By Peter Morgan
HAZOP identifies severity levels for accident outcomes (typically four of five).
5
Safety: three or added fatalities or abiding disabling injuries (PDIs)
Environmental: above impact, authoritative the civic news
Economic: losses greater than $10 million
4
Safety: one or two fatalities or PDIs, or austere abrasion to three or added people
Environmental: ample impact, authoritative the bounded news
Economic: losses amid $1 actor and $10 million
3
Safety: austere abrasion to one or two people, or accessory abrasion to three or more
Environmental: abstinent impact, charge be appear to ecology agency
Economic: losses amid $100,000 and $1 million
2
Safety: accessory abrasion to no added than two people. First aid.
Environmental: accessory impact
Economic: losses beneath than $100,000
1
Safety: no adverse bloom effects
Environmental: no apparent impact
Economic: negligible bread-and-butter impact
5
The accident has happened several times at the plant. Likelihood is added than already in 100 years.
4
The accident has occurred at the bulb and frequently in industry. Likelihood is amid already in 100 years and already in 1,000 years.
3
Incident has occurred at the plant, but is not accepted in industry. Likelihood is amid already in 1,000 years and already in 10,000 years.
2
Incident has occurred in industry. Likelihood is amid already in 10,000 years and already in 100,000 years.
1
The accident has a alien adventitious of accident and is exceptional of in industry. Likelihood is beneath than already in 100,000 years.
The HAZOP additionally identifies the abundance or likelihood of contest alignment from 1 in 100 years to 1 in 100,000 years (i.e., not adequate to anytime occur).
It is anon accessible that this ample assay of the abundance of admission contest does not accommodate the attention all-important to appraise the appeal on a assurance system. This, however, charge not backbite from the account of the HAZOP as continued as absolute accident frequencies (if they are accepted at the time of the HAZOP) are recorded in the LOPA.
The HAZOP activity provides an appraisal of the aftereffect of alone contest and their acknowledgment through absolute safeguards to actuate whether or not a architecture change or added layers of aegis are required. Objectively, back the bulb buyer establishes a ambition accident of, for example, 10-4 per year for contest of severity 4, this is the ambition accident for all contest combined. For example, for a burner administration arrangement (BMS) on a boiler, abortion of a feedwater valve or accident of agitation air are appropriate to shut off the ammunition supply. In one case, abortion of the BMS to act on appeal could account boiler or agent accident and, in the added case, a boiler explosion. The HAZOP activity treats these as absolutely abstracted events, but both actualize demands for activity from the BMS. The SIL adding cannot be done until the HAZOP has been completed, and all contest in this severity class accept been articular and assessed. It is important to agenda that although contest of a accurate severity may be mitigated by absolute safeguards (including the operator) so that the balance accident appears adequate afterwards added mitigation, if contest of this severity abode a appeal on the SIS, again the accident charge be included in the LOPA and consecutive SIL calculation.
HAZOP worksheet
The archetype HAZOP worksheet shows aloof two contest to authenticate the affiliation of the HAZOP activity and the LOPA. Agenda that it is not aberrant to accept to accede thirty or added contest as demands on a accurate SIS (e.g., in the case of a BMS).
The qualitative appraisal of the balance accident for these contest indicates that added aegis is appropriate in one case but not necessarily the other. However, the appraisal acknowledges that the accident will be added bargain for both contest by benumbed the boiler on apprehension of a aerial boom akin through the activity of an added band of aegis (i.e., a BMS in this case). Adding a cavalcade to the adequate HAZOP worksheet is a way to banderole that these contest can be added mitigated by a SIS and that the contest are to be included in the SIL calculation.
Note that a HAZOP assay agitated out to authorize the assurance requirements for a backup assurance arrangement cannot accommodate the absolute arrangement in assessing the demands placed on the backup system. This may be obvious, but it is a allurement calmly collapsed into by those absorbed in the accustomed operation of the bulb with all installed systems available.
LOPA worksheet
The LOPA worksheet uses account advertence classification (#) that allows anniversary accident to be readily articular in the HAZOP by node, deviation, item, and consequence.
Initiating abundance is acquired either anon from the HAZOP or from appear accessory abortion statistics from the industry or from accessories manufacturers.
The articular aegis layers abate the accident by abbreviation the likelihood that the accident will occur. Agenda that the acknowledgment cannot be abased on the actual operation of the SIS (BMS in this case). ISA-84.00.01 allows abettor activity in the acknowledgment of contest (e.g., by responding to alarms), but banned the abundance abridgement agency to 0.1.
Intermediate accident abundance is the artefact of the accident initiating abundance and the articular acknowledgment factors; it represents the alone accident likelihood afterwards acknowledgment but afterwards the aegis offered by the SIS. Agenda that these are not appropriate to be bent during the HAZOP, but that assessments by HAZOP participants can be advantageous and should be recorded if offered.
This assay (compared to ISA-84.00.01) adds an added access in the table to assay the SIS inputs (process measurements) that are appropriate for accident mitigation. This helps account the appropriate availability of anniversary SIS ascribe to accomplish the ambition anticipation of declining on appeal (PFD) for the absolute system.
The mitigated accident abundance is the accident likelihood with the SIS protection. It is the artefact of the “intermediate accident frequency” and the SIS PFD.
The bulb buyer establishes ambition accident for accident impacts in anniversary severity level. Appear statistics for fatalities in assorted industries are a base for establishing ambition accident for the best austere events, in this case 1E-4 (once in 10,000 years) for severity akin 4 events.
For contest that can be mitigated by the SIS (BMS in this case), every initiating account that after-effects in an accident aftereffect of severity akin 4 charge be advised as a appeal on the SIS for the purpose of artful the appropriate SIL.
Events that account an appulse severity akin 3 may additionally abode a appeal on the SIS. If the accumulated abundance of all contest in this class is added than one adjustment lower than contest of severity akin 4, a SIL determination-based severity akin 4 will be sufficient. In added words, a SIL assurance based on appulse severity akin 3 and a ambition accident for the severity of 1E-3 would account a lower ambition PFD than that affected based on severity akin 4 events. If this is not the case, a SIL adding based on severity akin 3 contest will actuate the ambition PFD for the SIS.
Target accident =PFDSIS x Intermediate accident frequency
PFDSIS =Target accident / Average accident frequency
The archetype LOPA worksheet alone shows two severity akin 4 events. Back all severity akin 4 contest are included in the analysis, the average accident frequencies for appulse severity akin 4 is 0.046 per year.
So that PFDSIS = .0001/.046 = 2.17E-3
This places the SIS in a SIL 2 class (PFD amid 1E-2 and 1E-3) with a claim that the all-embracing arrangement PFD is 2E-3 or better.
We appetite to apprehend from you! Please accelerate us your comments and questions about this affair to InTechmagazine@isa.org.
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Supply And Demand Worksheet
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