By Maklub Al Mostofa
Q. What is scavenging? Name the types of scavenge used for large two stroke engine. Describe the advantage of Uniflow scavenging.
It is the removal of residual exhaust gas and its replenishment with fresh air in an internal combustion of the engine. The fresh air intake and exhaust gas expel operation are not simultaneous fully but some degrees of overlap period are provided for better efficiency.
There are 3 types of scavenging process.
1. Loop scavenging
2. Cross scavenging
3. Uniflow scavenging
In this type of scavenging air passes over the piston crown and rises to form a loop. Ports are cut in the cylinder liner wall for this operation. The scavenge ports and exhaust ports are in the same side of the liner. Read more
By Maklub Al Mostofa
For the purpose of this Guide, an ‘Enclosed Space’ is defined as a space that has the following characteristics:
- Limited openings for entry and exit.
- Unfavourable natural ventilation.
- Not designed for continuous worker occupancy
Enclosed spaces include, but are not limited to:
- Cargo spaces
- Double bottoms
- Fuel tanks
- Ballast tanks
- Cargo pump-rooms
- Cargo compressor rooms
- Chain lockers
- Void spaces
- Duct keels
- Inter-barrier spaces
- Engine crankcases
- Engine scavenge air receivers
- Sewage tanks
By F. R. Chowdhury
Shipping is one of the oldest businesses in the world. Risk management and insurance is also equally old business. They are closely linked with one another. In fact the development of insurance took place in support of the shipping industry.
In the early days the ship-owner, trader and ship-captain was a single entity. A rich influential person got a ship built, procured some commodity that is readily available in his area and then sailed to another place for business. He would normally barter the goods in exchange of commodity available in the new land. Gradually gold and then coins and currency became medium of exchange. Fortune favours the brave. The pioneer in shipping gradually became a rich man. He was not anymore ready to undergo all the rolling and pitching at sea. He employed a trusted man as the captain of his ship. He still remained owner of the ship and the cargo. However, those days with no radio telecommunication there was no way for him to know anything until the ship was sighted on the horizon again. Some time the ship was never seen again – either lost at sea or hijacked by pirates.
Image Credit: Wärtsilä
The efficiency of the ship’s propeller is an important part of a ship’s overall propulsion efficiency. The Wärtsilä EnergoProFin has become a popular retrofit solution to improve the propulsion efficiency and hence improve the fuel efficiency of many ships already in service.The Wärtsilä EnergoProFin solution, a propeller cap with fins that rotates together with the propeller to produce fuel savings of up to 5%.
Ignition quality parameters:
- Maximum firing pressure.
4. Injection delay
5. Ignition delay .
1. Energy comparison
• The injection pump is a volumetric pump
• The higher the density the more energy it contains per volume unit
• The density difference between HFO and MDO is larger than the difference in net calorific value
2. Viscosity comparison
- The viscosity of MDO is lower than the viscosity of HFO (even HFO is heated)
• Lower viscosity fuels result in more internal leakage in the injection pump from the high pressure side to low pressure side.
• Internal leakage has to be compensated by giving more fuel rack
For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present of a combustible material, oxygen or air to support combustion and a source of heat in proportional ratio and within the flammable limits, the reaction which causes fire or explosion becomes cyclic.
Image Credit: www.brighthubengineering.com
Crankcase explosion normally occurs in trunk engine in which the lubricating oil used in the bearings is splashed around the crankcase and broken down into moderate size particles.
The main cause of crankcase explosions are the development of hot spots at various places in the crankcase. Due to the reciprocating motion of the piston the lubricating oil in the crankcase is splashed in the air.
For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present a combustible material, oxygen or air to support combustion and a source of heat at a temperature high enough to start combustion.
In the case of scavenge fires:
the combustible material is oil. The oil can be cylinder oil which has drained down from the cylinder spaces, or crankcase oil carried upwards on the piston rod because of a faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinders and other similar causes.
The oxygen necessary for combustion comes from the scavenge air which is in plentiful supply for the operation of the engines.
The source of heat for ignition comes from piston blow-by, slow ignition and afterburning, or excessive exhaust back pressure, which causes a blowback through the scavenge ports.
Cylinder lubrication in a low-speed main propulsion diesel engine:
Cylinder lubrication For marine diesel engines operating on residual fuels containing sulphur, cylinder lubrication must generally serve the following purposes:
■ Create and maintain an oil film to prevent metal to metal contact between the cylinder liner and piston rings.
■ Neutralise sulphuric acid in order to control corrosion.
■ Clean the cylinder liner, and particularly the piston ring pack, to prevent malfunction and damage caused by combustion and neutralisation residues.
Some common boiler problems are described below:
Cleanliness of the heat recovery surfaces after the boiler can often be judged by observing the gas pressure differential above and below. Any significant rise in this value should be attended to. Whilst good combustion conditions will minimise the risk, deposits allowed to accumulate in this area are a fire risk and, should fire take hold undetected, it can prove impossible to control and can wreck the heat exchanger, or even the whole boiler. There is plenty of evidence of soot fires leading on to hydrogen fires.
Image Credit: www.riceweightloss.com
Older loop scavenged engines may have a single injector mounted centrally in the cylinder head. Because the exhaust valve is in the centre of the cylinder head on modern uniflow scavenged engines the fuel valves (2 or 3) are arranged around the periphery of the head.
The pressure at which the injector operates can be adjusted by adjusting the loading on the spring. The pressure at which the injectors operate vary depending on the engine, but can be as high as 540bar.
In the draft “Code of practice for the design, safe operation, maintenance and servicing of boilers”, a requirement is made for regular water-quality monitoring of both limited-attendance boilers and unattended boilers.
Image Credit: Taybro Alloys Stainless Steel
Carbon is the most important component in commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite.
Image Credit: www.upc.edu
By Mohammud Hanif Dewan, IEng, IMarEng, MIMarEST, MRINA
Boilers are inspected to maintain the regulatory requirement. Regular internal and external examination during such survey constitutes the preventive maintenance schedule the boiler goes through to have a safe working condition.
IMO ship pollution rules are contained in the “International Convention on the Prevention of Pollution from Ships”, known as MARPOL 73/78. On 27 September 1997, the MARPOL Convention has been amended by the “1997 Protocol”;, which includes Annex VI titled “Regulations for the Prevention of Air Pollution from Ships”. MARPOL Annex VI sets limits on NOx and SOx emissions from ship exhausts, and prohibits deliberate emissions of ozone depleting substances.
The IMO emission standards are commonly referred to as Tier I…III standards. The Tier I standards were defined in the 1997 version of Annex VI, while the Tier II/III standards were introduced by Annex VI amendments adopted in 2008, as follows: