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

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Presentation on theme: "Risk assessment of relief streams on major hazard installations Stuart Ord Process Technologist."— Presentation transcript:

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

2 Hello! Stuart Ord Stuart Ord BSc, CEng, FIChemE, MEI, TechIOSH BSc, CEng, FIChemE, MEI, TechIOSH 7 years in Ministry of Defence 7 years in Ministry of Defence 20 years in ICI 20 years in ICI 3 years in BNFL 3 years in BNFL 4 years in Stopford Projects Ltd 4 years in Stopford Projects Ltd 5 years self-employed CEDCS Ltd 5 years self-employed CEDCS Ltd

3 Risk assessment Modern approach Modern approach Acceptance that all things can fail Acceptance that all things can fail Specification of Tolerability of Risk Specification of Tolerability of Risk Expectation that all risks are controlled Expectation that all risks are controlled Judgement by eye unlikely to be adequate for significant risks Judgement by eye unlikely to be adequate for significant risks

4 Safety Integrity Level Applies to E/E/PE systems Applies to E/E/PE systems Realistic assessment of a risk Realistic assessment of a risk Realistic estimate of its likelihood Realistic estimate of its likelihood Consideration of all protective systems Consideration of all protective systems In some cases, a layer of protection might be a relief valve In some cases, a layer of protection might be a relief valve If there is no need for an E/E/PE, it might be that a PRV is the ultimate protective measure and, if given a reasonable PFD, has then been shown to be adequate If there is no need for an E/E/PE, it might be that a PRV is the ultimate protective measure and, if given a reasonable PFD, has then been shown to be adequate

5 The legacy of learning Historical approach Historical approach Can the system be over-pressurised? Can the system be over-pressurised? Specify a pressure relief system Specify a pressure relief system Design, install and operate it properly Design, install and operate it properly Consider the risk dealt with Consider the risk dealt with Modern approach Modern approach We should assess risks to ensure they are tolerable We should assess risks to ensure they are tolerable We must ensure valves are installed in systems such that the risk of their (rare) failure is tolerable We must ensure valves are installed in systems such that the risk of their (rare) failure is tolerable

6 The site Large plants with toxic, corrosive, flammable and explosive materials Large plants with toxic, corrosive, flammable and explosive materials Some were designed over 40 years ago Some were designed over 40 years ago Culture of leading edge safety practices Culture of leading edge safety practices Hazard Studies had been applied Hazard Studies had been applied Best relief stream design and maintenance principles had been applied Best relief stream design and maintenance principles had been applied New owners determined to continue safety performance improvement (leading and lagging indicators) New owners determined to continue safety performance improvement (leading and lagging indicators)

7 Risk assessment at the site Consequence category model

8 Risk assessment Tolerability of Risk model

9 Relief streams at the site About 2850 relief streams About 2850 relief streams Consequence category 0: 500 Consequence category 0: 500 Consequence category 1: 1000 Consequence category 1: 1000 Consequence category 2: 600 Consequence category 2: 600 Consequence category 3: 400 Consequence category 3: 400 Consequence category 4+: 350 Consequence category 4+: 350

10 Relief streams at the site Consequence category 4+: 350 Consequence category 4+: 350 Toxic gasses Toxic gasses Liquefied toxic gases Liquefied toxic gases Explosive gasses Explosive gasses Corrosive gasses and liquids Corrosive gasses and liquids

11 Project scope (1) What has to be done What has to be done Retrieve calculation Retrieve calculation Retrieve current P&ID Retrieve current P&ID Retrieve any other paperwork Retrieve any other paperwork Internal and external independent reviews Internal and external independent reviews Workshop maintenance records Workshop maintenance records Check P&ID against existing plant Check 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 any Follow-up issues if any

12 Project scope (2) Review with Technical Plant Manager Review with Technical Plant Manager Frequency of operation of RST Frequency of operation of RST Plant practical issues (manning, etc) Plant practical issues (manning, etc) Causes of any RV unreliability Causes of any RV unreliability Risk assessment Risk assessment LoPA style LoPA style Project management Project management

13 Project scope (3) Estimated time ½ man-day per stream direct effort + ½ man-day support + implementation of any changes Estimated time ½ man-day per stream direct effort + ½ man-day support + implementation of any changes Hence ~ 2800 man-days for site review of all relief streams Hence ~ 2800 man-days for site review of all relief streams Hence approaching £1million + cost of any changes Hence approaching £1million + cost of any changes

14 Project scope (4) Estimated time ½ man-day per stream direct effort + ½ man-day support + implementation of any changes Estimated time ½ man-day per stream direct effort + ½ man-day support + implementation of any changes Hence ~ 350 man-days for site review of C4+ relief streams Hence ~ 350 man-days for site review of C4+ relief streams Hence ~ £100k + cost of any changes Hence ~ £100k + cost of any changes

15 Project scope (5) Review Cat 4+ streams Review 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 changes Implement changes Manage procedure Manage procedure Generate audit trail Generate audit trail Correct records if necessary Correct records if necessary

16 Project scope (5) Review Cat 4+ streams Review 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 changes Implement changes Manage procedure Manage procedure Generate audit trail Generate audit trail Correct records if necessary Correct records if necessary

17 How reliable is a relief device? Review of test results from maintenance workshop: Review of test results from maintenance workshop: Relief valve – fail to lift at 130% of set pressure - probability = 0.022. This could be regarded as PFD=0.022. Relief valve – fail to lift at 130% of set pressure - probability = 0.022. This could be regarded as PFD=0.022. Bursting disc – no field data, assumed that PFD = 0.01 Bursting disc – no field data, assumed that PFD = 0.01 Registered vents and overflows – PFD = 0.01 Registered vents and overflows – PFD = 0.01 These assume realistic pessimism These assume realistic pessimism Other authors Other authors Relief valve – clean service – 0.01 Relief valve – clean service – 0.01 dirty service – 0.1 dirty service – 0.1

