SAFETY-BARRIER DIAGRAMS FOR DOCUMENTING SAFETY OF HYDROGEN APPLICATIONS F. Markert and N.J. Duijm Systems Analysis Department, Risø National Laboratory, Technical University of Denmark, P.O. Box 49, DK-4000 Roskilde, Denmark

Paper nd ICHS, San Sebastian 11th-13th September Background Barrier diagrams serve two main purposes : 1)Evaluation of adequateness of safety measures (part of accident prevention) Are the barrieres reasonable and independent? Are barriers missing? 2) Communication to all stakeholders Illustrating the possible accident scenarios and safety measures taken to prevent them - Safety-barrier diagrams have been popular in Denmark as a risk analysis tool. - Safety-barrier diagrams are also useful for analysisng the new hydrogen technologies

Paper nd ICHS, San Sebastian 11th-13th September Definition of a safety barrier A barrier function is a function planned to prevent, control, or mitigate the propagation of a condition or event into an undesired condition or event; A safety barrier is a series of elements that implement a barrier function, each element consisting of a technical system or human action. Level indicator Alarm Operator ButtonPowered valve Signal DetectionDiagnosisAction

Paper nd ICHS, San Sebastian 11th-13th September Graphical presentation of a safety barrier Fault tree representation Safety barrier Condition on success is optional

Paper nd ICHS, San Sebastian 11th-13th September BARRIER DIAGRAMS Barriers can be of different types:  Active versus passive barriers  Automatic versus manual barriers Examples of barriers:  An alarm for high level in a tank.  A sprinkler system in a building to prevent fires in developing.  A dike surrounding a tank, designed to contain accidental spillage from the tank.

Paper nd ICHS, San Sebastian 11th-13th September CONSTRUCTION OF BARRIER DIAGRAMS The construction of barrier diagrams consists of 4 steps: 1.Construction of e.g. the event chains When constructing barrier diagrams one must start with ignoring all the existing barriers! The main structure of the barrier diagram is the event chains, which may consist of elements from both the event tree and the fault tree method.

Paper nd ICHS, San Sebastian 11th-13th September STEPS IN CONSTRUCTING BARRIER DIAGRAMS 2.Inclusion of the barriers.

Paper nd ICHS, San Sebastian 11th-13th September STEPS IN CONSTRUCTING BARRIER DIAGRAMS 3. Once the barrier diagram is finished, the level of safety should be evaluated  to determine whether there are sufficient barriers against the undesired events happening  When evaluating the diagram one must consider: The frequency/probability of the initiating events The severity of the end events (consequence assessment) The number, coverage and reliability of barriers in each of the event chains in the diagram 4. (optional) Classification of barriers according to type or evaluated reliability of the barrier..

Paper nd ICHS, San Sebastian 11th-13th September Safety-barrier diagrams are simpler than fault trees

Paper nd ICHS, San Sebastian 11th-13th September Dependency can be included via the ”Common Element” Common Element indicated: ”Single Operator”

Paper nd ICHS, San Sebastian 11th-13th September Barriers may not be bypassed Events/conditions and barriers are unique Paths through diagrams can converge and diverge; divergence can be exclusive (as in an event tree) or simultaneous (or parallel) Important properties of safety-barrier diagrams Exclusive Simultaneous/parallel DivergenceConvergence

Paper nd ICHS, San Sebastian 11th-13th September Important properties (continue) Diagrams can be split into connected sub-diagrams; Connected diagrams can be put together into a single diagram that fulfils the above conditions The probability of conditions in a safety-barrier diagram can be derived from the probability of the initial conditions and probabilities of failure on demand of the barriers.

