Bilge and ballast systems

The bilge system and the ballast system each have particular functions to perform, but are in many ways interconnected.

Bilge system

The bilge main is arranged to drain any watertight compartment other than ballast, oil or water tanks and to discharge the contents overboard.

The number of pumps and their capacity depend upon the size, type and service of the vessel. All bilge suctions must be fitted with suitable strainers, which in the machinery space would be mud boxes positioned at floorplate level for easy access. A vertical drop pipe would lead down to the bilge.

The emergency bilge suction or bilge injection valve is used to prevent flooding of the ship. It is a direct suction from the machinery space bilge which is connected to the largest capacity pump or pumps. An emergency bilge pump is required for passenger ships but may also be fitted as an extra on cargo ships, it must be a completely independent unit capable of operating even if submerged. A centrifugal pump with a priming device is usually used, driven by an electric motor housed in an air bell. The power supply is arranged from the emergency generator.

The various pumps and lines are interconnected to some extent so that each pump can act as an alternative or standby for another.

Ballast systems

The ballast system is arranged to ensure that water can be drawn from any tank or the sea and discharged to any other tank or the sea as required to trim the vessel. Combined or separate mains for suction and discharge may be provided. Where a tank or cargo space can be used for ballast or dry cargo then either a ballast or bilge connection will be required. 1'he system must therefore be arranged so that only the appropriate pipeline is in service; the other must be securely blanked or closed off. Where tanks are arranged for either oil or ballast a change-over chest must be fitted in the pipeline so that only the ballast main or the oil transfer main is connected to the tank.

II. Чтение и устный перевод

1. Boiler types

Two basically different types of boiler exist, namely the watertube and the firetube. In the watertube the feedwater is passed through the tubes and the hot gases pass over them. The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines.

In the firetube boiler the hot gases pass through the tubes and the feedwater surrounds them. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.

Water-tube boiler

A water-tube boiler is a type of boiler in which water circulates in tubes which are heated externally by the fire. Water-tube boilers are used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats up water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace in through a superheater in order to be-come superheated. Superheated steam is used in driving turbines. Since water droplets can severely damage turbine blades, steam is superheated to 730°F (390°C) or higher in order to ensure that there is no water entrained in the steam.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore downcom-er tubes', where it helps pre-heat the feedwater supply. (In 'large utility boilers', the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase the economy of the boiler, the exhaust gasses are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such water-tube boilers in thermal power station are also called steam generating units.

The older fire-tube boiler design-—in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space—is a much weaker structure and is rarely used for pressures above 350 psi (2.4 MPa). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: There is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

Firetube boilers

The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed, The name 'tank boiler' is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler' are also in use.

Most firetube boilers are now supplied as a completely packaged unit.

This will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings.

Scotch fire tube marine steam boiler

The single-ended return tube Scotch marine boiler consists of a cylindrical boiler shell of large diameter and short length, provided with two or more furnaces in corrugated fire-tubes. Each furnace ends in a combustion chamber, surrounded by water. The gases pass through a bank of flue-tubes from the combustion chamber to the smoke-box at the boiler front.

The Scotch marine fire-tube boiler contained a large quantity of water, about six times more than a water-tube boiler, and was therefore slow to steam up and to change the output capacity. Due to the Scotch boilers stiff construction it required also a long steaming up period to avoid leaks caused by thermal expansion of the material,

The double-ended Scotch fire-tube steam boilers were normally used in ships with many Scotch boilers. Space was saved even though two stokeholes were required. Normally a pair of furnaces shared one combustion chamber.

When the furnace door was open, cold air could hit the combustion chamber's opposite wall and cause tube leakage. To prevent that, a high baffle of firebrick was installed in the middle of the combustion chamber.

Twenty-four double-ended Scotch boilers with three furnaces at each end and five single-ended boilers with three furnaces were installed onboard R.M.S. Titanic. Electrically operated stoking indicators were used in the stokeholds to prevent that opposite furnace doors were open at the same time. These indicators also helped to minimize the total number of simultaneously open furnace doors.

Cochran boilers

The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid'. The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.

Composite boilers

A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs.

The Cochran boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.