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CA-3219407-C - MULTI-OBJECTIVE STEAM TEMPERATURE CONTROL

CA3219407CCA 3219407 CCA3219407 CCA 3219407CCA-3219407-C

Abstract

A control system for controlling a steam turbine power plant having multiple steam flow paths that converge to a combined steam path controls the final steam temperature of the steam input into the turbine by controlling one or more temperature control devices in each of the steam flow paths. The control system includes a multivariable controller, such as a multi-input/multi-output (MIMO) controller, that produces two control signals that control each of a set of downstream control valves in the split steam flow paths. The controller receives two inputs in the form of measured or calculated process variables including the final steam temperature and the inter-stage temperature difference between the steam being produced in each of the two split steam paths and performs multi-objective control based on these inputs. However, when one of the downstream control valves is placed into a manual mode, the controller shifts to being a single objective controller to control the final steam temperature of the system and to thereby perform better or more optimal control.

Inventors

  • Xu Cheng
  • Ranjit R. Rao
  • RICHARD J., JR. WHALEN

Assignees

  • EMERSON PROCESS MANAGEMENT POWER & WATER SOLUTIONS, INC.

Dates

Publication Date
20260505
Application Date
20170705
Priority Date
20160729

Claims (12)

  1. Claims What is claimed is: 1. A process control system for use in controlling a first fluid property of a fluid in a combined flow path of a process having two or more split flow paths that converge to form the combined flow path, comprising: a control element in each of the two or more split flow paths for controlling a particular fluid property within each of the split flow paths; a first sensor element that determines the first fluid property of the fluid in the combined flow path; a set of second sensor elements that determine a particular fluid property of the fluid within each of the split flow paths; and a multivariable controller coupled to the first sensor element and to the set of second sensor elements to receive values of a first controlled variable and a second controlled variable, that receives values of a first set point for the first controlled variable and a second set point for the second controlled variable, that includes a process model that relates changes in each of the control signals to changes in the controlled variables, and that uses the process model to simultaneously determine a plurality of control signals for controlling the control elements in the split flow paths based on the received values of the first and second controlled variables and based on the values of the first and second set points.
  2. 2. The process control system of claim 1, further including a manual mode block associated with each of the control elements that enables a user to place an associated control element in a manual mode in which the control element is not responsive to an associated one of the control signals, and further including a feedback tracking network that indicates when the control element in one or more of the split flow paths is in a manual mode, wherein the multivariable controller operates in a first mode when the feedback tracking network indicates that none of the control elements is in the manual mode to drive both the first controlled variable to the set point for the first controlled variable and the second controlled variable to the set point for the second controlled variable, and operates in a second mode when the feedback tracking network indicates that at least one of the control elements is in the manual mode to drive the first Date Re9ue/Date Received 2023-11-09 controlled variable to the set point for the first controlled variable without driving the second controlled variable to the set point for the second controlled variable.
  3. 3. The process control system of claim 2, wherein the multivariable controller operates in the first mode to apply a first weighting factor to control signal calculations associated with driving the second controlled variable to the set point for the second controlled variable and operates in the second mode to apply a second weighting factor to the control signal calculations associated with driving the second controlled variable to the set point for the second controlled variable, wherein the second weighting function is different than the first weighting function.
  4. 4. The process control system of claim 3, wherein the second weighting factor is zero.
  5. 5. The process control system of any one of claims 1 to 4, wherein the second controlled variable is combination of measurement values of the set of second sensor elements that determine the particular fluid property of the fluid within each of the split flow paths.
  6. 6. The process control system of any one of claims 1 to 5, wherein the second controlled variable is a difference between measurement values of the particular fluid property of the fluid within two of the split flow paths.
  7. 7. The process control system of claim 6, wherein the set point for the second controlled variable is zero.
  8. 8. The process control system of any one of claims 1 to 7, wherein the multi variable controller stores the set point for the second controlled variable.
  9. 9. The process control system of any one of claims 1 to 8, wherein the first fluid property is a temperature and the first controlled variable is the first fluid property. 36 Date Re9ue/Date Received 2023-11-09
  10. 10. The process control system of claim 9, wherein the particular fluid property is a temperature and the second controlled variable is a difference between the temperatures of the fluids in two of the split flow paths.
  11. 11. The process control system of any one of claims 1 to 10, wherein the multivariable controller is a model predictive controller.
  12. 12. The process control system of any one of claims 1 to 11, wherein the multivariable controller is process model based and the process model is a set of first principle equations. 37 Date Re9ue/Date Received 2023-11-09

