Turbine control valves dynamic interaction

Abstract

Method for controlling steam admission into a steam turbine, the turbine comprising a high pressure casing, at least one reduced pressure casing and an admission steam control system, the high pressure casing and at least one reduced pressure casing comprising control valves for steam admission. The admission steam control system manages the following steps: determining a steam flow demand; elaborating a high pressure control valve opening setpoint depending on the determined steam flow demand; imposing the elaborated high pressure control valve opening setpoint to the high pressure control valves; elaborating a reduced pressure control valve opening setpoint depending on the determined steam flow demand through the dynamic interaction between high pressure control valve opening setpoint and reduced pressure control valve opening setpoint; and imposing the elaborated reduced pressure control valve opening setpoint to the reduced pressure control valves.

Claims

1. A method for controlling steam admission into a steam turbine, the steam turbine comprising a high pressure casing, at least one reduced pressure casing, and at least one moisture-separator reheater to pull moisture from steam exhausted from the high pressure casing to yield dried steam and reheat the dried steam for admission to the at least one reduced pressure casing, the high pressure casing and the at least one reduced pressure casing comprising corresponding control valves for the steam admission, the corresponding control valves for the at least one reduced pressure casing positioned between the at least one moisture-separator reheater and the at least one reduced pressure casing, the method comprising: providing the at least one reduced pressure casing opposing the high pressure casing on a turbine shaft common to both the high pressure casing and the at least one reduced pressure casing; at an occurrence of a transient event during an operation of the steam turbine when there is a high volume of steam in the at least one moisture-separator reheater, determining a steam flow demand of the steam turbine, wherein the steam flow demand is determined using parameters that include a rotating speed of the steam turbine, a load of the steam turbine, a live steam pressure of the steam turbine, and a steam turbine operating mode; elaborating a high pressure control valve opening setpoint as a function of the determined steam flow demand; imposing the elaborated high pressure control valve opening setpoint to high pressure control valves associated with the high pressure casing; elaborating a reduced pressure control valve opening setpoint as a function of the determined steam flow demand; imposing the elaborated reduced pressure control valve opening setpoint to reduced pressure control valves associated with the at least one reduced pressure casing; and controlling the high pressure control valve opening setpoint and the reduced pressure control valve opening setpoint during the transient event, wherein the controlling includes continuing the elaborating and imposing of the high pressure control valve opening setpoint and the reduced pressure control valve opening setpoint to avoid overpressure in the at least one moisture-separator reheater during the transient event, wherein during the controlling of the high pressure control valve opening setpoint and the reduced pressure control valve opening setpoint, the reduced pressure control valves are more open than the high pressure control valves to reduce overpressure in the at least one moisture-separator reheater for protection thereof.

2. The method according to claim 1, wherein the imposing of the elaborated high pressure control valve opening setpoint to the high pressure control valves and imposing the elaborated reduced pressure control valve opening setpoint to the reduced pressure control valves are facilitated through use of control valve position-loop cards.

Description

(1) Other advantages and features of the invention will appear from the detailed description of an embodiment of the invention, which is a non limiting example, illustrated on the appended drawings of which:

(2) FIG. 1 is a schematic view of a power plant steam turbine for use in a method according to an embodiment of the invention; and

(3) FIG. 2 illustrates the steps performed by a steam admission control system of the steam turbine of FIG. 1.

(4) As shown on FIG. 1, a power plant steam turbine 1 comprises a high pressure steam casing 2 and at least one reduced pressure casings. In the illustrated example, the reduced pressure casings correspond to an intermediate steam casing 3 and three low pressure steam casings 4, 5 and 6. However, in another embodiment, the turbine may be provided, for example, with low pressure casings but no intermediate casing.

(5) The steam turbine is also provided with stop valves 7 and control valves 8 upstream of the high pressure casing 2, and stop valves 9 and control valves 10 upstream of the intermediate pressure casing 3.

