5 replies to this topic

[jprocess]

Dear All,
How cascade master and cascade slave loops should be in terms of speed order?
And what are common application of cascade control?
Cheers.

[Padmakar Katre]

Dear Jprocess,

I am replying your second point and regarding your first point even I am curious to know.

In process control cascade control is used for temperature control, level control, pressure control, and quality control. These are the primary control loops, while the secondary loop typically performs flow control or pressure control.This is not a thumb rule but its my general observation in last 3 yrs (My no of yrs experience)
e.g.

1.LEVEL-Vessel(Tower/tank/drum) Level control (Master) and flow control (Slave) in draw or take-off line

2.TEMPERATURE-Tower tray temperature(Master) and steam flow to reboiler (Slave)

3.PRESSURE-Tower pressure control (Master) and either cooling water flow or reboiler steam flow

control(Slaves)

4. QUALITY-TDS control in steam drum (Master) and Blowdown flow control(Slave)

And

[jprocess]

Dear Padmakar,
Thanks a lot for your valuable comments.
I asked about the speed form our Instrument specialist and he told me that the cascade slave should be at least 3 times faster than cascade master.
And about this item:
"2.TEMPERATURE-Tower tray temperature(Master) and steam flow to reboiler (Slave)"
I found a sample in our project that cascade master is the lower of tower tray temperature and kettle reboiler vapor return temperature and the cascade slave is steam flow rate.What is your idea about this configuration?
And you said about the common application of cascade control.I am interested of knowing about the benefits of it.
Thanks in advance.
Cheers.

[suhas]

Cascade is used where you want to control some variable (CV) using some manipulated variable (MV), but the response of the CV with the MV is sluggish - which means there is a time lag. Normally, as you have the feed back control strategy the MV will move only when the CV changes. Now consider there is a inherent change in the MV (not due to the control action) which may be due to some other effect; then, if there is no cascade then in that case that changed MV will slowly change the CV and then only your PID controller will come in action and changes the MV again. But this will invole a time lag due to sluggish response. While if you have the cascade then as soon as MV changes due to its inherent change it be corrected. Before affecting the CV
Consider the level controller cascaded with FIC. Here, LIC will give the set point to the FIC and then FIC will go to Control valve. Here LIC is master while FIC is slave.

Suppose there is no change in the Level but due to something Flow gets changed then here FIC will automatically changes the Control valve before Level gets affected. If here you don’t have cascade then change in flow will lead to the change in level and then LIC will take action and change the Control valve position

I hope you understand.

[Padmakar Katre]

Dear,
Just can you check at your summation box of these 2 temperature signals.Is it giving mean signal of TIC as a Set point to the FIC of steam flow or is it have a selector(low/high) for these 2 TTs.
You will get an idea by going through a document which we prepare during the dtail engg phase called DCS Functional Specifications.You will get all the calculations for the loop.
This is a common practice in industries to have this sort of loops.I found TIC at the reboiler outlet vapor line going back to tower as your master as well the tower control tray tempearture TIC as master or the combinations of these both in this configurations sometimes mean of it is given as a Sp to slave or through a selector switch ( it depends if you are running the column with different compositions spec of feed).
Addition to Suhas's Comment,(The reason why we should have Cascade Control Configuration)
One of the disadvantages of using conventional feedback control for processes with large time lags or delays is that disturbances are not recognized until after the controlled variable deviates from its set point. In these processes, correction by feedback control is generally slow and results in long-term deviation from set point. One way to improve the dynamic response to load changes is by
using a secondary measurement point and a secondary controller; the secondary measurement point is located so that it recognizes the upset condition before the primary controlled variable is affected.
One such approach is called cascade control, which is routinely used in most modern computer control systems

[jprocess]

Dear Padmakar,
My case between "mean signal of TIC as a Set point to the FIC of steam flow or having a selector(low/high) for these 2 TTs." is the second one and the lower of the 2 values is considered.In fact the tower is a condensate stabiliser.
And could you please explain more about:
"I found TIC at the reboiler outlet vapor line going back to tower as your master as well the tower control tray tempearture TIC as master or the combinations of these both in this configurations sometimes mean of it is given as a Sp to slave or through a selector switch (it depends if you are running the column with different compositions spec of feed)"
You stated about 4 different configuration.I am interested of knowing about their application.
Thanks in advance.