Patent Publication Number: US-6705086-B1

Title: Active thrust control system for combined cycle steam turbines with large steam extraction

Description:
BACKGROUND AND FIELD OF THE INVENTION 
     The invention is directed to a system for controlling axial steam turbine thrust to improve overall performance and reliability of a steam turbine. 
     Conventional steam turbines solve the large change in thrust load by increasing the thrust bearing area and avoiding a thrust load direction change from the active thrust bearing to the inactive thrust bearing. 
     SUMMARY OF THE INVENTION 
     The invention controls the axial steam turbine thrust by counteracting the thrust effect of a large steam extraction flow at the exhaust of the high pressure (HP) section. 
     Typically, the purpose of the extraction flow is: 
     a) to provide for steam injection into the gas turbine combustion system to augment the power output of the gas turbine, or 
     b) to provide for process extraction steam. 
     The active thrust control is achieved by a pipe and valve arrangement that controls the pressure at a packing step when the steam is extracted from the HP exhaust, thereby counteracting the increased stage thrust by an equivalent but opposing increased step thrust. This results in an overall reduced thrust load range and permits the use of smaller thrust bearings with reduced mechanical losses. 
     The proposed thrust control system solves two problems. First, the inventive thrust control system reduces the range of the thrust bearing load for a combined cycle machine that is designed for large extraction flows from the high pressure (HP) exhaust. With reduced thrust load range, the thrust bearing size and mechanical losses can be reduced, resulting in an overall improved machine efficiency. 
     Second, the inventive thrust control system avoids the condition of zero or indeterminate thrust load and decreases the risk of unstable thrust bearing operation and its potential impact on thrust bearing reliability. 
     The invention improves the overall performance and reliability of a combined cycle steam turbine by controlling the thrust load to a smaller range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in schematic form a thrust control system according to a first embodiment of the invention; 
     FIG. 2 illustrates in schematic form a thrust control system according to a second embodiment of the invention; 
     FIG. 3 shows in schematic form the control circuit for controlling the valves in the thrust control system; and 
     FIG. 4 shows in schematic form an alternative control circuit for controlling the valves in the thrust control system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show combined cycle steam turbines having single flow high pressure (HP) and intermediate pressure (IP) sections. The exhaust from the IP section flows to the low pressure (LP) section (not shown on FIG. 1) via a crossover pipe. A reheater  18  provides reheated steam exhausted from the HP section to the IP section. The system also provides for HP exhaust extraction steam flow to be used for other equipment such as a gas turbine or a process system. 
     As shown in FIG. 1, the thrust control system consists of pipes  10  and valves  12 ,  14  that are activated by a control signal to divert the N 1  packing leak-off pipe destination from a lower pressure stage to a higher pressure stage of the intermediate pressure (IP) section when the HP exhaust extraction flow is turned on via valve  16 . 
     The invention has several elements that when combined result in the reduced thrust load range. For example, the rotor has to be designed with a larger step at the N 1  leak-off point that generates a step thrust opposite to the direction of the HP stage thrust. 
     The N 1  leak-off has to be connected to two different points in the downstream steampath: (1) to the IP exhaust (existing connection); and (2) to a stage with higher pressure upstream of the IP exhaust point (new connection). The second connection requires a new shell penetration between the hot reheat bowl and the IP exhaust. 
     The two new motor operated valves  12 ,  14  (thrust control valves TCV 1  and TCV 2 ) are provided for redirecting the N 1  packing leak-off flow from the IP exhaust point A to the new higher pressure point B. 
     As shown in FIG. 3, a control system includes a controller  31  for sending a control signal to simultaneously operate valves  12 ,  14  based on a power control signal that activates valve  16  of the HP exhaust extraction flow, for instance a signal that controls the extraction flow for steam injection into a gas turbine combustion system (also referred to as “power augmentation”). The activation of valve  16  can be sensed and input to controller  31 , for example, by sensor  32 . Alternately, as shown in FIG. 4, the controller  31  can output the control signals to valves  12  and  14  in accordance with a preset pressure ratio of HP bowl pressure, sensed at point C by sensor  42  over HP exhaust pressure, sensed at point D by sensor  41 . 
     The operation of the inventive system will now be described with reference to FIG.  1 . When the HP exhaust extraction is turned on via opening valve  16 , the HP exhaust pressure decreases and the pressure ratio across the HP stages increases. At the same time the HP stage thrust increases and the steam turbine net thrust shifts towards the HP exhaust flow direction. 
     The thrust control system is activated as described above to counteract the increased stage thrust. The valve  12  closes while the valve  14  opens thereby redirecting the N 1  packing leak-off destination point to the higher pressure stage. This increases the step pressure at the N 1  Packing rotor step and as a result, the step thrust magnitude increases. This then directly counteracts the increased stage thrust and works towards limiting the range of total thrust load variation over the whole operating envelope. 
     The implementation of this control system requires a judicious selection of the N 1  packing rotor step diameter and the stage pressure for the second N 1  packing leak-off connection. When the demand for HP exhaust extraction steam is terminated, the valve  12  opens and the valve  14  closes. 
     As shown in FIG. 2, an alternate embodiment uses a two way diverting valve (TCV)  21  that combines the functions of valves TCV 1  and TCV 2  shown in FIG.  1 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.