The consequence calculations for event outcomes such as fires, explosions and toxic exposure are described in Part 3. For PRDs, failures to open upon demand will likely result in the protected equipment being exposed to significantly higher pressures than during normal operations. This methodology calculates the consequences for each PRD failing to open at sometimes significantly higher overpressure than the normal operating pressure of the equipment.
Table 7.13 shows the expected potential consequences of an overpressure event in a pressure vessel.
Table 7.13 is only provided for a qualitative discussion of the potential risks to equipment due to overpressure and is not intended to indicate any specific event outcome. The methodology accounts for the effects of overpressure on protected equipment by increasing the probability of loss of containment. At an overpressure of 4 times theMAWP, the probability of loss of containment is conservatively assumed to be equal to 1.0, see Section 7.2.4.j).
7.4.2 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, Df, 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 a RBI analysis of the PRD, then assumptions of the damage state of the protected equipment must be made as described in Section 7.2.5.b).
7.4.3 Overpressure Potential for Overpressure Demand Cases
For API RP 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 is 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.
ηmod
, prd l wgt
P
ηupd ,
prd l wgt
P
ηupd
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 7. 3. In many situations, the potential overpressure will approach the burst pressure (assumed to be 4 times theMAWP) 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 dead head 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 piping & 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,Po, 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 to 4 times theMAWP). 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 build-up is limited to the bubble point pressure of the contained fluid at the temperature of the heat/energy source being supplied to the process.
7.4.4 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,Fa, 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.
prd
a prd
total
F A
= A (1.37)
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 (1.38):
, ,
o j a o j
P = F P ⋅ (1.38)
The multiple installation adjustment factor, Fa, 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,FOP, which is used to determine the POFOD in Section 7.2.4.i.
7.4.5 Calculation of Consequence of Failure 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 7.4.1.
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 7.2.5. Secondly, the consequence of failure 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 vapor cloud explosions. 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.
The consequence calculations should be performed in accordance with Part 3 for each of the overpressure demand cases applicable to the PRD and for each piece of equipment that is protected by the PRD. The resultant consequence is Cf jprd, expressed in financial terms, ($/yr).
7.4.6 Calculation Procedure
The following procedure may be used to determine the consequence of a PRD failure to open.
a) STEP 1 – Determine the list of overpressure scenarios applicable to the piece of equipment being protected by the PRD under evaluation. Table 7.2 provides a list of overpressure demand cases specifically covered. Additional guidance on overpressure demand cases and pressure relieving system design is provided in API 521 [11].
b) STEP 2 – For each overpressure demand case, estimate the amount of overpressure,Po j, , likely to occur during the overpressure event if the PRD were to fail to open. Section 7.4.3 and Table 7. 3 provide guidance in this area.
c) STEP 3 – For installations that have multiple PRDs, determine the total amount of installed PRD orifice area,Atotalprd , including the area of the PRD being evaluated. Calculate the overpressure adjustment factor,
Fa, in accordance with Equation (1.37).
d) STEP 4 – Reduce the overpressures determined in STEP 3 by the overpressure adjustment factor in accordance with Equation (1.38).
e) STEP 5 – For each overpressure demand case, calculate the financial consequence, Cf jprd, , of loss of containment from the protected equipment using procedures developed in Part 3. Use the overpressures for the demand cases as determined in STEP 4 in lieu of the operating pressure, Ps.
f) STEP 6 – Using the values as determined above, refer to Section 7.6 to calculate the risk.