Presentation on theme: "Lesson 22 OIL POLLUTION PREVENTION ( 防止油污染 ). There are three major sources of oil pollution from the operation of ships: 1. the discharge into the sea."— Presentation transcript:
There are three major sources of oil pollution from the operation of ships: 1. the discharge into the sea of tank washings from tankers( 油轮 );
2. the ballast carried in oil cargo and bunker tanks and 3. the discharge from machinery space bilges.
To prevent pollution by oil from ships, the IMO has published ‘Regulations( 规则, 规章 ) for the Prevention of Pollution by Oil'
(MARPOL 73/78) that came into force( 开 始有效, 开始实行 ) in December 1983.
The first source is reduced by the discharge of tank washings to a slop tank( 污水舱 ) for settling,
and discharge overboard of the water while retaining the ‘load on top’ for pumping ashore to the refinery ( 炼油厂 ).
Crude Oil Washing (COW) eliminates the use of water and enables cargo residues( 残余, 剩余物 ) to be pumped ashore during cargo discharge because
because cleaning is carried out simultaneously( 同时地 ) with the discharge.
As well as requiring tankers of certain sizes to have segregated( 隔离 ) or clean ballast tanks (SBT or CBT) to reduce the ballast contaminated( 污染 ) with oil,
new regulations have strict requirements for control of discharge of oil from a tanker.
In brief, if oil cargo residue is to be discharged from a tanker it must: a) not be in a Special Area; b) be farther than 50 nautical miles( 海里 ) from land;
c) be on passage. Additionally; d) the instantaneous( 瞬间的 ) rate of discharge must not be more than 30 litres per mile; e) the total amount must be not more than 1/30,000 of the particular cargo, and
f) the oil discharge monitoring and control system and slop tank arrangement required must be in operation
Engine room bilge disposal( 处理 ) is treated separately in the regulations from discharges related to cargo/ballast operation.
In general, ships over 400 gt are permitted to discharge machinery space bilges into the sea provide :
a) the oil in the bilge discharge without dilution( 稀释 ) does not amount to more than 15 ppm;
b) this is achieved by the operation of an oily water separator and/or filtering system with discharge monitoring and control;
c) the ship is moving on passage; d) it is at least 12 nautical miles from land, and e) is not within a Special Area.
A tank or tanks of adequate capacity, having regard to(without regard to) the type of machinery and length of voyage, should be provided on board to receive the oil residues (sludge),
such as those resulting from the purification (purifier/clarifier) of fuel and lubricating oils and oil leakage in the machinery spaces.
The operations related to cargo oil, oil residue, oil and fuel tank ballast and washing, and bilge must be filled in the Oil Record Book.
The Oil Record Book (part one) for machinery space operations is the responsibility of the engine room department.
All constructions and equipment for oil pollution prevention must be proved in accordance with the regulations,
and issued with International Oil Pollution Prevention (IOPP) certificates.
OILY WATER SEPARATORS Oily water separators are used to ensure that ships do not discharge oil when pumping out bilge water, or oily water from oil tanks or any oil contaminated( 污 染 ) space.
According to international legislations( 国 际法规 —MARPOL 73/78 (International convention for the prevention of pollution from ships)) related to oil pollution,
depending upon the size of a ship a discharge purity of 100 or 15 parts per million (p.p.m)( 百万分之 …) of oil will be required.
Oily water separators using the gravity system can only achieve 100 ppm and
and must therefore be used in conjunction with some form of filter to discharge clean water with less than 15 ppm of oil.
Steam or electric heating coil are provided in the upper and and sometimes the lower parts of the separator, depending upon the type of oil to be separated.
Where greater purity is required, the almost oil-free water passes to a filter unit.
The water flows in turn through two filter stages and the oil removed passes to oil collecting spaces.
The first stage filter removes physical impurities present and promote some fine separation.
The second stage filter uses coalescer( 聚 合器、聚合元件 ) inserts to achieve the final de-oiling( 脱油 ).
Coalescence( 聚合 ) is the breakdown( 破坏 ) of surface tension( 表面张力 ) between oil droplets( 液滴 ) in an oil/water mixture which causes them to join and increase in size.
The oil from the collecting spaces is drained away manually, as required, usually about once a week.
The filter inserts will require changing, the period of useful life depending upon the operating conditions.
Drawing for oil/water separator & filter unit Catch plates Fine separating compartment Coarse separating compartment Oil collecting space Oily water mixture Separator unit Filter unit Discharge purity are 15 p.p.m. or less. 100 p.p.m or less
The latest legislative requirements are, where 100 p.p.m. purity is required, a monitoring unit which continuously records,
and where 15 p.p.m. purity is necessary, an alarm unit to provide warning when levels of discharge is in excess of 15 p.p.m.
Crude Oil Washing (COW) Crude oil washing of cargo tanks, although relatively new, has been installed on board a number of existing tankers.
The principle itself is simple, the tanks are washed by the cargo during discharge and only a water rinse( 用清 水刷, 冲洗, 漂洗 ) is necessary to enable the tanks to carry clean ballast.
With this system all crude oil washing must be completed before the ship leaves its final port of discharge.
The advantages are that the volume of oily water mixtures to be separated is considerably reduced compared with the conventional water washing.
Also constituents( 组成部分 ) of the crude oil which have settled out on the tank surfaces during the voyage are discharged along with the cargo
thereby increasing the total quantity of oil discharged.
Whilst COW may have the disadvantage that the discharged time is increased this is, one would imagine,
outweighed( 比 … 更重要 ) by the fact that tank washing during the ballast voyage, usually a tedious( 单调乏味的 ) and time consuming exercise is eliminated.
