Getting Loopy with Control Loops

This month's edition of Ask The Experts focuses in on manual control loops and offers guidance on which PID control mode should be used when, and what the range of tuning settings should be.

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0.5-2

Po

pH

100-500

5-25

1-10

Characterize

 

Note :
* typical range of setting
P:  control valve should have positioner
ER: external reset is recommended
Po or Ve: the digital algorithm used should be the positional or velocity type


SOME COMMENTS in addition to the figures given in the tabulation below:

  1. All controllers should have bumpless transfer. If the vendor’s system requires external reset to achieve bumpless transfer, or to prevent windup of the cascade master when cascade slave is not in “remote,” then ER is required, regardless of the application. Otherwise ER is required only on override control applications.
  2. The vendor’s system will determine whether a positional or velocity type algorithm is used. The type of application is immaterial, since in general they will provide equal performance.
  3. Tuning parameter ranges assume an ISA “ideal” form of PID algorithm is being used. If an interactive form is used, and the derivative mode is used, then the range of values for PB and for D should be doubled, and the range of values for I should be halved.

Harold Wade, Wade Associates, Inc. Hlwade@aol.com

 

CONTROLLED VARIABLES

PB

(%)

I  (minutes/repeat)

D

(Minutes)

OTHER

(See Note)

Level, Surge Drum

(HLA-LLA)-!0%

Not req’d

Not req’d

 

Level, Tight Cont

40% - 200%

1.5 – 15

Not req’d

P

Flow

100 – 250%

0.2 – 1.0

Never

 

Pressure, Gas

2% - 5%

May not be req’d

Not req’d

 

Pressure, Boilr Mstr

  or Dist Tower

10% - 50%

3.0 – 10.0

0.0 – 1.0

P (if not a cascade master)

Temperature (std.)

10 – 50%

5.0 – 20.0

0.0 – 2.5

P

Cascade Master

10 – 50%

5.0 – 20.0

0.0 – 2.5

ER

Cascade slave

50% - 200%

2.0 – 5.0

CW Cont on Exoth Reactor: 0.5 – 1.0

Otherwise: Not req

P

pH

100 – 1000%

3.0 – 20.0

0.5 – 2.0

P

(also linearize PV)




BE CAREFUL ABOUT general rules for specific PID numbers because there are different algorithms available. In general, use positioners on all the loops.  Positioners may be overkill on fast pressure loops.  Use External Reset on inner (slave) loops of cascades or if there are limiters or selectors on the other loops.  Velocity or positional algorithms will work; however there are often two distinct advantages to positional algorithms: fewer numerical problems and better anti-reset windup.  If you use a controller from any of the major DCS vendors, you will be fine.

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