Method and apparatus for performing diagnostics on conventional control valves

Abstract

The method comprises obtaining data samples, each data sample comprising a process variable and a controller output for adjusting the process variable to reach a set-point variable; and determining if there is valve jump or stiction by calculating a jump index or a static band index; wherein calculating the jump index comprises calculating a mean of a first set of angles, each angle from the first set of angles being computed from a ratio of: a difference between a peak process variable and its preceding process variable, and, a difference between a controller output corresponding to the peak process variable and its preceding controller output; and wherein calculating the static band index comprises calculating a mean of a second set of angles, each angle of the second set of angles being computed from a difference between a controller output corresponding to a flat process variable and its preceding controller output.

Description

technical field [0001] The present invention relates generally to performing diagnostics on conventional control valves in process plants, and in particular to determining valve hysteresis. Background technique [0002] Traditional control valves in process plants do not have smart positioners or other self-contained feedback mechanisms that allow users to discern how well such valves are performing. However, this also means that the cost of conventional control valves is low compared to smart valves. Therefore, it is expected that the use of conventional valves will account for the majority of valves used in process plants. [0003] It is known that one can use such as figure 1 A single-loop (automatic mode) control system as shown or as figure 2 The multi-loop (cascade mode) control system shown is used to control conventional control valves. Such as figure 1 As shown, the automatic mode control system 1000 generally includes only one control loop 1001 that controls ...

AI Technical Summary

Problems solved by technology

However, this leads to problems with the accuracy of the diagnostic results for cascade mode control systems because instead of receiving a constant setpoint variable, the secondary controller of the cascade mode control system receives a variable input OP1 from the primary controller 31

[0009] In particular, hysteresis is a problem where currently available valve diagnostic systems are not adequate for conventional control valves controlled in cascade mode control systems due to the variable input setpoint variables received by the secondary controller. Sure

With valve hysteresis, there is another problem: valve bounce, that is, when the controller output OP reaches a certain value, the valve finally responds, resulting in a process in the measurable process variable PV that far exceeds what would normally be expected based on the input OP value. Sudden jumpback of variable PV 302

When snapback occurs, the controller responds with a sharp drop in its output OP, causing a corresponding drop in PV

To avoid the PV dropping too much below the desired SV, the controller increases its OP again, but again the hysteresis of the valve causes the valve not to increase the flow rate accordingly until another back in the PV when the OP reaches the valve eventually to exceed the desired SV Jumping to respond to a certain value until valve bounce occurs

Images