WO2002017028A1

WO2002017028A1 - Method for detecting plug wear

Info

Publication number: WO2002017028A1
Application number: PCT/US2001/024898
Authority: WO
Inventors: Harry A. Burns; Brent H. Larson; Larry K. Brown
Original assignee: Fisher Controls International, Inc.
Priority date: 2000-08-22
Filing date: 2001-08-09
Publication date: 2002-02-28
Also published as: AU2001283197A1; AR030461A1

Abstract

A method for detecting a condition suggesting a potential for wear in a fluid control valve plug in a process control system having a microprocessor based valve positioner. A plurality of operating parameters including the flow rate, temperature, and pressure, and the valve position are monitored. The capacity of volume of the fluid control valve is determined based on the plurality of operating parameters. The deviation between the monitored valve position corresponding to the capacity of volume and a reference valve position is determined. If the deviation is greater than a predetermined value ten an indication of a condition of the plug suggesting a potential for wear is provided.

Description

METHOD FOR DETECTING PLUG WEAR

FIELD OF THE INVENTION

The present invention relates generally to plugs for sealing fluid control valves, and more particularly to a method for detecting plug wear in a fluid control valve .

BACKGROUND OF THE INVENTION Fluid control valves are used in environments that may cause significant plug erosion. Valves, for example, are used for controlling fluids in oil and gas pipelines and processing lines, chemical processing plants, nuclear power plants, etc. In particular, valves are implemented in oil -production where sand flows through the lines with hydrocarbons. The sandy fluid travels at a fast rate across the valve such that the plug erodes and wears out over time.

The purpose of valves is to control the rate or amount of flow in a line. Significant plug erosion may diminish the ability of a valve to accomplish such task. As the plug wears out in a fluid control valve, the valve position trends towards a more closed position to maintain similar flow rates across the valve. Plug erosion is a gradual process that is usually undetected until the process control limit has been reached and the valve must be immediately serviced to maintain control of the flow. A periodic maintenance plan may replace the plug on a schedule to prevent such erosion. However, such a plan may increase cost of premature maintenance of the plug.

Therefore, a need exists for a technique that monitors plug erosion to predict and reduce the downtime for repairs, and which is readily implemented to provide an early warning indication that the plug may be worn out to the point of loss in the desired precise control of the fluid flow through the valve.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, there is provided a method for detecting and indicating plug wear in a configuration where a microprocessor based valve positioner is controlling a valve actuator and a fluid control valve in a process control system.

In particular, a plurality of operating parameters and the position of the fluid control valve are monitored. The capacity of volume of the valve is determined based on the plurality of operating parameters. The monitored valve position is compared to a reference valve position corresponding to the capacity of volume. The deviation between the monitored valve position and the reference valve position is determined with the capacity of volume remaining constant. If the deviation is more than a predetermined value then an indication of a condition of the plug suggesting a potential for wear is provided.

A significant advantage of the present invention is in being able to detect wear in a plug of a fluid control valve without a scheduled shutdown of the process control system and thereby reduce cost of premature maintenance of the plug.

BRIEF DESCRIPTION OF THE DRAWINGS The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which:

Figure 1 is a schematic diagram illustrating a microprocessor based valve positioner operating an actuator and a fluid control valve in a process control system; Figure 2 is a flow diagram illustrating a method for detecting plug wear in accordance with the preferred embodiment of the present invention; and Figure 3 is a graphic plot derived from a method for detecting plug wear in accordance with the preferred embodiment of the present invention.

