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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-65
the burst pressure of the protected equipment). Other heat sources include steam reboilers to towers
and the hot side of heat exchangers.
d) Fluid Bubble Point Pressure—In many overpressure scenarios, the pressure buildup is limited to the
bubble point pressure of the contained fluid at the temperature of the heat/energy source being supplied
to the process.
Multiple Relief Device Installations
When the relief requirements for the process are such that multiple PRDs are needed to handle the required
relief capacity, there is a reduction of risk since the probability that all of the PRDs are in a failed state upon
demand will be reduced. The protected equipment will have a higher probability that some of the PRD
capacity is available on demand to minimize the amount of overpressure during an overpressure demand
case.
When a piece of equipment is protected by multiple PRDs, the calculated POFOD for any one specific PRD
in the multiple device installation will remain the same. However, an adjustment is made to the potential
overpressure as a result of the PRD failing to open on demand. This multiple device installation adjustment,
F a , takes into consideration common cause failures and also considers the likelihood that other PRDs of the
multiple device installation will be available to minimize the potential overpressure.
F
prd
A
a = (5.114)
prd
Atotal
This multiple device installation factor reduces the potential overpressure that is likely to occur by assuming
that some of the installed PRD relief area will be available if the PRD under consideration fails to open upon
demand. The multiple device installation adjustment factor has a minimum reduction value of 0.25. The
presence of the square root takes into consideration that the PRDs in a multiple device installation may have
common failure modes. The reduction in overpressure as a result of multiple PRDs is in accordance with
Equation (5.115):
Po,j = Fa ⋅ Po,j
(5.115)
The multiple installation adjustment factor, F a , is a ratio of the area of a single PRD (being analyzed) to the
overall areas of all PRDs in the multiple setup.
This reduced overpressure should be implemented when determining the protected equipment failure
frequency. However, it should not be considered when determining the overpressure factor, F op , which is
used to determine the POFOD in Section 6.2.4 i.
Calculation of COF to Open
Consequence calculations are performed for each overpressure demand case that is applicable to the PRD.
These consequence calculations are described in Part 3 of this document for each piece of equipment that is
protected by the PRD being evaluated and are performed at higher potential overpressures as described in
Section 6.4.1.
The overpressure for each demand case that may result from a failure of a PRD to open upon demand has
two effects. The probability of loss of containment from the protected equipment can go up significantly as
discussed in Section 6.2.5. Secondly, the COF as a result of the higher overpressures also increases. The
magnitude of the release increases in proportion to the overpressure, thus increasing the consequence of
events such as jet fires, pool fires, and VCEs. Additionally, the amount of explosive energy released as a
result of a vessel rupture increases in proportion to the amount of overpressure. Part 3 provides detail for the
consequences associated with loss of containment from equipment components.