Abstract:
Apparatus ( 10 ) controls net thrust in a steam turbine (T) in response to changes in the operating condition of the turbine. The turbine includes a thrust bearing (B) positioned between low and intermediate pressure sections (LP, IP) of the turbine and a high pressure section (HP) thereof. Sensors ( 14 ) for sense thrust loads on the thrust bearing. A number of control valve (CV 1 -CV 4 ) are used to balance pressures occurring at locations within the high pressure section of the turbine. A controller ( 16 ) is responsive to the sensors sensing a change within the turbine indicative of a significant change in net thrust to energize one or more of the control valves and to adjust the pressure within the high pressure section of the turbine so to maintain net thrust within an acceptable range of thrust values.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   None. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   This invention generally relates to steam turbines; and more specifically, to the development of a control system for stabilizing loading on thrust bearings within the turbine to maintain thrust levels within an acceptable range of values and avoid damage to the thrust bearings. 
   In a rotating turbomachine, thrust is an axial force acting on the rotating parts. Thrust is caused by unequal pressures acting over unequal surface areas, and changes in momentum of the fluid (steam) circulating through the machine. The sum of all axial forces acting on the rotating components of the turbine is referred to as “net thrust”. This net thrust is transmitted to a stationary thrust bearing which, in turn, is anchored to a foundation for the turbine engine. The thrust developed by the turbine has two components. These are: 
   (a) Stage thrust which is thrust resulting from the pressure distribution around a stage bucket (blade), a cover, a wheel, etc. Stage thrust is usually in the direction of steam flow. 
   (b) Step thrust which results from variations in the diameter of the shaft to which the buckets are mounted, and the local pressure at points along the length of the turbine. 
   Conventional methods for controlling thrust in a steam turbine include:  1 ) using a balance piston at the high pressure (HP) section,  2 ) varying the rotor diameter in each section,  3 ) varying the number of stages comprising each section, and  4 ) establishing an appropriate configuration for each the low pressure (LP) intermediate pressure (IP), and high pressure (HP) sections of the turbine. However, all currently available methods only control thrust under “normal” operating conditions. As an engine design is completed, and its operating conditions are fixed, the net thrust of the steam turbine is specified. The methods set out above cannot now dynamically or actively adjust the steam turbine&#39;s net thrust, either under normal conditions or during fault operations. 
   A previous attempt at controlling thrust in a steam turbine is shown U.S. Pat. No. 4,557,664 to Tuttle, where there is disclosed use of a sealed balance piston on an overhung shaft end. The piston can be vented to an ambient pressure to balance the thrust, or vented to another control pressure to counteract any other net unbalanced forces acting across the turbine. For gas turbines, positive pressure has been used to help equalize a pressure differential across a rotor shaft. Approaches using exhaust air or gas are described in U.S. Pat. No. 3,565,543 to Mrazek and U.S. Pat. No. 4,152,092 to Swearingen. 
   Though such pressure equalizing features help minimize axial thrust variations during normal operations, none control net thrust for turbines operating under fault conditions. This is because the above-mentioned approaches control thrust “statically” rather than “dynamically.” To control thrust dynamically, new techniques need be developed to satisfy the requirements of the power industry. 
   A number of fault operating conditions have the potential of creating large thrust forces. These include:
     a) Intercept valve closed condition   

   All reheat turbines have an intercept valve and a reheat valve connected in series between a reheater and the intermediate and low pressure sections of the steam turbine. Both valves are normally open to allow steam flow through the unit. The reheat valve acts to throttle steam flow through the reheat section following a loss of electrical load, this preventing an over speed trip of the turbine. If turbine speed continues to rise, the unit trips and the intercept valve shuts off to prevent steam flow from the reheater into a reheat turbine. An intercept valve closed condition also exists when either the intercept valve or reheat valve closes during full load operation, in response to a control system malfunction. This can result in a very large thrust load since both the intermediate and low pressure stage thrusts go to zero, while the high pressure stage thrust remains at its original level. The condition may cause a thrust reversal. That is, net thrust suddenly changes its direction from negative to positive producing a large impulse on the thrust bearing.
     b) Sudden opening of control valves   

   When a turbine is lightly loaded, flow through the high pressure and reheat sections is relatively small. Increase in load are normally accomplished through a slow and steady opening of the control valves at a specified rate. However, if the control valves malfunction and open quickly, a high flow through the high pressure section immediately occurs. Flow through the reheat section also builds up, but with a certain lag in time due to the volume of the reheater and its associated piping. Under this condition, the thrust in the high pressure section is much higher than the reheat thrust, resulting in a large thrust load acting on the thrust bearing in the direction of high pressure flow.