The system is to consist of permanent steel piping and is to be independent of the fire main or any system other than
other than for tank washing except that sections of the ship's cargo system may be incorporated in the crude oil washing system.
The tank washing machines for COW are to be permanently mounted and are to be of approved design.
Each machine is to be capable of being isolated by means of stop valves in the supply line.
The number and location of the tank washing machines is to be such that all horizontal and
and vertical areas are washed by direct impingement( 冲击, 撞击 ) or effectively by deflection or splashing of the impinging jet.
The three stages washing pattern is called a ‘programmable’ machine, that is one where the single nozzle can be controlled to give pre - set arcs.
A dual nozzle machine is not considered to be programmable since the nozzle is constantly rotating through 360 o.
Submerged( 浸 [ 沉, 淹 ] 没的, 水下的 ) machines are usually dual nozzle machines and therefore non- programmable but
but single nozzle machines are being developed which could be made programmable either by means of cams or possibly intrinsically( 本质地，固有 地 )safe electronic controls.
The pumps supplying crude oil to the machines should be either the existing cargo pumps or pumps specially provided for the purpose.
The pumping and piping arrangements are to be such that the COW system can be effectively operated with any one pump out of use.
The design and capacity of the cargo tank stripping( 清舱，扫线 ) system should ensure that
that the bottom of the tank being cleaned is kept free of accumulations of oil and sediment towards completion of the tank washing process.
A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process( 延时曝气过程 ).
Basically this consists of oxygenating( 氧 化，充氧 ) the liquor either by bubbling air through it or by agitating( 搅动 ) the surface.
By so doing a family of bacteria( 细菌 ) is propagated( 繁殖 which thrives( 兴旺, 繁 荣, 茁壮成长, 旺盛 ) on the oxygen content and
and digests( 消化 ) the sewage to produce an innocuous( 无害的, 无毒的 ) sludge.
The impression( 效果 ) that bubbling air through the sewage serves to oxidize(( 使 ) 氧 化 ) it thus reducing BOD(Biochemical Oxygen Demand 生化需氧量 ) is not strictly the case.
It is the bacteria that reduce the BOD by converting the organic( 有机物的 ) content of the sewage to a chemically and organically inert( 惰性的 ) sludge.
In order to exist, the bacteria need air and nutrient( 有营养的 ). The nutrient is in the form of body and galley wastes.
If the nutrient source is cut off, i.e. the plant shut down or by-passed for say an extended ocean passage,
the bacteria die and the plant cannot function correctly until a new bacteria colony( 菌群、菌落 ) is generated.
This process can taken from 7 to14 days. Bacteria which live in the presence of oxygen are said to be aerobic( 需氧的 ).
When oxygen is not present, the aerobic bacteria cannot live but a different family of bacteria is generated.
These bacteria are said to be anaerobic( 厌氧性的 ).
While they are equally capable of producing an inert sludge, in so doing hydrogen sulfide( 氢化硫 ), carbon dioxide and methane( 沼气 ) are formed.
Some sewage processes are designed to work anaerobically( 厌氧性地 ) but these are not usually adopted for shipboard use.
Reading Material Vacuum Distillation system ( 真空蒸馏系统 )
Distillation is the production of pure water from sea water by evaporation and re-condensing. On motor ships, distilled water( 蒸馏水 ) is usually produced as a result of evaporating sea water by a vacuum boiling or a vacuum flash( 真空闪发 ) process.
The machine used is called an ‘evaporator( 蒸发器、造水 机 )’, although the word ‘distiller’ is also used.
The saturation temperature( 饱和 温度 ) of water or steam is the temperature at which water evaporates or steam condenses and this depends upon the pressure, the higher the pressure the higher the saturation temperature, the lower the pressure the lower the saturation temperature.
The saturation temperature in the vacuum evaporator is quite low, around 40 ℃ for example, the energy from diesel engine jacket water is usually used to produce distilled water.
Vacuum boiling evaporator A single-effect vacuum boiling evaporator consists of a evaporating section and a condensing section. The lower part of the evaporating section is the heater which houses the stack( 管束 ) of sea water tubes.
Hot diesel engine jacket water passes over the stack and heats the sea water passing through the tubes. Because of the reduced pressure in the machine, some of the sea water boils as it flows through the tubes.
The steam produced rises to the upper part of the evaporating section, and passes through a water separator or demister( 除水器、去 雾器 ) which prevents water droplets passing through. In the condensing section the steam becomes pure water, which is drawn off by a distillate pump( 凝水泵 ).
The remainder of the sea water fed, the brine, is drawn out by a brine pump. An air ejector( 抽气 器 ) draws out the air from the evaporator to maintain the vacuum in it.
Flash evaporator( 闪发蒸发器 ) Flush evaporation is the result of liquid containing a reasonable amount of sensible heat at a particular pressure being admitted to a chamber at lower pressure.
The liquid immediately changes into steam, i.e. it flashes, without boiling taking place. A single stage flash evaporator consists of a flash chamber, a demister and a vapour condenser.
An air ejector or a vacuum pump is used to keep a very low pressure in the evaporator. When the sea water from the pre- heater enters the evaporator shell, some of the water flashes off into steam. The demister removes any water droplets from the steam as it rises.
The vapour is then condensed, collected and pumped out by the distillates pump. Any unevaporated sea water is pumped out by the brine pump.
More than one stage of evaporation can take place by admitting the sea water into chambers with progressively lower pressures, but usually, two stage flash evaporators are used on board ship.