Eliminating Air Bubbles in a pressure vessel under testing

[guideonl]

Hi,
What can be done to avoid trupped bubble air at the top of a pressure vessel without upper hole during hydraulic testing?
Also, once the bubble exist what can be done to release it?
Thanks

 

[Lnewqban]

Could you run an internal tube up to the top of the vessel?

 

[guideonl]

I guess no. The tank is full with water

 

[anorlunda]

Devices like that have been used since the days of steam locomotives. Will something similar do what you want?

 

[Baluncore]

If it is a gas pressure cylinder, then I expect it has a liquid drain at the base.
You could;
1. Redesign it so it could be safely tested.
2. Or; Turn the vessel upside down so the hole is at the top.
3. Or; Fill it with hot distilled water that will cool and disolve the remaining gas.

 

[Lnewqban]

I guess no. The tank is full with water

Could you explain a little further? ⁉

 

[guideonl]

Hi everyone,
I'll try explain...

I am a qualified inspector for pressure vessels, and hydraulic testing means filling the tank with liquid (water) to the top of the tank, than using a hand pump raizing the internal tank pressure up to 150% of the design pressure for 30 min.

Most of the designed pressure vessels I've tested are equipped with such openning at the top (plug/pipe/valve..) that can be open/dismantled to enable releasing the air from the tank during filling it with water. Few tanks (vertical/horizontal) are not equipped with such opennings at the top (the opennings are locatted below the top e.g at the shell of vertical tank, so during filling the tank with water - the inside air can be released as long as the openning is above the water level, once the water level pass the openning - the air is trupped between the upper water level and the (inside) top of the tank and could not be released = trupped bubble air).

The ideas to redesign or turn the vessel upside down so the hole is at the top are impractical when it is already installed at the plant..

 

[Baluncore]

A tube with a float should be introduced that will reach the highest point and vent air from the very top of the tank.

You may need a coaxial adaptor with a valve where you introduce the test fluid. That valve will initially vent air through the tube.

Depending on materials and pressures, the tube and float may need to be replaced after a single use.

 

[Lnewqban]

I agree with @Baluncore post above.
As there will not be pressure during the time of air removal, even a very light plastic hose could be used, in such a way that the float does not need to be too bulky.

If practical, the interior of the top portion of the vessel could be filled with a bladder bag, pressurized first and then deflated and removed prior the test.

A few bubles will not affect the hydraulic test or will create a hazard of explosive rupture, the volume they can expand is very small.

 

[guideonl]

Thank you,
It is little bit comlicated to arrange at the field, though it makes sense. The openings I can use usually and practically are ∅1/2", sometimes up to ∅2", but I'll try to look for a solution in the direction you offered above.
Thank you all

 

[anorlunda]

As there will not be pressure during the time of air removal, even a very light plastic hose could be used, in such a way that the float does not need to be too bulky.

Good point. Perhaps the air removal tube could be removed before the pressure test.

 

[Lnewqban]

Good point. Perhaps the air removal tube could be removed before the pressure test.

Even better.
For frequent tests of the same problematic tank, I would suggest the plant engineer to permanently install a simple manual vent valve at the highest point, if at all possible.

 

[Baluncore]

What is the range of capacity of the vessels?
What is the pressure range?
What are the fluids contained?

For compressed air, I would consider a flexible 1/4” silicone tube, with a float of closed cell foam, maybe neoprene. Clear Vinyl Tube, CVT, would be cheaper than silicone but less flexible.

I would fit a 1/2” Tee adaptor at the access point. The test water would enter and drain through the stem of the Tee. A 1/4" metal tube would pass freely through the cross of the Tee into the tank where it is connected to the flexible tube. The metal tube would have a compression fitting to the outer end of the Tee, and a valve outside the vessel. That valve would control the air bleed during the venting.

 

[guideonl]

Thank you Baluncore,
I think the concept is correct, but it would be better if you can attach a sketch to clarify the tubes connections ( 1/4” silicone tube & 1/4" metal tube ) and the way there are inserted ( with a float of closed cell foam ) into the ∅1/2" opening of the tank. Does a small (less than 1/2") float is sufficient to raise the tube?
Also, I am not sure I understood your compression fitting to the outer end of the Tee recommendation, sketch would be helpful.

