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

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5-50 API RECOMMENDED PRACTICE 581The initiating event frequencies for any or all of the overpressure demand cases as shown in Table 5.2are default values that may not be applicable in all situations. Owner–users may have operatingexperience with a particular process system that may warrant using other event frequencies.Additionally, a PRD that protects multiple pieces of equipment may legitimately see an increaseddemand for a particular overpressure scenario. For example, a PRD located on a crude distillation towermay also protect the desalted preheat exchanger train. Since the PRD protects equipmentencompassing a much greater area of the unit, an increase in the event frequency for the fire case maybe appropriate. In general, where a PRD protects multiple pieces of equipment, the initiating eventfrequencies should be evaluated to determine if an increase is justified.PRD POF on DemandThe next step is to obtain the probability that the PRD will fail to open upon demand in service.a) GeneralAPI 581 provides default failure on demand failure rates developed from industry data. These defaultvalues are expressed as default Weibull curves that are modified by several factors based on thefollowing procedure.1) STEP 1.1—Determine default Weibull parameters, β and η def , based on category of service severity(Section 6.2.4 b), selection of the default POFOD curve (Section 6.2.4 c), and type of PRD(Sections 6.2.4 d through 6.2.4 f).STEP 1.2—Apply an adjustment factor, F c , for conventional valves discharging to closed system or to flare(Section 6.2.4 g).STEP 1.3—Apply an adjustment factor, F env , for environmental factors (Section 6.2.4 h).STEP 1.4—The result of the procedure outlined above will be a modified characteristic life, η mod , as definedin Equation (5.92).η= F ⋅F⋅ η(5.92)mod c env defSTEP 1.5—At this point, the modified characteristic life, η mod , needs to be updated to the updatedcharacteristic life, η upd , based on the PRD’s specific inspection and testing history (Section 5.2.4 i).STEP 1.6—This updated characteristic life, η upd , is then used to calculate the POFOD as a function of time, t,for the specific PRD in accordance with Equation (5.93).Pfod⎡β⎛ t ⎞ ⎤= 1−exp⎢ ⎥⎢−⎜ ⎟⎝η⎥upd⎢⎠⎣ ⎥⎦(5.93)STEP 1.7—The POFOD should be adjusted based on the overpressure scenario with Equation (5.94). Theoverpressure factor, F OP,j , is an adjustment for overpressure scenarios higher than 1.3 times the setpressure (Section 6.2.4 j). The subscript j identifies the specific overpressure and accounts for the fact thateach has a different potential overpressure.Pfod , j = Pfod ⋅ FOP, j(5.94)

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-51b) Categories of Service SeverityThe failure rates of PRDs are directly related to the severity of service in which they are installed.Different categories of service are established in the PRD module as a function of the fluid tendency toinduce PRD failure due to corrosion, fouling, plugging, or other effects. Temperature has also been foundto be a factor in determining the severity of service. The categories of service severity (MILD,MODERATE, or SEVERE) are linked to specific failure tendencies (and default Weibull cumulative failuredistribution curves) and are described in Table 6.5.It is important to note that a fluid that is classified as being a MILD service group for the fail to openfailure mode is not necessarily a MILD service for the leakage failure mode. As an example, industryfailure data show that cooling water, which is known to be a dirty/scaling service, has one of the highestfailure rates for the fail to open case and therefore may be classified as SEVERE for the FAIL case.Conversely, PRDs in cooling water service have not demonstrated a significant amount of leakagefailures and therefore may be classified as MILD for the leak case. Another example is steam, whereindustry data indicate that steam should be classified as MILD for the fail to open case but classified asSEVERE for the leak case. Steam is known to be a leaking service due to the erosive nature of the hightemperaturesteam.c) Default POFOD vs Time in Service1) GeneralTable 6.6 provides the default Weibull parameters for failure to open for conventional spring-loadedpressure-relief valves (PRVs), balanced bellows PRVs, pilot-operated PRVs, and rupture disks.These parameters were determined using industry failure rate data. The majority of the availabledata indicated successful performance during the interval that the PRD was in service. Thesuccessful test points are referred to as suspensions and were included with the failure data indetermination of the Weibull parameters.Weibull parameters are provided for the three categories of PRD service severity—MILD,MODERATE, and SEVERE—as discussed in Section 6.2.4 b. These values, when substituted intothe Weibull cumulative failure density function, F(t), given by Equation (5.90), provide the defaultPOFOD curves for each of the PRD types listed in the table.For example, Figure 6.2 provides the default Weibull cumulative failure distribution curves used forspring-loaded conventional PRVs using the Weibull function to describe the three categories ofservice severity.Note that the units for the POFOD data presented in Figure 6.2 are failures/demand as these datawere established from bench tests of actual PRDs, not from continuous service data. POFODshould not be confused with POF (failures per year) that includes the demands on the PRD (seeSection 6.2.3) and the probability that the protected equipment will fail (see Section 6.2.5).The cumulative failure distribution curves shown in Figure 6.2 and the Weibull parameterspresented in Figure 6.6 are the default values based on the category of service severity of the PRDbeing evaluated. These base values are defaults and should be overridden if the owner–userprovides site-specific data as explained in Section 6.2.4 c3.Presence of an Upstream Rupture DiskRupture disks are often installed in combination with PRVs to isolate the valve from processconditions and corrosive or fouling fluids that can reduce the probability that the valve will openupon demand. API 520, Parts 1 and 2 provide additional information related to the use andinstallation of rupture disks upstream of PRVs.

