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Risk assessment of Risk assessment of relief streams on major relief streams on major hazard installations hazard installations Stuart Ord Stuart Ord Process Technologist Process Technologist
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Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Mar 26, 2015

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Page 1: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Risk assessment of relief Risk assessment of relief streams on major hazard streams on major hazard

installationsinstallations

Stuart OrdStuart OrdProcess TechnologistProcess Technologist

Page 2: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Hello!Hello!

Stuart OrdStuart Ord BSc, CEng, FIChemE, MEI, TechIOSHBSc, CEng, FIChemE, MEI, TechIOSH 7 years in Ministry of Defence7 years in Ministry of Defence 20 years in ICI20 years in ICI 3 years in BNFL3 years in BNFL 4 years in Stopford Projects Ltd4 years in Stopford Projects Ltd 5 years self-employed CEDCS Ltd5 years self-employed CEDCS Ltd

Page 3: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Risk assessmentRisk assessment

Modern approachModern approach Acceptance that all things can failAcceptance that all things can fail Specification of Tolerability of RiskSpecification of Tolerability of Risk Expectation that all risks are Expectation that all risks are

controlledcontrolled Judgement “by eye” unlikely to be Judgement “by eye” unlikely to be

adequate for significant risksadequate for significant risks

Page 4: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Safety Integrity LevelSafety Integrity Level

Applies to E/E/PE systemsApplies to E/E/PE systems Realistic assessment of a riskRealistic assessment of a risk Realistic estimate of its likelihoodRealistic estimate of its likelihood Consideration of all protective systemsConsideration of all protective systems In some cases, a layer of protection might In some cases, a layer of protection might

be a relief valvebe a relief valve If there is no need for an E/E/PE, it might If there is no need for an E/E/PE, it might

be that a PRV is the ultimate protective be that a PRV is the ultimate protective measure and, if given a reasonable PFD, measure and, if given a reasonable PFD, has then been shown to be adequatehas then been shown to be adequate

Page 5: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

The legacy of learningThe legacy of learning

Historical approachHistorical approach Can the system be over-pressurised?Can the system be over-pressurised?

Specify a pressure relief systemSpecify a pressure relief system Design, install and operate it properlyDesign, install and operate it properly Consider the risk dealt withConsider the risk dealt with

Modern approachModern approach We should assess risks to ensure they are We should assess risks to ensure they are

tolerabletolerable We must ensure valves are installed in systems We must ensure valves are installed in systems

such that the risk of their (rare) failure is such that the risk of their (rare) failure is tolerabletolerable

Page 6: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

The siteThe site

Large plants with toxic, corrosive, Large plants with toxic, corrosive, flammable and explosive materialsflammable and explosive materials

Some were designed over 40 years agoSome were designed over 40 years ago Culture of leading edge safety practicesCulture of leading edge safety practices

Hazard Studies had been appliedHazard Studies had been applied Best relief stream design and maintenance Best relief stream design and maintenance

principles had been appliedprinciples had been applied New owners determined to continue safety New owners determined to continue safety

performance improvement (leading and performance improvement (leading and lagging indicators)lagging indicators)

Page 7: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Risk assessment at the siteRisk assessment at the site

Category 1 Significant Consequences

Category 2 Serious Consequences

Category 3 Severe Consequences

Category 4 Major Consequences

Injury incident (on-site effects)

Minor/classified Low probability OTD

OTD Low probability of major injury

Major injury Multiple injuries

Fatality or few employee fatalities

Injury incident (off-site effects)

People affected- short term minor Few people require hospital treatment

Serious injury, 10s in hospital

Health incident (on-site effects)

Possible exposure Few cases of employee ill-health

Harmful conditions Several employees affected

Distressing exposure Serious health effects, eg carcinogen release

Fatality directly linked to exposure

Category 5 Extremely Serious Consequences

Category 6 Disaster

Category 7 Major Disaster

Injury incident (on-site effects)

Several fatalities (5 - 10), including some off site. many injuries

Many fatalities (10 - 100)

