Probabilistic Framework for Onboard Fire Safety

Transcription

1 1 Probabilistic Framework for Onboard Fire Safety Reliability and Effectiveness Models of Passive and Active Fire Safety Systems (D1.3) Philipp Lohrmann (BMT), Apurba Kar (IRS), Antoine Breuillard (BV) BMT - document author - - organisation name - Kostas Spyrou document approved by - - revision number - 13 January 2011 PU - submission date - - distribution level - 1

2 2 Document history Revision number Date Remarks /07/2009 Skeleton document /10/2010 Final draft for revision by partners /10/2010 Final version /01/2011 Revised version 2

3 3 Contents 1 Introduction Overview Current SOLAS regulations for passenger ships Essential systems in SOLAS Chap II-2 Part C Emergency procedures Objectives Analysis of Fire Safety Systems Automatic Fire Detection Systems System set-up Smoke Detector Heat detector Fault tree analysis of a Detector System Sprinkler Systems System set-up Sprinkler Types and their working principles Regulatory requirements (Reference FSS code) Fault tree analysis of a Sprinkler System Water Mist Systems System set-up Fault tree analysis of a Water Mist System/Hi-Fog System CO 2 Systems System set-up Fault tree analysis of a CO2 system

4 4 2.5 Foam Systems System set-up Fault tree analysis of a Foam System Reliability model of detection and suppression systems The generic model Propagation of error Maintenance intervals Application to concrete systems Heat detector Photoelectric Smoke Detector Ionisation Smoke Detector Detector panel Sprinkler system Foam System Water Mist System Hi-Fog System CO2 System Conclusions References...66 Appendix - Fault trees and diagrams used in the failure rate analysis

5 5 Executive summary A ship fire is a complex scenario that depends on a large range of parameters. While the location and cause of the fire are crucial, the sequence of events immediately after the ignition is of equal importance. In particular, the reaction of the crew and the influence of the fire suppression systems decide whether or not the fire escalates to dangerous proportions. To realistically model a ship fire, it is important to have a quantitative measure for the influence of these factors. We present a methodology to analyse the reliability of fire suppression systems. For a given (constant) failure rate, we use an exponential curve to describe the reliability as a function of time. The domain of this curve is given by the maintenance interval of the system. To determine the failure rate of each system, we refer to historical data if possible. In case such data is not available, we perform a detailed fault tree analysis of the system, taking into account redundancies and manual actions. The failure rate derived in this way is then used as input for the exponential model. For each system, we compute the average and standard deviation of the reliability over the maintenance interval. These figures might be referred to in subsequent scenario analyses carried out later in the project. 5

6 1 Introduction Overview Although the days of wooden ships are long gone, fire remains one of the most dangerous threats to seafaring vessels. In fact, accident statistics (Figure 1) show that fire is the most frequent incident on cruise ships, outstripping collisions and grounding. Figure 1- Most frequent hazards on passenger ships (1) Since 1970, the size of cruise ships has roughly doubled every decade, with the latest generation the Oasis or Genesis class (2) carrying over 5,000 passengers. This dramatic increase in size poses new challenges for the avoidance and mitigation of fire risk. To the present day, fire safety regulations, including SOLAS (3), have been based on prescribed designs and performance based approaches. The weakness of this methodology is twofold: (i) it does not provide a systematic framework for testing fire scenarios and (ii) it might be too rigid to accommodate novel designs. The aim of FIREPROOF is to provide a probabilistic framework that allows a risk-based approach to fire safety. The aim of work package 1 (WP1) is to systematically generate fire scenarios based on the set up of the vessel, including space type and fire safety systems. Each of these scenarios comes with a probability that determines its weight in the risk analysis. The total fire risk of the ship is then given by R = i σ p i C i 6

7 7 where σ denotes the set of all scenarios, p i is the probability for scenario i, and C i is a measure of the consequences (or cost) of this scenario (e.g. loss of life). In theory, this sum runs over all possible scenarios. In practice, however, it is impossible to consider all scenarios, and hence one has to preselect. In particular, all scenarios with low probability and low cost would be excluded. These scenarios constitute the input for the fire and evacuation models developed under work package 2, which in turn will be used for benchmarking in work package Current SOLAS regulations for passenger ships The IMO convention SOLAS Chap II-2 (see (3)) has been one of the first maritime chapters to be rewritten (December 1996 Amendments) in a goal based manner and with support of a Fire Test Procedures code. The goals and functional requirements are given in the first place. Then the sub-sections are given. The fire safety objectives of SOLAS Chapter II-2 are to: prevent the occurrence of fire and explosion; reduce the risk to life caused by fire; reduce the risk of damage caused by fire to the ship, its cargo and the environment; contain, control and suppress fire and explosion in the compartment of origin; provide adequate and readily accessible means of escape for passengers and crew. In order to achieve the fire safety objectives, the following functional requirements are embodied in the regulations of Chapter II-2 as appropriate division of the ship into main vertical and horizontal zones by thermal and structural boundaries; separation of accommodation spaces from the remainder of the ship by thermal and structural boundaries; restricted use of combustible materials; detection of any fire in the zone of origin; containment and extinction of any fire in the space of origin; protection of means of escape and access for fire fighting; ready availability of fire-extinguishing appliances; and minimization of possibility of ignition of flammable cargo vapour As mentioned above, the fire risk is composed of a frequency of ignition and a consequent severity. The frequency of ignition can be reduced by the prevention of fire ignition and several theoretical possibilities exist (See (4)). The severity is in fact a probabilistic distribution of severity depending on the random nature of activation and efficiency of fire safety systems as well as the random nature of the protection of persons. 7

