Response time calc for using longer impulse tube

[njp8332]

Hello all,

apologies for my wordings as English is not my first language.

we are installing level transmitter, gauge and DP Transmitter combo assembly in distillation column with 2" nozzle. we came to know that for some reason, bottom nozzle doesn't have any liquid, actual collection tray has been rearranged lately and it is in opposite direction.

we devised alternative to block off isolation valve for bottom nozzle (use it only as support) and get another 1/2" SS impulse tube from location where liquid is collected in tray. I understand that this might add some measurement delay due to length and diameter of impulse tube. Level measurement may not be very responsive to actual level changes.

how can I quantify this measurement delay against unit step change in actual level ? which theory or formula i need to consider ?

Thanks,

 

[georgeverghese]

Only on startup will bottom impulse line show some time delay. Once this line is flooded / primed, there should be no delay. I have seen LT - dp cell with long impulse lines and these seem to work without any problems.
But you do need a reasonable residence time in the collection sump on that side draw tray. Also, if the liquid is close to boiling point / bubble point temperature, or if column pressure is subject to some fluctuations, then liquid in the bottom impulse line can boil, resulting in erroneous readings. In such cases, it is better to installed diaphragm sealed impulse lines for both vapor and liquid side of dp cell.