To prevent the introduction of harmful aquatic species into new environments by shipping which have been thought to be a major threat to marine ecology and have been devastating in many areas of the IMO adopted the Ballast Water Management Convention in 2004 at a diplomatic conference in London and eventually the convention entered into force on 17th September 2017.
More than 65 states representing more than 70% of the total world tonnage signed the BWM Convention.
Nitrogen gas generators are used to produce clean and dry nitrogen of higher purity for inerting, purging and cargo padding.
Mainly there are two types of nitrogen generators are widely used on board:
Nitrogen may also be supplied from ashore in nitrogen cylinders and stored on board, but nitrogen generators are most convenient way for inerting, purging and cargo padding.
Requirement of nitrogen used as inert gas on chemical tankers:
On tankers nitrogen is not used for fighting fire but for cargo quality control.
Some cargoes are very much oxygen sensitive to react with and may turn to out of specification. Some cargo may also react with carbon di oxide in the flue gas. Other cargoes are highly sensitive to moisture and may liable to discoloration. The cargoes require inert gas of extreme purity to exclude oxygen from the tanks.
Oil fired flue gas contains less oxygen but at the same time it contains soots and carbon di oxide which may react with cargoes. For these reasons oil fired flue gas systems are rarely used on chemical carriers.
Inerting:
The introduction of inert gas into a tank with the object of attaining the inert condition.
Purging:
The introduction of inert gas into a tank already in the inert condition with the object of:
(1) further reducing the existing oxygen content; and/or
(2) reducing the existing hydrocarbon gas content to a level
Topping up: The introduction of inert gas into a tank which is already in the inert condition with the object of raising the tank pressure to prevent any ingress of air.
Nitrogen generator can be operated in two modes, based on purity
Nitrogen generator can also be operated on fresh air mode where it supplies air from the atmosphere to the cargo tanks.
Components of N2 generator:
Air compressor unit: It consists of a number of motor-driven screw compressors. This compressor unit must be supplied with the cooling arrangement.
Air dryer: It removes moisture from the compressed air. To ensure proper functioning of the separating unit moisture must be drained out.
Air filter unit: This unit consists of air filter and an activated carbon tower. The air filter separates oil and impure substances from the air delivered by the air compressor. It may also be fitted with auto drain to facilitate drainage. Sometimes additionally an air heater is also fitted. This is of vital importance to remove oil from the air to protect the carbon molecular sieve or membranes. The activated carbon tower prevents any oil, moisture, and foreign material to enter the PSA unit in the event of malfunction of air dryer and air filter units.
PSA (Pressure swing adsorption unit):
The adsorbent packed in the adsorption tank selectively adsorbs oxygen gas under the pressurized condition and separates nitrogen gas.
Adsorption is a process in which a substance accumulates on the surface of a solid to form a very thin film. The principle of oxygen separation is that the major constituents of air (nitrogen and oxygen) are adsorbed to a different extent when passed over a carbon-molecular sieve material.
The sieve adsorbs most of the oxygen from the air, allowing the nitrogen to pass through and be collected. The oxygen is desorbed and vented to atmosphere. The carbon-molecular sieve material also adsorbs a number of other gases, i.e.: carbon dioxide and water vapor.
Buffer tank is provided to ensure constant flow of pure nitrogen gas throughout the operation.
The cycle of adsorption, desorption and filling is repeated by every minute to produce nitrogen gas continuously.
Membrane unit:
Selective permeation is the basic principle of gas separation by membrane modules. Permeation of a gas is the ability to dissolve and diffuse through a membrane. Gases are separated as their permeability. Gases with higher diffusion rate and lower molecular sizes can penetrate the membranes faster than gases with larger molecular size and lesser diffusion rate.
The ‘slow’ gases are methane, nitrogen and carbon monoxide, the ‘medium’ gases are argon and oxygen, and the ‘fast’ gases are water vapour, hydrogen and carbon dioxide. O2, H2O, CO2
The “fast” gases, permeate through the membrane wall much faster than the “slow” gases, thus separating the original mixture into two streams.
The membrane unit is made up from bundles of thin hollow fibers which give a large wall area for separation. Each membrane module contains millions of hollow fibers. The membrane bundles are enclosed in pressure vessel pipes of about 100 to 200 millimeters diameter; several of these bundles may be arranged in parallel.
As the two main components of air are nitrogen and oxygen and they have different permeation rates means they can be separated by membranes once they pass through.
Hence the high velocity gases pass through the membranes and vented to atmosphere allowing only nitrogen to come out from the membranes.
An oxygen sensor is fitted at the outlet of purified product gas that indicates the level of purity. i.e. oxygen content. When the oxygen level comes down to desired level it allows pure nitrogen gas to pass to the deck and close the vent valve leading to atmosphere. The efficiency of the separation depends on the flow rate through the membranes; a control valve is used to regulate the flow and thereby the oxygen content. Oxygen enriched air is vented as a waste gas, which must be exhausted to a safe area. To ensure reliability of the measured values oxygen analyser needs periodic calibration. AIR point calibration is sufficient when using for tasks such as measuring oxygen concentration in an inert gas which does not contain a degrading gas. SPAN point calibration needs if the sensor has been degraded or the sensor unit has been replaced.
MARPOL:
Annex VI 0.50% Sulphur Limit: The limit for sulphur in fuel oil used on board ships to be reduced to 0.50% m/m (mass by mass).
