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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-85
Table 6.5—Categories of PRD Service Severity (Fail Case Only)
PRD
Service
Severity
Characteris
tic MTTF
Characteristic of
Failure
Expected Stream
Characterization
Typical
Temperature
Examples of Service
Mild
Failure is
characterize
d by a long
(25 years)
MTTF
Failure is strongly
characterized as a
“wear out” type of
failure, in which the
failure occurs due to an
accumulation of damage
over a long period of
time
— Clean hydrocarbon
products at moderate
temperature
— No aqueous phase
present
— Low in sulfur and
chlorides
Low
temperature,
always
<< 500 °F
Examples include:
product hydrocarbon
streams (including
lubricating oils), liquefied
petroleum gas (LPG,
BFW, low-pressure
steam, and clean gasses
such as nitrogen and air
— Hydrocarbons that may
contain some
Moderate
Failure
occurs at an
average (15
years) MTTF
Failure is weakly
characterized as a
“wear out” type of
failure, in which the
failure occurs due to an
accumulation of damage
over a long period of
time
particulate matter
— A separate aqueous
phase may be present,
but is a minor
component
— Clean, filtered, and
treated water may be
included in this category
— Some sulfur or
Up to
500 °F
(may exist)
Examples include:
intermediate hydrocarbon
streams, in-service lube
and seal oils, process
water (NOT cooling water
or boiler feed water), and
medium- to high-pressure
steam
chlorides may be
present
— High-temperature
hydrocarbon streams
with significant
Failure is characterized
tendency to foul.
Severe
Failure is
characterize
d as a
relatively
short (7
years) MTTF
as a “random” type of
failure, in which the
failure can occur due to
a variety of mechanisms
(such as corrosion or
plugging)
— Sulfur and chloride
concentrations may be
high
— Monomers processed at
any temperature that
can polymerize are in
this group as well
> 500 °F
Examples include: heavy
hydrocarbon streams
such as crude, amine
services, cooling water,
corrosive liquids and
vapors, and streams
containing H2S
— Sometimes included are
aqueous solutions of
process water, including
cooling water
NOTE 1
NOTE 2
MTTF does not reflect replacement history, where the history indicates a renewal of the asset without a failure noted.
Refer to Table 5.11 for the categories for the LEAK case.