DETAILED DESCRIPTION

Figure 1 illustrates a microprocessor based valve positioner 110 including a current to pressure converter 112 and a microprocessor 114, a set point circuit 115, and a process proportional/integral/derivative (PID) unit 116. The valve positioner 110 is coupled to a supply pressure 117 and receives a drive signal on an input line 118 for providing a corresponding drive pressure on an output line 120 to a valve actuator 122. The output of the valve actuator 122 is coupled to a valve stem 124 of a fluid control valve 126. The valve 126 is coupled into a pipeline 128 of a fluid process system. The valve stem 124 is coupled to a valve plug 130, which seals a valve seat 132. The valve plug 130 controls the flow of fluid from a valve inlet 134 through the valve seat 132 in the valve opened position and to a valve outlet 136. In typical operations of a fluid process control system, a set point circuit 115 provides a drive signal on the input line 118 to the microprocessor 114 and thereby moves the valve stem 124 and the plug 130 through the actuator 122 to provide a desired process value at the output 136 of the valve 126. This process value, which may be a desired flow rate, is determined by a process value measurement device 140 for coupling, for instance, to a well known process proportional/integral/derivative (PID) unit 116 to reset the set point 115. A well known feedback configuration between the valve stem 124 and the valve positioner 110 is provided by a feedback linkage 144 coupled to the valve stem 124 to provide information as to the valve stem position to a valve position sensor 146. A valve position feedback signal from the valve position sensor 146 is coupled to the microprocessor 114 on a feedback line 148. In addition to monitoring the valve position, the microprocessor may monitor a plurality of upstream values by coupling to an upstream value measurement device 150. The process value and the plurality of upstream values determine the capacity of volume of the valve as one of ordinary skills will readily recognize. The capacity of volume is the maximum potential of flow through the valve. In this typical fluid process control system, a standard valve positioner cannot easily detect wear in the plug 130. As the plug 130 erodes over time, a standard positioner will adjust the valve position to a more closed position to maintain similar flow rates across the valve. In some conditions, the plug 130 may be eroded to a point beyond where the valve 126 can control the process . Plug wear may not be detected without taking the system off-line to manually inspect the plug 130. Monitoring a plurality of operating parameters and the position of the valve 126 to detect wear in the plug 130 alleviates the unplanned shutdowns required when the plug 130 needs immediate service for repair or replacement . The capacity of volume based on the plurality of operating parameters and the monitored valve position are determined to detect whether a change in the valve position exceeds a predetermined value. A potential for plug wear is suggested with the capacity of volume remaining constant, and the monitored valve position moving towards a more closed state. The fluid process control system would cause the valve to close more to compensate for the increased fluid flow area caused by the plug wear. Reference may be made to Figure 2 in which there is indicated a method for presenting an indication of a condition of a plug suggesting a potential for wear, which can be used to suggest repair or replacement should take place. In the valve positioner 110 of the present invention, the microprocessor 114 includes a plug wear detection algorithm 200 performing the steps shown in Figure 2.

As shown in Figure 2, a plurality of operating parameters of a fluid process control system are monitored at step 210. The plurality of operating parameters may be, but is not limited to, a flow rate, a temperature, and a change in pressure. For example, the process value measurement device 140 monitors the flow rate at the valve outlet 136 and the upstream value measurement device monitors the upstream pressure and temperature of a fluid flowing through the valve 126 from the valve inlet 134. At step 220, the capacity of volume of the valve 126 is determined based on the plurality of operating parameters as one of ordinary skill in the art will readily recognize. The capacity of volume is the maximum potential of flow through the valve 126. At step 230, the position of the valve is monitored by the valve position sensor 146. The valve position is adjusted according to a desired rate or amount of flow through the valve 126. At step 240, the monitored valve position is compared to a reference valve position corresponding to the capacity of volume determined at step 220. The reference valve position is a predetermined valve position for a particular capacity of volume. In the preferred embodiment, a deviation between the monitored valve position and the reference valve position is determined. If the deviation between the monitored valve position and the reference valve position is more than a predetermined value then at step 250, an indication of a condition of the plug suggesting a potential of wear is provided. The indication may be, but is not limited to, an alarm alerting the operator of the process control system that wear on the plug is beyond the desired operating level. If the deviation between the monitored valve position and the reference valve position is within the predetermined value then that indicates wear on the plug does not warrant an immediate repair or replacement .

