The design of a boiler feed pump turbine features some unique characteristics that presents certain challenges in terms of efficiency management, varying operating ranges, and many other features. In order better understand the accepted designs of Boiler Feed Pump Turbines (BFPTs), it is important to know how the operation of steam turbines used to drive boiler feed pumps can fundamentally improve fossil and nuclear plants. Much like the design of mechanical drive turbines, feed pump turbines also feature the same thermodynamic objectives as the main turbine and all of the engineering difficulties with optimal blade design, rotor and bearing harmonic conditions, ideal flow path definitions, and so on. However, some distinctions can make a BFPT design particularly distinct from a regular mechanical drive turbine. Figure 1 shows a basic heat balance diagram for a plant using a boiler feed pump turbine arrangement.
Inherent in its name, the BFPT must be fully compatible with the boiler feed pump. In other words, the necessary power and speed of the BFPT are determined by the requirements of the pump. In a fully integrated and dynamic system such as this, a large portion of the design requires developing a proper heat balance that will optimize the plant performance. In general, the boiler feed pump turbine uses both steam from the boiler and the main turbine to drive the mechanical shaft connected to the boiler feed pump. This arrangement has proven highly successful in efficiently applying the steam’s thermal energy throughout the plant. In certain arrangements, the BFPT can instead accept steam from cold reheat lines, main unit crossover piping lines, and different extractions from the main turbine. Regardless of the source, one distinction specifically unique to the BFPT is that it must accept steam from two separate sources.
In reference to schematic in Figure 1, the BFPT accepts steam at different pressures from both the boiler and the main turbine. The low-pressure steam extracted from the main turbine, generally between the high-pressure (HP) turbine and intermediate pressure (IP) turbine sections, will range from 75 psig to 250 psig. On the other hand, high-pressure steam directed from the boiler can reach pressures as high as 2400 psig, even 3500 psig in supercritical plants. The ability to utilize two vastly separate steam sources is made possible with the use of two separate inlet designs for the BFPT. The inlet designs for both the high pressure and the lower pressure sections of the BFPT consist of a series of valves driven by an actuator. The percentage in which each of these valve sections are open controls the different operating conditions of the plant. Three main operating points are considered for the feed pump turbine based on solely the lower pressure steam conditions coming from the main turbine. The conditions with these valves wide open (VWO), 40% of the main unit load (MUL), and the run out point (65% of MUL) all define the operating ranges of this section of the turbine. The range associated with each of these points allow the engineer to size the correct areas of the LP nozzles.
High-pressure steam from the boiler can be used to start the BFPT without using an auxiliary steam source. These start up requirements determine the nozzle sizing for the HP steam inlet section. As seen above, in order to achieve an optimal and efficient design for a BFPT, a number of different intermediate design points must be considered due to the expansive operating range that this particular turbine experiences. The analysis of different off-design curves becomes crucial in the design of boiler feed pump turbines and is a must for any engineers looking to improve their axial turbine design for boiler feed pump turbines. To learn more about the full design of process of SoftInWay’s AxSTREAM®, please click here.