     c) Bottled up   

   When a turbine trips, the intercept valve and main stop valves of the turbine shut off at approximately the same time. All flow to the turbine stops. The high pressure and reheat sections eventually empty out into a condenser and the pressures in these sections decrease to that of the condenser. If, however, steam in the high pressure section becomes trapped between the stop valve and intercept valve, a “bottle up” occurs. Initially, the bottled up pressure equals the mean reheat pressure for normal operation. But, due to stored heat in the boiler, the pressure of the bottled up steam rises until reheat safety valves open. The opening pressure of these valves is about 1.25 times the cold reheat pressure and is the highest possible pressure in the high pressure section of the turbine.
     d) Seismic event   

   Seismic thrust is a force acting on the thrust bearing when the turbine experiences seismic vibrations. Seismic activity is described by the maximum acceleration as a fraction of the gravity of acceleration            . This seismic thrust is superimposed on the normal thrust.
   To meet useful life requirements for a thrust bearing, its loading is kept within certain limits. Under normal operating conditions, thrust bearing loading must be lower than 400 psi (for a pivoted type thrust bearing) but larger than 50 psi. A setting of 50 psi avoids thrust reversal if temporary changes within the turbine upset the normal balance of forces. Second, if an intercept valve closes, the maximum allowable loading increases to 600 psi. Third, for seismic events, the maximum allowable loading is 1,800 psi. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly stated, the present invention is directed to the control of axial thrust loads in a steam turbine. This is accomplished by controlling a pressure differential across a balance piston in a high pressure section of the turbine in response to variations in net thrust. An apparatus of the invention controls net thrust in the turbine in response to changes in the operating condition of the turbine. The turbine includes a thrust bearing installed between the low and intermediate pressure sections of the turbine and the high pressure section. Load sensors installed on opposite side of the thrust bearing sense thrust loads on the bearing. A plurality of control valves act to balance pressures occurring at locations within the high pressure section. A controller is responsive to the sensors sensing a change within the turbine indicative of a significant change in net thrust to activate one or more of the control valves so to adjust the pressure within the high pressure section and maintain the net thrust within an acceptable range of thrust values. 
   Although primarily designed for controlling axial thrust in the high pressure section of the turbine, the invention can be implemented in other sections of the turbine as well. 
   The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings which form part of the specification: 
       FIG. 1  is a simplified representation of a steam turbine; 
       FIG. 2  illustrates a control valve arrangement of the present invention for thrust load control; 
       FIG. 3  is a graph illustrating control valve operation under different conditions; 
       FIGS. 4   a  and  4   b  are graphs depicting thrust ranges under normal operating conditions of a turbine and under fault conditions; and, 
       FIG. 5  is a flow diagram for the control system. 
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
   In accordance with the present invention, the net thrust load of a steam turbine is controlled by controlling the pressure differential across a balance piston in a high pressure section of the turbine in response to net thrust variation. Referring to  FIG. 1 , a turbine T is shown to be comprised of a high pressure section HP, an intermediate pressure section IP, and an adjacent low pressure section LP. Each section may be comprised of one or more stages. The rotating elements housed within these various stages are commonly mounted on an axial shaft or rotor S. As shown in  FIG. 1 , high pressure section HP is arranged opposite to the intermediate and low pressure sections IP and LP of the turbine. This is done to balance stage thrusts. Further, a thrust bearing B is installed between sections HP and IP. The size (area) of thrust bearing B is selected to ensure that under a wide range of operating conditions (e.g., the turbine&#39;s load, operating speed, temperature, and pressure levels within the turbine, etc.), the thrust pressure will fall within a predetermined range of values. 
   For the turbine of  FIG. 1 , step thrust is primarily developed in four packing regions: a packing N 1  at the downstream end of low pressure section LP, a packing N 2  at the upstream end of intermediate pressure section IP, and packings N 3  and N 4  at the respective upstream and downstream ends of high pressure section HP. The packings (or steam seals) are typically labyrinth type seals as is well known in the art, although other types of seals can be used. Further, as shown in  FIG. 2 , the packing for a particular section of the turbine comprises a number of sealing elements such as the labyrinth seals N 3 - 1  to N 3 - 7  shown in the Figure. 
   The step thrusts produced in sections IP and LP are relatively small because the pressures in these sections are relatively low (from atmosphere pressure to about 50 psi in section LP, up to about 400 in section IP). The largest step thrust occurs in the packing N 4 . This is because the diameter of rotor S sharply decreases at the transition from a last stage of high pressure section HP to the packing N 4 . Step thrust at packing N 3  is subject to the next highest level of thrust due to the high pressure at this section. Because net thrust can build up to levels beyond the capability of thrust bearing B, the step thrust present at a specified location within the turbine has been used to equalize the thrust differential across rotor shaft S. This allows the thrust bearing to be of a reasonable size. 