5-50 API RECOMMENDED PRACTICE 581

The initiating event frequencies for any or all of the overpressure demand cases as shown in Table 5.2

are default values that may not be applicable in all situations. Owner–users may have operating

experience with a particular process system that may warrant using other event frequencies.

Additionally, a PRD that protects multiple pieces of equipment may legitimately see an increased

demand for a particular overpressure scenario. For example, a PRD located on a crude distillation tower

may also protect the desalted preheat exchanger train. Since the PRD protects equipment

encompassing a much greater area of the unit, an increase in the event frequency for the fire case may

be appropriate. In general, where a PRD protects multiple pieces of equipment, the initiating event

frequencies should be evaluated to determine if an increase is justified.

PRD POF on Demand

The next step is to obtain the probability that the PRD will fail to open upon demand in service.

a) General

API 581 provides default failure on demand failure rates developed from industry data. These default

values are expressed as default Weibull curves that are modified by several factors based on the

following procedure.

1) STEP 1.1—Determine default Weibull parameters, β and η def , based on category of service severity

(Section 6.2.4 b), selection of the default POFOD curve (Section 6.2.4 c), and type of PRD

(Sections 6.2.4 d through 6.2.4 f).

STEP 1.2—Apply an adjustment factor, F c , for conventional valves discharging to closed system or to flare

(Section 6.2.4 g).

STEP 1.3—Apply an adjustment factor, F env , for environmental factors (Section 6.2.4 h).

STEP 1.4—The result of the procedure outlined above will be a modified characteristic life, η mod , as defined

in Equation (5.92).

η

= F ⋅F

⋅ η

(5.92)

mod c env def

STEP 1.5—At this point, the modified characteristic life, η mod , needs to be updated to the updated

characteristic life, η upd , based on the PRD’s specific inspection and testing history (Section 5.2.4 i).

STEP 1.6—This updated characteristic life, η upd , is then used to calculate the POFOD as a function of time, t,

for the specific PRD in accordance with Equation (5.93).

P

fod

⎡

β

⎛ t ⎞ ⎤

= 1−exp

⎢ ⎥

⎢

−⎜ ⎟

⎝η

⎥

upd

⎢

⎠

⎣ ⎥⎦

(5.93)

STEP 1.7—The POFOD should be adjusted based on the overpressure scenario with Equation (5.94). The

overpressure factor, F OP,j , is an adjustment for overpressure scenarios higher than 1.3 times the set

pressure (Section 6.2.4 j). The subscript j identifies the specific overpressure and accounts for the fact that

each has a different potential overpressure.

Pfod , j = Pfod ⋅ FOP, j

(5.94)

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