Last updated 26 September 2017

Control Loop Case History 17 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1. When your plant is running in a steady-state condition, are over 90% of the loops in automatic?

  •  a) Yes  b) No

 

2. During plant start up or shutdown, do the Operators put quite a few or even many loops into manual?

  •  a) Yes  b) No

 

3. How is tuning done in your plant?

  •  a) Self tuning controllers.
     b) Tuning package.
     c) "Scientific" or semi-scientific  methods (e.g. Ziegler - Nichols).
     d) Trial and error.

 

4. If an Operator complains that a loop is not controlled properly, do your instrument technicians or engineers

  •  a) Adjust the tuning parameters to  try and improve matters?
     b) Carry out a full loop analysis involving a series of open and closed loop tests to  firstly determine why the loop is not  behaving correctly?

 

5. Are there many filters on the process variable signals on the loops in your plant?

  •  a) Yes  b) No

 

6. The relationship between the Process Operators and instrument and control (I&C) department is

  • a) Good. The Operators trust the  I&C people.
    b) Bad. There is mistrust, and lack of co-operation.

 

7. The operation of a modern digital controller is complex, and most have numerous selectable options and features.  Do your I&C people fully understand the PID functioning of the controller, including the algorithm, how it will effect tuning, and whether each of the P and the D actions operate on error or process variable? 

  •  a) They understand all these things.
     b) They don't understand the terms  used in the question.

 

8. What equipment do your I&C department have to undertake loop analysis?

  •  a) Some form of high speed, high  resolution recorder.
     b) Nothing special. They use the  tuning screen of their SCADA  system.

 

9. Is loop optimisation regarded as a discipline in your plant that must be regularly carried out?

  •  a) Yes  b) No

 

10.  If you are using digital controllers and smart transmitters, is the scan rate of the controller set to be at least 2 to 5 times slower than the transmitter's scan rate?

  •  a) Yes  b) No

 

11.  If your control loops use a digital controller with a scan rate of 1 second or longer, are anti-aliasing filters set on your transmitters, particularly on very noisy measurements? (Note this does not refer to process variable filters set in the controller).

  •  a) Yes  b) No

 

12.  Have your Operators received some elementary training on the practical operation of PID control?

  •  a) Yes  b) No

 

13.  Have your I&C personnel performing loop optimisation received detailed training on the operation of the processes in your plant?

  •  a) Yes b) No

 

14.  Do the process people and the I&C people work as a team on optimising the loops?

  •  a) Yes  b) No

 

15.  Are measuring instruments and transmitters calibrated and checked on a regular basis?

  •  a) Yes  b) No

 

16.  Are valves "stroked", and associated equipment, including pressure regulators, I/P converters, and positioners, checked on a regular basis?

  •  a) Yes  b) No

 

SCORING THE QUIZ

 

Score one point for each of the following answers:

 

1. b

2. a

3. d

4. a

5. a

6. b

7. b

8. b

9. b

10.  b

11.  b

12.  b

13.  b

14.  b

15.  b

16.  b

 

HOW YOUR CONTROLS RATE

 

If you have scored between 10 and 16:

 

You have serious problems in the feedback control loops in your plant, of which you may not even be aware. If this is the case, ask yourself why all that money was originally spent on automating the plant, if it is not working properly.  Also enumerate the obtainable benefits if control was improved. These would include increased productivity, better quality, and decreased running costs. Would it not be worthwhile considering investing some time and money to improve the control?  It will require commitment on the part of senior and middle management. You will also have to invest in some equipment and training.

 

If you have scored between 4 and 9:

 

Your controls are probably running better than most plants.  However the score does show that improvements need to be made.

 

If you have scored between 0 and 3:

 

Your plant is definitely committed to optimisation, and your controls will be running really well. Your staff will be competent and well trained.

 

 

Index to articles

 

HOW WELL ARE THE FEEDBACK CONTROL LOOPS IN YOUR PLANT WORKING?

 

Our experience shows that over 65% of feedback control loops in automatic are operating ineffectively, and less than 15% have anything close to optimum tuning parameters. It costs approximately R50,000 to automate a control loop. In spite of this, the poor state of the controls in a plant are generally accepted as normal. In fact very few people are even aware that the loops are not working properly.

 

To help you to judge approximately how well the loops in your plant are operating, we have devised the following quiz. This is based on experience taken from several hundreds of plants over a wide range of industries in various countries. Take a few minutes to study the questions. If necessary research them, and answer them as honestly as possible.