   In steam turbine T, the packings N 1 -N 4  work either as pressure packings to prevent higher pressure steam from leaking out into a room (not shown) where the turbine is housed, or as a vacuum packing preventing air from leaking into the turbine. As the operating load on turbine T increases, pressure in the high and intermediate sections, HP and IP respectively, of the turbine increases. Packings at the ends of these sections (the packings N 2 -N 4  shown in  FIG. 1 ) are now act as pressure packings. When the turbine is operating to cause gears to turn and a vacuum to be pulled, all of the packings (packings N 1 -N 4 ) act as vacuum packings and function to minimize steam leakage loss. 
   Referring to  FIG. 2 , the high pressure inlet to turbine section HP is indicated L and has a general bowl shape. As leakage flow passes a component of a seal packing (e.g., packing N 3 - 1 ), a pressure differential builds up across the packing element. For example, if steam turbine T has a bowl pressure P bowl  of 1930 psi at inlet L, a pressure P 1  on the downstream side of packing element N 3 - 1  may be, for example, on the order of 920 psi, or P 1 ˜920 psi. Similarly, the pressure on the downstream side of the next packing element N 3 - 2  may be, for example, 540 psi, or and P 2 ˜540 psi. Conventionally, the balance piston P in the high pressure section HP is used to control thrust of a steam turbine. Since balance pistons are known in the art, its construction and operation is not described. 
   Those skilled in the art will further understand that a pressure P 3  on the downstream side of packing element N 3 - 3 , a pressure P 4  on the downstream side of packing element N 3 - 5 , and a pressure P 5  on the downstream side of packing element N 3 - 6  reflect similar changes in pressure through the high pressure section of the turbine. At the outlet end of the section, at the downstream side of packing element N 3 - 7 , the pressure P atm  reflects the pressure at a drain port. Utilizing the various pressures, and ambient pressure, the net thrust of turbine T is controlled within allowable regions. 
   A net thrust control system of the present invention is indicated generally  10  in FIG.  2  and includes a plurality of solenoid control valves CV 1 -CV 3 , and an optional control valve CV 4 . As is well known in the art, solenoid valves are control devices used to automatically control pressures at packing components in the thrust control system of turbine T. When electrically energized or de-energized, the valves allow steam to either flow or stop. Each valve has an inlet I and an outlet O. 
   In  FIG. 2 , three solenoid valves CV 1 -CV 3  are shown connected to components of packing N 3 . A first solenoid valve CV 1  has its outlet connected to the drain portion of the turbine where the pressure is P atm . The inlet of valve CV 1  is connected to both the downstream side of balance piston P and its associated packing element N 3 - 2  where the pressure is normally P 2 , and to the outlet of control valve CV 2 . The inlet of control valve CV 2  is connected to bowl L of the high pressure section HP of the turbine where the pressure is P bowl . The third control valve CV 3  has its inlet also connected to bowl L, and its outlet is connected to the downstream side of packing element N 3 - 1  (the upstream side of balance piston P) where the pressure is P 1 . Optionally, a fourth control valve CV 4  is connected across balance piston P. It will be noted that there is a series/parallel arrangement of the control valves and that, in accordance with the invention, one or more of the control valves can be opened at one time to control net thrust of the turbine. 
   The control valves are normally closed and do not impact steam turbine operation. As shown in  FIG. 4   b , there are four identified regions of net thrust. Regions I and II which extend from −400 psi to 0, and from 0 to +400 psi respectively represent a normal operating range for the turbine. In Region I, thrust is toward the intermediate and low pressure sections IP and LP of the turbine, while in Region II, thrust is toward high pressure section HP. Those skilled in the art will understand that the point 0 psi may be crossed over from one direction to the other during operation of turbine T, but the transition is typically a gradual transition. 
   Under a fault condition, however, such as when an intercept valve (not shown) is closed, the load on thrust bearing B changes sharply. Referring to  FIGS. 4   a  and  4   b , during the time it takes for the intercept valve to close (times t 1  to t 3  in the Figures), net thrust decreases significantly. Without thrust control system  10 , net thrust will not only keep moving from a minus psi value toward zero, but will rapidly pass through the 0 psi crossover point and change its direction from negative (i.e., toward intermediate and low pressure sections IP and LP) to positive (toward high pressure section HP). This is indicated by the dashed line in  FIG. 4   b . The result is the thrust load switching from one side of thrust bearing B to the other, and producing a large force impulse on the thrust bearing. This, in turn, can cause a crash between rotating and stationary components of the turbine due to the resulting axial displacement. 
   In operation, control valve CV 1  of control system  10  is activated when net thrust falls to between 10-30% of its original value, but with the thrust still being within Region I of  FIG. 4   b . Referring to  FIG. 2 , it will be seen that with control valve CV 1  open, the P 2  at the downstream side of balance piston P will approximate the drain pressure P atm . As a result, the step thrust toward intermediate and low pressure sections IP and LP of the turbine can double or triple to balance the change in thrust. 
