Functional Safety Concept to Support Hazard Assessment and Risk Management in Water-Supply Systems
Abstract
:1. Introduction
2. Functional Safety
- In the field of industrial processes—IEC 61511;
- In the field of machines—IEC 62061;
- In the field of nuclear energy—IEC 61513.
3. The Safety Integrity Level—SIL
- The average per-demand probability of the safety functions’ failure (PFDSYS)—for safety systems operating on demand;
- The average per-hour probability of a condition failure (PFHSYS)—for safety systems operating continuously.
- Hazardous detectable failure
- Hazardous undetectable failure
- Safe (non-hazardous) detectable failure
- Safe (non-hazardous) undetectable failure.
- The short MTTF whose value is in the range 3–10 years;
- The average MTTF whose value is in the range 10–30 years;
- The long MTTF whose value is in the range 30–100 years.
4. The Multi-Barrier System
- A system for the analysis of water quality at the water intake or at the WTP, augmented by biomonitoring that assesses water quality in terms of overall pollution;
- An early warning system—information on water quality provided in advance, and based on automatic and continuous water-quality analysis carried out at a protection-and-warning station located upstream;
- A late warning system—analysis of the quality of water taken into the distribution subsystem run by both the supply company and the relevant department of Sanepid (Poland’s sanitary and epidemiological service).
5. Water Protection and Contamination Hazards
- Microbiological contaminants;
- –
- Protection of the intake against cattle and human centres (protection zones of water intakes);
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- Use of early warning systems (e.g., guard stations);
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- Stopping water abstraction during periods of high pollution, such as after storms;
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- Increasing the reliability of treatment by introducing back-up (alternative) systems;
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- Automatic closure systems preventing the supply of inadequately treated water;
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- Devices preventing flow returns.
- Chemical contaminants;
- –
- Optimisation of chlorine dosing to reduce trichloromethane;
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- Isolation of the system from potential leaks;
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- Risk assessment for suppliers of chemical agents.
- Physical contaminants;
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- Flushing the water supply network of waterworks;
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- New standard maintenance procedures to prevent sludge re-suspension;
- –
- Anti-flow backflow measures.
- Modifications in the type or the dose of coagulant or flocculant;
- The use of granular or powdered activated carbon;
- Increasing the dose of disinfectant to eliminate biological contamination.
- Raw water storage tanks;
- Clean water storage tanks.
- Time of retention;
- Frequency and scale of changes in level;
- Seasons and weather conditions;
- Type and state of the tank’s inner surface;
- Operational and functional conditions;
- Type of flow inside the tank (mixing and replacement of water, absence, or non-absence of zones of dead water).
6. The Risk Reduction Requirements and Methodology
- Hardware and software redundancy for control-system components;
- The separation of alarm systems;
- Advanced visuals of the processes relevant to the WSS;
- The training of operators on emergency management;
- The commissioning of current-process diagnostic systems and devices permitting automation;
- The introduction of Fault-Tolerant Control Systems (FTCSs), whereby [37]
- –
- The place of hardware redundancy in FTC systems is taken by analytical (information) redundancy, and therefore some software redundancy;
- –
- The place of the dynamic redundancy used primarily in controllers of automation systems is mainly taken by an approach that identifies failures of measuring and actuates devices;
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- Other diagnostic methods are deployed: these can be computer-related regarding controllers and process-related regarding measuring lines and actuators;
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- Dynamic redundancy should be considered at the earliest stages of the design.
- C—size of probable losses:
- –
- C1—small;
- –
- C2—medium;
- –
- C3—large.
- F—frequency and/or duration of the threat:
- –
- F1—rare; fairly frequent and/or short exposure time;
- –
- F2—frequent; continuous and/or long exposure.
- E—possibility of the threat being counteracted:
- –
- El—possible;
- –
- E2—impossible.
7. The Case Study
7.1. Characteristics of the Water System Analysed
7.2. An Example of the Method Being Applied
7.3. Discussion of the Obtained Paths
- Magnitude of possible consequences—C2;
- Frequency of occurrence of threat/duration of exposure to risk—F2;
- Possibility of the threat being counteracted—E1.
- More-frequent monitoring of the water supply;
- Raising the level of stability of the treated water;
- Possible ozonation and filtration through granular activated carbon
8. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Risk Parameters | Qualitative Classification | Quantitative Classification | Points Scale | |
---|---|---|---|---|
Frequency of occurrence of threat/duration of exposure to risk—F | F1 | incredible/negligible | <1 time in 30 years/ <10% of the time | 1 |
F2 | unlikely/average | 1 in >10 to 30 years/ 10–20% of the time | 2 | |
F3 | sporadic/frequent to permanent | 1 time in 10 years/≥20% of the time | 3 | |
Magnitude of possible consequences—C | C1 | noticeable organoleptic changes in water, a nuisance that is not a health hazard, few consumer complaints | less than 0.01 | 1 |
C2 | quality standards breached slightly, health problems and complaints as regards quality (e.g., odour) among consumers | 0.01 to 0.1 probable fatalities per event | 2 | |
C3 | hospitalisation of those exposed is required, information is supplied in public media | >0.1 to 1.0 probable fatalities per event | 3 | |
C4 | threat to the health or lives of consumers, serious toxic effects on indicator organisms, mass hospitalisation, fatal cases, media headlines | >1 probable fatality per event | 4 | |
Possibility of the threat being counteracted—E | E1 | routine periodic monitoring of water quality and online monitoring of selected indicators | >90% probability of hazard being avoided | 1 |
E2 | routine periodic monitoring of water quality | ≤90% probability of hazard being avoided | 2 |
Risk Category | Quantitative Gradation of Risk | PLr |
---|---|---|
Inadmissible | 16–24 | 4 |
Unacceptable | 8–12 | 3 |
Controlled | 3–6 | 2 |
Tolerable | 2 | 1 |
Negligible/No safety requirements | 1 | 0 |
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Tchórzewska-Cieślak, B.; Pietrucha-Urbanik, K.; Eid, M. Functional Safety Concept to Support Hazard Assessment and Risk Management in Water-Supply Systems. Energies 2021, 14, 947. https://doi.org/10.3390/en14040947
Tchórzewska-Cieślak B, Pietrucha-Urbanik K, Eid M. Functional Safety Concept to Support Hazard Assessment and Risk Management in Water-Supply Systems. Energies. 2021; 14(4):947. https://doi.org/10.3390/en14040947
Chicago/Turabian StyleTchórzewska-Cieślak, Barbara, Katarzyna Pietrucha-Urbanik, and Mohamed Eid. 2021. "Functional Safety Concept to Support Hazard Assessment and Risk Management in Water-Supply Systems" Energies 14, no. 4: 947. https://doi.org/10.3390/en14040947