Large number of fatalities (> 100)

Injury incident (off-site effects)

Fatality(s), many injuries

Health incident (on-site effects)

Major health effects threaten operation Health risk unacceptable

- -

Consequence category model

Page 8: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Risk assessmentRisk assessment

10-7

ExtremelyUnlikely

2 4

Event Frequency (/yr)

10-6 10-5 10-4 10-3 10-2 0.1 1 10

5 6 7 431

VeryUnlikely

UnlikelyQuite

UnlikelySomewhatUnlikely

FairlyProbable

Probable

1

2

3

4

5

6

7Category 7Major Disaster

Category 6Disaster

Category 5Extremely Serious Consequences

Category 4Major Consequences

Category 3Severe Consequences

Category 2Serious Consequences

Category 1Significant Consequences

INTOLERABLE

TOLERABLE

TIFALARP

TIFALARP

10-7

ExtremelyUnlikely

2 4

Event Frequency (/yr)

10-6 10-5 10-4 10-3 10-2 0.1 1 10

5 6 7 431

VeryUnlikely

UnlikelyQuite

UnlikelySomewhatUnlikely

FairlyProbable

Probable

1

2

3

4

5

6

7Category 7Major Disaster

Category 6Disaster

Category 5Extremely Serious Consequences

Category 4Major Consequences

Category 3Severe Consequences

Category 2Serious Consequences

Category 1Significant Consequences

INTOLERABLE

TOLERABLE

TIFALARP

TIFALARP

Tolerability of Risk model

Page 9: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Relief streams at the siteRelief streams at the site

About 2850 relief streamsAbout 2850 relief streams Consequence category 0: 500Consequence category 0: 500 Consequence category 1: 1000Consequence category 1: 1000 Consequence category 2: 600Consequence category 2: 600 Consequence category 3: 400Consequence category 3: 400 Consequence category 4+: 350Consequence category 4+: 350

Page 10: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Relief streams at the siteRelief streams at the site

Consequence category 4+: 350Consequence category 4+: 350 Toxic gassesToxic gasses Liquefied toxic gasesLiquefied toxic gases Explosive gassesExplosive gasses Corrosive gasses and liquidsCorrosive gasses and liquids

Page 11: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (1)Project scope (1)

What has to be doneWhat has to be done Retrieve calculationRetrieve calculation Retrieve current P&IDRetrieve current P&ID Retrieve any other paperworkRetrieve any other paperwork

Internal and external independent reviewsInternal and external independent reviews Workshop maintenance recordsWorkshop maintenance records

Check P&ID against existing plantCheck P&ID against existing plant Review with TPM (see next slide)Review with TPM (see next slide) Complete Risk Assessment (see next slide)Complete Risk Assessment (see next slide) Follow-up issues if anyFollow-up issues if any

Page 12: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (2)Project scope (2)

Review with Technical Plant ManagerReview with Technical Plant Manager Frequency of operation of RSTFrequency of operation of RST Plant practical issues (manning, etc)Plant practical issues (manning, etc) Causes of any RV unreliabilityCauses of any RV unreliability

Risk assessmentRisk assessment LoPA styleLoPA style

Project managementProject management

Page 13: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (3)Project scope (3)

Estimated time ½ man-day per Estimated time ½ man-day per stream direct effort + ½ man-day stream direct effort + ½ man-day support + implementation of any support + implementation of any changeschanges Hence ~ 2800 man-days for site review Hence ~ 2800 man-days for site review

of of all relief streamsall relief streams Hence approaching £1million + cost of Hence approaching £1million + cost of

any changesany changes

Page 14: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (4)Project scope (4)

Estimated time ½ man-day per Estimated time ½ man-day per stream direct effort + ½ man-day stream direct effort + ½ man-day support + implementation of any support + implementation of any changeschanges Hence ~ 350 man-days for site review of Hence ~ 350 man-days for site review of

C4+ relief streamsC4+ relief streams Hence ~ £100k + cost of any changesHence ~ £100k + cost of any changes

Page 15: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (5)Project scope (5)

Review Cat 4+ streamsReview Cat 4+ streams Consider:Consider:

Is the design basis still sound?Is the design basis still sound? Is the paperwork up to date and correct?Is the paperwork up to date and correct? Is the risk properly managed?Is the risk properly managed?

