Abstract:
The present invention is a method of minimizing steam boiler pressure changes or turbine power changes during turbine control valve operational safety test stroking. The method of the present invention uses control valve positions as feedback into a compensation algorithm to minimize flow disturbance caused by the closing and reopening of a turbine control valve during periodic operational testing. By maintaining the total mass flow through several parallel turbine inlet control valves constant, the steam generator pressure is maintained constant, and the inlet pressure regulator is unaffected during inlet control valve testing. Maintaining the total mass flow through several parallel turbine inlet control valves constant also minimizes turbine power changes during inlet control valve testing. In addition, the monitoring of additional process parameters is not needed. The position (valve stem lift) of the individual parallel valves is used for closed loop control of inlet valve position, and is sufficient for the purpose of maintaining constant flow.

Description:
The present invention relates to turbines, and, in particular, to a method of minimizing flow disturbance caused by the closing and reopening of turbine control valves during periodic operational testing, and specifically, to using control valve positions as feedback to minimize such flow disturbance. 
   BACKGROUND OF THE INVENTION 
   Required operating procedure for turbines includes periodic operational testing (closing and reopening) of parallel inlet flow control valves used in turbines. The testing is done to confirm operability of turbine safety mechanisms. One problem with such testing is changes in the turbine steam boiler pressure or changes in turbine power as a result of the closing and reopening of the turbine control valves during the periodic operational test. Steam boiler pressure changes or turbine power changes must be minimized during turbine control valve operational safety test stroking. When present, the turbine inlet pressure regulation or turbine power feedback must not be affected or modified to achieve the compensation. 
   One pre-existing method to minimize inlet pressure excursions uses turbine inlet pressure in a proportional regulator. The inlet pressure regulator design is defined and required by the steam boiler design and, thus, cannot be modified. Other methods that have been used to compensate for turbine power disturbances caused by flow changes that occur during operational testing of inlet control valves are the use of electrical power feedback in a proportional plus integral regulator, or the use of turbine-stage pressure feedback in a proportional regulator. Neither of these methods may be applied to the inlet pressure problem because they both allow inlet pressure to change. Some of these methods also involve the monitoring of additional process parameters. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is a method of minimizing steam boiler pressure changes or turbine power changes during turbine control valve operational safety test stroking. The method of the present invention uses control valve positions as feedback to minimize flow disturbance caused by the closing and reopening of a turbine control valve during periodic operational testing. By maintaining the total mass flow through several parallel turbine inlet flow control valves constant, the steam generator pressure is maintained constant, and the inlet pressure regulator is unaffected during inlet control valve testing. Maintaining the total mass flow through several parallel turbine inlet control valves constant minimizes turbine power changes during inlet control valve testing. The position (valve stem lift or stroke) of the individual parallel valves is already present because it is used for closed-loop control of the inlet control valve positions. The valve position is sufficient, and results in improved performance, for the purpose of maintaining constant total flow when the method described herein is utilized. The monitoring of the available or additional process parameters for the purpose of reducing flow disturbance during inlet control valve testing, is not needed. 
   The flow is determined as a function of control valve position, i.e., valve stem lift. The flow change due to closure of one of the several parallel flow paths during valve testing, results in a change to the system that is controlling pressure from N valves to N- 1  valves. The flow characteristic for each valve of the system with N valves, and for the system with N- 1  valves, is determined during the turbine design process. The flow characteristics thus determined are based on total flow and individual valve stem lift. For any given valve not under test, the difference in the flow-lift characteristic between the N and N- 1  condition is known. This difference is applied to the total flow demand to each of the N- 1  valves on the basis of the total N valve demand derived from the position of the valve under test. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph showing the total flow characteristic for a system when controlling with N valves and when controlling with N- 1  valves for various valve lift values. The graph also shows the flow difference between the N and the N- 1  condition as a function of valve lift. 
       FIG. 2  is a block diagram of a control circuit for controlling the flow through the input control valves of a turbine showing the interfacing of such circuit with the flow control circuit for one valve of a total of N valves present in the turbine. 
       FIG. 3  is a block diagram of an exemplary flow control circuit with control valve test compensation for one valve of a total of N valves present in a turbine. 
       FIG. 4  is a graph of the control valve test flow compensation showing additional flow demand required for three valves to equal mass flow through four valves. 
       FIG. 5  is a graph of a control valve test with an inlet pressure regulator and without the flow compensation function. 
