Fire and gas detection basics
Updated 11 Oct 2019
Modern ships may not be made of easily flammable wood but fire is still one of the most feared events onboard a ship. Although a ship afloat is surrounded by the means to fight most fires, the construction of a ship and the flammable and explosive material that may be on board make fire-fighting a very hazardous venture. Prevention and detection is therefore of the utmost importance.
SOLAS requires regular checks for fire which today is mostly done by automatic systems of various types instead of the traditional means of fire patrols, but very often a fire will be discovered by a crewman or passenger before an automatic alarm system activates. For this reason it is important that manual fire alarm points are strategically located throughout the vessel and their locations well signed. Fire patrols are still required by some administrations even if automatic systems are fitted. Engine rooms, personal cabins and cargo spaces are where most fires begin.
Fire detection systems are compulsory in ships that have periodically unattended machinery spaces. In addition to the general fire alarm system, there are other devices on board that will alert crew when a situation that could lead to a fire or explosion is occurring. Temperature sensors on machinery and oil mist detectors fitted to the main engine both fall into this category. So too do gas detectors that can monitor the build-up of explosive gases from oil and gas cargoes.
The vast majority of fire detection and alarm systems are those which make use of detectors and call points. Detectors are usually of three types sensing heat, smoke or flame. The first is by means of temperature sensors, the second by an ion chamber and the third by using infrared light to detect flicker patterns caused by flames.
For cargo holds, a different type of smoke detection is used. Sampling smoke detectors draw air from the cargo hold continually using fans and a pipework system that can also be used to send CO2 gas to extinguish a fire in the hold. In the smoke detection system, the air is tested for smoke and other combustion products before being vented to air.
A fire detection system can be in one of two configurations. These are usually described as conventional or addressable with the latter being able to pinpoint the exact area where a fire has started or the alarm has been raised. The choice between the two systems often comes down to a question of cost but there is no doubt that on a large vessel, particularly on a passenger ship, the addressable systems are far more preferable.
In operation, both systems work in much the same way and will use common components. The difference between the two systems is the way in which the detectors and call points are connected to the control panel. In a conventional system, the detectors and call points are linked to the control panel using individual wire connections, whereas in an addressable system a number of detectors and call points will be linked together using what are referred to as loops. This means that in a conventional system, although there are more wires and consequently a more expensive connection operation, the system is not as accurate as the addressable system. Each device along the loop has its own unique identifiable address but in the conventional system this is not the case.
Over the years there have been several incidents involving leakage of hydrocarbon gases from tanker cargoes. Leaks into pump rooms and other machinery spaces present a risk of explosion while leaks into void spaces and ballast tanks present a danger to crew entering those spaces. Gas detectors are used to identify the presence of these gases allowing appropriate action to be taken.
As with much of the equipment on ships, the gas detection equipment is not unique to the marine industry and is identical to devices used in several shore-based applications. The devices can be obtained from several sources including direct from manufacturers, from ship chandlers and specialist safety equipment suppliers.
Gas detection equipment measures a gas’s concentration against a calibration gas that acts as a reference point. In the early days of gas detection, monitors detected a single gas, but a modern unit can detect several toxic or combustible gases, or even combinations of them.
Detection systems for ships come in three basic types: fixed, portable and personal. Fixed gas detectors are set up in much the same way as a fire detection system with strategically-placed detectors linked to a central control panel. At least one leading manufacturer markets a combined fire and gas detection system.
Portable gas detectors consist of a device that can be carried to any location for testing its atmosphere. They will be used for a variety of purposes such as fire patrols and checking before entry into enclosed spaces is attempted. Personal devices are intended to be worn by crew when carrying out their duties in areas likely to be affected by gas leaks.
All types of gas detector require regular testing and calibration. Because of their obvious importance to safety, it is advisable for testing to be carried out before each use. SOLAS II-2 regulation 4 requires tankers to be equipped with at least one portable instrument for measuring flammable vapour concentrations, together with a sufficient set of spares and a means for calibrating such instruments. Where the atmosphere in double-hull spaces cannot be reliably measured using flexible gas sampling hoses, the spaces must be fitted with permanent gas sampling lines configured with the design of the spaces taken into account.
In addition, the pump rooms of tankers carrying cargoes with a flashpoint of below 60°C are required to be fitted with a system for continuous monitoring of the concentration of hydrocarbon gases. Sampling points or detector heads must be located in suitable positions in order that potentially dangerous leakages are readily detected.
When the hydrocarbon gas concentration reaches a pre-set level – no higher than 10% of the lower flammable limit – a continuous audible and visual alarm signal must be automatically activated in the pump- room, engine control room, cargo control room and navigation bridge to alert personnel to the potential hazard.
Details of the equipment and performance standards for fixed gas detection systems were formulated some years after the FSS Code was first published and are contained in a new Chapter 16 that was added to the code in 2007. An updated version was approved in 2010 and can be found in MSC.1/Circ.1370.
In September 2013, the IMO amended SOLAS regulation XI-1/7 relating to the carriage requirements for portable instruments to test the atmosphere of enclosed spaces for oxygen, flammable products, H2S and CO. There is something of an overlap with the requirement for portable instruments and the requirement for enclosed space entry drills and training every two months that became mandatory as of January 2015 under amendments to SOLAS (Regulation III/19). The IMO has urged flag states to accelerate the requirement for testing equipment and it may be that for some ships the need to carry equipment is already in place.
To accompany the new IMO regulation, there is an MSC circular on guidelines to facilitate the selection of portable atmosphere testing instruments for enclosed spaces. As well as a need to purchase suitable equipment, the regulation will also mean that most companies will need to amend their working practices and safety systems.
There is currently no requirement for a portable instrument on any ship type to be able to detect the potential gases that might be found in ballast tanks as a consequence of treating ballast water. Considering that some ballast systems – even explosion- proof versions – can produce highly flammable hydrogen or toxic chlorine, some concerns have been expressed as to what may happen if a fault develops in those systems.
An aspect of the type-approval for the explosion-proof ballast treatment systems is the requirement for alarms when hydrogen levels exceed certain levels and for the system to shut down if levels reach a point below the lower explosive limit, although the exact level can vary from system to system.
For systems where chlorine gas may be present, the main concern of the IMO is ensuring the gas is neutralised before or during discharge to protect the environment. However, for crew required to enter a ballast tank in a possible emergency, determining the level of either gas as well as those already covered may be a matter of life or death.