 

[DaveE]

OK, I know it may not be practical, but, how about evacuating the air with a vacuum pump before filling with liquid. This would have the advantage of being a set up that doesn't require anything related to the idiosyncrasy of the individual tank. A pump, some valves, gauges, and plumbing that you bring and then connect to the tank with whatever port is available.

 

[Tom.G]

how about evacuating the air with a vacuum pump before filling with liquid.

Yeah, I thought about that too; then realized that:
1) Tough to find water that doesn't have some dissolved gasses in it
2) That would create a huge vacuum vessel that would likely collapse, versus a pressure vessel

Oh well.

 

[Baluncore]

It is pointless trying to design a solution, while not knowing the range of operating parameters. We need to know...
1. The test fluid. Often water, maybe hydraulic oil, and what else ?
2. The height of the top of the vessel above the access port ?
3. The distance of the access port from the highest point in the vessel ?
4. The test pressure range ?
5. The range of vessel volume involved ?
6. What else is in the tank that may obstruct a vent tube ?
7. What thread forms and sizes are used to attach to the access port ?

 

[guideonl]

It is pointless trying to design a solution, while not knowing the range of operating parameters. We need to know...

1. The test fluid is water
2. The height of the top of the vessel above the access port range is about 250-500 mm
3. The distance of the access port from the highest point in the vessel seems to me to the same question/answer as 2.
4. The test pressure range is betwwen 15- 20 atm.
5. The range of vessel volume involved mostly between 250 - 2000 liter
6. What else is in the tank that may obstruct a vent tube - nothing.
7. What thread forms and sizes are used to attach to the access port - mostly ∅1/2" BSP

 

[jrmichler]

Also, I am not sure I understood your compression fitting to the outer end of the Tee recommendation, sketch would be helpful

Here's a sketch:

A pipe cross provides connections for filling and a pressure gauge. The steel air vent tube is bent so that it goes to the top of the tank. Loosen the compression fitting so the cross fitting assembly can spin, install the assembly, then tighten the compression fitting. Fill the tank until water runs out of the tube, then close the valve in the tube (not shown). Run the pressure test.

This is a simplified sketch - the compression sleeve and nut are not shown. The fill line is not shown. The shutoff valve in the tube is not shown.

Steel tube is cheap, so you can make a custom assembly for each different tank size. You should be able to use 1/4" OD steel tube in 1/2" pipe fittings. That tube size is easy to cut and bend by hand. You need to make sure that the tube is properly aligned inside the tank, possibly an external bend in the same plane as the inside bend.

 

[Baluncore]

As @jrmichler shows in the diagram, with such a small tank there is no need for a flexible tube and float. The flexible tube and float would only be needed for large industrial reservoir tanks where the access port is some distance vertically and horizontally from the high point.

I would use annealed copper tube that is easy to bend into a long gentle curve. Cut a slight Vee notch in the end of the tube so it cannot accidentally seal flat against the upper inside surface of the vessel.

The tube might be extended for larger vessels by sliding a length of clear vinyl tube onto the end, maybe with a float.

Place a swivel connection where the Tee assembly connects to the vessel, not at the compression fitting. I would use a Tee connector so I could be sure of the orientation of the internal tube. You should be able to hit the inside top of the vessel with the tip of the tube, and feel on the outside where it is contacting the inner surface.

The compression fitting should have a brass olive that clamps onto the tube. Normally the tube end would rest against an internal step in the fitting, but since in this design the tube passes through the assembly, you will need to carefully drill or bore out that step without damaging the olive seat. The olives should not be reused once they have been clamped onto a tube.

A valve to control the venting of air will be connected to the external end of the tube. Close that valve when water flows clear without air bubbles.

Check that the pressure ratings of the Tee, compression fittings, and valve all meet the required 20 bar or 300 psi. Most low cost pipe fittings will fall short of that. For higher pressures I would select all parts from the range of standard 1/2” and 1/4" BSP hydraulic fittings.

 

[guideonl]

Thank you Jrmichler & Baluncore, it is helpfull.