8 8 SOLAS ChapII-2 provides the minimum rules to decrease the frequency of ignition onboard in the second part B (Prevention of fire and explosion), to manage the fire in the third part C (Suppression of fire) and finally to manage the exposed with the fourth part D (Escape). Ro-ro passenger ships have special requirements included in Part G (Special requirements for the protection of vehicles, special category spaces and ro-ro spaces). In this part, the interest is to exemplify part C with essential systems mandatory onboard passenger ships, including Ro-Pax vessels. The emergency procedures requirements are also presented Essential systems in SOLAS Chap II-2 Part C Detection and alarm (Regulation 7) Smoke detection in service spaces, control stations and accommodation spaces, including corridors, stairways and escape routes within accommodation spaces. Smoke detectors need not be fitted in private bathrooms and galleys and in spaces having little or no fire risk. (regulation 7-5.2) Manually operated call points throughout the accommodation spaces, service spaces and control stations. (regulation 7-7) An efficient patrol system shall be maintained so that an outbreak of fire may be promptly detected (Regulation 7-8) Fire detection alarms for the systems required by paragraph 5.2 centralized in a continuously manned central control station (regulation 7-9) Control of smoke spread (Regulation 8) Release of smoke from machinery spaces Draught stops. Air spaces enclosed behind ceilings, panelling or linings shall be divided by close-fitting draught stops spaced not more than 14m apart. In the vertical direction, such enclosed air spaces, including those behind linings of stairways, trunks, etc., shall be closed at each deck. (regulation 8-4) Atriums equipped with a smoke extraction system. The smoke extraction system shall be activated by the required smoke detection system and be capable of manual control. The fans shall be sized such that the entire volume within the space can be exhausted in 10 min or less Containment of fire (Regulation 9) Main vertical zones bounded by "A-60" class divisions. (regulation ) Bulkheads within a main vertical zone required to be "A" class divisions shall be at least "B" class or "C" class divisions as prescribed in the tables in regulation (regulation ) Protection of stairways and lifts in accommodation area as per regulation Penetrations in fire-resisting divisions and prevention of heat transmission as per regulation 9-3 Openings in bulkheads and decks in passenger ships as per regulation Ventilation systems as per regulation 9-7 Among others, the ventilation fans shall be so disposed that the ducts reaching the various spaces remain within the main 8

9 9 vertical zone; and stairway enclosures shall be ventilated and served by an independent fan and duct system which shall not serve any other spaces in the ventilation systems Fire fighting (Regulation 10) Automatic sprinkler, fire detection and fire alarm system in all control stations, accommodation and service spaces, including corridors and stairways. Alternatively, control stations, where water may cause damage to essential equipment, may be fitted with an approved fixed fire-extinguishing system of another type. Spaces having little or no fire risk such as voids, public toilets, carbon dioxide rooms and similar spaces need not be fitted with an automatic sprinkler system. Water supply systems, portable fire extinguishers, Fixed fire-extinguishing systems, Fire-fighter's outfits as per regulation Structural integrity (Regulation 11) The hull, superstructures, structural bulkheads, decks and deckhouses shall be constructed of steel or other equivalent material (regulation 11-2) Protection of vehicles, special category and ro-ro spaces (Regulation 20) Detection should take into account the effect of ventilation of those spaces Sample extraction smoke detection systems may be used Special category spaces shall be patrolled Structural fire protection insulation should be A60 all around the spaces except when spaces of category (5), (9) and (10) are on the other side sealable ro-ro spaces : CO2 or inert gas or high expansion foam as per resolution A123 (V) MSC.1/ Circ 1272 other : water spraying system as per resolution A123 (V) MSC.1/ Circ 1272 scuppers and possibly drainage/pumping Portable fire extinguishers Casualty threshold, safe return to port and safe areas (Regulation 21) and Design criteria for systems to remain operational after a fire casualty (Regulation 22) For new ships of 120m in length or having three or more fire zones the new regulation II-2 reg. 21 applies: Definition of casualty threshold defined as the loss of any space of origin of fire up to the nearest A class boundaries being part of the space if fitted with a fixed fire fighting system (sprinklers) or being part of the adjacent spaces otherwise. Safe return to port if fire breaks out but no casualty threshold is exceeded: a list of systems should remain operational and safe areas should be designed to accommodate the passengers. Design criteria for systems if fire breaks out and one casualty threshold is exceeded in one MVZ: listed FSSs should be so arranged and segregated as to remain operational during 3 hours in other MVZs and so that the orderly evacuation and abandonment of the ship is possible. 9