Description

MULTI-OBJECTIVE STEAM TEMPERATURE CONTROL Technical Field [0001] This patent relates generally to the control of boiler systems and, more particularly, to the control and optimization of steam generating boiler systems using a multi-objective controller. Background [0002] A variety of industrial as well as non-industrial applications use fuel burning boilers which typically operate to convert chemical energy into thermal energy by burning one of various types of fuels, such as coal, gas, oil, waste material, etc. An exemplary use of fuel burning boilers is in thermal power generators, wherein fuel burning boilers generate steam from water traveling through a number of pipes and tubes within the boiler, and the generated steam is then used to operate one or more steam turbines to generate electricity. The output of a thermal power generator is a function of the amount of heat generated in a boiler, wherein the amount of heat is directly determined by the amount of fuel consumed (e.g., burned) per hour, for example. [0003] In many cases, power generating systems include a boiler which has a furnace that bums or otherwise uses fuel to generate heat which, in tum, is transferred to water flowing through pipes or tubes within various sections of the boiler. A typical steam generating system includes a boiler having a superheater section (having one or more sub-sections) in which steam is produced and is then provided to and used within a first, typically high pressure, steam turbine. While the efficiency of a thermal-based power generator is heavily dependent upon the heat transfer efficiency of the particular furnace/boiler combination used to bum the fuel and transfer the heat to the water flowing within the superheater section and any additional section(s) of the boiler, this efficiency is also dependent on the control technique used to control the temperature of the steam in the superheater section and any additional section (s) of the boiler. [0004] As will be understood, the steam turbines of a power plant are typically run at different operating levels at different times to produce different amounts of electricity based on energy or load demands. For most power plants using steam boilers, the desired steam temperature set points at final superheater outlets of the boilers are kept constant, and it is necessary to maintain steam temperature close to the set points (e.g., within a narrow range) at all load levels. In particular, in the operation of utility ( e.g., power generation) boilers, control of steam temperature 1 Date Re9ue/Date Received 2023-11-09 is critical as it is important that the temperature of the steam exiting a boiler and entering a steam turbine is at an optimally desired temperature. If the steam temperature is too high, the steam may cause damage to the blades of the steam turbine for various metallurgical reasons. On the other hand, if the steam temperature is too low, the steam may contain water particles, which in tum may cause damage to components of the steam turbine over prolonged operation of the steam turbine, as well as to decrease the efficiency of the operation of the turbine. Moreover, variations in steam temperature also cause metal material fatigue, which is a leading cause of tube leaks. [0005] Typically, each section (i.e., the superheater section and any additional sections such as reheater sections) of the boiler contains cascaded heat exchanger sections wherein the steam exiting from one heat exchanger section enters the following heat exchanger section with the temperature of the steam increasing at each heat exchanger section until, ideally, the steam is output to the turbine at the desired steam temperature. For example, some heat exchanger sections include individual primary superheaters that are connected in parallel, and which may in tum be connected in series to a final superheater. In such parallel connected or cascaded arrangements, steam temperature is controlled primarily by controlling the temperature of the water at the output of the first stage of the boiler which is primarily achieved by changing the fuel/air mixture provided to the furnace or by changing the ratio of firing rate to input feedwater provided to the furnace/boiler combination. In once-through boiler systems, in which no drum is used, the firing rate to feedwater ratio input to the system may be used primarily to regulate the steam temperature at the input of the turbines. [0006] While changing the fuel/air ratio and the firing rate to feedwater ratio provided to the furnace/boiler combination operates well to achieve desired control of the steam temperature over time, it is difficult to control short term fluctuations in steam temperature at the various sections of the boiler using only fuel/air mixture control and firing rate to feedwater ratio control. Instead, to perform short term ( and secondary) control of steam temperature, in many cases saturated water is sprayed into the steam a