wave overpressure can be
calculated using blast curves. An acceptable curve for estimating the overpressure is the Hopkinsonscaled
curved that is reproduced by CCPS [17] . Equation (3.170) is a curve fit of the Hopkinson-scaled
data that provide a closed form solution for determining the side-on overpressure (units are bar):
P
⎛
⎞
⎜
1.1288697 7.9625216
− 0.059965896 + − + ⎟
⎜
2
ln ⎡RHS , n
⎤
( ln ⎡RHS , n
⎤
⎟
⎜
⎣ ⎦ ⎣ ⎦)
⎟
⎜ 25.106738 30.396707 19.399862 ⎟
= C ⋅⎜
− + −⎟
⎜( ln ⎡RHS , n
⎤) ( ln ⎡RHS , n
⎤) ( ln ⎡RHS , n
⎤)
⎟
⎜
⎣ ⎦ ⎣ ⎦ ⎣ ⎦
⎟
⎜ 6.8853477 1.2825511 0.097705789 ⎟
+ −
6 7 8
⎜( ln ⎡⎣RHS , n
⎤⎦) ( ln ⎡⎣RHS , n
⎤⎦) ( ln ⎡⎣RHS , n
⎤⎦)
⎟
⎝
⎠
SO, n 26 3 4 5
(3.170)
For use in Equation (3.171), the Hopkinson-scaled distance, R , presented above requires units of
HS , n
m/kg 1/3 and is calculated from the actual distance from the blast center as follows:
xs
R = C ⋅ (3.171)
W
vce
n
HS , n 27 13
TNT
5.8.5.6 VCE Safe Distance and Consequence Area
The consequence areas for serious injury to personnel and component damage can be determined once the
overpressure as a function of distance from the blast is known. For serious injuries to personnel, the
consequence area can be based on the following probit equation provided by Eisenberg [28] .
Pr =− 23.8 + 2.92⋅ln ⎡C ⋅PSO n
⎤
⎣ 28 , ⎦ (3.172)
This probit equation provides the probability of process building collapse due to structural damage as a result
of overpressure. Data show that personnel can withstand much higher overpressures (15 to 30 psi
overpressure for lung hemorrhage) when out in the open and that typical serious injury occurs as a result of
the collapse of buildings.
For component damage, an overpressure of 34.5 kPa (5.0 psi) has proven to be a good value to use when
evaluating the consequence area to equipment as a result of overpressures from explosions.
Note that the side-on overpressure is a function of the distance from the blast source to the target. An iterative
approach is required to determine the acceptable distance away from the explosion.
This procedure is carried out for personnel injury as well as component damage for each of the release hole
sizes selected as described in Section 4.2. Once the safe distances, vce
vce
xs and
cmd , n
xs , are determined,
inj,
n
Equation (3.173) and Equation (3.174) are used to calculate the VCE consequence areas.
CA
CA
vce
( xs ) 2
cmd n
= π ⋅ (3.173)
vce
f , cmd , n
,
vce
( xs ) 2
inj n
= π ⋅ (3.174)
vce
f , inj, n
,