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

5-64 API RECOMMENDED PRACTICE 581
Damage State of the Protected Equipment
A direct link to the current condition, or damage state, of the protected equipment is critical to the evaluation
of the consequence of PRD failure. Damage for each protected vessel is measured by a DF, D f , which is
calculated considering each of the damage mechanisms (corrosion, cracking, creep, etc.) that are applicable
to the protected equipment. The higher the overall DF of the protected equipment, the more likely the
equipment is to experience undesirable consequences as a result of a PRD that is in a failed state (stuck)
upon demand. Part 2 of this document provides details on calculation of the DF and the probability of loss of
containment from fixed equipment.
Where damage assessment has not been completed in conjunction with an RBI analysis of the PRD, then
assumptions of the damage state of the protected equipment must be made as described in Section 6.2.5 b).
Overpressure Potential for Overpressure Demand Cases
For API 581 to provide a relative ranking of risk between PRDs, the analysis must include an assessment of
the overpressure demand cases (overpressure scenarios) that are applicable to each PRD. In other words,
what process upsets are the device protecting against and how critical would the effect on the protected
equipment be if the device were to fail to open upon demand.
The PRD methodology makes a clear distinction between criticality of the overpressure demand cases that
the device is protecting against, i.e. why the device is there. For example, a PRD that protects equipment
and piping for the blocked discharge demand case downstream of a pump is considered to be less critical
than a device that is protecting a reactor from a runaway chemical reaction since the amount of overpressure
expected as a result of a PRD failure to open upon demand would be much less. Likewise, a device that is
only protecting piping against thermal relief is much less critical than a device that is protecting low-pressure
equipment from gas breakthrough from a high-pressure source due to control valve failure.
For most of the overpressure demand cases, the potential overpressure that results when a PRD fails to
open upon demand from an overpressure event may be calculated. The logic for determining the potential
overpressure for each of the overpressure demand cases is provided in Table 6.3. In many situations, the
potential overpressure will approach the burst pressure (estimated to be design margin times the MAWP ) of
the protected equipment since the overpressure demand case is not self-limiting. In other overpressure
scenarios, such as a blocked discharge downstream of a centrifugal pump, the potential overpressure will
limit itself to the deadhead pressure of the pump, which is typically 1.3 times the normal discharge pressure
of the pump.
This part of the analysis requires a thorough review of the unit pressure-relief study and piping and
instrumentation diagrams (P&IDs) and should be performed by personnel qualified and experienced in the
design and installation of pressure-relief systems.
In general, the determination of the potential overpressure, P o , as a result of PRD failure to open upon
demand is a function of the following.
a) Type of Upstream Overpressure Source—For example, centrifugal pumps, steam supply headers,
upstream pressure vessels, etc.
b) Upstream Source Pressures—These include the steam supply pressure, control valve upstream
pressure, pressure from the high-pressure side of a heat exchanger, and deadhead pressure for
centrifugal rotating equipment. Additionally, credit for PRDs on upstream equipment can be assumed to
be available to limit overpressure.
c) Heat Sources, Types, and Temperatures—In cases of blocking-in equipment, the heat source supplying
energy to the system has a significant impact on the potential overpressure. For example, solar
heat/energy supplied in a thermal relief scenario will typically result in flange leaks and the overpressure
ends up nominally being the normal operating pressure of the system. On the other hand, if the heat
source is a fired heater, the overpressure can build until a rupture occurs (i.e. overpressure exceeding