why using this control scheme?? 1

[mahmmoud30]

hi all
tha attached pic is our stripper at Naphtha hydrotreating unit (NHT),after u see the pic plz answer me for?
1- why try no. 5th from top is selected as a temp. control while the common practice is the top tray temp will be controlled by the reflux flow.
2- why the TIC-1036 is cascaded to FIC1014(this product(LPG) we send it to another unit) and not with reflux flow
3- why the level of the reflux drum is cascaded to the reflux flow (LIc1008 with FIC1013)?
4- also the temp. control of the reboiler furnace (F-71102) has a selector TS1001 between 2 controls, the TIC1020 is the calssic but i want to know more about PDIC1001
thanks for being helpfull

 

[JLSeagull]

Using a slow control loop output as the setpoint for a faster loop improves control. Properly tuned, such cascade loops can limit reduce the process variable swings and improve stability. Flow loops are fast; level and temperature loops are slow.

I agree if you are suggesting that the level should set the product flow and temperature set the reflux. I don’t have time for a study and I have not played with distillation control for a while. Others can address why temperature toward the top of the tower was selected for the product flow instead of the reflux flow. Internal reflux control used to be popular using differential temperature such as a high tray and the overhead. I cannot comment on why tray 5 from the top was selected. The graphic is not clear regarding the PDIC measurement points. One is the reflux line but the graphic does not reflect the other pressure in a manner that is clear to me. Perhaps the furnace temperature and PD selection is an automatic override.

 

[mahmmoud30]

thanks JLSeagull for passing over and appreiate your response.
the PDIC measurement is a typical orifice plate that installed at the reboiler furnace outlet line but itsn't reading a flow, it reads PD across that line.

 

[jmw]

Just curious, is it the Graphic editor that is a demo or the control scheme displayed?
If this is a demo control scheme, it doesn't necessarily have to be fully accurate to be a demonstration.
If the scheme is an actual plant under control then it ought to be accurate and any anomalies fully understood.

JMW

http://www.ViscoAnalyser.com

 

[mahmmoud30]

Mr.Jmw
the scheme is an actual plant under control and operation for 3 years. this snapshot was taken during first configuration and if you like, I will upload the actual DCS snapshot which is the same and with actual operating values.
thanks for passing over.

 

[JLSeagull]

Does the graphic actually reflect the control configuration?

Assume that the control is as shown. If the level is too high then perhaps it just sends liquid back to the column depending upon the output action. The cooling could knock the vapor down so perhaps that could work. And if the temperature is too high does it increase or decrease the overhead product draw?

 

[mahmmoud30]

Does the graphic actually reflect the control configuration?
Yes the scheme is an actual plant under control and operation for 3 years. this snapshot was taken during first configuration and if you like, I will upload the actual DCS snapshot which is the same and with actual operating values.
we needs to answer your questions to well understand the system. thanks

 

[JLSeagull]

The graphic display is a completely separate issue from the control configuration. One must check the output of the controllers to see what valve is operated. This could be done in manual and just watching the valve instead of checking the configuration. This should confirm whether temperature or level controller operates the product draw but you would want to see the actual valve move not the output on the screen. The valve action, direct or reverse etc. can be seen on a detail display.

 

[mahmmoud30]

as I promised, the attached pic is a snapshot from the real DCS on july 16th 2009 for the mentioned column.

 

[dcasto]

A unique feature is your fuel take off, a partial condensor. That may be why the designers selected the control loops. you could reverse FIC1014 and FIC1013 (the valves) and arrive at the same control.

PDIC1001 is watch differential pressure across the tower. If it goes to high, then the tower is flooding and you need to cut heat input or reflux, which will cut heat input.

 

[mahmmoud30]

thank you dcasto
about using DP to detect flooding plz see the attached file