3 replies to this topic

[Meysam_45000]

Dear Friends,

 

I want to know whether there is any Standard for the place of Level Control Connection on the bottom of distillation tower or not?

 

We have a distillation tower which for measuring its bottom level, we a level gauge and transmitter on the bottom of this distillation tower. As you can see from the attached picture, there are two 2” connections on the side of tower for this instrument; top tap and bottom tap.

 

One of these 2” connections is above the last downcomer of tower and a half cylinder baffle is around it. The other connection is in the bottom liquid. The reboiler is thermosyphon. Our transmitter and level gauge are installed on the same place. Our transmitter is displacer type.

 

It seems that since the 2” connection is in downcomer, and liquid accumulate in the downcomer, liquid spill into the level gauge from 2” connection with a speed higher than the liquid exit from the bottom tap of level gauge, and therefore we have a malfunction in reading level gauge and transmitter indication; sometimes the level gauge is full of liquid.

 

To sum up, we want to change the location of 2” connection and place it a bit upper, exactly under the downcomer of upper tray. I want to know that if this is right and if any standard is available?

 

 

Attached Files

•  Level Gauge.jpg   16.13KB   9 downloads

[aroon]

Meysam,
 
Increasing top level bridle (nozzle) height is not going to cause any harm for the level gauge. You can place it anywhere in column above the present connection (assuming the present connection is set by using suitable residence time for the calculations).
 
You can refer any of the chemical engineering Handbook where you will find the criteria for the residence time for all type of equipment. Here you have to refer column bottom. There must be multiple options for residence time like column bottom with pump or without pump, going to storage or other process units, etc. you have to select proper which fits your system.
 
You may face problem in covering level transmitter range due to limited probe, but you can manage it by using suitable piping arrangement from shifted top nozzle to existing LT. please see the attached extract of one of the book on level settings.
 
Hope this will be helpful for you.

Attached Files

•  Level Setting Guidance.jpg   33.73KB   7 downloads

[narendrasony]

Dear Meysam,

The LP (upper) tapping should be in the vapor space. Liquid should enter only from the HP tapping and not from the LP tapping.

 

If the LP tapping is really close to the downcomer area as shown in your sketch (you may  verify it again in the column GA drawings) and if there is a chance that liquid can enter the LP tapping during normal or upset conditions, tapping should definately be elevated.

 

Creating a new nozzle on the column  will call for ASME-VIII recertification ("R" stamp).

 

Regards

Narendra

[rabindra]

The Level transmitter operates in the following principle. It measures the pressure difference at two points and from the delta Pressure it calculates the height of the liquid between the two tapping (using density and acceleration due to gravity) as other inputs.

 

When you put the LP tapping inside the down comer area, the pressure at that point will be a little higher than the actual pressure on that tray because of the additional height of liquid in the down comer. Hence the transmitter may show wrong values for level.

 

Another problem with having a LP tapping above the bottom tray is that there is a chance of column bottom level reaching the reboiler outlet nozzle even when the level shows < 100%. This can have devastating consequences since reboiler outlet vapor may splash the liquid and damage the bottom trays. In view of the above, ideally, the column bottom LP tapping should be below the reboiler outlet nozzle.

 

In case your tapping is that of Level Gauge (and not LT), chances are that the LG will always show 100% full due to ingress of liquid from down comer.

 

Hope this helps.