Michael Brown Control Engineering CC 

Practical Process Control Training & Loop Optimisation

 

CASE HISTORY 193

 

THE BIG DIPPER

 

This article’s example is of an important  flow control of Flare Gas in a petro-chemical refinery.  The control was suffering from really serious problems, often cycling severely, and at other times the PV (flow process variable) would suddenly move quite a lot away from the SP (setpoint).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1

 

 

Figure 1 is the “Closed Loop Test As Found”.  This is a test where SP step changes are made with the controller in automatic with the same tuning parameters that were in the controller when we started the tests.
This is a very interesting test.  It showed that when the SP was stepped upwards in the valve opening direction, the tuning was in fact quite good with pretty fast response which is typical of a normal fast control of a flow process.  However once at SP the flow would sometimes suddenly drop violently by sometimes over 20% and then after about 5 seconds would almost instantly recover.  On examining the PD (controller output) trace, it can be seen that controller did not react very quickly to these swings.  This would immediately lead one to suspect that there was a huge problem somewhere either in the positioner or the valve.
Another observation on the first part of the test where the SP changes were increasing is that the valve could be two or three times oversized.  (This is based on the ratio of the magnitudes of the step in PV to that of the PD.   This ratio is referred to as PG (process gain).  On a self-regulating class of process like flow one can judge a valve is properly sized if the PG is unity.  If it is greater than unity then the valve may be oversized.  If it is less than unity it indicates that the transmitter range is too wide.  Explanations on the “science” behind this are given in my Loop Signature series.
Now we come to the second part of the test where the SP steps are in the opposite direction reducing the flow, and the valve is moving in the closing direction.  The behaviour of the process is now very different, with the flow not reacting in the same way as before. Instead the process almost appears to be cycling badly with the PV frequently performing the same type of violent swings  that were observed on the first part of the test.
The behaviour of the PD was also different.  It appears to be ramping down fairly slowly but with a fairly small, almost “saw-tooth” cycle superimposed on it.  It no longer responded quickly to the SP steps.  This could mean that the valve was very sticky when closing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2

 


Figure 2 is the recording made when we performed the Open Loop test.  The controller was placed in manual and the PD was stepped.  Again it was really fascinating to see what the valve did.
The following could be observed On the steps of PD moving upwards  ;
1. The PV responded quickly to the steps but with an absolutely huge spiked overshoot of almost 40%.
2. The PG on each of the steps was about 2 which indicate the valve is about two times oversized (which is in fact not terribly serious).
3. The installed valve characteristics were linear which is good.  (The PG was the same for each of the steps).  If it had varied, it would indicate non-linear installed valve characteristics, which can cause control difficulties, as a loop is tuned for only one value of PG).
The steps of PD moving downwards showed a completely different story:
1. The valve stuck many times after the PD had been stepped.
2. It sometimes took several steps of the PD before the valve actually responded and did move the PV down.  This confirmed the previous observation that the valve was very sticky when closing.
3. The valve would frequently move in big spikes down and then back up.
4. On one occasion when the PD was stepped down, the valve, instead of closing, actually opened for a few seconds before starting to move in the closing direction again.
The final conclusion of course is that the open loop test confirmed the observations made after the closed loop test, and that there was an absolutely major problem occurring in the valve or positioner. This needed to be sorted out if good control was required.
I called this study ‘The Big Dipper” as it reminds one of a “ride” in fairgrounds where you suddenly and violently go up and down.
Once again we have an excellent example of how important it is to properly analyse a loop’s behaviour, before even thinking of tuning.