18 Why are valves unreliable? Normal failures: Normal failures: Faulty manufacture Faulty manufacture Random failures Random 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 design Wrong design Transcription error on specification sheets Transcription error on specification sheets Administrative error in the manufacture process, making the valve supplied not fit with the design Administrative error in the manufacture process, making the valve supplied not fit with the design Incorrect installation Incorrect installation Incorrect spring setting Incorrect spring setting

19 Why are bursting disks unreliable? Cannot be non-destructively tested - manufacturer makes a batch of disks and then tests a sample Cannot be non-destructively tested - manufacturer makes a batch of disks and then tests a sample Possible to fit the wrong disk, or to fit it wrongly (e.g. backwards!) Possible to fit the wrong disk, or to fit it wrongly (e.g. backwards!) Blockage is still possible on dirty service Blockage is still possible on dirty service However, it is believed that the reliability of a bursting disk is greater than that of a relief valve. However, it is believed that the reliability of a bursting disk is greater than that of a relief valve.

20 Risk targets

21 Results Number of C4+ streams reviewed - 111 Number of C4+ streams reviewed - 111 Risk results summary: Risk results summary: Existing installation not necessary on risk grounds * : 9 (8%) Existing installation not necessary on risk grounds * : 9 (8%) Existing installation necessary and adequate : 90 (81%) Existing installation necessary and adequate : 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 criteria # Consequently modified to meet risk criteria

22 Example 1 Liquefied toxic gas stock tank bursting disk relief Liquefied toxic gas stock tank bursting disk relief Over-pressurised to failure and LOC of liquid due to incorrect operation and failure of other protective measures Over-pressurised to failure and LOC of liquid due to incorrect operation and failure of other protective measures Widespread personnel exposure & death of several workers Widespread personnel exposure & death of several workers Consequence – Category 6 Consequence – Category 6 Tolerable frequency of occurrence – E-6 per year Tolerable frequency of occurrence – E-6 per year Probable frequency of occurrence (without BD) – E-7 per year Probable frequency of occurrence (without BD) – E-7 per year Conclusion – not needed Conclusion – not needed

23 Example 2 Process evaporator (corrosive material) spring relief valve Process evaporator (corrosive material) spring relief valve Steam tube failure causes body to fail and LOC of hot corrosive liquid, personnel exposure & death of 1 Steam tube failure causes body to fail and LOC of hot corrosive liquid, personnel exposure & death of 1 Consequence – Category 4 Consequence – Category 4 Tolerable frequency of occurrence – E-4 per year Tolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without RV) – 5E-2 per year Probable frequency of occurrence (without RV) – 5E-2 per year Conclusion – PFD required 0.002 Conclusion – PFD required 0.002 Single relief valve is inadequate Single relief valve is inadequate Introduced steam tube thickness checks, repositioned RV discharges Introduced steam tube thickness checks, repositioned RV discharges

24 Example 3 Explosive gas compressor 1st stage spring relief valve Explosive gas compressor 1st stage spring relief valve Blockage of outlet causes LOC of gas; explosion & death of 1 Blockage of outlet causes LOC of gas; explosion & death of 1 Consequence – Category 4 Consequence – Category 4 Tolerable frequency of occurrence – E-4 per year Tolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without RV) – 1.2E-4 per year Probable frequency of occurrence (without RV) – 1.2E-4 per year Conclusion – PFD required 0.83 Conclusion – PFD required 0.83 Single relief valve is adequate Single relief valve is adequate

25 Example 4 Corrosive liquid stock tank vent causes tank failure, rapid LOC of liquid breaches bund; personnel exposure & death of 1 Corrosive liquid stock tank vent causes tank failure, rapid LOC of liquid breaches bund; personnel exposure & death of 1 Consequence – Category 4 Consequence – Category 4 Tolerable frequency of occurrence – E-4 per year Tolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without vent) – 9E-3 per year Probable frequency of occurrence (without vent) – 9E-3 per year Conclusion – PFD required 0.011 Conclusion – PFD required 0.011 Single vent is adequate Single vent is adequate

26 Example 5 Liquefied toxic gas export line thermal relief valve Liquefied toxic gas export line thermal relief valve Over-pressurised to failure and LOC of liquid due to incorrect operation and failure of other protective measures Over-pressurised to failure and LOC of liquid due to incorrect operation and failure of other protective measures Widespread personnel exposure & death of 1 Widespread personnel exposure & death of 1 Consequence – Category 4 Consequence – Category 4 Tolerable frequency of occurrence – E-4 per year Tolerable frequency of occurrence – E-4 per year Probable frequency of occurrence (without BD) – E-3 per year Probable frequency of occurrence (without BD) – E-3 per year Conclusion – PFD required 0.1 Conclusion – PFD required 0.1 Single spring relief valve is adequate Single spring relief valve is adequate

27 Conclusions Pressure relief is mandatory where over-pressure is possible Pressure relief is mandatory where over-pressure is possible Assessment cost is about 1 man day per stream Assessment cost is about 1 man day per stream Remediation costs if stream is found inadequate will vary Remediation costs if stream is found inadequate will vary Traditional approach of just add a relief device of your choice is not necessarily meeting acceptable risk criteria Traditional approach of just add a relief device of your choice is not necessarily meeting acceptable risk criteria LoPA style risk assessment is applicable to deciding whether a relief meets TOR criteria LoPA style risk assessment is applicable to deciding whether 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!


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