hey guys,

we have two reactor almost identical. in each reactor we have a dp level transmitter (HP direct mount and LP Capillary type) in 3rd floor and in 12th floor. the transmitters are measuring almost identical tanks around 1 meter height. having water as process fluid inside and being purged by nitrogen. the problem is the transmitters at the 12th floor is reading differently during night and day about 10-15% difference. while the transmitters in the 3rd floor are reading good. we tried insulating the tank and the capillary but still have different reading during night and day. anybody have any suggestions?

 

I assume, because it wasn't stated, that the level is supposedly constant and the 10-15% change is a measurement artifact, and that the level change is not due to someone on the night shift taking some process water to make a pot of tea.

It is normally recommended to use dual remote capillary seals in the hope that the temperature change will equally affect both capillaries and the DP action of subtracting low side from high side will cancel out the temperature effect on the transmitter's pressure/level output (that doesn't always happen as anyone who's dealt with remote capillary seals will confirm).

But the temperature change to a single remote capillary seal has nothing to supposedly cancel its effect on the other DP port so the temperature effect is obvious, as you've described.

Nice try, insulating the tank and capillary but the problem is, insulating the capillary will only slow heat transfer, it won't prevent heat transfer. Eventually the capillary will heat up or cool down and the temperature changes will appear as pressure/level changes.

I've never heat traced a remote seal capillary, but if you did and it held a constant temperature, then you could apply an offset at the level readout for whatever offset is created by a fixed temperature effect from the heat trace.

Or, you might try to determine the source of the convection or radiant heat on the 14th floor that isn't on the 3rd floor (because the 3rd floor does not exhibit the level change) and alter the heat source affecting the capillary somehow, you might stabilize the level reading like it is on the 3rd floor.

Or you could put up a video recorder and see who's making tea.