10 Emergency procedures General Emergency procedures are of primary importance with regards to safety. An effort was undertaken to identify systems required with respect to an emergency situation. In the following, II-2 means SOLAS Chapter II-2; FSS/n Fire Safety Code Chapter n, and LSA/n Life Saving Appliances Code Chapter n Detection and alarm Fire detection and fire alarm systems, according to SOLAS Chapter II-2 and FSS Code, rely on fixed fire detection (smoke detection and manual operated call point) and automatic sprinkler. The requirements are mainly encapsulated in: SOLAS Chapter II-2 Regulation 7 Detection and alarm SOLAS Chapter II-2 Regulation 10 Fire fighting FSS Code Chapter 8 - Automatic sprinkler, fire detection and fire alarm systems FSS Code Chapter 9 - Fixed fire detection and fire alarm systems The following tables summarises main requirements related to fire detection and fire alarm systems. More details, as well as technical implementations, are described in the next chapter. Smoke Manual Operated Call Point Sprinkler Location II-2/7-5.2 In service spaces, control stations and accommodation spaces, including corridors, stairways, escape routes within accommodation spaces and ro-ro spaces (II-2/20.4.1) II-2/7-7 Throughout the accommodation spaces, service spaces, control stations and ro-ro spaces (II- 2/ ) II-2/ In all control stations, accommodation and service spaces, including corridors and stairways Section FSS9/ Grouped into sections System with individually identifiable fire detectors: a section may serve spaces on both sides of the ship and on several decks, but shall not be situated in more than one main vertical zone. FSS9/ Grouped into sections FSS8/ Grouped into separate sections. Each of which shall contain not more than 200 sprinklers. Shall not serve more than two decks and shall not be situated in more than one main vertical zone. System control: Indication FSS Visual and audible fire signal at the control panel and indicating units. As per smoke FSS8/ Visual and audible alarm signal automatically at one or 10

11 11 signal more indicating units System control: location FSS9 / The control panel: on the navigation bridge or in the continuously manned central control station. / Indicating unit: - At least one, be so located that it is easily accessible to responsible members of the crew at all times. Ditto FSS8/ On the navigation bridge or in the continuously-manned central control station. In addition, in a position other than on the aforementioned spaces to ensure that the indication of fire is immediately received by the crew. - One be located on the navigation bridge if the control panel is located in the main fire control station. System control: identificatio n FSS9/ Indicating units shall, as a minimum, denote the section. Ditto FSS8/ units shall indicate in which section served by the system a fire has occurred Table -1: Alarm and indication In addition, a fire patrol shall be maintained so that an outbreak of fire may be promptly detected (II-2/7-8-1) Control and monitoring of systems General Active fire protection systems should be controlled and monitored. On passenger ships, this is mainly centralized in a continuously manned central control station (CMCCS). According to SOLAS Chapter II-2 Regulation 3 Definition : Continuously manned central control station is a central control station which is continuously manned by a responsible member of the crew Central control station is a control station in which the following control and indicator functions are centralized: 1. fixed fire detection and fire alarm systems; 2. automatic sprinkler, fire detection and fire alarm systems; 3. fire door indicator panels; 4. fire door closure; 5. watertight door indicator panels; 6. watertight door closures; 11

12 12 7. ventilation fans; 8. general/fire alarms; 9. communication systems including telephones; and 10. microphones to public address systems. Control stations are those spaces in which the ship's radio or main navigating equipment or the emergency source of power is located or where the fire recording or fire control equipment is centralized. Spaces where the fire recording or fire control equipment is centralized are also considered to be a fire control station. Additionally, new ChapII-2 reg. 23 imposes New Passenger ships (>1 st July 2010) to have one safety centre be part or be close to the navigation bridge and should 1. be ergonomic 2. have communication means with CMCCSs, bridge, storage rooms for FSSs and fire equipment lockers 3. be capable to control and monitor of functionalities of FSSs: all powered ventilation systems; fire doors; general emergency alarm system; public address system; electrically powered evacuation guidance systems; watertight and semi-watertight doors; indicators for shell doors, loading doors and other closing appliances; water leakage of inner/outer bow doors, stern doors and any other shell door; television surveillance system; fire detection and alarm system; fixed fire-fighting local application system(s); sprinkler and equivalent systems; water-based systems for machinery spaces; alarm to summon the crew; atrium smoke extraction system; flooding detection systems; and fire pumps and emergency fire pumps. Control and monitoring from the continuously manned central control station In addition to previously discussed requirements on fire detection alarms, according to II-2/9-3 Passenger ships carrying more than 36 passengers shall have the fire detection alarms centralized in a continuously manned central control station. The following table summarises control and monitoring requirements for ventilation fans and fire doors. Location Ventilation fan II-2/ In general, ducts should remain in the MVZ Fire door MVZ Stairway enclosure 12

