I need your help_level transmitter_DP type

[eng1carlos]

Hi,

Gentlemen, I need your help in the following issue:

we have butanizer(C4) vessel, and we have displacer transmitter to measure the level across the vessel, the transmitter was reading fine, then suddenly plugged, we drop it and cleaned the chamber and calibrated the Trx then return it back, then it worked for 2h then start fluctuating again

after that we changed the transmitter type with DP capillary type
then it worked smooth for 3 days the start fluctuating.(in the linke below you can see the process trend).

now I am looking for a proper solution for these problem.

I advance appreciated your support.
with best regards,

 

[KiwiMace]

Are these nozzles on a standpipe? You mentioned a displacer which leads me to believe they might be, and I have commented accordingly.

If this was getting less responsive, I might point at measurement issues, but abruptly more responsive/unstable I would tend to think a process issue.

Is anything else connected to the standpipe/tapping that might cause level instability? An analyser tapping, solenoid operated something. I assume you have looked into any changes or maintenance that might have happened at the time - opened IV's up to standpipe... that sort of thing.

Can you blow down the standpipe? I don't know this process so I don't know the characteristics of the fluid.

 

[danw2]

The period seems to be 18 minutes or little less at ~10 cycles over 3 hours. The cycling hovers around the nominal level value, increasing in amplitude, like the loop gets mistuned with too much gain and overshoots.

I've looked through my list of pressure xmtr failure modes and none covers this.

When a diaphragm gets crudded over, sensitivity decreases, the opposite of what you're seeing.

The only measurement thing I can come up with is temperature induced cycling. I have seen cyclic temperature cycles with remote seal gauge pressure transmitters used for liquid level. Mounted outdoors, alternating sun/cloud or day/night exposure would heat or cool the capillary and be reflected as a pressure change.

Could one of the DP capillaries run over a heat source that turns on for 9 minutes, then turns off for 9 minutes, heating the capillary and producing an increase in pressure in the capillary and then a subsequent decrease in pressure when the heat source turns off?

Have you checked the controller settings - is there a 2nd set of PID tuning constants that somehow got switched in?

 

[eng1carlos]

Thanks for everyone

some comments to your replies;

KiwiMace: yes it's external transmitter (smillar to the picture in the linke below).
- great point, you are right it might be a process issue, I'll check from the process side if there is some thing changed in the material properties.
- only the trx connected to the tapping.
- the maintenance, they did several overhauling for the Trx, but they do the same periodically for all transmitters.

danw2: good point, in capillary type the pressure change with ampiant temperature, I agree with 100%. but for displacer type it will not effect, So i would know, why the displace suddenly fluctuate, while it was working fine since long back?, and why the DP type worked smooth for days the start fluctuating?
- regarding the controller setting, no body touch it, and I think if there is some thing changed in PID that will lead to continuous fluctuating.




Thanks again,

 

[ietech12]

How long as it been since the vessel was been cleaned. If both devices share the similar issue I can assume that it may be as simple as plugged taps. If the lower nozzle is located directly under the vessel or close the bottom of the vessel, you may have pookie (What the techs refer to as residuals)layered on the bottom. Operations or maintenance probably blew the taps with steam of N2 to clear the nozzles and temporarly made a clean path thru the pookie for a short time until the pookie slowly fills back in. Hope this helps.

Regards,
Aldo