Re: Pressure-Compensating TCs at Regulatory Level ?

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Posted by Doug B Robertson on June 17, 1999 at 17:08:41:

In Reply to: Re: Pressure-Compensating TCs at Regulatory Level ? posted by Don Snowden on June 10, 1999 at 00:12:13:

Guys,

Could this type of problem with a regulatory-level PCT be a result of dynamic mis-match between the pressure and the temperature signals? Maybe a small almount of lag on one of the inputs to the PCT calc would remove any unwanted dynamic behavior such as an inverse response.

I have never run into this problem with a PCT, but on an Alky application I had a similar problem with the iso to olefin ratio calc. This calc was the result of several flow and analyzer signals, and the analyzers had a significant lag. I was able to model the dynamic behavior of the analyzers and lag the flows to match that response. This resolved a vicious inverse response in the calculation and allowed me to control I:O acceptably well with a PID loop!

Regards,
Doug

PS - I would like to point out that the DIB PCT controller that Dan described was a project saving stroke of genius... and that it was completely Dan's idea. We all tried and failed to get the regulatory tuning to perform well on that tower and it was beating us. The pressure and temperature controllers would not stop interacting. The PCT controller stopped the interaction completely, although I remember Dan tuned the PCT compensation on-line to get the thing to work right. This was one of many experiences working with Dan that gave me an enduring respect for him as one of the best engineers I have ever worked with.

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: Guys,

: Could this type of problem with a regulatory-level PCT be a result of dynamic mis-match between the pressure and the temperature signals? Maybe a small almount of lag on one of the inputs to the PCT calc would remove any unwanted dynamic behavior such as an inverse response.

: I have never run into this problem with a PCT, but on an Alky application I had a similar problem with the iso to olefin ratio calc. This calc was the result of several flow and analyzer signals, and the analyzers had a significant lag. I was able to model the dynamic behavior of the analyzers and lag the flows to match that response. This resolved a vicious inverse response in the calculation and allowed me to control I:O acceptably well with a PID loop!

: Regards,
: Doug

: PS - I would like to point out that the DIB PCT controller that Dan described was a project saving stroke of genius... and that it was completely Dan's idea. We all tried and failed to get the regulatory tuning to perform well on that tower and it was beating us. The pressure and temperature controllers would not stop interacting. The PCT controller stopped the interaction completely, although I remember Dan tuned the PCT compensation on-line to get the thing to work right. This was one of many experiences working with Dan that gave me an enduring respect for him as one of the best engineers I have ever worked with.

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