13 13 Separated system for stairway enclosures and dangerous areas Galley boundaries Ro-ro spaces boundaries CMCCS Control II-2/7-9.3 Remote shutting down II-2/7-9.3 Remote closing and be capable of reactivation CMCCS Indicator Control panel Other requirements II-2/7-9.3 closed or off status II-2/ Controls so grouped that that all fans may be stopped from either of two separate positions II-2/7-9.3 & II-2/ Open or closed positions II-2/ A door closed remotely from the central control station shall be capable of being re-opened from both sides of the door by local control. After such local opening, the door shall automatically close again Table -2: Control and monitoring: ventilation fans and fire doors Other Control and monitoring The following systems could also been controlled and/or monitored from the CMCCS. Location Control Indicator Fire damper II-2/ Where it is necessary that a ventilation duct passes through a main vertical zone division, a fail-safe automatic closing fire damper shall be fitted adjacent to the division II/9-7.3 Ducts with a free cross-sectional area exceeding 0.075m 2 shall be fitted with fire dampers through "A" class bulkheads or decks Fail-safe automatic Be capable of being manually closed from each side of the division Shall be fitted on at least one side of the division with a visible indicator showing whether the damper is in the open position Smoke extraction system II2/8-5 Atrium By the required smoke detection system (*) Be capable of manual control Table -3: Control and monitoring: Fire damper and smoke extraction system (*) According to IMO MSC/Circ.1034 (see (5)): Guidelines for smoke control and ventilation systems for internal assembly stations and atriums on new passenger ships, The automatic activation of the smoke extraction system, by means of the smoke detectors, may be delayed for up to 2 min from the activation of the first detector, if not 13

14 14 acknowledged. This 2 minute delay is intended to allow for crew verification of the smoke detector alarm. This Circular is not a mandatory text but should be brought to the attention of interested parties. 1.3 Objectives As stated above, the aim of work package 1 is to create a methodology for the systematic generation of fire scenarios and their incident probabilities. The aim of task 1.3 is to develop a probabilistic model for the reliability and effectiveness of active and passive Fire Safety Systems (FSS). In this context, active FSS are any systems that are activated in the event of a fire and include detection systems (smoke and heat detectors), automatic suppression systems (sprinklers, CO 2 systems etc) as well as any manual measures. In contrast, passive FSS include structural features like fire walls and fire doors. The analysis done within this task focuses on the reliability of automatic FSS. A methodology is developed that can in principle applied to any system, provided that its setup is known and the failure rates of its components are available. We will analyse the following systems: Smoke detectors; Heat detector; Alarm panel/detector system; Sprinkler; Foam system; Water mist system and Hi-Fog system; CO 2 system. For each system, we perform fault tree analyses as necessary and express the reliability as a function of time over the maintenance interval. 14

15 15 2 Analysis of Fire Safety Systems An important part of this task is the analysis of the Fire Safety Systems used on cruise ships. The reliability and efficiency of these systems are crucial input parameters for any fire simulation model that massively influence the incidence probabilities and consequences of fire scenarios. In this chapter, we describe in technical detail some typical FSS used on cruise ships. In particular, we give an overview over the functionality of these systems, list its components and briefly describe regulatory requirements. We analyse fault trees of the relevant systems to obtain values for their failure rates. 2.1 Automatic Fire Detection Systems System set-up Definition An Automatic Detection System comprises of detectors, manually operated call points, a control panel displaying the location of the source of a fire and a power supply. Its purpose is to detect a fire at its initial stage and to automatically activate an alarm. Application as per SOLAS and FP 53/3/1 convention (6): Table 4 - SOLAS regulation for detection systems Operation Any required fixed fire detection and fire alarm system with manually operated call points shall be capable of immediate operation at all times (This does not require a back-up control panel). In passenger ships, the fixed fire detection and fire alarm system shall be capable of remotely and individually identifying each detector and manually operated call point. Fire detectors fitted in passenger ship cabins, when activated, shall also be capable of emitting, or cause to be emitted, an audible alarm within the space where they are located. There shall be not less than two sources of power supply for the 15

16 16 Power Supply Total operation Smoke detector Heat detector electrical equipment used in the operation of the fixed fire detection and fire alarm system, one of which shall be an emergency source of power. Separate feeders reserved shall provide the supply solely for that purpose. Such feeders shall run to an automatic changeover switch situated in or adjacent to the control panel for the fire detection system. There shall be sufficient power to permit the continued operation of the system with all detectors activated, but not more than 100 if the total exceeds this figure Required in all stairways, corridors and escape routes within accommodation spaces shall be certified to operate before the smoke density exceeds 12.5% obscuration per meter, but not until the smoke density exceeds 2% obscuration per meter. Smoke detectors to be installed in other spaces shall operate within sensitivity limits to the satisfaction of the Administration having regard to the avoidance of detector insensitivity or over sensitivity. Heat detectors shall be certified to operate before the temperature exceeds 78ºC but not until the temperature exceeds 54ºC, when the temperature is raised to those limits at a rate less than 1ºC per min. At higher rates of temperature rise, the heat detector shall operate within temperature limits to the satisfaction of the Administration having regard to the avoidance of detector insensitivity or over sensitivity. Operating temperature The operation temperature of heat detectors in drying rooms and similar spaces of a normal high ambient temperature may be up to 130ºC, and up to 140ºC in saunas 1.Control and monitor input signals from all connected fire and smoke detectors and manual call points 2. Provide output signals to the navigation bridge, continuously manned central control station or onboard safety centre to notify the crew of fire and fault conditions 3. Monitor power supplies and circuits necessary for the operation of the system for loss of power and fault conditions Control panel 4. May be arranged with output signals to other fire safety systems including - paging systems, the fire alarm or public address system, fan stops, fire doors, fire dampers, the sprinkler system, the smoke extraction system, the low-location lighting system, the fixed local application fire-extinguishing system, the CCTV system, other fire safety systems; and fuel system quick closing valves. 16

