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'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.