       FIG. 6  is a graph of a control valve test with an inlet pressure regulator and with the flow compensation function. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a method of using control valve position as feedback into a compensation function to minimize flow disturbance caused by the closing and reopening of a turbine control valve during periodic operational testing. According to the method of the present invention, total mass flow for N parallel flow valves is calculated as a function of control valve position (valve stem lift). The flow change due to closure of one of the N parallel flow valves during valve tests, results in change of the system that is controlling pressure from N valves, to N- 1  valves. The flow characteristic for each valve of the system with N valves, and for the system with N- 1  valves, is determined during design. The flow characteristics are based on total flow (valve) demand. For any given valve not under test, the flow difference characteristic between the N and the N- 1  condition is known. 
     FIG. 1  is a graph  10  showing the difference in flow characteristics between N and N- 1  turbine flow control valves. The bottom horizontal axis of graph  10  represents flow in pounds mass per hour (lbm/hr). The left vertical axis represents stem lift (valve opening) in inches, while the right vertical axis represents the percentage (position-%) of a valve opening with respect to the maximum opening of which the valve is capable of providing. The top horizontal axis of graph  10  represents the percentage of power of a steam turbine taking steam from a nuclear power source (Rx power-%). 
   Curve  12  shows the total level of flow (lbm/hr) versus stem lift (inches), for a total of four turbine control valves. Curve  14  shows the total level of flow versus stem lift for three of the four turbine control valves, where one of the control valves has been closed for test purposes. Curve  16  represents the actual difference between the total mass flow for four turbine control valves and the total mass flow for three of the turbine control valves where one of the control valves has been closed. Thus, for example, if each of the control valves in a four-valve set had a stem lift of 1″, the corresponding flow for all four valves being open would be approximately 5.5E+06 lbm/hr. Conversely, if one of the four control valves were closed, the remaining three valves would produce a corresponding flow of 4.0E+06 lbm/hr where each of the three valves had a stem lift of 1″. This difference is reflected in graph  16  where a stem lift of 1″ on graph  16  corresponds to a flow difference of approximately 1.5E+06 lbm/hr. 
   Curve  18  represents a “smoothing out” of curve  16  to provide a more appropriate curve to control flow change of the three control valves remaining open to minimize flow disturbance of the fourth valve is closed and then reopened. Thus, for example, if the flow through four valves were 8.0E+06 lbm/hr, curve  12  in graph  10  indicates that each of the valves has a stem lift of approximately 1.4″. If one of the valves is then closed for test purposes, to compensate for the loss of flow through the closed valve, the remaining three valves would require additional lift of approximately 0.6″ per valve to maintain a flow of 8.0E+6 lbm/hr. Curve  18  can be obtained on a visual approximation basis or by using a mathematical approach, such as regression analysis. 
     FIG. 2  is a block diagram  20  generally showing the manner in which the mass flow through each of several parallel turbine inlet control valves is controlled. As shown in  FIG. 2 , a turbine  22  includes several process sensors relating to the operation of the turbine. These sensors include a load sensor  24 , a speed sensor  26  and a pressure sensor  30 , the latter of which is connected to a control valve  28  controlling the flow of process fluid to turbine  22 . The outputs of sensors  24 ,  26  and  30  are provided as inputs  25 ,  27  and  31 , respectively, to a load controller  38 , a speed controller  36  and a pressure controller  32  used to control the operation of turbine  22 . The outputs  34 ,  35  and  40 , respectively, of pressure controller  32 , speed controller  36  and load controller  38 , in combination, constitute turbine  22 &#39;s processor controller&#39;s flow demand. Outputs  34 ,  35  and  40  are fed into a selector  42 , and in combination, produce an output  44  which is the selected total flow demand used by the process controller to control the flow through the control valves providing mass flow into the inlet of turbine  22 . Output  44  of selector  42  is referred to as “TCV Reference”, which is a signal that effectively establishes the total flow demand for the valves to produce. In normal operation, the TCV Reference signal is fed into a test control circuit  48  which includes the means to convert the TCV reference into the required valve position and generates an output  49  that establishes Valve Position Demand. Output  49  is received by a valve servo position loop  47  which provides closed-loop position control of the lift of valve  28 . 
