Section 7 Flashcards
Classification of tankers
- General purpose tanker (10,000-24,999 DWT)
- Medium range tanker (25,000-44,999 DWT)
- Large range 1 (45,000-79,999 DWT)
- Large range 2 (80,000-159,999 DWT)
- Very large crude carrier (160,000-319,000 DWT)
- Ultra large crude carrier (320,000-549,999 DWT)
Short notes on alfra scale and world scale
Alfra-scale, population between most companies due to logistical reasons, are the ALFRAMAX vessels with DWT between 80,000 and 120,000T.
LR1 and LR2 fall in the ALFRA-scale.
World scale, an international freight index for tankers that provides a method of calculating the freight applicable to transporting oil by reference to a standard vessel on a roundtrip voyage from one or several load ports to one or several discharge ports.
I.G diagram
i. Boiler uptake gases are drawn to the scrubber until via flue gas isolating valves.
ii. Within the scrupper unit, the gas is cooled, cleaned and dried.
iii. 2 I.G. blowers supplies the treated gas to the tanks.
iv. Regulation of gas delivered to the deck is managed by the gas control valves and the deck pressure is controlled by a pressure controller.
v. Before entering the deck line, the gas passes through non-return valves which automatically prevents the back-flow of gases.
vi. After the deck seal, the I.G. relief is mounted to balance built-up deck water seal pressure when the system is shut down.
vii. An oxyfen analyzer is fitted after the blower to analyze the oxygen content to ensure that there is >8%.
Use of scrubber
A cleaning chamber to cool the gas and remove most of the sulphur dioxide and particular soot. Seawater is used to scrub or remove containments from the gas before it passes to cargo toxics.
Deck seals
Are safety devices fitted in the I.G. system on deck and prevents the back flow of flammable gases from cargo tanks into non-hazardous area.
PV Valve
Provides the flow of the small volumes of vapour, air, or inert gas mixtures caused by thermal variations in a cargo tank.
PV Breaker
A liquid-filled breaker connected to the cargo tank venting it I.G. system, set to operate whenever pressure increases or decreases beyond the set points.
Mast Riser
The primary means for venting out gases. This is a pipe extending up to at least 6 metres above the main deck, fitted with an approved flame arrestor.
An isolation valve is provided between the cargo tank venting/inert gas main and the mast riser.
During loading the mast riser valve is left open and the vapours are expelled to air.
During discharge, this valve is kept closed, so that inert gas is allowed to replace the tank atmosphere.
Inerting, purging and gas-freeing
Inerting is the process of pumping I.G. into the tanks to create an atmosphere incapable of burning. Achieved by overall o2 content <8% by vol.
Purging when the I.g. inside the tanks are already having <8% O2 with the intent of lowering the O2 and or Hydrocarbon content to ever lower levels.
Gas-freeing is the process of creating a normal atmospheric condition inside the tank whereby O2 is at least 21%.
Simple sketch and explanation of the flammability diagram
- Line AB represents the concentration of O2 & HC only, left of AB includes IG in the mixture, right of AB no IG.
- CED, only a mixture of HC and O2 inside the shaded region is capable of burning.
- The mixture above UFL is too rich in fuel to burn, sometimes referred to as UEL.
- Mixture below LFL is too lean in fuel to burn, sometimes referred to as LEL.
- An inert mixture is represented by F, diluted by air, the composition moves along the line FA, and enters the shaded area. This means the composition will be in a flammable condition.
- When IG is continually introduced, the flammable range decreases progressively until the O2 content reaches approximately 11% by volume(POINT E) at which no mixture can burn.
- For the practical purposes to allow a safety margin, 8% is taken as a level of O2 with no flammable circumstance.
- A dilution with IG is possible to move a mixture to a point represented by H.
- Line GA passes by just touching the flammable portion, if a mixture at F would have been purged to the point just above H, the mixture would be very close or in the flammable area.
- The critical dilution limit is the minimum amount of purging required to ensure that the mixture doesn’t fall in the flammable range while diluting with air.
- To prevent fire or explosion in the tank containing HC or O2 mixture, therefore, necessary to produce and supply IG having an O2 content not normally exceeding 5% and to displace the existing air in the tank until the resultant O2 level throughout the tank does not exceed by 8% by volume.
What is critical dilution
The min. amt. of I.G. required to ensure the hydrocarbon mixture doesn’t get into the range of the flammable region.
Difference between dilution and displacement method.
Dilution mixes the air while displacement replaces the air inside the tank with I.G.
With displacement method, O2 content of 8% from the initial 21% is achieved 3x quicker than dilution method.
C.O.W precautions
- Pre-arrival checks performed.
- The whole operation to be discussed between crew and shore personnel.
- Agree on a comm channel.
- Singal and emergency signs to be agreed upon.
- I.G. plant to be working and O2 content <5%
- Fixed O2 analyzer to be checked and calibrated for operation.
- Portable O2 analyzer checked and calibrated for operation.
- O2 readings must be taken from both sides of the bulk head.
- All tanks must be checked for + I.G. pressure.
- Assign all person to check for leakage as soon as operation starts.
- check all equip under C.O.W system for operation.
- Check and set the line and valves for the ship to shore under C.O.W system.
Different types of piping systems on tankers
- Direct Line
- Ring Main
- Single Line to Single tank system
- Free flow system
Different types of chemical tankers
Type 1: Carries chemicals that pose the most serious threats.
Type 2: Carries checmicals that pose a secure threats.
Type 3: Carries chemicals that pose considerate threats.