(6) Two moisture-separator reheaters 11 and 12 are located upstream of the intermediate pressure stop valves 9 and control valves 10.

(7) The steam, supplied by a boiler (not shown here), enters the high pressure steam casing 2. The admission in the high pressure casing is done through high pressure stop valves 7 and high pressure control valves 8.

(8) After steam expansion in the high pressure casing 2, the steam is sent to moisture-separator reheaters 11 and 12.

(9) The steam is thus admitted in the intermediate pressure casing 3, through the intermediate pressure stop valves 9 and the intermediate pressure control valves 10, and then admitted in the low pressure casings 4, 5 and 6.

(10) Furthermore, the steam turbine 1 comprises a steam admission control system 13 which, when the turbine is running and advantageously when a transient occurs, is activated.

(11) As illustrated in FIG. 2, in a first step, the control system 13 determines a steam flow demand 14 in function of various turbine parameters 15. The parameters 15 may include, for example, the rotating speed of the turbine, the load, the live steam pressure, the turbine operating mode, limitations and runbacks.

(12) On one hand, the control system 13 elaborates a high pressure control valve opening setpoint 16 depending on the determined steam flow demand 14 and imposes the elaborated high pressure control opening setpoint 16 to the high pressure control valves 8.

(13) The control system 13 converts directly the steam flow demand 14 into high pressure control valve opening setpoint 16 thanks to a predefined law.

(14) On another hand, the control system 13 elaborates a reduced pressure control valve opening setpoint 17 and imposes the elaborated reduced pressure control valve opening setpoint 17 to the reduced pressure control valves 10. In the illustrated example, the reduced pressure control valves correspond to the control valves 10 of the intermediate pressure casing 3. Thus, the control system 13 elaborates an intermediate pressure control valve opening setpoint 17 and imposes the elaborated intermediate pressure control valve opening setpoint 17 to the intermediate pressure control valves. If, according to another embodiment, the turbine does not comprise intermediate pressure casing but only low pressure casing, that step applies to the low pressure control valves.

(15) In the illustrated embodiment, the steam admission control system 13 comprises one position-loop card per control valve. The position-loop cards 18 for the high pressure control valves 8 and the position-loop cards 19 for the intermediate pressure control valves 10 are respectively configured to perform the steps of imposing the opening setpoint to the high pressure control valves 8 and imposing the opening setpoint to the intermediate pressure control valves 10.

(16) The control system 13 elaborates the intermediate pressure control valve opening setpoint 17, depending on the determined steam flow demand 14, through the dynamic interaction 20 between high pressure control valve opening setpoint and the intermediate pressure control valve opening setpoint, which results in a dynamic interaction 20 between the high pressure control valves 8 position and the intermediate pressure control valves 10 position.

(17) The dynamic of evolution of the steam flow demand is used to smoothly move from a static control of the control valves 8, 10 used in normal operation of the turbine 1 to a dynamic control used to pass over a transient.

(18) The present invention allows a fast and stable control of the steam admission into the casings 2, 3, 4, 5, 6, and hence a fast and stable control of the turbine 1 generator unit speed and power by controlling the control valves 8, 10 position at any time.

(19) In normal operation of the turbine 1, the method allows, for example, avoiding overpressure in the moisture-separator reheaters, imposing that the intermediate pressure control valves 10 be more open than the high pressure control valves 8 and fully open from a load between 5 and 40%.

(20) A further advantage of the invention is, during a transient such as a load variation, a grid fault or a switch to house load operating mode, containing the high volume of steam of the moisture-separator reheaters 11, 12 and avoiding unacceptable overspeed and stress of the shaft-line that interconnects the rotor with the steam turbine 1. This is possible by controlling the high pressure control valves 8 position and the intermediate pressure control valves 10 position in parallel to adapt the thermal power regarding the electrical load, while remaining the intermediate pressure control valves 10 slightly more open than the high pressure control valves 8 in order to avoid overpressure in the moisture-separator reheaters 11, 12.