   To minimize steam boiler pressure changes or turbine power changes during turbine control valve operational safety testing, the present invention uses a test compensation circuit  50 . This compensation circuit uses control valve positions as feedback and compensates by adjusting the flow through parallel control valves to minimize flow disturbance caused by the closure and reopening of turbine control valve  28  during testing. Test compensation circuit  50  is shown in greater detail in  FIG. 3 . According to the present invention, the test compensation circuit  50  would be reproduced along with test control circuit  48  and valve servo position loop  47  for each valve of several parallel turbine inlet control valves used to control the mass flow through turbine  22 . In this regard, output  44  of selector  42  would be provided as signals  41 ,  43  and  45  to control valves  2 ,  3  and N, respectively, as shown in  FIG. 2 . 
     FIG. 3  is a more detailed block diagram of the test control circuit  48  commonly used to control mass flow through parallel turbine inlet control valves. Test compensation circuit  50  is also shown in more detail in  FIG. 3 . In particular, circuits  50 A and  50 B shown in  FIG. 3  together constitute test compensation circuit  50  shown in  FIG. 2 . 
   Referring to block diagram  50 A in  FIG. 3 , signal  46 , TCV Reference, is input to a test compensation array  52  and a summing circuit  59 . Signal, TCV Reference, is indicative of the mass flow demand for all of the parallel inlet control valves to achieve a desired level of total mass flow through turbine  22 . Test compensation array  52  is essentially a “look up table” that provides the flow compensation, for the mass flow difference demanded by TCV Reference, for the three input control valves not being tested, where a fourth one of the control valves is being closed for testing. As noted above, the flow compensation required for a given TCV reference comes from curves  16  and  18  shown in  FIG. 1 , which show the difference in total mass flow for three turbine control valves versus four turbine control valves for different values of valve stem lift. 
     FIG. 4  is a graph effectively representing the function performed by Test Comp Array  52 . The compensation array, Test Comp Array  52 , is based on the mass flow being demanded (“TCV Reference”). This then skews the graph  18  shown in  FIG. 1  to look like curve  74  in graph  75  of  FIG. 4 . The bottom horizontal axis of graph  75  represents mass flow demanded (“TCV Reference” in percentage) that is input to Test Comp Array  52 . The left vertical axis represents flow compensation (in percentage) that is output from Test Comp Array  52 . 
   The output of Test Comp Array  52  is fed into a sample and hold circuit  54 , which receives a signal  55  identified as “CVx Test State”. The signal, “CVx Test State”, is a logic “True/False” signal generated by the activation of a test switch (not shown), which indicates whether the particular input valve controlled by circuit  48  shown in  FIG. 3  (here, valve # 1 ) is in test mode. If it is, “False” (meaning that valve # 1  is not being tested) signal “CVx Test State” enables sample and hold circuit  54  to pass the output of Test Comp Array  52  into a multiplier circuit  56 . Sample and hold circuit  54  provides the flow compensation for the three input control valves not under test (which include valve # 1 ) with respect to the mass flow demanded by the TCV Reference signal. 
   Also inputted into multiplier circuit  56  is a second signal  70 , identified as “CVx Comp Ref”, which is generated by the circuit of block diagram  50 B. “CVx Comp Ref” is the amount of flow compensation needed at a given TCV Reference for the for the three valves not under test. 
   Referring now to  FIG. 50B , an input signal  60 , identified as “Position From CV Servo Regulator For CVm”, is input into a Lift Flow Array  62 . The signal “Position From CV Servo Regulator For CVm” is dynamic signal that indicates the lift position of the valve (here, valve # 1 ) being controlled by circuit  48  shown in  FIG. 3  and the valve servo position loop ( 47  in  FIG. 2 ). Lift Flow Array  62  is also essentially a “look up table” that provides, for the stem lift of valve # 1 , a translation to a total flow demand value for use by the three input control valves not being tested (which include valve # 1 ), when a fourth one of the control valves is being closed for testing. As noted above, the translation to total flow demand value comes from curve  12  shown in  FIG. 1 , which show the total mass flow for four turbine control valves for different values of valve stem lift. 
   Sample and Hold Circuit  64  receives a signal  71  identified as “CVm Test Select”, which is the logic “True/False” signal generated by the activation of the test switch (not shown), which selects the particular input valve controlled by test control circuit  48  shown in  FIG. 3  (here, valve # 1 ) for testing. If “CVm Test Select” is “False”, it enables Sample and Hold Circuit  64  to pass the flow demand value from Lift Flow Array  62  to a Divider Circuit  66 . When “CVm Test Select is “True”, the flow demand value from Lift Flow Array  62  is held and passed to Divider Circuit  66 . Lift Flow Array Circuit  62  also provides Divider Circuit  66  with a varying flow demand signal for the other three input control valves not under test, as the stem lift of such tested valve, such as valve # 1 , varies. 