17 17 The maximum spacing of detectors Table 5 - Spacing of detectors Type of detector Maximum floor area per detector (m2) Maximum distance apart between centers (m) Maximum distance away from bulkheads (m) Heat Smoke Regulatory Requirements (FSS code, FP 53/3/1 ): Fixed fire detection and fire alarm systems with individually identifiable fire detectors shall be so arranged that: 1. means are provided to ensure that any fault (e.g., power break, short circuit, earth, etc.) occurring in the section will not prevent the continued individual identification of the remainder of the connected detectors in the section; 2. all arrangements are made to enable the initial configuration of the system to be restored in the event of failure (e.g., electrical, electronic, informatics, etc.); 3. the first initiated fire alarm will not prevent any other detector from initiating further fire alarms; and 4. no section will pass through a space twice. When this is not practical (e.g., for large public spaces), the part of the section which by necessity passes through the space for a second time shall be installed at the maximum possible distance from the other parts of the section. 17

18 18 Cables used in the electrical circuits shall be flame retardant according to standard IEC On passenger ships, cables routed through other main vertical zones that they serve, and cables to control panels in an unattended fire control station shall be fire resisting according to standard IEC 60331, unless duplicated and well separated. Detectors shall be located for optimum performance. Positions near beams and ventilation ducts, or other positions where patterns of airflow could adversely affect performance, and positions where impact or physical damage is likely, shall be avoided. Detectors shall be located on the overhead at a minimum distance of 0.5 m away from bulkheads, except in corridors, lockers and stairways. The activation of any detector or manually operated call point shall initiate a visual and audible fire detection alarm signal at the control panel and indicating units. If the signals have not been acknowledged within 2 min, an audible fire alarm shall be automatically sounded throughout the crew accommodation and service spaces, control stations and machinery spaces of category A. This alarm sounding system need not be an integral part of the detection system. Functionality A typical detection system has some sensing elements which can sense the presence of smoke or heat caused by a fire in or near to the affected area. Subsequently these sensors send an electrical signal to the fire alarm panel or to specific safety equipment for activation. When this signal activates the audio-visual alarms on the panel, the concerned crew members are alerted. The panel helps them to quickly identify the fire-affected zone and thus to take appropriate countermeasures like fire extinguishing or evacuation. The responsible crew member is expected to acknowledge the alarm by pressing a cancel button within two minutes of its activation which silences the hooter/alarm. If this is not done, then the alarm gets activated in all areas/zones. The control panel has alarms for the following fault situations: On detection of fire and smoke Failure of main supply or low voltage of the battery backup system. Excessive current (fuse gets blown). Fault in the electrical power cable. 18

19 19 Technical details of the Fire Control Panel Primary Power supply: Commonly a non-switched 120 or 240 V AC source supplied from a commercial power utility. In non-residential applications, a branch circuit is dedicated to the fire alarm system and its constituents. "Dedicated branch circuits" should not be confused with "Individual branch circuits" which supply energy to a single appliance. Secondary (backup) Power supplies: This component, usually sealed, lead-acid storage batteries or other emergency sources including generators, is used to supply energy in the event of a primary power failure. Initiating Devices: This component acts as input to the fire alarm control unit and are either manually or automatically activated. Notification appliances: This component uses energy supplied from the fire alarm system or other stored energy source to inform crew members of the need to take action, usually to evacuate. Safety Interfaces: This interface allows the fire alarm system to control aspects of the built environment and to prepare the place for fire and to control the spread of smoke fumes and fire by influencing air movement, lighting, process control, human transport and exit Smoke Detector Definition A smoke detector is a device that monitors the smoke concentration in the surrounding air. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionisation), while others use both detection methods (hybrid system) to increase sensitivity to smoke. Smoke detectors are usually powered by a central fire alarm system, which in turn is powered by the main power supply system with a battery backup. Types of Smoke Detectors Optical This detector type is based on the fact that smoke particles scatter light. It typically consists of a light source (bulb or infrared LED), a lens to collimate the light into a beam, and 19

20 20 a photodiode or other photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in a straight line and misses the detector. When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm. Ionisation This type of detector is based on the principle that α-radiation is attenuated by smoke particles. A typical source of radiation is a small sample of Americium-241 which has a half-life of over 430 years. The α-particles emitted by the sample pass through the ionisation chamber, an air filled space between two electrodes. The radiation ionises gas molecules within the ionisation chamber, which allows a small electric current to flow between the electrodes. When smoke particles enter the ionisation chamber, they absorb the radiation and hence reduce this current, which triggers the alarm. While ionisation smoke detectors tend to be cheaper than optical ones, they are also more prone to false alarms Heat detector Definition A heat detector is a device which responds to the changes in ambient temperature in the vicinity of the detector. A predetermined threshold value is set in the detector system. When the ambient temperature exceeds this threshold value, it sends out an electrical signal to trigger the alarm. In some cases the automatic sprinkler and detection functions are combined in one single device, such that when the temperature exceeds the threshold limit both the fusible bulb of the sprinkler gets fused causing discharge of water and the detector alarm gets activated simultaneously. Heat detectors are generally of three basic types, depending on the physics of operation: rate of rise type, fixed temperature and rate compensation. Yet again, to increase sensitivity any combination of these two types is used. Types of heat detectors in comparison Table 6 - Types of detectors Criterion Rate of rise detector Fixed temperature Detector Rate compensation detector 20

