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Interlocked isolation valves - less is more

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The problems with the trend of increasing the number of interlocked valves.

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Interlocked isolation valves - less is more

  1. 1. Tel: (+44) 01492 879813 Mob: (+44) 07984 284642 andy@abrisk.co.uk www.abrisk.co.uk 1 Interlocked isolation valves – less is more Andy Brazier
  2. 2. “Those bloody interlocks” “They are fine if you are starting in exactly the right place, doing exactly the right task and everything is working perfectly. Most of the time they are a complete nightmare”. Process operator talking about a pig launcher. 2
  3. 3. Paper overview Use of interlocks to reduce human error risk Trend to more interlocks Additional complexity Purchase and maintenance costs Over reliance Could they increase risks? 3
  4. 4. What are interlocks? Means of making the state of components dependent on each other One item cannot be operated unless another item is in its required state Can be electrical, electronic or mechanical Primarily concerned with ‘trapped key’ interlocks Not computer controlled sequences such as ‘start-up permissives.’ 4
  5. 5. Use of trapped key interlocks Ensure a spared item remains Relief valves Filters Ensure an item is in its correct status before carrying out a task Pig launcher is fully isolated before opening Filter Ensuring an item is fully isolated before carrying out a task Steps performed in correct sequence. 5
  6. 6. Simple example – spared relief valves 6 Vessel Emergency vent header RV-A RV-B Vessel Emergency vent header RV-A RV-B Interlocked valves Interlocked valves
  7. 7. Human factors of interlocking Appears to provide a ‘fail safe’ control “Remove the ‘human factor’ by ensuring dangerous processes happen only in a designated sequence” Vendor of proprietary interlock system “Enforce and guarantee a pre-defined sequence of operation and so eliminate human error.” Vendor of proprietary interlock system. 7
  8. 8. Is this true? An interlock is only as good as the person maintaining it Edmonds 2016 When an interlock fails the operator is usually the only line of defence Marsden et al 2003 Interlocks do fail They can be defeated. 8
  9. 9. A critical look at the human factors Assuming it is safe to operate a valve because the key fits. Prone to design and maintenance failures Being forced to continue with a sequence of steps that may not be the most effective or safe. Proving isolation integrity Using ‘master’ keys to override interlocks due to equipment problems or non-routine activities Paying less attention because the perception is that a task cannot be done incorrectly. 9
  10. 10. Increased use of interlocks Advances in technology and software to design More interlocked steps More complex interactions Key exchange units Integrated with PLC, DCS and SIS Risk aversion Can’t be criticised for interlocking May be criticised for not. 10
  11. 11. Current guidance “Interlock arrangements may be provided as safety systems, particularly where they prevent inadvertent operation. The guideline to the Pipelines Safety Regulations 1996 (HSE 1996) “Pig launcher and pig receiver shall be equipped with an interlock system to prevent opening of isolation valves around the launcher when the launcher door is open.” The NORSOK Standard P-100 for Process Systems (NORSOK 2001) 11
  12. 12. Simple example Goal is to open door ‘D’ To do this safely: Close inlet valve ‘I’ Close outlet valve ‘O’ Open vent valve ‘V’ Monitor the vent to confirm successful isolation. 12 I V O D
  13. 13. Potential errors I left open – vessel not isolated – process release when D is opened O left open – vessel not isolated – process release when D is opened V is left closed – vessel remains pressurised – release of trapped material when D is opened V is not monitored – integrity of isolation is not confirmed – possible release if I or O pass. 13 I V O D
  14. 14. Error likelihood – no interlocks Any of the potential errors is possible Operator needs to think what they are doing Will be very focussed on opening the vent before opening the door May feel that monitoring the vent is not necessary. 14 I V O D
  15. 15. Error likelihood – I and O interlocked Cannot open the door until the vessel is isolated Once both isolation valves are closed the operator will have the key to open the door May be more inclined to overlook the requirement to open and monitor the vent. 15 I V O D
  16. 16. Error likelihood – I, O and V interlocked Cannot open the door until the vessel is isolated and vented Task requires very little thought – can’t be done incorrectly Will the operator even think about monitoring the vent? 16 I V O D
  17. 17. Error likelihood – V interlocked Isolation valves may be left open Errors will be discovered when vent is opened May feel that monitoring the vent is not necessary. 17 I V O D
  18. 18. Quantifying human error likelihood Do not believe any of these numbers!!! 18
  19. 19. Suggested rates (per operation) Operator fully engaged in task and fully understands the need for the step - 0.001 Operator engaged in task but may not appreciate step significance - 0.01 Operator not engaged in task - 0.1 19
  20. 20. Error likelihood calculations None I+O I+O+V V Vent not opened 0.001 0.01 0 0 Vent not monitored 0.01 0.01 0.1 0.01 Error likelihood 0.011 0.02 0.1 0.01 20 0.001 – Fully engaged 0.01 – May not appreciate significance 0.1 – Not engaged
  21. 21. Interlock failure Very likely that if an operator obtains Key D they will assume valves are in the correct state Mechanical failures do occur Someone may have used a Master Key 21 None I+O I+O+V V Vent not opened 0.001 0.01 0.1 0.1 Vent not monitored 0.01 0.01 0.1 0.01 Error likelihood 0.011 0.02 0.2 0.11 0.011 0.02 0.1 0.01
  22. 22. Conclusion Simple example illustrates interlocks do not “Remove the human factor” “Eliminate human error” Quantification illustrates how human error likelihood may increase due to interlocks Especially when interlock failure is taken into account Don’t believe the numbers 22
  23. 23. That was a very simple example 23 Pipeline Kicker line Closed drain Process vent header Pig receiver Relief valve Emergency vent header Purge medium Local vent
  24. 24. What is the solution? We need to define strategies for interlocking Fully Partial Minimal We need to be very careful about managing interlock failures and overrides What do you think? Increased use of interlocks has improved safety? Less is more? 24
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