Fluid handling – Processes
Reexamination Certificate
2000-06-09
2001-08-21
Lee, Kevin (Department: 3753)
Fluid handling
Processes
C251S129040, C137S554000
Reexamination Certificate
active
06276385
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to positioners for moving a valve to a desired position and, more particularly, relates to control systems for such positioners.
BACKGROUND OF THE INVENTION
In the transmission of fluids through pipelines, processing facilities, and the like, it is necessary to control valves within the system accurately and quickly. For example, in petroleum processing plants or chemical manufacturing facilities integrated control schemes often require various valves throughout the system to be opened and closed in sequence, and to various degrees, depending upon a given set of operating parameters. Those operating parameters may include a desired flow rate, a timed release of fluid, the particular fluid being processed, etc.
As such control systems become increasingly automated, it is imperative that the control valves be positioned accurately and responsively. Any lag time between receipt of a position command signal, and the actual positioning of the valve to the desired location detrimentally affects the process in terms of productivity and profitability.
As a result, systems have been developed for responsively positioning a control valve. For example, in one of my previous inventions, disclosed in U.S. Pat. No. 4,509,403, assigned to the present assignee, a positioner system is provided which employs a single feedback loop for dynamically controlling the position of a valve. In addition, U.S. Pat. No. 5,884,894 discloses a positioning system employing first and second control loops for controlling the position of a spool valve. However, spool valves typically exhibit high leakage, and if designed to exhibit less leakage, result in a relatively large “dead zone” in the operating range of the valve. This detrimentally affects responsiveness of the valve. Moreover, such spool valves need to be manufactured to close tolerances, increasing the cost of the valve.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a control valve assembly is provided which comprises a control valve, a plug and seat valve, a first position sensor, a second position sensor, and a comparator. The plug and seat valve is operatively associated with the control valve. The first position sensor is adapted to monitor the position of the control valve and generate a first position signal. The second position sensor is adapted to monitor the position of the plug and seat valve and generate a second position signal. The comparator is adapted to compare the first position signal to a first setpoint and generate a second setpoint, and is further adapted to compare the second position signal to the second setpoint and generate a correction signal. The correction signal is communicated to the plug and seat valve for positioning the control valve.
In accordance with another aspect of the invention, a valve control system is provided which comprises a control valve, a control valve actuator, a first position sensor, a plug and seat valve, a first comparator, a second position sensor, a second comparator, and a transducer. The control valve actuator is operably associated with the control valve and is adapted to modify the position of the control valve. The first position sensor is proximate the control valve and is adapted to sense the position of the control valve. The first position sensor generates a signal indicative of the control valve position. The plug and seat valve is in fluid communication with the control valve actuator and is adapted to generate a pressure signal for use by the control valve actuator. The first comparator is adapted to compare the control valve position to a control valve setpoint and generate a plug and seat valve position setpoint. The second position sensor is proximate the plug and seat valve and is adapted to sense the position of the plug and seat valve and generate a signal indicative of the plug and seat valve position. The second comparator is adapted to compare the plug and seat valve position to the plug and seat valve position setpoint and generate a plug and seat valve correction signal. The transducer is adapted to receive the plug and seat valve correction signal and generate a corresponding pressure signal for communication to the plug and seat valve.
In accordance with another aspect of the invention, a system for positioning a control valve driven by a control valve actuator, wherein the control valve actuator is in communication with a plug and seat valve, comprises a first control loop and a second control loop. The first control loop compares a signal indicative of control valve position to a first setpoint to generate a second setpoint. The second control loop compares a signal indicative of plug and seat valve position to the second setpoint to generate a control valve actuator drive signal, the control drive actuator drive signal being communicated to the control valve actuator for positioning the control valve.
In accordance with another aspect of the invention, a system for positioning a control valve driven by a control valve actuator is provided wherein the control valve actuator is in communication with a control valve actuator valve. The system includes a first control loop, a second control loop, and a tunable gain adjustment circuit. In the first control loop a signal indicative of control valve position is compared to a first setpoint to generate a second setpoint. In the second control loop a signal indicative of control valve actuator valve position is compared to the second setpoint to generate a control valve actuator drive signal, the control valve actuator drive signal is communicated to the control valve actuator for positioning the control valve. The tunable gain adjustment circuit is in the second control loop and is adapted to modify the signal indicative of control valve actuator valve position.
In accordance with another aspect of the invention, a method for positioning a control valve is provided which comprises the steps of monitoring the position of the control valve, comparing the position of the control valve to a first setpoint, generating a second setpoint based on the comparing step, monitoring the position of a plug and seat valve, comparing the position of the plug and seat valve to the second setpoint, generating a correction signal based on the comparison of the plug and seat valve position to the second setpoint, and positioning the control valve based on the correction signal.
These and other aspects and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4509403 (1985-04-01), Gassman et al.
patent: 4747424 (1988-05-01), Chapman
patent: 5318064 (1994-06-01), Reinicke
patent: 5532925 (1996-07-01), Gassman
patent: 5564470 (1996-10-01), Denmark et al.
patent: 5884894 (1999-03-01), Smith
patent: 5950668 (1999-09-01), Baumann
patent: 0 827 056 A2 (1998-03-01), None
Fisher Controls International Inc.
Lee Kevin
Marshall O'Toole Gerstein Murray & Borun
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