Paper nd ICHS, San Sebastian 11th-13th September Comparison with other (graphical) risk analysis methods Barrier diagrams are developed from cause-consequence diagrams Cause-consequence diagrams combine fault trees and event trees Barrier diagrams simplify the presentation of safety systems (”and” gates in fault trees) A ”Bowtie” diagram is a safety-barrier diagram with a single ”Critical Event”

Paper nd ICHS, San Sebastian 11th-13th September Example safety-barrier diagram Process flow diagram for a hydrogen refuelling facility with cryogenic delivery

Paper nd ICHS, San Sebastian 11th-13th September Safety-barrier diagram for the unloading of a LH2-truck Safety-barrier diagram for the unloading of a liquid hydrogen truck at a refuelling station on the basis of the FMEA study by Venkatesh S., et.al.; Failure modes and effects analysis for hydrogen fueling options. California Energy Commission;2004.

Paper nd ICHS, San Sebastian 11th-13th September Barrier diagram evaluation & type classification Barrier Name Barrier Type Description Unloading Inspection Activated Barrier - Procedural (Observation of local conditions not using instruments) On arrival of the truck, the truck is inspected visually for defects by driver or station operator (need to be decided who). The mitigation action what to do in case defects are noted need to be included. Hydrogen sensors Activated Barrier - Warned (Human Action based on passive warning) Hydrogen sensors are located near the unloading facility The sensors themselves do not mitigate the presence of an explosive atmosphere, so a follow up has to be found in terms of alarms, evacuation, close down of unloading operation, close down of potential ignition sources Inspection of hoses Activated Barrier - Procedural (Observation of local conditions not using instruments) Inspection of hoses before unloading/connection

Paper nd ICHS, San Sebastian 11th-13th September Barrier diagram evaluation & type classification Inspection of connections Activated Barrier - Procedural (Observation of local conditions not using instruments) Connections are inspected before unloading action is started Supervision during unloading Activated Barrier - Procedural (Observation of local conditions not using instruments) The unloading action is monitored by personnel. Note that the personnel may become disabled by freeze burn - consider additional remote monitoring Deviations of the unloading lead to aborting the unloading action

Paper nd ICHS, San Sebastian 11th-13th September Barrier diagram evaluation & type classification Barrier Name Barrier Type Description Personal protection Temporary Passive Barrier - Put in place (and removed) by person Unloading operator and truck driver have donned protective clothing against cryogenic burn ("Nomex suit") Limited accessActivated Barrier - Warned (Human Action based on passive warning) A safety distance around the truck and unloading facility where access is prohibited to third parties and personnel not involved in the unloading action. Venting prior disconnection Activated Barrier - Procedural (Observation of local conditions not using instruments) Unloading hoses need to be vented prior to disconnection Collision prevention Temporary Passive Barrier - Put in place (and removed) by person /Permanent Passive Barrier/Activated Barrier - warned Temporary Passive Barrier The unloading facility can be protected against external impacts such as third party traffic by: a) caution cones, b) concrete poles, and c) impact resistant movable fences/barriers

Paper nd ICHS, San Sebastian 11th-13th September Conclusions The methodology of safety-barrier diagrams has been introduced and exemplified by the safety analysis of two sections of a hydrogen refueling station. Safety-barrier diagrams offer a good overview of the safety precautions that are included in the different sections, and the consequences of the failure of these precautions. Safety-barrier diagrams support hazard analysis; they do not support or replace the preceding phase of hazard identification, for which exist a range of more suitable methods, such as FMEA or HAZOP The logic framework used for safety-barrier diagrams and the use of a classification for the different safety barriers forces the analysts to consider the completeness of the barriers (in terms of the detect- diagnose-act sequence) and the role of the safety barrier in the system.

Paper nd ICHS, San Sebastian 11th-13th September Conclusions The safety-barrier diagrams allow both quantitative and qualitative assessments to be made. The presentation by means of safety-barrier diagrams is simpler, and thereby easier to understand by non-experts than other graphical methods such as fault trees or event trees. Therefore safety-barrier diagrams are excellent means for documenting system safety and for communication with authorities and other stakeholders.

Paper nd ICHS, San Sebastian 11th-13th September Thank you for your attention