Multi-objective steam temperature control

A method of controlling a process having two or more split flow paths that converge to form a combined flow path, with one or more control elements in each of the two or more split flow paths for controlling a fluid property of fluid within the split flow paths, the method comprising:

• receiving indications of a first and a second controlled process variable;
• receiving set points for each of the first and second controlled process variables;
• determining, via a processor, a plurality of control signals, each control signal to be used to control one of the one or more control elements in one of the split flow paths, and
• receiving a feedback indication of when the control elements in one or more of the split flow paths is in a manual mode in which the control element is not responsive to an associated one of the control signals;
• wherein determining the plurality of control signals includes determining the plurality of control signals in a first mode, in which the feedback indication indicates that none of the control elements is in the manual mode, using each of the received first and second controlled process variable indications and the set points for each of the first and second controlled process variables to determine the control signals, and determining the plurality of control signals in a second mode, in which the feedback indication indicates that at least one of the control elements is in the manual mode, using only one of the first and second determined controlled process variable indications and only one of the first and second set points for the controlled process variables.

A process control system for controlling a process having two or more split flow paths that converge to form a combined flow path, with one or more control elements in each of the two or more split flow paths for controlling a fluid property within the split flow paths, comprising:

• a multivariable controller that includes a plurality of controlled variable inputs, which each receives a value of a different controlled variable, and a set point input for each of the plurality of controlled variables associated with the controlled variable inputs, wherein the multivariable controller operates via a processor to simultaneously determine a multiplicity of control signals with each control signal for use in controlling one of the control elements in one of the split flow paths, wherein the multivariable controller determines the values of the multiplicity of control signals to implement two or more objectives, the first objective being to drive a first controlled variable to the set point for the first controlled variable and the second objective being to drive a second controlled variable to the set point for the second controlled variable;
• and
• a feedback tracking network that indicates when the control elements in one or more of the split flow paths is in a manual mode in which the control element is not responsive to an associated one of the control signals;
• wherein, in a first mode, when the feedback tracking network indicates that none of the control elements is in the manual mode, the multivariable controller determines the control signals to implement both of the first and second objectives and, in a second mode, when the feedback tracking network indicates that at least one of the control elements is in the manual mode, the multivariable controller determines the control signals to implement only one of the first and second objectives.

A method of controlling a process having two or more split flow paths that converge to form a combined flow path and including one or more control elements in each of the two or more split flow paths for controlling a fluid property of fluid within the split flow paths, comprising:

• receiving indications of a multiplicity of controlled variables;
• storing set points for each of the multiplicity of controlled variables;
• determining, via a processor, a plurality of control signals, each control signal to be used to control one of the one or more control elements in one of the split flow paths, including, determining the values of the multiplicity of control signals to implement a multiplicity of objectives, each objective related to driving an associated one of the controlled variables to the set point for the associated one of the controlled variables, and
• receiving a feedback indication of when the control element in one or more of the split flow paths is in a manual mode in which the control element is not responsive to an associated one of the control signals;
• wherein, in a first mode when the feedback indication indicates that none of the control elements is in the manual mode, determining the plurality of control signals includes determining values for the control signals that implement all of the multiplicity of objectives and, in a second mode when the feedback indication indicates that at least one of the control elements is in the manual mode, determining the plurality of control signals includes determining values for the control signals that implement less than all of the multiplicity of objectives.

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;
• 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;
• and
• 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 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.