21 21 Type of detection Rate-of-rise detectors sense the rate of change in ambient temperature. Fixed-temperature heat detectors operate when the temperature of the operating elements reaches a specific point. This type of detector also activates on sensing the rate of rise of ambient temperature. However, at the lower temperature range it acts as a Fixed temperature type and at the higher range of ambient temperature, in particular close to the set value, it works as rate-ofrise type. Thus it provides more warning time before the temperature actually reaches the set value. Operating range Operating range is about 12 to 15 F (7-8 C) per min. Operating temperature is about 135 F (58 C) (up to 5 F per minute) Areas where it is used Applied in areas subjected to high ambient temperature, i.e. boiler rooms or places which require detectors that operate only if close to fire. Used mainly in unattended spaces to detect smoldering fires that increase the temperature of a detector above its design value, usually 135 F to 145 F or 185 to 200 F. Places where there are low rates of temperature change Combination type heat detectors This type of detector has multiple sensor elements to respond to the presence of fire and/or smoke. There are many variations in the design of such detectors such as: response from any element, response from a partial combination of all elements or complete response from all elements. Generally, the principle of operation of the first type is to respond to both the rateof-raise and fixed-temperature scenario. The operation based on rate-of-rise of temperature is very useful and effective in case when fire develops rapidly because it requires much less response time. In contrast, the elements which operate on the principle of fixed-temperature respond slowly to the development of fire because time is required to reach the set temperature, as such this is useful for the case of slowly developing fire. In general, the common features of the construction of a combination detector include a vented air chamber and a flexible diaphragm for sensing the rate-of-rise of temperature. On the other hand, the elements based on the fixed-temperature principle usually consist of a leaf-spring restrained by 21

22 22 a eutectic metal which fuses on reaching the temperature at the element to its preset temperature, closing and hence activating the circuit Fault tree analysis of a Detector System We use the fault tree analysis to obtain a value for the failure rate of an Automatic Detector System. To obtain failure rates for individual components, the OREDA handbook (7), (8) has been used. In cases where it didn t provide the required information, RIAC data (9) has been referred to. The type of the failure rate distribution for various components has been assumed to be Exponential as indicated in the OREDA handbook. All abbreviations used in the table below are same as those in the OREDA and RIAC data books. Item Failure Mode Mean (λ) per 10 6 hrs Standard Dev. Mean time to failure (hrs) Data Source Remarks Pump Centrifugal, Sea Water MD- Critical OREDA This item has been considered for all sea water lifting pumps in the fault tree. Valve General Critical OREDA For all valves same value has been used. Compressor general Pump centrifugal General Fire and Gas Critical, breakdow n MD- Critical, breakdow n UD- Critical, OREDA OREDA For pumps other that sea water lift these values have been used OREDA 22

23 23 detector General spurious operation WB- Critical PrC- Critical Table 7 - Failure rates for components from OREDA The data for the following items has been taken from the RIAC handbook. We have assumed that these items also follow an exponential distribution like those given in OREDA. 23

24 24 ITEMS Part Description Quality Level Hooter Alarm (annunciator, buzzer) Application Environment (λ) per 10 6 hrs Mean time to failure (hrs) Data Source Remarks COM GB RAC No item in name of Hooter in the RAC, data for Alarm considered Ventilator Ventilator, air UNK GM RAC - circulator Timer Timer(summary) RAC - Relay switch or other failure in panel Panel,fire control system MIL GM RAC - Switch Fail Push Button (Summary) COM GM RAC Push button type switch has been considered. Cable connection Cable, electrical UNK GM RAC, - Audio visual Device Alarm (Summary) RAC - Battery DB, BL- Battery Battery, Rechargable COM N RAC - 24

25 25 Power Supply MS-Uninterrupted COM GF RAC For main supply Pressure switch ES-Diesel Generator (summary) Switch, Pressure (Summary) COM GF RAC For emergency supply RAC - Pressure Gauge Gauge, Pressure UNK GM RAC - Level Indicator Indicator, liquid level UNK GM RAC - Strainer Strainer, Water COM GF RAC - Nozzle Nozzle (Summary) RAC - Solenoid Valve Valve, Solenoid (Summary) UNK GM RAC - Pipe Pipe, water COM GF RAC - 25

26 Actuator Pipe UNK GM RAC - Pipe(AP) Fiber optic cable Assumed This Item is supposed to be reliable. We have assumed its reliability Table 8 - Component failure rates taken from RIAC value as 0.85 over 8000 hrs. of mission time and the MTTF value estimated accordingly

