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Steam Propulsion

Conventional steam ship platforms are each configured in their own unique way to support the mission of the ship. Some ships have separate machinery spaces for steam generation and turbine operations, while others combine all main propulsion machinery in one machinery space.

On ships using steam propulsion, boilers produce steam that is used by various components. The steam that goes to the main engines, ship's service turbine generators, boiler internal desuperheater and main feed pumps (on certain ships) is called main steam.

Steam generation in a conventional steam plant begins with the boiler. The D-type boiler has been installed in US Navy ships since 1950. Whether 600 psi or 1200 psi, D-type boiler construction is basically the same with a few exceptions, such as number of fuel oil burners and overall size and volume. Fuel oil burners are located on the boiler front and extend into the furnace to provide a means of firing the boiler. Depending on boiler design two to six burners are installed in the boiler.

The ship's main propulsion turbines are designed to efficiently convert the thermal energy of steam into useful mechanical energy to propel the ship through the water. As work is extracted from steam its pressure decreases. The high pressure turbine is designed to efficiently extract work out of the high pressure steam as it initially enters the main propulsion turbines. The low pressure turbine is designed to efficiently extract work out of steam which is exhausting out of the high pressure turbine at a lower pressure.

The propulsion turbines are designed to operate at high speeds. If the ship's propellers were to operate at such a high speed, cavitation around the propeller would occur and the ship would make little progress through the water. To allow both the turbine and the propeller to operate at their most efficient speeds, a main engine reduction gear is used to reduce the high rpms of the main propulsion turbines into a lower rpm for the main shaft.

Main propulsion boilers provide steam to the main propulsion turbines and auxiliary services in order to supply all shipboard steam systems in accordance with demand. It is designated as a D-type boiler because of the relative positions of the drums and side header which form the letter D. All D-type boilers are designated as uncontrolled superheat boilers because all the steam generated by the boiler must pass through the superheater. Superheater outlet temperature is a result of the combustion gas flow in proportion to the total amount of steam flow through all ranges (0 - 120%). The design characteristics ensure that the temperature will stabilize at set point.

The D-type boiler uses the principle of accelerated natural circulation to circulate water through the boiler. To enable this principle to work, relatively cool water will naturally circulate through large diameter pipes to distribution points low in the boiler. The downcomers are large diameter pipes connecting the steam drum with the water drum and lower headers to ensure proper circulation by delivering water from the steam drum to the water drum and lower headers. The downcomers are located between the inner and outer air casing to protect them from the direct radiant heat of the furnace.

The water drum is located at the bottom of the boiler below the main generating bank and acts as a lower reservoir of water for distribution to the main generating bank. Also, this large drum serves as a collection point for solids (sludge) that precipitate to the bottom that are removed by bottom blowdown.

The fuel oil service system provides the required amount of fuel oil to the boiler for operation. Its design allows operators to recirculate fuel for system testing and boiler light-off, redirect contaminated fuel, adjust for abnormalities in system pressure, and to quickly secure the system in case of a fuel leak or a casualty.

The combustion air system draws air from the outside atmosphere and directs it to the boiler to facilitate combustion. The proper amount of air to fuel is critical for the complete combustion of fuel. Too much or too little combustion air causes exhaust gases to become excessively white or black; situations that bring unique hazards to plant operation. The fireroom watch team must be able to monitor the exhaust gases to help maintain a clear smoke free stack. Smoke indicators and periscopes are installed to allow monitoring of the stack gases leaving the boiler. The smoke indicator is an electro-mechanical device and the periscope is an optical device. All ships have periscopes and many have electro-mechanical smoke indicators or stack gas analyzers. These devices are located above the economizer at the base of the stack so that combustion gases leaving the boiler must pass through its line of sight or the sensing element. From monitoring the stack gases, the combustion process can be adjusted for maximum efficiency or a casualty situation can be detected.

Automatic Boiler Controls (ABCs) are used extensively on modern surface ships to control the operation of the boiler and its auxiliaries under all load conditions from minimum to 120 percent. The ABCs include the various sub-systems necessary in maintaining both combustion rate and sufficient feed water supply to answer all steaming requirements within allowable tolerances.

The major piece of equipment which must be placed in operation in order to get the ship underway is the main engine. This requires placing the main condenser in operation and then testing and warming the main engine. If another boiler is needed to get underway, it should be lit-off early enough to ensure that it is on the line and the plant is stable prior to stationing the sea and anchor detail.

Before lighting fires in a boiler, there are several systems that must be checked and aligned so that electricity, atomizing air, combustion air, and fuel are available at the firing aisle. Power is needed for the electric forced draft blower, fuel oil service pump, and low pressure air compressor (LPAC). Low pressure air is required for automatic boiler control (ABC) systems and for fuel atomization. In addition, the boiler must have the proper level of water in the steam drum for light-off and the feed system must be tested for proper operation in preparation for supplying feedwater to the boiler after light-off. The following sections proceed step by step in the alignment and testing of these systems and ultimately for igniting the torch and establishing controlled combustion in the furnace.

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Page last modified: 07-07-2011 12:57:59 ZULU