Column high level protection .. above reboiler return worthwhiles

[jamesbanda]

Dear sirs,

We have regonised that we should consider high level switch protection on the base of one column to protect the trays via a level switch. However, the only pratical nozzle location (glass column)is above the vapour feed. It is below the tray support..

but i am questioning if this would offer any protection? some of our snr enginers believe it would.. but iam less convinced.. if it is above the vapour feed surely we would have vapour bubbles(like a coke bottle) and most likely surge the trays..? or does the liquid need to cover the trays to risk damange.

 

[epoisses]

It could be a lack of knowledge from my side, but I'm not sure in what sense a tower bottoms level higher than the lowest tray would lead to tray damage. What EXACTLY should be prevented? If you can answer that question maybe the response would give you a better indication of where and how to install a level switch.

 

[pennpiper]

Jamesbanda,
You did not give us much information to work with, however I think that something is possible.
You need to find a nozzle that is lower on the vessel and in the liquid zone. This might be a TW (thermowell) nozzle or a steam-out connection, etc. I will assume you have a TW nozzle low in the liquid.
Now make a flanged TEE to install on this nozzle at the next shut-down. The thermowell will be reinstalled in the straight run of the TEE. The branch, installed in the horizontal will have a block valve.
Now on the upper connection you noted in your question you also add a block valve.
You now have a valved connection below and a valved connection above your target high level switch. From these two connections construct a vertical pipe stilling well. At the proper point on the vertical stilling well pipe make the connection(s) for the high level swich.
By creating this stilling well you reduce the influence of of the reboiler return flow. You will also tend to reduce the effect of surface foam.

 

[jamesbanda]

Thankyou penn piper,

I think you are basically suggesting a stand pipe. I know this would adeuqatly work - normally. However in our application the fluid is subject to polymerisation in dead legs so i really need to use a switch (if pratical) and would ideally like to use the nozzle above the reboiler return but below the base packing supports..but i'm not sure if this gives me protection

so do you think the switch located above the reboiler return is not ok? or it should work..

 

[pennpiper]

jamesbanda,
If I am reading your objective corrently you want to end up with a high liquid level alarm and/or shut-down switch.
You apear to have a vessel with a liquid level in the surge (lower) section. You also have a reboiler working on the bottom of this vessel. The reboiler return in a situatuion like this is normally a vapor.
You are suggesting that you would use a nozzle located above the reboiler return for your level switch.
Based on my experience you should never allow the liquid level to rise above the reboiler return nozzle.
If I am right in my understanding of your situation then you could be asking for trouble.

 

[jamesbanda]

Yes, sorry for the confusion

We want a high liquid level switch to protect the column base from overfill into the packing.

I want to understand if overfill into the packing/trays is the main consern or is the consern filling above the reboiler return nozzle.... and once liquid is above the reboiler return it is certain to cuase damage..before it gets to the trays / packing...

 

[djack77494]

jamesbanda,
I think you need to worry about both. I'm assuming you have a thermosiphon reboiler. If so, the return line, carrying a 2 phase fluid, MUST be above the liquid level. Also, again with some inherent assumptions, your packing must be above the liquid level. Besides performance issues, the packing supports are unlikely to be designed for the much greater momentum that upflowing liquid would exert.
Doug

 

[jamesbanda]

So,

we have a thermosiphon reboiler. (sorry for not mentioning above).

If the reboiler return is not above liquid level is it really that big a deal because it is mostly 80% liquid anyhow (i know the vapour rates are higher)...

do you have any references (articles) etc that would confirm this requirement

 

[djack77494]

I have never seen or heard of a thermosiphon reboiler return not entering a vapor space region. I do think it is unadvisable (or worse) to do so. You are depending on differences in fluid density to move your fluids through the reboiler, overcoming all equipment and piping hydraulic resistance in the process. That fact alone would cause me to exercise extreme caution in deviating from known practice. Despite the above, I do not see anything that would definitely make me think that a submerged return could not work. I hope someone else will chime in with an opinion on this.
Doug

 

[sshep]

I think a switch above the reboiler return is going to be fine. It is certainly better than nothing. I have witnessed covered reboiler returns at the column. The effect can be a violent bumping action- not something you would want to continue for any longer than neccessary. If the primary level indiaction fails, the heat input and levels can continue to increase with no direct indication inside that something is dangerously wrong.

Some sort of independent level shutdown (trip the heat) is advisable for safety reasons. The recent fatalities at the Texas BP plant show how dangerous it can be to continue to fill a column while applying heat. If they had the shutdown you suggest it could have saved many lives.

best wishes,
sshep

 

[jamesbanda]

thanks sshep,

i am not proposing running above the reboiler return just suggesting i should install a switch to reduce the risk of tray damange from overfill. My assumption is the risk of tray damage from overfill above the reboiler return is low provided a level switch catches the level rise before the lowest tray sees liquid.

 

[sshep]

I knew your intent is simply as protection, I was only pointing out that the consequences can be more severe than mere tray damage and you are wise to consider high level.

I am curious as to what you envision your switch will be connected to- i.e. alarm only, shutdown of heat input, general column shutdown (close feed, reflux, heat, etc). If your switch will close valves, will it be by latching solenoid (field reset) or simply by "force manual close" logic in DCS/PLC. Again, just getting some data on alarm and shutdown philosophy- since this is a retrofit and you don't have nozzles for redundant high level switches, you will obviously have to consider the relatively high probability and consequence of spurious trips.

best wishes my friend,
sshep

 

[hazel]

dear jamesbanda
you have to identify your level transmitter type first.
If the level transmitter type is float, it is easily to install the magnetic level switch on the chamber.You can consult with instrument maintenace people.

I don't think the tray will seriously damaged by the reboil return line comapred with feed line. If there is a nozzle above the return nozzle, I suggest you can use this nozzle to install the switch to protect trays from feed surge.

 

[jamesbanda]

Shepp,

to answer your question we will configure this as PSD. DCS Safety based trip. I understand the BP texas incident. and the pressure configuration. We have independed SIS(Hardwired), and PSD - SIL1 protection for pressure that event. If we hit the high level i will trip feeds/ reboil and reboil heat