Control method and control system for a flow control valve
A method and system for controlling a flow control valve, comprising measuring a flow at a suitable point of a flow process and calculating a pressure drop across the valve from a C v curve of the valve, a measured valve opening α m , and the measured flow (Q m ). From this calculated pressure drop, a flow set point (Q sp ), and an inverse C v curve of the valve, a new valve opening (α c ) providing the desired flow is calculated. The control is thus adapted to each operating point and its prevailing flow gain and linearizes the dependence between flow control and flow.
1. A method for controlling a flow control valve, the method comprising:
measuring a present opening of the valve;
measuring a flow through the present opening of the valve;
determining a C v value of the valve, using said present opening of the valve, on a C v curve of the valve, wherein the C v value is a flow coefficient value and the C v curve describes C v values as a function of valve openings;
calculating a pressure drop across the valve by means of the measured flow and the determine C v value;
calculating a flow deviation as a difference between a desired flow set point and the measured flow;
integrating said flow deviation;
calculating a corrected flow set point by adding the integrated flow deviation to the desired flow set point;
calculating a new C v value of the valve by means of the corrected flow set point and the calculated pressure drop;
determining a new valve opening, using said new C v value of the valve, on an inverse C v curve of the valve, wherein the inverse C v curve describes valve openings as a function of C v values;
moving the valve to a new opening position in response to the determined new valve opening; and
continuing said determining of a new valve opening and said moving the valve to a new opening position until the measured flow is set to the desired flow set point.
2. The method as claimed in claim 1 , further comprising prestoring values of the C v curve of the valve and also a value of the inverse C v curve in a memory.
3. The method as claimed in claim 1 , further comprising prestoring values of the inverse C v curve in a memory.
4. A control system for a flow control valve, comprising:
a controller configured to receive a measurement of a present opening of the valve;
a flow indicator configured to measure a flow through the present opening of the valve;
the controller being configured to determine a C v value of the valve, using said present opening of the valve, on a C v curve of the valve, wherein the C v value is a flow coefficient value and the C v curve describes C v values as a function of valve openings;
the controller being configured to calculate a pressure drop across the valve by means of the measured flow and the determined C v value;
the controller being configured to calculate a flow deviation as a difference between a desired flow set point and the measured flow;
the controller being configured to integrate said flow deviation;
the controller being configured to calculate a corrected flow set point by adding the integrated flow deviation to the desired flow set point;
the controller being configured to calculate a new C v value of the valve by means of the corrected flow set point and the calculated pressure drop;
the controller being configured to determine a new valve opening, using said new C v value of the valve, on an inverse C v curve of the valve, wherein the inverse C v curve describes valve openings as a function of C v values;
the controller being configured to move the valve to a new opening position in response to the determined new valve opening; and
the controller being configured to continue said determining of a new valve opening and said moving the valve to a new opening position until the measured flow is set to the desired flow set point.
5. The system as claimed in claim 4 , wherein the controller comprises a memory configured to prestore values of the C v curve of the valve.
6. The system as claimed in claim 4 , wherein the controller is implemented in one of the following apparatuses: a control valve guide, a valve positioner, an intelligent field device, a controller of a distributed control system, a process control computer, or a control room computer.
7. The system as claimed in claim 4 , wherein the controller further comprises a memory configured to prestore values of the inverse C v curve.
8. A controller for controlling a flow control valve, comprising:
the controller being configured to receive a measurement of a present opening of the valve;
the controller being configured to receive a measurement of a flow through the present opening of the valve;
the controller being configured to determine a C v value of the valve, using said present opening of the valve, on a C v curve of the valve, wherein the C v value is a flow coefficient value and the C v curve describes C v values as a function of valve openings;
the controller being configured to calculate a pressure drop across the valve by means of the measurement of the flow and the determined C v value;
the controller being configured to calculate a flow deviation as a difference between a desired flow set point and the measurement of the flow;
the controller being configured to integrate said flow deviation;
the controller being configured to calculate a corrected flow set point by adding the integrated flow deviation to the desired flow set point;
the controller being configured to calculate a new C v value of the valve by means of the corrected flow set point and the calculated pressure drop;
the controller being configured to determine a new valve opening, using said new C v value of the valve, on an inverse C v curve of the valve, wherein the inverse C v curve describes valve openings as a function of C v values;
the controller being configured to move the valve to a new opening position in response to the determined new valve opening; and
the controller being configured to continue said determining of a new valve opening and said moving the valve to a new opening position until the measurement of the flow is set to the desired flow set point.
9. The controller as claimed in claim 8 , wherein the controller comprises a memory configured to prestore values of the C v curve of the valve.
10. The controller as claimed in claim 8 , wherein the controller comprises a programmable processor or a computer and a program code to be executed in the computer or the processor.
11. The controller as claimed in claim 8 , wherein the controller further comprises a memory configured to prestore values of the inverse C v curve.
12. A non-transitory computer-readable storage medium having computer-executable components that perform:
receiving a measurement of a present opening of a valve;
receiving a measurement of a flow through the present opening of the valve;
determining a C v value of the valve, using said present opening of the valve, on a C v curve of the valve, wherein the C v value is a flow coefficient value and the C v curve describes C v values as a function of valve openings;
calculating a pressure drop across the valve by means of the measurement of the flow and the determined C v value;
calculating a flow deviation as a difference between a desired flow set point and the measurement of the flow;
integrating said flow deviation;
calculating a corrected flow set point by adding the integrated flow deviation to the desired flow set point;
calculating a new C v value of the valve by means of the corrected flow set point and the calculated pressure drop;
determining a new valve opening, using said new C v value of the valve, on an inverse C v curve of the valve, wherein the inverse C v curve describes valve openings as a function of C v values;
moving the valve to a new opening position in response to the determined new valve opening; and
continuing said determining of a new valve opening and said moving the valve to a new opening position until the measurement of the flow is set to the desired flow set point.
13. An apparatus comprising a processor and a memory storing executable instructions configuring the processor to perform:
receiving a measurement of a present opening of a valve;
receiving a measurement of a flow through the present opening of the valve;
determining a C v value of the valve, using said present opening of the valve, on a C v curve of the valve, wherein the C v value is a flow coefficient value and the C v curve describes C v values as a function of valve openings;
calculating a pressure drop across the valve by means of the measurement of the flow and the determined C v value;
calculating a flow deviation as a difference between a desired flow set point and the measurement of the flow;
integrating said flow deviation;
calculating a corrected flow set point by adding the integrated flow deviation to the desired flow set point;
calculating a new C v value of the valve by means of the corrected flow set point and the calculated pressure drop;
determining a new valve opening, using said new C v value of the valve, on an inverse C v curve of the valve, wherein the inverse C v curve describes valve openings as a function of C v values;
moving the valve to a new opening position in response to the determined new valve opening; and
continuing said determining of a new valve opening and said moving the valve to a new opening position until the measurement of the flow is set to the desired flow set point.