If not, what changes should be made?If not, what changes should be made? Implement changesImplement changes

Manage procedureManage procedure Generate audit trailGenerate audit trail Correct records if necessaryCorrect records if necessary

Page 16: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Project scope (5)Project scope (5)

Review Cat 4+ streamsReview Cat 4+ streams Consider:Consider:

Is the design basis still sound?Is the design basis still sound? Is the paperwork up to date and correct?Is the paperwork up to date and correct? Is the risk being adequately controlled?Is the risk being adequately controlled?

If not, what changes should be made?If not, what changes should be made? Implement changesImplement changes

Manage procedureManage procedure Generate audit trailGenerate audit trail Correct records if necessaryCorrect records if necessary

Page 17: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

How reliable is a relief How reliable is a relief device?device?

Review of test results from maintenance Review of test results from maintenance workshop:workshop: Relief valve – fail to lift at 130% of set pressure - Relief valve – fail to lift at 130% of set pressure -

probability = 0.022. This could be regarded as probability = 0.022. This could be regarded as PFD=0.022. PFD=0.022.

Bursting disc – no field data, assumed that PFD = 0.01Bursting disc – no field data, assumed that PFD = 0.01 Registered vents and overflows – PFD = 0.01Registered vents and overflows – PFD = 0.01 These assume “realistic pessimism”These assume “realistic pessimism”

Other authorsOther authors Relief valve – “clean” service – 0.01Relief valve – “clean” service – 0.01 “ “dirty” service – 0.1dirty” service – 0.1

Page 18: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Why are valves unreliable?Why are valves unreliable?

““Normal” failures:Normal” failures: Faulty manufacture Faulty manufacture Random failuresRandom failures Damage, contamination or deterioration in the field Damage, contamination or deterioration in the field

In practice, human errors can also contribute:In practice, human errors can also contribute: Wrong designWrong design Transcription error on specification sheets Transcription error on specification sheets Administrative error in the manufacture process, Administrative error in the manufacture process,

making the valve supplied not fit with the designmaking the valve supplied not fit with the design Incorrect installationIncorrect installation Incorrect spring settingIncorrect spring setting

Page 19: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Why are bursting disks Why are bursting disks unreliable?unreliable?

Cannot be non-destructively tested - Cannot be non-destructively tested - manufacturer makes a batch of disks manufacturer makes a batch of disks and then tests a sample and then tests a sample

Possible to fit the wrong disk, or to fit Possible to fit the wrong disk, or to fit it wrongly (e.g. backwards!)it wrongly (e.g. backwards!)

Blockage is still possible on dirty Blockage is still possible on dirty serviceservice

However, it is believed that the However, it is believed that the reliability of a bursting disk is greater reliability of a bursting disk is greater than that of a relief valve.than that of a relief valve.

Page 20: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Risk targetsRisk targets

Page 21: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

ResultsResults Number of C4+ streams reviewed - 111Number of C4+ streams reviewed - 111 Risk results summary:Risk results summary:

Existing installation not necessary on risk Existing installation not necessary on risk grounds grounds ** : 9 (8%) : 9 (8%)

Existing installation necessary and adequate Existing installation necessary and adequate : 90 (81%): 90 (81%)

Existing installation inadequate : 12 (11%) Existing installation inadequate : 12 (11%) ##