27 27 In some cases, no data is available. In these cases expert judgement has been used to obtain approximate failure rates. The components in question are given in the table below. ITEMS for which Static Reliability has been assumed* Assumed Reliability Operator fails to take action 0.85 DOOR 0.85 Cylinder 0.85 Battery general 0.85 Table 9 - Static reliabilities *Static reliability is independent of mission time. It is considered as availability on demand. The fault tree for a Detector System is given in the appendix. We have used the software BlockSim to analyse the tree. We obtain the following failure rate for the whole system: λ = days 2.2 Sprinkler Systems Definition A sprinkler system is a fixed installation that discharges water in case of a fire. It is generally used for fighting and suppressing fire in control stations, accommodation and service spaces, as well as corridors and stairways. 27

28 28 Precautionary note: In the spaces where water may cause damage to essential equipment, it may be fitted with an alternative approved fixed fire-extinguishing system (SOLAS Regulation ) System set-up A typical sprinkler system consists of three parts: pressure tank, air compressor and sprinkler pump. A sprinkler system is generally installed for the purpose of fire extinguishing in accommodation areas and service spaces. These are fixed type installations. The system consists of a network of sprinkler heads located in the areas that need to be protected, with an appropriate spacing. Due care needs to be taken for placing the heads so that these do not get damaged by mechanical contacts or impacts. There are different types of sprinkler heads, differing by their mechanism of operation. One very common type is a glass bulb head, used for automatic activation. The glass bulb gets fused at a predetermined temperature in the wide range of 50 C to 300 C (10), (11). Usually the network of sprinkler heads is divided into sections. As per FSS regulations, each of these sections must not have more than 200 sprinkler heads. The sections are equipped with valves which trigger signals for activation of both audible and visual alarms when water flows through them. Thus it is possible to identify which section of the ship has been affected by fire. There is a system fitted in the main control room which automatically keeps track of all the sections of sprinkler heads. Functional diagrams of the system are given below. 28

29 29 Is o la tio n V a lv e s W a te r c o n n e c tio n fro m s h o re T o S p rin k le r H e a d s to a u to m a tic sprinkler system C o m p re s s o r P re s s u re S w ic h e s re d u c e r C o m p re s s e d a ir T o B ilg e F re s h w a te r F ilte rs S a fe ty V a lv e h y d ro p h o re ta n k L e v e l L e v e l S w ic th J o c k e y P u m p P in d ic a to r P u m p P u m p s L e v e l in d ic a to r P re s s u re T a n k T o B ilg e E n g in e R o o m F ire p u m p T o B ilg e F w d. P u m p R o o m S e a C h e s t F w d. P u m p R o o m S e a C h e s t P P re s s u re C o n tro l P S ta rt M P a P S to p M P a P re s s u re S w itc h e s 1 : C o m p re s s o r S ta rtin g P s ta rt :- 0.7 M P a, P s to p :- 0.8 M P a 2 : S p rin k le r P u m p S ta rtin g P start :- 0.5 MPa 3 : L o w P re s s u re S ig n a llin g P start : MPa Figure 2 - Functional diagrams of a sprinkler system 29

30 30 The regulations demand that each section of sprinklers can be isolated by one stop valve only. The stop valve in each section should be readily accessible in a location outside of the associated section or in cabinets within stairway enclosures. Additional isolation valves may be fitted for the purpose of servicing, maintenance or for refilling of antifreeze solutions in the sprinkler piping. Audio-visual alarms are provided for these additional isolation valves when fitted. The quantities of spare nozzles carried on the ships are as follows (FSS Code): Table 10 - Nozzles Total number of nozzles Required number of spares < to > The number of spare nozzles of any type need not exceed the total number of nozzles installed of that type Sprinkler Types and their working principles As per Resolution A.800(19), the types of Sprinklers Systems are (12): Wet pipe systems Dry pipe systems Deluge systems Antifreeze systems Pre-action system Water-based extinguishing medium Wet pipe systems This type of sprinkler is the most widely used system. It is the simplest type of sprinkler and also the only type which can be operated automatically. Its active components are an automatic sprinkler head and usually (but not always) an automatic alarm check valve. This simplicity makes the system very reliable as compared to the other types. The pipelines of the 30

