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

5-58 API RECOMMENDED PRACTICE 581
Protected Equipment Failure Frequency as a Result of Overpressure
Where risk analysis has been completed for equipment components being protected by PRDs, each piece of
protected equipment has a damage adjusted POF calculated as the equipment’s GFF multiplied by a DF,
see Section 6.1 and Equation (5.89). The DF is determined based on the applicable damage mechanisms for
the equipment, the inspection history, and condition of the equipment. The DFs for the protected equipment
are calculated as a function of time. This is very important when evaluating the inspection interval for the
PRD. As the PRD inspection interval is extended, the damage related to the vessel increases as does the
risk associated with the PRD.
a) Damage Factor Calculation Procedure for PRD with Fixed Equipment
The damage adjusted POF are calculated at the normal operating pressure of the equipment and are
adjusted when evaluating PRDs as follows. When a PRD fails to open upon demand, the pressure in
the protected equipment rises above the operating pressure and in many cases, significantly above the
MAWP. The protected equipment damage adjusted POF (P f (t) from Equation (5.89)) is adjusted based
on the calculated overpressure for the overpressure demand case under consideration. The damage
adjusted POF, which is equal to the probability of loss of containment from the protected equipment, at
the overpressure is calculated as follows:
⎛
P o,j ⎞
⎜3.
464837⋅
MAWP
⎟
⎝
⎠
f , j = ⋅ total ⋅ f ⋅ MS ⋅ (5.109)
( 0 0312881
)
P . gff D F e
The above equation is set up so that at normal operating pressure (≤ MAWP), the probability of loss of
containment from the equipment, P f,j , is equal to the damage adjust failure frequency, P f , calculated in
fixed equipment RBI for the protected equipment using Equation (5.89). At elevated overpressures
when the PRD is being evaluated, the probability of loss of containment in the protected equipment
increases. As an upper limit, for an undamaged piece of equipment (D f = 1.0), the probability of loss of
containment will equal 1.0 when the overpressure is equal to the burst pressure, or the failure pressure
of the vessel. The burst pressure of the vessel can be estimated using the design margin times the
MAWP (the design margin of the equipment, also known as safety factors, constructed in accordance
with various codes are shown in Table 6.4). Alternatively, the burst pressure can be more accurately
calculated using a more advanced analysis such as Svensson’s method [11] . For a damaged piece of
equipment (D f 1.0), the probability of loss of containment can reach 1.0 at pressures much lower
than the damaged equipment burst pressure, see Figure 6.6 for further clarification.
The probability of occurrence of any of the four holes sizes (i.e. small leak to rupture) is increased at
elevated overpressures due to the increased probability of loss of containment and may be calculated
as follows:
n ⎛ gff n ⎞
Pf ,j = Pf ,j ⎜
⎝ gff
⎟
total ⎠
(5.110)
See Section 6.2.2 for initial discussion on the discrete hole sizes; Part 2, Table 3.1 for gff n and gff total ;
and Part 3, Table 4.4 for definitions of the hole and actual representative sizes.
b) Selection of DF Class when PRD RBI Is Performed Without Fixed Equipment
If fixed equipment risk analysis has not been performed, then the DFs for the protected equipment that
normally would be calculated for fixed equipment will have to be specified. The DFs may be determined
quantitatively using a DF class as shown in Table 6.11. This method should be considered to be less
quantitative than when an RBI analysis is conducted to determine fixed equipment DFs.