* Retained because still required by law* Retained because still required by law # Consequently modified to meet risk # Consequently modified to meet risk

criteriacriteria

Page 22: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Example 1Example 1

Liquefied toxic gas stock tank Liquefied toxic gas stock tank bursting diskbursting disk relief relief Over-pressurised to failure and LOC of liquid due to Over-pressurised to failure and LOC of liquid due to

incorrect operation and failure of other protective measuresincorrect operation and failure of other protective measures Widespread personnel exposure & death of several workersWidespread personnel exposure & death of several workers Consequence – Category 6Consequence – Category 6 Tolerable frequency of occurrence – E-6 per yearTolerable frequency of occurrence – E-6 per year Probable frequency of occurrence (without BD) – E-7 per Probable frequency of occurrence (without BD) – E-7 per

yearyear Conclusion – Conclusion – not needednot needed

Page 23: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Example 2Example 2

Process evaporator (corrosive material) Process evaporator (corrosive material) spring relief valvespring relief valve Steam tube failure causes body to fail and LOC of hot Steam tube failure causes body to fail and LOC of hot

corrosive liquid, personnel exposure & death of 1corrosive liquid, personnel exposure & death of 1 Consequence – Category 4Consequence – Category 4 Tolerable frequency of occurrence – E-4 per yearTolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without RV) – 5E-2 per yearProbable frequency of occurrence (without RV) – 5E-2 per year Conclusion – Conclusion – PFD required 0.002PFD required 0.002 Single relief valve is Single relief valve is inadequateinadequate Introduced steam tube thickness checks, repositioned RV Introduced steam tube thickness checks, repositioned RV

dischargesdischarges

Page 24: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Example 3Example 3

Explosive gas compressor 1st stage Explosive gas compressor 1st stage spring relief spring relief valvevalve

Blockage of outlet causes LOC of gas; explosion & Blockage of outlet causes LOC of gas; explosion & death of 1death of 1

Consequence – Category 4Consequence – Category 4 Tolerable frequency of occurrence – E-4 per yearTolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without RV) – Probable frequency of occurrence (without RV) –

1.2E-4 per year1.2E-4 per year Conclusion – Conclusion – PFD required 0.83PFD required 0.83 Single relief valve is adequateSingle relief valve is adequate

Page 25: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Example 4Example 4

Corrosive liquid stock tank Corrosive liquid stock tank ventvent causes tank causes tank failure, rapid LOC of liquid breaches bund; failure, rapid LOC of liquid breaches bund; personnel exposure & death of 1personnel exposure & death of 1

Consequence – Category 4Consequence – Category 4 Tolerable frequency of occurrence – E-4 per yearTolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without vent) – Probable frequency of occurrence (without vent) –

9E-3 per year9E-3 per year Conclusion – Conclusion – PFD required 0.011PFD required 0.011 Single vent is adequate Single vent is adequate

Page 26: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

Example 5Example 5

Liquefied toxic gas export line Liquefied toxic gas export line thermal relief valvethermal relief valve Over-pressurised to failure and LOC of liquid due to Over-pressurised to failure and LOC of liquid due to

incorrect operation and failure of other protective measuresincorrect operation and failure of other protective measures Widespread personnel exposure & death of 1Widespread personnel exposure & death of 1 Consequence – Category 4Consequence – Category 4 Tolerable frequency of occurrence – E-4 per yearTolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without BD) – E-3 per Probable frequency of occurrence (without BD) – E-3 per

yearyear Conclusion – Conclusion – PFD required 0.1PFD required 0.1 Single spring relief valve is adequate Single spring relief valve is adequate

Page 27: Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist.

ConclusionsConclusions Pressure relief is mandatory where over-pressure is possiblePressure relief is mandatory where over-pressure is possible

Assessment cost is about 1 man day per streamAssessment cost is about 1 man day per stream Remediation costs if stream is found inadequate will varyRemediation costs if stream is found inadequate will vary

Traditional approach of “just add a relief device of your Traditional approach of “just add a relief device of your choice” is not necessarily meeting acceptable risk criteriachoice” is not necessarily meeting acceptable risk criteria

LoPA style risk assessment is applicable to deciding LoPA style risk assessment is applicable to deciding whether a relief meets TOR criteriawhether a relief meets TOR criteria

Penalty costs of stream failing to protect could be much higher!Penalty costs of stream failing to protect could be much higher!