A steam turbine is an external-combustion rotary machine that converts the thermal energy of steam into mechanical work. After steam from the boiler enters the turbine, it sequentially passes through a series of annular nozzles and moving blades, converting the steam’s thermal energy into mechanical energy that drives the turbine rotor. In turbines, different methods of energy conversion give rise to turbines with various operating principles. So, what is the role of small-scale steam turbines?

　　 What is the function of a small steam turbine?

　　Small steam turbines are auxiliary equipment used in thermal power plants. Simply put, a 600-MW power turbine requires a boiler with a capacity of over 1,000 tons, and the boiler is supplied with water by two feedwater pumps. The combined capacity of these two feedwater pumps must exceed 12 MW. Therefore, if electric motors were chosen instead, the auxiliary power requirements of the thermal power plant would increase significantly, making the solution economically inefficient. Moreover, starting and operating ultra-large-scale electric motors places higher demands on supporting equipment, thereby increasing investment costs. For this reason, we have opted for small steam turbines with capacities ranging from 6 to 20 MW to drive the boiler feedwater pumps. These turbines are driven by intermediate steam extracted from the main generator turbine, achieving the goal of energy conservation.

　　 What role does the coupling play in a small steam turbine?

　　1. Regardless of the equipment or type of machinery, there is often a need for a single device to serve as a crucial link—connecting the upper and lower components. In the case of small steam turbines, the coupling plays this vital role. Although its technical term is “coupling,” professionals working with power equipment often prefer to call it a “back-to-back wheel.” Functionally, it acts like a central hub, as it’s used to connect the rotor of the steam turbine-generator set and then transmit the power from the small steam turbine to the generator.

　　2. When classifying couplings for small steam turbines, they are generally divided into rigid couplings, semi-flexible couplings, and flexible couplings. What exactly is a rigid coupling? Typically, due to its simple structure and compact size, it is much cheaper. However, it has high requirements for alignment accuracy, as rotor vibrations often lead to thermal expansion. A semi-flexible coupling can effectively address some of the shortcomings of small steam turbines because it does not demand such strict alignment. Nevertheless, its cost is relatively higher.

　　 What is the primary function of the oil supply system for small steam turbines?

　　1. Supply oil to each bearing in the unit to lubricate and cool the bearings;

　　2. Provide stable and sufficient pressurized oil to the regulating system and protective devices to ensure their normal operation;

　　3. Provide lubricating oil for all transmission mechanisms. The steam turbine lubrication system mainly consists of a main oil pump, an oil injector, a standby heavy-oil pump, an AC oil pump, a DC oil pump, an oil cooler, a filter screen, and an oil tank. The main oil pump serves as the power source for the oil system and continuously supplies oil to both the lubrication system and the governing oil system during normal operation. The AC oil pump and the DC oil pump, also known as auxiliary oil pumps, are used to supply lubricating oil when the main oil pump fails to function properly during the start-up or shut-down of the steam turbine.