Level transmitter range 1

[Jack Nicholson]

Hi everyone! I have a question.
In our basic design (phase 1) for our kettle type heat exchanger, level transmitter legs (N4, N5) were designed on the side of exchanger (right below and above tube sheets) as illustrated on picture. But in the final design these legs were transferred to the top and bottom of exchanger. So here is the question. Should we consider our lever transmitter range based on phase 1 or phase 2? Our instrument engineer insist on considering L trans. range based on phase 1, because the operator could raise the level unintentionally way above HLL (LT send signal to LV).
Thanks in advance ...

 

[pierreick]

hi ,
Some people are using radiation based technology , check for it @ Vega

https://www.vega.com/en-us/industries/chemical/reboiler

note : A weir is missing in your sketch?
Good luck
Pierre

 

[LittleInch]

What are you using for the level transmitter?

A float type or radar?

Why not use a pressure based one on N5 only?

But if the entry nozzle is above NLL then you're going to get some amount of splashing and no fixed level.

The smaller the range the better the accuracy of the level will be so phase 1 range would be better in that respect and once you go above the top of N4 you have no idea what the level is so where is the HH level trip?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[danw2]

I assume the Nx locations are the locations of remote seal diaphragms for differential pressure hydrostatic head measurement.

When the control action, based on triggering an alarm or whatever at HLL, fails then someone can see how much higher than HLL the level is with the phase 2 N5/N4 diaphragm location.

If phase 2 N3/N2 scaling does not match N5/N4 scaling, then an operator might assume the volume above N2 is useable volume, for instance if scaling for N3/N2 is in meter/mm and scaling for N5/N4 is in percent.

Is there high limit point level sensor at the HLL elevation to back up the continuous measurement?

Remote seals on pressure transmitters make the measurement a pretty good (unintended) thermometer reflecting temperature changes. Has that been considered?

 

[TiCl4]

"Our instrument engineer insist on considering L trans. range based on phase 1, because the operator could raise the level unintentionally way above HLL (LT send signal to LV)."

Clarification needed: Is he referencing the instrument range or the span? What type of instrument are you installing? I suspect he is talking about the span, or the 0-100% setting, not the range, which is a fixed value that is determined by the instrument type and model. My next statement is assuming he is wanting to set the instrument span to be 0-100% with the original N4 position as the 100% mark.

If so, I believe that the statement quoted above is the absolutely wrong way to look at setting an instrument's span. Changing the 100% mark from the phase 1 N4 position to the phase 2 N4 position should not alter what kind of setpoint an operator can enter - you should already have in place programming that disallows setpoint entry at or above the HLL.

I had the same issue with an instrument engineer before with storage tanks - he was setting the span to read 100% when our storage tanks were only 90% full. His view was that the tanks should not be filled more than 90% full, so why have the instrument read above that? Of course, this loses visibility on what is happening inside the vessel should the limit be exceeded. If a startup, shutdown, or emergency event took the level above 90% of tank volume (100% on span), then operators had NO visibility as to what was occurring in the vessel or if their mitigation measures were having any effect. If the instrument has the range, my view is USE IT!

One supporting point to my view is the BP Texas refinery explosion (

https://www.youtube.com/watch?v=goSEyGNfiPM&ab_channel=USCSB).

The raffinate tower had a level gauge that only read 0-9', and operators routinely filled the tower above that level, losing visibility on what the actual level was. If you intentionally limit the span of an instrument to only read part of the vessel level when it has the capability of reading the entire vessel height, you are gimping yourself and your operations in much the same way the BP operators had an insufficient level device.

 

[Jack Nicholson]

Dear TICl4, thanks for your reply.

quoted:'' you should already have in place programming that disallows setpoint entry at or above the HLL.".
As a matter of fact, No!, we don't have any logic that restrict setpoint entering above HLL!, and I don't think there would not any around the world. The operator can enter any setpoint (for instance operator can enter level set between 0 - 100%, although the HLL is 80%).
Thanks for sharing of that video, that's absolutely valuable.

 

[georgeverghese]

Would agree with your instr. engineer: smaller range implied by phase 1 gives better sensitivity. The additional static head error imposed by the gas, with the nozzles in phase 2, is negligible.

 

[Jack Nicholson]

You agree with instr. Engineer?? Because of sensitivity?
We don't have sensitivity problem. Our first issue is that we can't observe the exchanger level beyond N4 in phase 1.

 

[georgeverghese]

How can the level go higher than N4, when HLL should have tripped liq C3 feed before N4 level to the kettle? Some thermal expansion of the liquid is possible, but this increase in level beyond N4 is possibly small.
If HLL is only alarm without trip, then there may be some problems with the operation of the LIC at this kettle - slow response at this LIC to sudden loss in tubeside feed?
If you really must know level beyond HLL, then install a second LT parallel with existing controller LT. This 2nd LT may be calibrated to see the entire level range. Or better, see if the existing LT has dual range capability : control range signal to LIC and wider range signal to new DCS LI only.