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.

 

[Bambie]

Sorry about the attachment - try this one.

 

[Bambie]

And here is the reservoir attachment.

 

[danw2]

Presumably you've got a DP transmitter, low side connected on the constant pressure cover gas, and high side connected to the constant flow pressure outlet.

In a right-side up bubbler, when people say their level indication gradually drifts downwards, it's usually condensate liquid collecting on the low side port of the DP transmitter creating a gradually increasing offset which is interpreted as a higher cover gas pressure resulting in a lower level indication. But the concept still applies.

Is the low side port collecting condensate liquid? Is it being drained (deliberately or coincidentally) when the tank is emptied, thereby restoring a 'true' level indication?

 

[Bambie]

danw2,

Your presumption about the DP is correct and we have never found condensate in he LP lines because the LP PRV is constant flow (not constant pressure as shown on the schematic) so the line is being continually purged with dry helium.

 

[3DDave]

Just to think it through -

As time goes by the actual depth of fluid is increasing even though there's a constant delta-p to maintain the same volume of helium flow?

Is there some other factor, such as changing fluid density that can happen?

I'd look at adding a precision flow meter on the constant volume side and a precision pressure gauge to both sides for a while to see exactly what is happening as well as a temperature gauge to the tank. If liquid density isn't changing then one or both of the regulators may be drifting.

 

[hacksaw]

How much drift are you seeing, the time scale involved, the sp. gravity of the liquid being measured and how are you maintaining a liquid full condition in the 1" hp pipe?.

 

[Bambie]

3DDave and hacksaw,

I have left many of the instrument details off the schematic like pressure and temperature gauges and two 'traditional' bubblers that provide comparative levels - I will add them after Thanksgiving.

The upside-down bubbler level begins to drift down after a few months (indicating a lower level than the two 'traditional' bubblers) and the error is incremental, reaching 18" after one year.

We have ensured that all instruments are calibrated and level adjusted for specific gravity changes with temperature (sg = 1.1 nominal).

The drain line has a 1% slope (1/8" per foot), so there is some low frequency pulsing as bubbles coalesce and surge up the drain, so maintaining a 'liquid full' condition would only be momentary at best.

 

[hacksaw]

Using a small bore drain line as a bubble path will surprize you even in clean fluids. Things can occur that would seem to defy the laws of physics, and degrade the measurement.

Been there done that...

 

[danw2]

1) Why do you call it a reservoir/valve? It appears to me to be sealed 'tin can' with an internal weir on one axial opening (vessel side) and a second offset axial opening (gas) . There are no moving parts in the reservoir, are there?

2) > specific gravity changes with temperature

What's the range of SG change? plus or minus on the nominal 1.1?

3) Is there an annual change-over when the tank is emptied and then refilled?
If so, are the bubbler pipes and reservoir drained and then re-filled at that time?

4) thought: The 45 ft of bubble path pipe is dead-headed at the reservoir. If the vessel contents SG changes, the piping and reservoir's contents change only by 'new' going into equilibrium with 'old' over time. That could mean some 'old' contents remain in the bubble pipe.

Is the effect you are seeing due to a relatively lower SG liquid in the 45 ft bubble pipe (a lower high side DP pressure creates a lower level reading) than the liquid SG in the vessel?

Could the vessel's SG vary 15% ? (an 18" error in 10 feet/ 120 inches)

 

[3DDave]

Still need a reliable flow measurement.

Is the liquid a pure liquid or a mixture? It seems to me that some components of a mixture could be bubbled out over time. Something along those lines would explain a difference between the inverted bubbler and conventional ones in the same tank.

If that could be the case a second line from the bottom of the tank to beneath the inverted bubbler should establish a slow recirculation, as is seen in fish-tank bubble filtration, to avoid a stratification of the fluid in the drain line.

 

[Bambie]

danw2, 3DDave and hacksaw,

1) I call it a (control) valve because the helium flow path closes and opens in response to tank level changes.

2) S.G. changes a maximum of 1%; the tank temperature is maintained at 25 degC and the fluid is very pure - not a mixture.

3) The tank is drained and re-filled annually via other drain lines however there is no drain on the reservoir so it starts measuring each 'new' tank contents half filled with last year's contents.

4) If the reservoir and drain line S.G. dropped due to some chemical change then buoyancy forces should drive it into the tank.

It seems improbable, but could tiny helium bubbles plate-out on the stainless steel over time, slowly reduce the flow resistance in the line which would require less pressure to maintain flow? Perhaps tank draining and de-pressurization pops these bubbles off the surface?

 

[3DDave]

The bouyancy is offset by the restricted movement from the small diameter over the long distance, allowing for an inversion. There's a stream of low-density bubbles interfering with material entering the vertical tube.

If it is bubbles, then a whack with a rubber mallet should knock them loose.

Is the fluid temp in the pipe carefully maintained?

 

[Bambie]

3DDave,

The drain line and reservoir are at ambient room temperature (25 degC), the same as the tank and contents.

What you are describing should happen immediately - why would an inversion form gradually?

 

[3DDave]

If some characteristic of the fluid in the dead-end vertical tube changes slowly this would match what you are seeing.

Obviously it can't be an ideal steady state system that is also varying over time, so something is changing. The only thing mentioned that resets this is to drain the tank, which should also drain the line, so there is apparently something in the line that is interfering with the measurement. If there is a bubble-trap that is allowing part of the column to be replaced with gas, that would certainly do it.

Tapping with a rubber mallet may free up the bubbles, so that's one way to examine it. If the flow rate is small or the differential of bubbles-in to bubbles-out it will take time for the gas column to form.

 

[Bambie]

3DDave,

Perhaps a slow accumulation of friable corrosion material around and over the tank drain hole could cause this interference with bubbling action.

The dynamic pressure increase due to the debris restriction at the tank entry point would probably be small compared to the reduction in static head from a partially helium filled line.

I will certainly recommend 'whacking' the drain line after Thanksgiving.

I suppose an immediate 'apparent' level rise could mean only one thing - release of helium slugs with an equivalent cumulative vertical height.

 

[3DDave]

Please have a containment plan in place.

If it turns out that that's the problem, then consider adding a second loop to recirculate fluid in the drain line to keep the passage clear or add a sight-glass to the top of the run where it gets to the tank.

 

[Bambie]

3DDave,

What is and why would I need "a containment plan"?

Your recirculating pipe idea would be difficult to install due to accessibility. A second drain line entering the reservoir below the helium level would ensure a helium-free static head reference.

I might achieve the same end with a 1/4"dia x 50 foot drain snake coil inserted at the reservoir and pushed up the line until the end was proud of the debris in the tank. By locating the snake adjacent to the reservoir weir, the inner 1/8"dia snake coil hole would provide a preferential flow path for the helium and the annulus would remain solid with fluid from tank to reservoir.

Is there any chance this would work, if 45' was snakeable?

 

[3DDave]

If there is a problem with corrosion and someone is careless with how hard the pipe is hit, one might discover an unexpected weak spot. It will take a while, but the fluid will come out if the drain line breaks. The containment plan is what to do with all the fluid if the tank dumps its contents, including running from the building, if the material is dangerous to be around.

A second line entering the reservoir will allow recirculation. The bubbles will entrain fluid up the existing drain line which will be replaced by flow in the second line.

I have not enough information to determine if that is snakeable. It is cheap enough to try. Leaving it in place may make it more prone to blockage.

 

[Bambie]

3DDave,

Perhaps periodically increasing helium flow would clear debris and trapped slugs?