Hazardous areas of liquid cargo ships are areas where explosive gas atmospheres are present or expected to occur in quantities sufficient to require specialized measures for the structure, installation and use of electrical equipment. According to the IEC 60092-502:1999 standard, the hazardous areas of the ship are divided and the hazard level is defined to provide the basis for the subsequent installation of electrical equipment or protective measures on the ship. Correct hazardous area demarcation is of great significance to improve the operational efficiency and operational safety of ships.
For the danger zone, the countries of the world divide it differently, but it can be roughly divided into two major factions: China and most European countries adopt the classification method of the International Electrotechnical Commission (IEC), while other countries represented by the United States and Canada use the North American classification method.
For the requirements for the division of hazardous areas, the SOLAS Convention, the IBC Rules and the relevant domestic rules all refer to standards such as IEC60092, IEC60079 or the equivalent GB3836, so the requirements for the division of hazardous areas are also requirements of the SOLAS Convention.
The comparison at home and abroad is as follows:
According to the frequency and duration of the occurrence of explosive gas environments, China divides explosive gas environments into the following three regions:
Zone 0: Area where explosive gases occur continuously or for a long time.
Zone 1: An area where explosive gas atmospheres may be possible during normal operation.
Zone 2: An area where an explosive gas atmosphere is unlikely to occur during normal operation, and even if it does occur, it is an area that occurs occasionally and exists only for a short time.
The purpose of hazardous area demarcation
The presence of explosive gases in a certain area is often caused by normal release or accidental release, so the division of the hazardous area is inseparable from the source of release. A release source is a place or location in an explosive atmosphere that can be released into the air under normal operating conditions and form a gas, vapor, fog or liquid. Such as cargo tanks, dirty oil tanks and cargo tubing ventilation or openings, cargo oil pumps, valves, flanges and other connections and pipe accessories, etc.
The purpose of hazardous area division is to identify which areas of the ship are most likely to have combustible explosive substances according to the location of the release source, to determine the dangerous area of the ship, and to further determine the area that requires the use of installed special electrical equipment.
(Figure: Schematic diagram of the hazard area division)
The form of hazardous area demarcation
According to IEC60092-502:1999, there are three forms of demarcation of hazardous areas. For ease of separation, we use the following illustration to represent Zones 0, 1, and 2.
★ The first form: divided according to the internal release source and ventilation
The demarcation of the hazardous area is only a matter of principle, and in actual implementation, it is determined according to the ventilation of the area, the presence or absence of release sources, and the type of hazardous area in the adjacent area. In addition, some premises may be treated as an exception if measures are taken on the bulkhead structure to ensure that positive pressure is maintained on the dangerous premises in terms of ventilation.
In the open area, the natural ventilation is usually good, and the gas diffusion is relatively fast and does not accumulate for a long time. However, in sheltered places, if the combustible gases are to disperse quickly without forming a build-up, strong ventilation must be used. In the first case, the corresponding hazard zone type caused by the type of release source is based on ventilation, and if the ventilation is unqualified, the level of the hazardous area is increased. If there is a Class 2 release source in the premises, if the ventilation is unqualified, it will be reduced to a class 1 hazardous area, and the level of the danger area can be appropriately reduced when the ventilation frequency and ventilation volume are increased.
★ The second form: according to the internal positive pressure protection and ventilation division;
There is an opening to the danger area, with an automatic closing device of the airtight closure facility or the area of the airtight door, if you want to be a non-hazardous area, you can use another airlock protection, the airtight door and equipped with an automatic closing device; or the use of overpressure ventilation, there are certain requirements for the ventilation rate, if the ventilation system fails, an audible and visual alarm should occur, and automatically close the non-explosion-proof electrical equipment.
An enclosed premises located in the hazardous area of the open deck, which does not contain a source of release and has an opening to the dangerous area on the open deck, shall not be considered a dangerous place if the following measures are taken. These measures include: (1) the use of airlocks at the entrance; (2) the airlocks are equipped with two self-closing steel doors without any locking devices; and (3) both the airlock premises and the enclosed places should be mechanically ventilated by normal safety areas in accordance with the regulations. (4) The air gate place maintains positive pressure relative to the external hazardous area, and the closed place also maintains positive pressure relative to the air gate place; (5) Each side of the air gate should have a sound and light indicator, if the two doors are opened at the same time, the indicator should work; (6) to monitor the immersion of liquid vapor in the air gate; (7) if the mechanical ventilation fails, an audible and visual alarm should be issued, and the power supply of all non-qualified explosion-proof equipment should be automatically cut off.
