Air diffusing into water during hydrotest 1

[Tazweld]

I have read several forums on the problem with air trapped in a hydro test. I know in fresh water typically water diffuses into water at 1200psi.
Our issue is thus, what is the lowest pressure that air will start to diffuse into water and to add another factor not fresh water but into produced water. I don't have the salt content available at this time.
The pipeline we are testing has been in service since 2004 and is a OD-112.5mm WT-7.4 Star 2000 fibreglass line 380m in length
any and all help greatly appriciated

 

[racookpe1978]

Even an open-air static pond or lake has "air" dissolved in it from what is essentially a 0.0 "atmospheric" differential pressure: If oxygen were not continuously dissolving in either fresh or salt water, the fish would die within minutes.

Rate of solution (for nitrogen and oxygen) into static (non-sprayed, no waves, no "bubbling brooks" of splashing water) room-temperature water will be proportional to area of the water, will increase slightly as temperature goes down (and will be zero when the water is boiling), and will increase with air pressure (more collisions of air molecules O2 and N2 with the water surface per unit area.

 

[LittleInch]

What's the issue here? whatever the diffusion rate is it will be very low and not normally an issue for hydrotest.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

I think the original poster is "over-thinking" a non-important problem.

The volume of trapped air in a hydrotest is important to minimize - to eliminate completely if in ANY way possible! - but the dissolved gasses in the water in a hydrotest? I have never seen that be a problem in any systems flush or hydrotest or operational test.

Dissolved gasses affects water chemistry of course, but that is an operation issue settled over time well after the initial fill and flushes and hydro.

 

[weldstan]

Tazweld,
As the others have indicated, you are creating an issue where there isn't one.

 

[btrueblood]

Not going to agree that this is a non-trivial problem for a buried pipeline. Presumably there may be a significant amount of trapped air due to the pipe following earth contours, with no way to vent the air pockets, and not enough water volume or flow rate available to purge them. The hydrotest will then see an exponential decay rate of pressure vs. time, or alternatively, the hydrotest pump will need to keep pumping an exponentially decaying volume of water per unit time, as the air goes into solution in the water and equilibrium is established. If the air pocket volumes are unknown or not well estimated, the decay rate could be mistaken for a leak. One would expect (hope) that at some point in time, the decay rate drops off below the acceptable leak rate; this is how anybody who's really done a hydrotest does it, as there is no way to eliminate ALL of the air in any hydrotest.

 

[weldstan]

btrueblood,
Agreed that the trapped air could pose a problem (maintaing pressure and safety related) but not its diffusion into the water. This is 122 mm (4") diameter FRP pipe. It should be no real problem in filling & venting.

 

[rconner]

I think bimr supplied some pretty fair advice (as he usually does!) potentially applicable to testing in the presence of some air or other things in the article he provided link to on the current thread

http://eng-tips.com/viewthread.cfm?qid=370704.

[Read particularly the last paragraph about repeating timed pressurizations, perhaps even multiple times, and then comparing all aspects of results. If the results are exactly the same each time, it is more than likely a leak as opposed to air effects!]