Control scheme of reboiler & outlet of distillation column

[koba78]

Dear Eng-tips member :

A distillation column outlet with PT and FT connected through interlock
,PIC, FIC to control the FV (hot water supply to the vertical re boiler).

The question in the following is hope that you can help:

1. As the flow signal low low, and pressure signal high high, it will initiate the
interlock to close FV (hot water supply to the re boiler.)

But, Is the Flow soft link signal is first into the interlock, then FY better
than first into the FY then interlock?

2.It make sense that pressure high high at topping outlet, the column should stop
heating the process fluid at bottom of the column, so FV should be closed.
I wonder that this FIC, PIC control is inverse control(the signal increase the valve close)

regards
thank you very much.

 

[sshep]

Dear Koba,

I see that your english is struggling, so your questions were not exactly clear. I would like to say that what is drawn does not seem very reliable for modern control.

If the system is adequately protected by a relief valve to uncontrolled hot water flow, then pretty much any system will provide "feel good" redundancy because it is not the final protection. Usually these type systems (for safety) either trip a solinoid valve on the instrument air to the hot water control valve causing it to close, or "force manual close" the control valve directly. Working through a cascade control system is not reliable.

For a real safety instrumented system, you do not even want to depend on closing the control valve. The reason is that a failure of the valve itself can be the cause of the need to trip. Instead a dedicated (high integrity, self testing) trip valve which is completely independent of the control valve is used.

best wishes,
Sean Shepherd

 

[koba78]

Dear

I am sorry not to put my question clear, and also post the drawing
again in the attached file. The FY-A is in cascade with FY-B, amd FY-B takes the signals for the hot water supply header temperature, and hot oil supply from FT-B to caculate the hot water duty in the reboiler vs the duty required by reboiler. The querry in my mind is why the high range/ low range control to be set up ? As I know that, there is two train in this plant, in oder to preven over reboiling the column and over pressure releif from the psv, there will be a large change in the reboiler duty requrement so it is necessary that the control action to be quick to react to a step change. So is it the reason why we put the high range/low range control on the hot water supply feed.

Thank you very much.