How To Measure And Combat Valve Stiction On Line

How To Measure And Combat Valve Stiction On Line

By: John Gerry of ExperTune Inc.

and

Michel Ruel of TOP Control

Presented at ISA2001, Houston, Texas on September 11, 2001 at 10:15 AM.

Abstract

Valve stiction in a control loop always causes problems. In a control loop, stiction causes the loop to cycle. Any amount of stiction will degrade the performance of the control loop. Identifying stiction as a problem is the first step to combating it. Once identified there are several methods for improving loop performance. This paper discusses a method for measuring stiction while the loop is operating and then methods to combat this.

Introduction

Stiction is a combination of STIck and friCTION. Combining these two words gives stiction. Stiction is represented as the force necessary to start a body in motion. In a control loop the stiction is in the valve stem. A change in the amount of force applied to the stem is required for it to move.

Figure 1 shows the time response flow loop under closed loop control when the loop contains stiction. Notice the cycle. Integral action in the positioner or controller will cause the controller output to increase or decrease until the valve overcomes the stiction band. Once overcome, the valve moves suddenly causing the process to react too far. The controller output again increases or decreases until the valve overcomes the stiction band, thus repeating over and over causing the stiction cycle.

By measuring the stiction of a control loop online it is possible to pinpoint and identify if stiction is degrading loop control. Once identified, it is best to repair the equipment. However, repairs often time cannot be performed until a shut-down or other future date. Techniques for combating stiction are discussed in the second part of this paper.

Figure 1: Closed loop cycle caused by stiction.

On-line vs. Offline

On-line measurements of stiction are much more valuable than off-line measurements of stiction since when the valve is in service the line pressure and material in the line effect the stiction and mechanics of the valve. These are not available off-line.

In a bench test, there is no lubrication of the valve or vibration present. There is also no force required by the valve to push against that of the process pressures.

Steps to measure stiction on-line

to measure stiction-line, requires some verify small controller output changers with the controller in manual. Here are the steps:

* Put the controller in manual with the output near the normal operating range.

* Start recording data using a strip chart recorder or computer system

* Change the controller output by 5 to 10% to overcome the hysteresis on the loop. If the process variable does not move from this change, repeat it until the process variable moves.

* Wait for the process variable to settle

* Make a small change in the controller output - about .2% in the same direction as the last step. Wait for the same amount of time as the previous step to see if the process variable moves

* Repeat the last step until the process variable moves

The stiction in the loop is the total amount of controller output change required to make the process variable move.

Figure 2 shows this series of tests performed on a flow loop. There are 2 vertical lines on the plot. These vertical lines mark the "stiction band" or the difference in controller output indicating the amount of stiction present in the valve. Figure 1 shows the result of this stiction in a closed loop system.

Figure 2: Data collected for stiction check.

Combating Stiction

The best solution for increasing performance in a control loop containing stiction is to repair the valve or positioner to eliminate the stiction. In many cases, however, this is not possible because of the economics of keeping production running. In these cases, methods for combating the stiction to reduce the effects are beneficial. The negative effects of stiction cannot be totally eliminated without repairing the valve. However, there are techniques for reducing the effects of stiction on control loop performance.

Conditions when the valve cannot be repaired

Sometimes the valve cannot be feasibly repaired. This condition may occur for these reasons:

1-economically not feasible to stop production,

2-valve/actuator type is the problem and it is necessary to use this type of valve/actuator for fail safe considerations

3-replacing the valve actuator could be too expensive

4-the process imposes this type of valve where a lot of friction is present, ex Gate valve

Techniques for combating stiction on-line

These tuning techniques can be used to keep the plant running:

1-Tune the positioner using a large proportionnal gain, and no Integral action. If Derivative action is available, use some to make the valve continuously move. With integral action in the positioner, the positioner may wind up, causing the valve to seemingly have a mind of its own. After some period of time, the stem will jump, after the positioner has wound up enough. By removing integral action from the positioner, this windup problem is eliminated.

2-If a smart positioner is used, adjust the parameters. Some positioners do not use PID but special algorithms to send a burst of pressure each time a new position is requested. The positionner action is to stop the valve at the requested position.

3-Use a PID controller (for the control loop) where the Integral action has a variable strength. If absolute error is smaller than some value then take out the integral action otherwise use it: For error<x Ki=o (or Ti=infinite); if not Ki = normal value. Using this method, when the valve is within the stiction band, the integral action is missing from the controller, the controller output will not integrate, having the end effect of removing the stiction cycle from the loop.

4-Use a PID with gap; if the absolute error is smaller than x, the controller output is frozen; if not, the amount of error from the gap is used as the controller input.