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

3-30 API RECOMMENDED PRACTICE 581
tox
2) Ammonia and Chlorine—Calculate CA
inj,
n
using Equation (3.65) for a continuous release or Equation
(3.66) for an instantaneous release. The constants used in these equations are from Table 4.12.
tox
3) For Toxic Fluids Listed in Section 4.9.8—Calculate CA
inj,
n
using Equation (3.65) for continuous and
instantaneous releases (using 3 minute release for instantaneous releases). The constants used in
these equations are from Table 4.13.
e) STEP 9.5—If there are additional toxic components in the released fluid mixture, STEPs 9.2 through 9.4
should be repeated for each toxic component.
f) STEP 9.6—Determine the final toxic consequence areas for personnel injury in accordance with
Equation (3.68).
4.10 Determine Nonflammable, Nontoxic Consequence
4.10.1 General
Consequences associated with the release of nonflammable, nontoxic materials are not as severe as with
other materials; however, they can still result in serious injury to personnel and damage to equipment.
4.10.2 Consequence of Steam Leaks
Steam represents a hazard to personnel who are exposed to it at high temperatures. Steam leaks do not result
in a component damage consequence. In general, steam is at 100 °C (212 °F) immediately after exiting a hole
in an equipment item. Within a few feet, depending upon its pressure, steam will begin to mix with air, cool and
condense. At a concentration of about 20 %, the steam/air mixture cools to about 60 °C (140 °F). The approach
used here is to assume that injury occurs above 60 °C (140 °F). This temperature was selected as the
threshold for injury to personnel, as this is the temperature above which OSHA requires that hot surfaces be
insulated to protect against personnel burns. This recommended practice assumes that injury occurs as a
result of a 5 second exposure [2] to temperatures of 60 °C (140 °F).
To determine an equation for the consequence area of a continuous release of steam, four release cases
(0.25 in., 1 in., 4 in., and 16 in.) were run through atmospheric dispersion software for varying steam pressures.
A plot of the release rate vs the area covered by a 20 % concentration of steam shows a linear relationship in
accordance with Equation (3.69).
CA = C ⋅ rate
(3.69)
CONT
inj, n 9
n
For instantaneous release cases, four masses of steam were modeled: 4.5 kg, 45.4 kg, 454.0 kg, and
4,540 kg (10 lb, 100 lb, 1,000 lb, and 10,000 lb), and the relationship between release mass and consequence
area to 20 % concentration was found to be in accordance with Equation (3.70).
INST
inj, n 10
( ) 0.6384
CA = C mass
(3.70)
n
For nonflammable releases of steam, the continuous/instantaneous blending of results should be performed
as described in Section 4.8.5. The blending factor, fact , for steam leaks is calculated using Equation (3.71).
IC
n
fact
IC
n
⎡⎧rate
⎫ ⎤
n
= min ⎢⎨
⎬, 1.0⎥
⎢⎣⎩
C5
⎭ ⎥⎦
(3.71)