Upside Down Bubbler Design

[Bambie]

In my plant there is an ancient upside-down bubbler system that provides level indication for a large tank (bubbler schematic attached).

A constant flow regulator pushes helium 25’ down to a reservoir/valve (drawing attached), which then bubbles it through 45’ of 1”nps drain pipe located in the tank bottom. Another regulator and relief valve maintains a constant helium cover gas pressure and the difference in pressures provides level indication.

My understanding is that when tank level rises, it also rises in the reservoir which prevents helium from bubbling out past the ‘V’ shaped weir. This flow reduction causes the supply regulator to increase pressure in order to maintain flow.
When tank level drops, it also drops in the reservoir which releases more helium dropping regulated pressure to maintain flow.

This system always starts out providing accurate level indication after draining the tank, however, as time passes it gradually begins to underestimate tank level.
Has anyone has seen this type of weir design in a gas/liquid reservoir and what could cause the characteristic drift.

 

[StoneCold]

My initial thought when I saw the Reservoir/weir item was that it is really just a sight glass to see that the helium bubbler is pushing enough gas down the tube to keep it clear so you get an accurate reading.
I think you need to slightly increase the gas flow on this leg and let it run for a while and see if you get drift. If you have other bubbler designs on other tanks that work without drift, why don't you just change this one to be like the other ones and forget it?

Regards
StoneCold

 

[Bambie]

StoneCold,

Access is problematic and by compensating for the predictable drift we have been able to get reasonable agreement between the three level indications for this process (and hope to continue doing just that).