   The denominator “B” of the divider circuit  66  is the flow demand value from Lift Flow Array  62 . This value remains the same during the test closing of a given valve. The numerator “A” of the divider circuit  66  is the varying flow demand value from Lift Flow Array  62  that changes as the tested valve is closed and reopened. The output of the divider circuit  66  is a fraction that starts at 1 (meaning no compensation) and gets progressively closer to 0 (meaning 100% compensation) as the tested valve is closed. 
   The output of the divider circuit  66  is then fed into a summing circuit  68  which also receives an input signal identified as “K One”, a reference signal with a constant value of “1”. The output from Divider Circuit  66  (initially 1 for no compensation) is subtracted in Sum Circuit  68  from the fixed constant of “1” constituting signal “K One”. For a given valve being tested, this subtraction produces an output of “0” that is fed into Multiplier Circuit  56  of the valves not being tested, as the signal “CVx Comp Ref”. Signal “CVx Comp Ref” begins at 0, and, as the tested valve is closed, the numerator “A” in Divider Circuit  66  changes as the varying value of the lift position of the tested valve changes as the tested valve is closed and then reopened. As the output of Divider Circuit  66  gets smaller and smaller as the tested valve is closed, the output of Sum Circuit  66  increases from 0 to 1. As the tested valve is reopened, the output of Sum Circuit  66  decreases from 1 to 0. The output of summing circuit  68  is output signal  70 , “CVm Comp Reference”, which, as noted above, is input into multiplier circuit  56 . 
   As also noted above, CVx Comp Ref” is an indication of the amount of flow compensation needed for the for the three valves not under test. Thus, by way of example, if valve # 4  is being tested, and each of valve #s  1 ,  2 , and  3  need to be opened from 1-inch to 1½ inches to compensate for the mass flow lost by the full closing of valve # 4 , the additional ½-inch″ of lift is the result of the flow compensation value multiplied by a compensation factor that&#39;s going to move the lift for valves  1 ,  2  and  3  from 1″ to 1-½″ as valve # 4  closes. Thus, as valve # 4  is closed, the flow compensation for each of valves  1 ,  2 , and  3  would be multiplied by “CVx Comp Ref”, which is a changing signal starting out initially at 0 and increasing to 1 or 100% as valve # 4  is fully closed. 
   The output of multiplier circuit  56  is fed into a Select Circuit  58 , which also receives a second signal “K Zero”, a reference signal with a constant value of “0”, and a third signal from valve test control circuit  48  that determines whether reference signal “K Zero” or the output of multiplier circuit  56  is fed into Sum Circuit  59 . In Sum Circuit  59 , either the “0” output of Select Circuit  58  or the valve stem lift compensation signal output of Select Circuit  58  is summed with the signal “TCV Reference” and fed into a Flow Lift Array  73  that determines the valve lift of valve # 1 , as controlled by test control circuit  48 . The logic of the test control circuit is such that the Select Circuit  58  will output the value of multiplier circuit  56  only when a valve, other than itself, is being tested. 
   To test the method and system of the present invention, a turbine system to be controlled was mathematically modeled, thermodynamically accurate, and simulated in real time. The model system consisted of source and sink with four parallel control valves individually controlling flow through four nozzles. The simulated system was connected to the embodiment of the control system of the present invention described above. The control system contained the algorithms for compensation of flow during valve testing as described above. For comparison, the control system was configured to include flow compensation and not use flow compensation. The overall control strategy requires control of pressure ahead of the valves using a proportional regulator. The use of the control valve test compensating control of the present invention reduced the pressure excursion of the turbine inlet main (throttle) steam pressure by 95%, as shown in  FIGS. 5 and 6 , respectively.  FIG. 5  is a graph  80  that shows the results of a control valve operative test without the flow compensation of the present invention, while  FIG. 6  is a graph  82  that shows the results of a control valve test with the flow compensation of the present invention. In both tests, valve # 3  was the valve closed for test purposes. The position of valve # 3  is shown as curve  84  in both  FIGS. 5 and 6 , while the pressure change in the steam pressure of the system when valve # 3  is originally open, closed, and then reopened, is shown as curve  86 . The position of each of valve # 1 ,  2  and  4  is shown as curves  81 ,  83  and  85 , respectively, in both  FIGS. 5 and 6 . 
   While the invention has been described in connection with what is presently considered to be the 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.