★ The third form: according to the situation of substances that may form an explosive gas atmosphere. There are four categories of substances that may create an explosive gas atmosphere (as follows):
01
Flammable liquids with flash point (closed cup test) not exceeding 60 °C;
02
Flammable liquids with a flash point (closed cup test) exceeding 60 °C and the temperature rising to within 15 °C below its flash point (closed cup test) due to the surrounding environment or the heating of the goods;
03
Combustible gases in gaseous or liquid state;
04
Reacts with other materials or products to produce flammable gaseous substances (e.g. acids)
(The division is more complicated, and the specific division can refer to the requirements of IEC 60092-502:1999.) ）
Category I: Liquid cargo ships carrying flammable liquids with flash points not exceeding 60 °C, such as oil tankers and chemical tankers. The hazard areas of such liquid cargo ships are mainly:
Zone 0: Liquid cargo compartment, oil tank, and internal space containing release sources such as cargo pipe systems (Figure 1). In view of the high risk of transporting flammable liquids and gases with flash points not exceeding 60 ° C, the entire cabin section is a danger zone 0. The hazard areas of such liquid cargo ships are mainly:
Figure 1
Zone 1: Isolated empty cabins and ballast nacelles adjacent to Zone 0 and areas within 2.4 m of height from the main deck (Figure 2). The upper part of the liquid cargo deck is generally equipped with openings for the external communication of the cargo compartment, such as the washing hatch cover, the ventilation opening of the liquid cargo compartment, and the opening of the bathymetric tube in the cabin.
Figure 2
Zone 2: The area on the main deck that is within 1.5 m around Zone 1, etc. (Figure 3).
Figure 3
It should be noted that some liquefied ships are equipped with special permeable devices, when the pressure in the liquid cargo compartment is too high, these devices will release the vapor substance in the cargo compartment into the outside air, stipulating the vertical position of the permeable pipe of this ventilation device, divided into 1 zone within the vertical cylinder with an infinite height of 6 meters as a radius, and 4 meters outside the 1 zone divided into 2 zones (figure 4).
Figure 4
The second category: liquid cargo ships carrying flammable liquids with a flash point of more than 60 °C, and due to the surrounding environment or the heating of the cargo, their temperature will not rise to 15 °C below their flash point, such as asphalt boats. Only the internal space of the liquid cargo tank, the oil tank and the release source containing the cargo pipe system can be considered as Zone 2, and the others are non-hazardous areas.
Category III: Combustible gases displayed in a gaseous or liquid state. Its Zone 0, Zone 1 and Zone 2 are equivalent to the division of Zone 2 in liquid cargo ships that transport liquid cargo flash point (closed cup test) not exceeding 60 °C combustible liquid (excluding liquefied gas). The location of the separate liquid cargo compartment where the secondary screen wall is set up is divided as follows:
Category IV: Can react with other materials or products to produce flammable gaseous substances, such as acids. It only has the division of District 1 and District 2, and the division can be seen as the area of District 0 and District 1 of the first category becoming the area of District 1 and District 2 of the fourth category.
Typical location analysis
On a liquid cargo ship, there are three typical locations, namely the release source location, the adjacent release source location and the safety position, such as the liquid cargo compartment, the adjacent area with personnel operations, and the superstructure.
Figure Division of hazardous areas
In the figure, due to the nature of the cargo, the entire section of the liquid cargo compartment is a Class 0 danger zone. Adjacent to the liquid cargo compartment, with a cargo pipe system with flanges installed, the compartment without mechanical ventilation is a Class 1 hazard zone, such as an isolated empty compartment, an empty space or a ballast compartment. The cargo pump compartment with mechanical ventilation is also a Class 1 hazard zone due to the large number of pipe systems. Such areas are prone to dangerous gas accumulation, and should be equipped with combustible gas detection alarm devices or increase explosion-proof ventilation devices to reduce the accumulation of dangerous gases or reduce the concentration of dangerous gases.
The open deck area of the liquid cargo compartment is generally equipped with an opening for the external interconnection of the cargo compartment, such as the cargo compartment outlet, gas or vapor outlet, cargo pipe flange or cargo valve, etc., and the area within 3m from the opening is a class 1 danger zone, and the area within 1.5 meters from the 1 area is divided into 2 zones. In the main deck area without an opening to the external interchange, if natural ventilation can be guaranteed to be unrestricted, the area within 2.4 m of the main deck height can be identified as Zone 2.
Some liquefied ships are equipped with special ventilation devices, such as the cargo tank ventilation port and breathing valve, when the pressure in the liquid cargo compartment is too high, these devices will release the steam material in the cargo compartment into the outside air, the vertical position of the permeable pipe of this ventilation device, divided into 1 zone within the vertical cylinder with an infinite height of 6 meters radius, and the 4 meters outside the 1 zone is divided into 2 zones.
For superstructures, since it is a place where people work and live, to avoid being affected by hazardous areas, they should be designed as non-hazardous areas. In order to ensure the safety of the living area, the empty warehouse and the cargo pump compartment are usually set up between the liquid cargo compartment, with a comprehensive monitoring and alarm system, laying "A-60" class fireproof materials, maintaining watertight separation, etc.