So far we have classified turbines into two general groups: IMPULSE TURBINES and REACTION TURBINES, depending on the method used to cause the steam to do useful
Figure 5-6.-Impulse main propulsion turbine.
work. Turbines may be further classified according to the following:
• Type and arrangement of staging
• Direction of steam flow
*Repetition of steam flow
*Division of steam flow
A turbine may also be classified by whether it is a condensing unit (exhaust to a condenser at a pressure below atmospheric pressure) or a noncondensing unit (exhausts to another system such as the auxiliary exhaust steam system at a pressure above atmospheric pressure).
CONSTRUCTION OF TURBINES
Other than the operating and controlling equipment, similarity exists in both the impulse and reaction turbines. These include foundations, casings, nozzles, rotors, bearings, and shaft glands.
Turbine foundations are built up from a structural foundation in the hull to provide a rigid supporting base. All turbines are subjected to varying degrees of temperature-from that existing during a secured condition to that existing during full-power operation. Therefore, means are provided to allow for expansion and contraction.
At the forward end of the turbine, there are various ways to give freedom of movement. Elongated bolt holes or grooved sliding seats are used so that the forward end of the turbine can move fore and aft as either expansion or contraction takes place. The forward end of the turbine may also be mounted with a flexible I-beam that will flex either fore or aft.
The materials used to construct turbines will vary somewhat depending on the steam and power conditions for which the turbine is designed. Turbine casings are made of cast carbon steel for nonsuperheated steam applications. Superheated
Figure 5-7.-Turbine assembly in a machine shop.
Figure 5-8.-Typical sliding surface bearing.
applications use casings made of carbon molybdenum steel. For turbine casings used on submarines, a percentage of chrome stainless steel is used, which is more resistant to steam erosion than carbon steel. Each casing has a steam chest to receive the incoming high-pressure steam. This steam chest delivers the steam to the first set of nozzles or blades.
The primary function of the nozzles is to convert the thermal energy of steam into kinetic energy. The secondary function of the nozzles is to direct the steam against the blades.
Rotors (forged wheels and shaft) are manufactured from steel alloys. The primary purpose of a turbine rotor is to carry the moving blades that convert the steam's kinetic energy to rotating mechanical energy.
The rotor of every turbine must be positioned radially and axially by bearings. Radial bearings carry and support the weight of the rotor and maintain the correct radial clearance between the rotor and casing.
Axial (thrust) bearings limit the fore-and-aft travel of the rotor. Thrust bearings take care of
Figure 5-9.-Labyrinth packing gland.
any axial thrust, which may develop on a turbine rotor and hold the turbine rotor within definite axial positions.
All main turbines and most auxiliary units have a bearing at each end of the rotor. Bearings are generally classified as sliding surface (sleeve and thrust) or as rolling contact (antifriction ball or roller bearings). Figure 5-8 shows a typical sliding surface bearing.
Shaft Packing Glands
Shaft packing glands prevent the leaking of steam out of or air into the turbine casing where the turbine rotor shaft extends through the turbine casing. Labyrinth and carbon rings are two types of packing. They are used either separately or in combination.
Labyrinth packing (fig 5-9) consists of rows of metallic strips or fins. The strips fasten to the gland liner so there is a small space between the strips and the shaft. As the steam from the turbine casing leaks through the small space between the packing strips and the shaft, steam pressure gradually reduces.
Figure 5-10.-Carbon packing gland.
Carbon packing rings (fig 5-10) restrict the passage of steam along the shaft in much the same manner as labyrinth packing strips. Carbon packing rings mount around the shaft and are held in place by springs.
Three or four carbon rings are usually used in each gland. Each ring fits into a separate compartment of the gland housing and consists of two, three, or four segments that are butt-jointed to each other. A garter spring is used to hold these segments together. The use of keepers (lugs or stop pins) prevent the rotation of the carbon rings when the shaft rotates. The outer carbon ring compartment connects to a drain line.
In this chapter, you have learned about the components inside a steam turbine casing. You have also learned the basics of how the steam turbine works. For more information on steam turbines, refer to Machinist's Mate 3 & 2, NAVEDTRA 10524-F1, chapter 2.