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

5-46 API RECOMMENDED PRACTICE 581
⎯ leakage past device (LPD),
⎯ spurious or premature opening (SPO),
⎯ device stuck open (DSO).
The FAIL modes are the primary concern since their failure results in the potential of overpressuring
protected equipment and loss of containment. Included in this case is the instance when a device only opens
partially (DPO), where consequences are not as severe, but equipment may be overpressured nonetheless.
API 581 conservatively considers any data indicating a partially open device as a failure to open as
discussed in Section 5.2.2.
A device OASP is included in the FAIL failure mode. The POF curves are based on bench test data where a
failure is defined as any test requiring a pressure greater than 1.3 times the set pressure. A value that opens
above set pressure during a test but does not exceed 1.3 times the set pressure is considered a successful
test and is not included in the FAIL case.
Consequences associated with the FAIL failure mode include the effects of fires and explosions on
personnel and equipment and the exposure of personnel to toxic fluids as a result of loss of containment.
These consequences and their effect on personnel and equipment are further described in Part 3 of this
document.
A secondary concern is a failure of the PRD to contain the process while operating at normal conditions. The
API 581 methodology groups the remaining three failure modes together into the LEAK category. LPD, SPO,
and DSO are failures that will not result in overpressure or loss of containment from the protected equipment
but will result in potential unacceptable leakage from the process system. The consequences of leakage
through a PRD can range from a minor nuisance, causing some loss of product, to a more severe
consequence resulting in a process shutdown to repair or replace the PRD. If the PRD discharges to the
atmosphere, additional consequences may be environmental impact and potential for fires, explosions, and
toxic exposure.
Use of Weibull Curves
It is necessary to express the POF to open and the probability of leakage as a function of time for risk-based
planning of inspections and tests. Weibull functions are suitable for this task with the added advantage that
they may be used to evaluate large populations of data points to seek trends. In the absence of large sets of
failure data, the functions are still useful as a starting point since the parameters involved describe both the
manner of failure and the time to failure.
Using a two-parameter Weibull distribution [4] , the cumulative failure density function, F(t), sometimes
referred to as Unreliability, is expressed in Equation (5.90) as shown in Section 5.1.3.
The Weibull η parameter or characteristic life is equivalent to the MTTF when the Weibull β parameter is
equal to 1.0. Throughout this document, discussions are made related to the adjustment of the Weibull η
parameter. Adjustments are made to the η parameter to increase or decrease the POFOD and leakage
either as a result of environmental factors, PRD types, or as a result of actual inspection data for a particular
PRD. These adjustments may be viewed as an adjustment to the MTTF for the PRD.The assumption used to
determine the default Weibull parameters is that PRDs in similar services will have a similar POFOD, P fod ,
and similar probability of leakage, P l . Therefore, industry failure rate data may be used as a basis for
establishing the initial (or default) probabilities of failure for a specific device. The POFOD of the specific
device is related to identifiable process and installation conditions. Such conditions may include process
temperature, process corrosivity, and the tendency of the process to foul, polymerize, or otherwise block the
PRD inlet or prevent the PRD from reseating during operation. Also associated with failure are conditions
such as rough handling during transportation and installation and excessive piping vibration. Increased
demand rates and improper installations that result in chatter may also increase the POFOD and leakage.