SOLAS:
Amendments to SOLAS regulations III/3 and III/20 to make mandatory the requirements for maintenance, thorough examination, operational testing, overhaul and repair of lifeboats and rescue boats, launching appliances and release gear.
Amendments to SOLAS regulation II-2/13 to extend the requirements for evacuation analysis to all passenger ships, not just RO-RO passenger ships.
In less than a week time, From 1st January 2020, the maximum permissible global Sulphur content of marine fuels is going to be reduced from
3.5 % to 0.5% according to MARPOL Annex VI. Ships operating outside
Emission Control Areas (ECAs) will have three options to achieve compliance:
A greenhouse gas (GHG) study by the International Maritime Organization (IMO) estimates the total carbon dioxide emissions (CO2) by the shipping industry in 2012 was 2.7% of global CO2 emissions. Unless the shipping industry takes some control measures, by 2050 these emissions are expected to increase by up to 250% from their 2012 levels. Although the cost-reducing effects of some measures and new technologies are well established, shipping companies appear reluctant to adopt them.
What is Maintenance?
The technical meaning of “maintenance” involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery and supporting utilities in industrial and residential installations.
Objectives of Maintenance
Author: Md Taifur Rahman
A toxic atmosphere may cause various acute effects including impairment of judgment, unconsciousness and death.
Can be lacking for:
A flammable atmosphere present a risk of fire or explosion. Such atmosphere can arise from the presence of flammable liquid or gas or suspended combustible dust in air inside the enclosed space.
Implementation of “Permit-to-enter” system for entry into enclosed spaces:
After completion of work designated person should ensure entry persons and related equipment’s are safely withdrawn.
Author: Md Taifur Rahman
Author: Usman Karneh
The function of an electrical system is to safely convey the power from the point of generation (source) to where it is required the various loads or equipment connected to it. The electrical source, like the load may either be a single phase (2-wire) or three phase (3 to 4- wire) system
An electrical system can take any of two forms:
Neutral Insulated system, or
Neutral Earthed system.
For single phase neutral insulated system, the neutral of the source is insulated. In other words, the neutral is not connected to the earth. While for the neutral earthed system, the neutral of the source is connected to the earth.
For three phase star neutral insulated system, the common point of the source is not connected to earth. Hence there are only 3 phase wires R,Y,B emerging from the source. In neutral earthed system, the star point is earthed to ground. So 4 wires emanate from source (3 phase and 1 neutral).
Three (3) types of fault may occur in an electrical system:
Open circuit fault: is the result of a break in the conductor so that no current flow through the load.
Short circuit fault: is due to break in insulation and two conductors (line and neutral) are directly touching each other resulting into a short path to current flow and allowing a very large current to bypass the load.
Earth Fault: This is also due to insulation breakdown which somehow allow the conductor to come into direct contact with the metal enclosure or body frame of the equipment.
A single earth fault occurring in the line of an earthed neutral system, would be equivalent to a short circuit fault since this will creates a closed path for the earth current to flow through to the ground as a result of minimal resistance, this earth current may increase to a very large extent. If the earth current increases beyond the current rating of the generator, the entire system may collapse causing irreparable damage. To limit this earth current, a Neutral Earthing Resistor is connected to the earthed neutral of the source. This resistor is of sufficient ohmic value to limit the earth current within rating of the generator. However, the magnitude of earth current is sufficiently large to operate the tripping mechanism of the faulted equipment immediately isolating it from supply and rendering it safe.
A single earth fault in a neutral insulated system, would not cause any earth current to flow. This is because a single earth fault current does not provide a complete circuit for earth current to flow. So no protective trip will operate and system will continue to function normally.
However, if a second earth fault occurs on another line in the insulated system, the two earth faults together will be equivalent to a short circuit and the resulting earth fault current will operate the available protection devices and cause disconnection of services.
For system where the priority requirement is to maintain continuity of the electrical supply to essential equipment in event of a single earth fault occurring the isolated neutral (neutral insulated) system is used. For system where the priority requirement is the immediate isolation of earth-faulted equipment is automatically achieved by a neutral earthed system.
To understand this better, a case study is used; If the earth fault occurs in an essential system like that of steering gear of a ship, then in case of: Insulated neutral system, no earth fault current will flow and the steering gear will continue to operate until there is a second earth fault in any equipment present onboard. Hence, with a single earth fault, the essential service will continue to operate.
If the system would have been an earthed neutral, then a single earth fault would have caused heavy earth current to flow and operate the tripping mechanism causing shutting down of steering gear. This will seriously compromise the safety of the ship navigation. Hence, it is well understood that onboard a ship, a neutral insulated system is to be used while, in an industry or on shore installations neutral earthed system is used.
Shipboard main LV systems at 440 V are normally provided with neutral insulated system. On the other hand HV system (1000 V to 3.3 KV) are usually provided with neutral earthed system via a neutral earthing resistor.
In a HV system, certain essential loads can be supplied by a transformer with it’s secondary insulated to ensure no earth fault current flows in the equipment in order to maintains the continuity of service.
Shipping regulations require that the hazardous areas of tanker such as the cargo area, pump room should have a neutral insulated system to prevent any stray earth current from flowing in the hull and causing explosion hazard.
However an exception is included in case if tanker has a 3.3 KV system, the earthed system is permitted provided that the earthed system does not extend forward of engine room bulkhead and into the hazardous area.
Both the Insulated neutral and Earthed neutral system have got their own advantages and disadvantages. Where it is easier in the earthed neutral system to detect any earth faults in system, it is easier in isolated neutral system to maintain the continuity of service.