A customer has a cascade set of PID function blocks written in Honeywell's Experion PKS system.

I have suggested changing the scan rate of the outer controller to 5 or 10 seconds (it's a temperature control) for better operation. Looking at the details offered by Honeywell, we can only raise the maximum scan rate to 2 seconds. Does anyone know how to increase this to 5 or 10 sec?

Honeywell has been no help (Non-English speaking country and not one of the languages I speak).

 

I'm going to stick my neck out here and say "why would you want to do this ?".

Too Slow a scan rate can be a problem, too fast a scan rate would normally only be an issue if you have not tuned correctly to account for deadtime in the process.

While temperature control loops are usually frustratingly slow to respond, you should also consider looking for process issues in you plant like a badly tuned slave loop, a sticking valve, a poor temperature transmitter installation etc.

But the more significant problem with a 10 second scan time on your loop would more likely be the HMI interaction. When you slow down the execution of a block, you typically also slow down the processing rate of everything including HMI processing. A 10 second scan time would mean the block takes anywhere up to 10 seconds to respond to an operator command or display changes on the HMI.

Rob
www[.]lymac.co.nz

 

Certainly the only thing you loop need is tune. temperature loops have low PV changes, so just adjust your loop parameters there are lots of variable in loop configuration to handle out that, GOD.

 

Regardless the reason, changing the controller scan rate is a very big issue. You need to shutdown the controller and reload all points with new controller scan rate.

I think what you need is reconfiguring the controller. If you are using PID controller, try to reduce the gain (P), reduce the integration value (I) and if required you can add the derivative value (D). It will make your controller not reactive and provide slower response to the temperature change.