31 31 water supply system for this type of sprinkler arrangement always remain filled with fresh water (in order to prevent corrosion) under pressure and get activated automatically with the fusing of the sprinkler heads. The sprinkler heads get fused and open for discharge of water only when the temperature reaches a threshold value. Working Principle: When the sprinkler heads exposed to the threshold temperature, the heat sensitive element (glass bulb or fusible link) is released, allowing water to flow from that sprinkler. The glass bulb or link applies pressure to a cap which acts as a plug preventing water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler. Because each sprinkler activates independently when the predetermined heat level is reached, the number of sprinklers that operate is limited to only those near the fire, thereby maximising the available water pressure over the location of the fire Dry pipe systems On vessels operating in areas where the ambient temperature is below the freezing point of water, the wet pipe system is not suitable to use because the static water inside the pipe might freeze, causing blockage of the water flow. In such cases a dry pipe system is a suitable option. In dry pipe systems, the pipes are kept pressurised with air or nitrogen instead of water when the system is in idle state. Water is discharged only after the air/nitrogen is released on fusing of the sprinkler bulb when exposed to the threshold temperature. Hence in the initial stage only air/nitrogen is vented out of the sprinkler head which does not contribute to extinguishing the fire. The actual effect of fire suppression begins when water is discharged through sprinkler nozzles. Due to this delay this system is not as effective as the wet pipe system in the initial stages of a fire. Working Principle: When the temperature at the sprinkler head reaches the threshold level, it opens out, allowing the air or nitrogen to vent out through the sprinkler head. Thus the pressure in the pipeline drops. This leads to a pressure gradient across the dry pipe valve, allowing water to enter in to the sprinkler piping system. The venting of air or nitrogen from the sprinkler in the initial stage causes the delay in the venting of water from the sprinklers needed to control the fire. Therefore, during the initial stages of fire the dry pipe systems are usually not as effective as wet pipe systems for the suppression of fire Comparison of Dry Pipe fire Sprinkler systems and Wet Pipe Sprinkler system Table 11 - Different types of sprinkler systems Criterion Dry Pipe System Wet Pipe System 31

32 32 Complexity Dry pipe systems require additional control equipment and air pressure supply components which increases its complexity. Wet pipe systems are simple, with the only operating components being the automatic sprinkler heads. Pipes are filled with water under pressure so no extra components are required. Installation and Maintenance costs The added complexity impacts the overall dry-pipe installation costs, and increases maintenance expenditure primarily due to added service labour costs. Its simplicity and wet pipe reduces maintenance and installation costs. Corrosion potential After operation or testing, the dry-pipe sprinkler system s piping is drained, but residual water may collect in low spots, and moisture is also retained in the atmosphere within the piping. This moisture, coupled with the oxygen increases pipe internal wall corrosion rates, possibly leading to leaks. Wet pipe systems are filled with water all the time, so oxygen exposure is reduced, hence less chance of corrosion or leakage. Application Dry pipe systems are usually only used in spaces in which the ambient temperature may be cold enough to freeze water. Wet pipe systems cannot be used in places where the ambient temperature drops below the freezing point of water Deluge Systems As per IMO Resolution A.800(19), "Deluge" systems are systems employing open sprinklers attached to a piping system connected to a water supply through a valve that is opened by the detection system installed in the same areas as the sprinklers. In these systems, the sprinklers fitted are of open type (i.e. without any bulb) and the sprinkler network piping system is not filled with liquid or gas under normal condition. This type of system is suitable to rapidly 32

33 33 cover large areas affected by fire. Sometimes it is also used to prevent fire from entering from other adjacent locations. Under normal conditions (i.e. when there is no fire), the ingress of water into the sprinkler network piping is prevented by a deluge valve fitted at the inter-connection of the water supply line with the sprinkler network piping. Working Principle: On activation of the automatic detector or manual call point the corresponding signal is transmitted to the control panel, which in turn signals the deluge valve to open, allowing water to pass into the sprinkler network piping system and water is instantly discharged through the sprinkler heads Antifreeze Systems This type of system is a wet pipe sprinkler system employing automatic sprinklers attached to a piping network containing an antifreeze solution and connected to a water supply (Resolution A.800(19)). Working principle: When an automatic sprinkler is exposed to sufficient heat, the heat sensitive element (glass bulb or fusible link) is triggered, allowing antifreeze solution to discharge. Immediately after this, water flows from the sprinkler head Pre-action Systems This type of sprinkler system employs automatic sprinklers attached to a piping system containing air that may or may not be under pressure, with a supplemental detection system installed in the same area as the sprinklers (Resolution A.800(19)). Working Principle: Actuation of the detection system opens a valve that permits water to flow into the sprinkler piping system and to be discharged from any sprinklers that may be open Construction and Working Principle of a Sprinkler Head Fusible sprinkler heads can be used for both detection and extinguishing of fire. A typical fusible sprinkler head is covered by a semicircular quartzoid bulb, which contains an expandable liquid. A deflector plate is attached at the end of the bulb. This plate serves the purpose of spraying the water over a larger area (Figure 3). When the bulb is exposed to heat, it causes the liquid inside the bulb to expand, leading to breakage of the bulb, and thus allowing the water to flow from the sprinkler head. The water supply system for this sprinkler arrangement features a tank pressurised with compressed air to keep the water in the pipes at a constant pressure. When the bulb fuses and the water flows out, the pressure in the pipe lines drops, which causes a salt water pump to start automatically. This salt water pump provides a continuous supply of water to the sprinkler system. In case of a wet sprinkler system, the piping and heads are initially filled with fresh water under pressure in order to protect the system from corrosion. 33

34 34 Figure 3 - Sprinkler head Regulatory requirements (Reference FSS code) Table 12 - Regulations on sprinklers Type of operation -Automatic in operation, -Capable of both detecting the fire and suppressing the fire. Pressure Tank capacity -A pressure tank should have a volume of at least twice that of the charge of fresh water, equivalent to the amount of water which would be discharged in 1 min by the pump. -Means of replenishing the air under pressure and of replenishing the fresh water charge in the tank -Provision to prevent passage of sea- water into the tank. 34