Gas carriers have specific cargo containment and handling requirements due to the highly volatile nature of their cargo. In the event that flammable gases are ignited, extinguishing solutions that can be deployed quickly and effectively whilst minimising the risk to personnel and the environment is required.
Dry Chemical powder: The Dry Chemical is a powder composed of very small particles, usually of sodium bicarbonate, potassium bicarbonate, potassium sulphates, urea-potassium based bicarbonate, potassium chloride, or mono ammonium phosphate with added particulate material supplemented by special treatment to provide resistance to packing, resistance to moisture absorption (caking), and the proper flow capabilities
Dry Power Content on ship: Potassium Sulphate
What is an expellant?
It is an inert gas that provides the internal pressure in a fire extinguisher to expel the extinguishing medium e.g. Dry chemical powder, water, foam. The expellant for a stored pressure (STP) fire extinguisher is the gas inside the unit that pressurises the extinguisher. The expellant gas pressure is recorded on the STP units gauge. The cartridge operated type fire extinguisher has the expellant gas inside a cartridge which is released manually into the extinguisher when pressure is required to expel the extinguishing medium.
Why Nitrogen is used as Expellant?
- Nitrogen (N2) is extensively used for pressuring STP fire extinguishers as it is a dry gas and will not contaminate a dry powder extinguishing medium.
- It is also the cheapest of all the acceptable gases
- Max water content is less 0.006 m/m
- N2 is transported in very high pressure cylinders (200 Bar)20000Kpaand a pressure
- reducing Regulator must be used to pressurise an STP fire extinguisher to its
- specified Internal pressure (normally 14Bars) 1400Kpa
Why Co2 not used as Expellant?
CO2 is a wet gas and Dry Chemical Powder (DCP) is hydroscopic (attracts moisture), this causes the DCP to go lumpy or hard. An STP, DCP fire Extinguisher will then fail to operate successfully.
Advantages of DCP Extinguisher
- Breaks down the chemical reaction within the fire and suppresses the flames almost immediately.
- system is an effective
- non-toxic alternative which is harmless to people and the environment
- Powder penetrates into the smallest and even partially covered locations
- Can be operable at low temperatures also
General system principles
The powder is stored in a pressure tank and is discharged by using a propelling gas, normally nitrogen & other gases can also be used.
Separately stored propellant the principle most frequently used in fixed and larger mobile systems is to have the propellant (normally Nitrogen) stored in separate tanks. The powder is stored in a tank at atmospheric pressure and pressurised when the system is activated.
Pressurizing through a pressure regulator is the normal principle for fixed installations and the basis for the systems further described here. This method gives the best control of the discharge and ensures a steady discharge rate through the whole discharge period. Endothermic reaction of the dry chemical’s main component (usually a bicarbonate of sodium, potassium or other salt) that causes absorption of heat and, consequently, favours extinction; production of carbon dioxide, consequent to the scission of the extinguishing dry chemical, that reduces the quantity of oxygen involved in combustion and formation, in some cases, of water vapour that once more reduce oxygen, When projected onto fire, extinguishing dry chemicals actually interfere with the combustion chain reactions to hinder their propagation: the larger the surface area on contact between flames and a dry chemical, i.e. the smaller the powder particles, the more effective is the dry chemical itself as a fire extinguishing agent. When the above-indicated phenomena occur and free radicals are reduced, there is corresponding progressive decrease of the combustion reaction; the whole process culminates in complete extinction.
IGC Code 2016 Regulation:
11.4.1 Ships in which the carriage of flammable products is intended shall be fitted with fixed dry chemical powder fire-extinguishing systems, approved by the Administration based on the guidelines developed by the Organization, for the purpose of fire fighting on the deck in the cargo area, including any cargo liquid and vapour discharge and loading connections on deck and bow or stern cargo handling areas, as applicable.
11.4.2 The system shall be capable of delivering powder from at least two hand hose lines, or a combination of monitor/hand hose lines, to any part of the exposed cargo liquid and vapour piping, load/unload connection and exposed gas process units.
11.4.3 The dry chemical powder fire-extinguishing system shall be designed with not less than two independent units. Any part required to be protected by 11.4.2 shall be capable of being reached from not less than two independent units with associated controls, pressurizing medium fixed piping, monitors or hand hose lines. For ships with a cargo capacity of less than 1,000 m3, only one such unit need be fitted. A monitor shall be arranged to protect any load/unload connection area and be capable of actuation and discharge both locally and remotely. The monitor is not required to be remotely aimed, if it can deliver the necessary powder to all required areas of coverage from a single position. One hose line shall be provided at both port- and starboard side at the end of the cargo area facing the accommodation and readily available from the accommodation.
11.4.4 The capacity of a monitor shall be not less than 10 kg/s. Hand hose lines shall be non-kinkable and be fitted with a nozzle capable of on/off operation and discharge at a rate not less than 3.5 kg/s. The maximum discharge rate shall allow operation by one man.
The length of a hand hose line shall not exceed 33 m. Where fixed piping is provided between the powder container and a hand hose line or monitor, the length of piping shall not exceed that length which is capable of maintaining the powder in a fluidized state during sustained or intermittent use, and which can be purged of powder when the system is shut down. Hand hose lines and nozzles shall be of weather-resistant construction or stored in weather resistant housing or covers and be readily accessible.
Maintenance Guidelines: MSC.1/Circ.1432
Monthly testing and inspections
5.6 Fixed dry chemical powder systems
Verify all control and section valves are in the proper open or closed position, and all pressure gauges are in the proper range.
Annual testing and inspections
7.9 Fixed dry chemical powder systems
1 visually inspect all accessible components for proper condition;
2 verify the pressure regulators are in proper order and within calibration; and
3 agitate the dry chemical powder charge with nitrogen in accordance with system manufacturer’s instructions.
(Note: Due to the powder’s affinity for moisture, any nitrogen gas introduced for agitation must be moisture free.)
Two-year testing and inspections
8.2 Fixed dry chemical powder systems
1 blow dry nitrogen through the discharge piping to confirm that the pipe work and nozzles are clear of any obstructions;
2 operationally test local and remote controls and section valves;
3 verify the contents of propellant gas cylinders (including remote operating stations);
4 test a sample of dry chemical powder for moisture content; and
5 subject the powder containment vessel, safety valve and discharge hoses to a full working pressure test.
10.3 Fixed dry chemical powder systems
Subject all powder containment vessels to hydrostatic or non-destructive testing carried out by an accredited service agent.
C/E PRAVIN KUMAR VL
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