In an alternate embodiment, the upstream pressure of the valve 126 is held constant, and the flow rate of the valve 126 is monitored along with the valve position. A potential for plug wear may be suggested with the valve position being required to move towards a more closed state as the flow rate remains constant . Figure 3 is a graphic plot of the capacity of volume corresponding to the valve position in a fluid control valve. In particular, each point represents the monitored valve position corresponding to a capacity of volume determined based on the plurality of operating parameters at a particular time. Accordingly, the monitored valve positions of Points 1, 2 , 3 , and 4 correspond to a capacity of volume determined based on a plurality of operating parameters during times T_1; T₂, T₃, and T₄, respectively. Each of the monitored valve positions corresponding to Points 1-4 are compared to a reference valve position with the capacity of volume held constant. As noted above, the reference valve position is a predetermined valve position corresponding to a particular capacity of volume. A deviation between the monitored valve position and the reference valve position is determined for comparison with a predetermined value. The deviation is compared to a predetermined value to detect a condition of the plug suggesting a potential for wear. As illustrated by the reference line, there is a direct correlation between the capacity of volume and the valve position such that the capacity of volume increases along with the valve position. An increase in valve position results in a more open state in the valve, which results in a greater flow rate and amount of flow through the valve and thereby increases the capacity of volume. As noted above, the deviation between the monitored valve position and the reference valve position is determined. If the deviation is more than a predetermined value then an indication of a condition of the plug suggesting a potential for wear is provided. For example, the position of the valve is measured in sixteenths of an inch and the monitored valve position corresponding to Point 2 deviates from the reference valve position corresponding to Point C by two-sixteenths of an inch (2/16) . If the predetermined value is one-sixteenth of an inch then the deviation between the monitored valve position corresponding to Point 2 and the reference valve position corresponding to Point C exceeds the predetermined value. As a result, an indication of a condition of the plug suggesting a potential for wear is provided. However, if the predetermined value is three-sixteenths of an inch (3/16) then the deviation does not exceed the predetermined value, and no indication of a condition of the plug suggesting a potential for wear is provided.

As shown in Figure 3, the monitored valve position corresponding to Point 1 suggests the least potential for plug wear relative to the valve positions corresponding to Points 2-4. The deviation between the monitored valve position corresponding to Point 1 and - li the reference valve position corresponding to Point D is one-sixteenth of an inch, and the deviation between the monitored valve positions of the other points and its respective reference valve positions are two- sixteenths of an inch (2/16) , five-sixteenths of an inch (5/16), and seven-sixteenths of an inch (7/16), respectively. Accordingly, the monitored valve position corresponding to Point 4 suggests the most potential for plug wear in a valve. The deviation between the monitored valve position corresponding to Point 4 and the reference valve position corresponding to Point F is the greatest in comparison to the deviations between the monitored valve positions corresponding to Points 1-3 and their respective reference valve positions.

The foregoing detailed description has been given for clarity of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims

WHAT IS CLAIMED;

1. In a microprocessor based valve positioner having a set point input and providing a drive signal for controlling a valve actuator with corresponding drive pressure for operating and positioning a valve stem and plug with respect to a valve seat of a fluid control valve in response to the valve stem position feedback to the valve positioner, a method for detecting plug wear in the control valve comprising the steps of: monitoring a plurality of operating parameters ; monitoring a position of the fluid control valve and providing a corresponding monitored valve position; detecting a condition of the plug suggesting a potential for wear; and providing an indication of the condition of the plug suggesting a potential for wear.

2. The method of claim 1, wherein the plurality of operating parameters comprises one of a flow rate, a temperature, and a change in pressure.

3. The method of claim 1 further comprises the step of determining a capacity of volume of the valve based on the plurality of operating parameters.

4. The method of claim 3 further comprises the steps of determining a reference valve position corresponding to a determined capacity of volume and comparing the monitored valve position to the reference valve position with the capacity of volume remaining constant.

5. The method of claim 4 further comprises the step of determining a deviation between the monitored valve position and the reference valve position.

6. The method of claim 5 further comprises the step of comparing the deviation to a predetermined value .

7. The method of claim 1, wherein the step of providing an indication of the condition of the plug suggesting a potential for wear comprises the step of providing an alarm in response to detecting a condition of the plug suggesting a potential for wear.