   It may be that in some situations, the generated step thrust will still not balance the thrust. In these circumstances, control valve CV 3  is also opened so to increase pressure P 1  to pressure P bowl , and produces a large pressure drop across balance piston P. By controlling the extent to control valves CV 1  and CV 3  are opened, net thrust can be precisely controlled within the allowable operation region (Region I in the above example). At this time, control valves CV 2  and CV 4  (if control valve CV 4  is used) remain closed. 
   If the opposite situation to that described above occurs; that is, net thrust in the direction of the intermediate and low pressure sections IP and LP becomes too large, system  10  operates to open control valve CV 2 . This has the effect of making balance piston P nonfunctional (since the pressure differential ΔP across the balance piston becomes very small). Alternatively, instead of using control valve CV 2 , if control valve CV 4  is used, opening this control valve has the same effect as opening control valve CV 2 . 
   In other situations, it may be desirable to open control valves CV 1  and CV 2 , which are connected in series, so to connect bowl L of high pressure section HP to the environment or drain of the turbine. Because of the series/parallel connections of the control valves, different combinations of the control valves can be opened at any one time as operating circumstances warrant to control net thrust load. 
   Most commercially available solenoid valves open and close substantially instantaneously. This can cause very large shock pressures within control system  10 , and potentially damage the control valves, especially at high flow velocities. To address this problem, control valves CV 1 -CV 4  include dampeners  12  by which the valves can be opened and closed in a predetermined manner during a time interval Δt. This is accomplished by inputs to the control valves from a controller  16 . In  FIG. 3 , three possible paths to open and close a control valve are illustrated. These paths include linear, exponential, and logarithmic paths. While each path may have certain advantages with respect to the others, it has been found that the greatest sensitivity and effectiveness in operating a control valve, the logarithmic path is preferable. Those skilled in the art will appreciate, that certain of the control valves can be opened in accordance with one path while others are opened using a different path. Also, paths other than the three shown in  FIG. 3  may be implemented without departing from the scope of the invention. 
   Referring again to  FIGS. 4   a  and  4   b , they illustrate the variation of the steam turbine net thrust as control system  10  acts in response to an intercept valve closing. For purposes of understanding operation of system  10 , it is assumed that the closing rate of an intercept valve follows the logarithmic function of ƒ(t)=ƒ(t o,lV )+b log a (t) for b&lt;0, and the opening rate of a control valve CV follows the logarithmic function of ƒ(t)=ƒ(t o,CV )+b log a (t) for b&gt;0. In  FIG. 4   a , the intercept valve begins to close at time t 1  and reaches its fully closed position at time t 3 . As the thrust reduction is detected; for example by sensors  14  shown in  FIG. 1  positioned on opposite sides of thrust bearing B and supplying inputs to controller  16 , control valve CV 1  is commanded by the system to start opening at time t 2  (using one of the paths shown in  FIG. 3 ) and to complete opening by time t 4 . As shown in  FIG. 4   b , during the interval from time t 1  to time t 2 , net thrust is changing, but for the entire interval from time t 1  to time t 4 , the net thrust remains is in region I. This is important, because by actively or dynamically responding to an abrupt change of conditions within turbine T, the resulting forces imparted to the turbine are constrained within acceptable limits, and the turbine does suffer any damage resulting from the change. 
     FIG. 5  is a flow diagram for system  10  and illustrates processing of the thrust load control. As noted, thrust load sensors  14  are installed at opposite sides of thrust bearing B to monitor and diagnose changes in thrust. During steam turbine T operation, only one side of thrust bearing B is loaded at any one time. Variation of thrust load is calculated from sensor  14  measurements as 
         η   =         F     t   +   1       -     F   t         F   t         ,         
where F t  is the sensed force at a point in time and F t+1  is the sensed force at the next point in time.
 
   When |η| is between 10-30%, control system  10  is activated. According to the sign of the thrust differential (i.e., F t+1 −F t &gt;0 or F t+1 −F t &lt;0), one or more of the control valves are opened to balance the thrust. Again, this dynamic response to changed conditions avoids a thrust reversal with thrust load changing from one side of thrust bearing B to the other, and provides necessary time for steam turbine T to shut down following a normal procedure. 
   While the invention has been described in connection with a fault condition (intercept valve closing), those skilled in the art will recognize that control system  10  of the invention can also be used with a steam turbine under normal operation of the turbine. Further, while control system  10  has been described with respect to high pressure section HP of turbine T, the control system can also be employed in either or both the intermediate and low pressure sections IP and LP of the turbine. 
   In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.