Removing entrained air in water process lines

[harry222]

Hello,

I'm searching for a easy way of removing entrained air from seawater piping (2" up to 30"). More than 2000 m3/h is not uncommon.

In all simpleness one can say that seawater is pressurized with air upstream pump(s). There will be pressure drops on the way, giving entrained air on pump(s) suction side. Worst case might get around 7-10 vol% air.

I have read elsewhere that one can fit a high point auto vent, ball and float arrangement, but can someone direct me to a more detailed way of design?

Other ways of design?

Many thanks

 

[JimCasey]

Automatic air eliminators are not really unlike steam traps except they open whe they are full of gas instead of being full of liquid.

Most of the steam-trap companies offer products in this categoty, and their application handbooks will give you guidelines for how big, how many, and where to install them in your system.

Here is one:

http://www.spiraxsarco.com/pdfs/TI/p017_02.pdf

For seawater you might want to use bronze/monel, or you might search a bit more for a plastic-bodied valve. Google is a good thing.

 

[CountOlaf]

I don't know about an open system like this with that fairly hefty amount of flow (8800 gpm!), but in closed loop hydronic systems, even if an automatic float type air vent is installed at the high point of the mech room, there is usually also an air separator (ASME stamped vessel) below this which utilizes centrifugal/tangential force to move the liquid to the outside of the vessel while air stays in the center of the vortex and travels up to the auto air vent.

Check out perhaps Wessels Company's "Severe Service SS air separators" or ITT Bell & Gossett "Rolairtrol". Both offer up to 24" flanged connection sizes for their separators.

 

[harry222]

Thank you for your guidance.

Been in contact with Spirax Sarco, but unfortunately they were not unable to meet the volumes of seawater (and nitrogen) we can have.

I was hoping to solve this without going down the road of a separator vessel as this would increase complexity somewhat. (This application is both off- and onshore in Oil&Gas Business, and complexity adds costs at greater pace by adding equipment).

Any new ideas?

Many thanks,

 

[djack77494]

harry,
Excuse me if I get a bit off track, but how/why do you expect to get so much entrained air into seawater? Is this really seawater or produced water? Might you be coming from a deoiling unit where you've entrained air to aid in floating dispersed oil droplets? Though I've worked with those situations, I haven't seen the concerns you are raising. Ultimately however, it is garvity (buoyancy) and time that will separate entrained air from a liquid. Temperature will help with more favorable equilibrium conditions AND lower viscosity.
Doug

 

[harry222]

Perhaps I should have given a bit more details on this, so I perfectly understand the question to "why so much entrained air in the water?"

First;
I wrote air, but really it's nitrogen. Just simplified.

Anyway:
This is seawater injection for on- or offshore application. Seawater is lifted topside, treated and pumped/injected into oil reservoirs. In our case we treat the seawater by mechanically stripping the seawater with nitrogen, to removed dissolved oxygen (preventing corrosion and bacterial growth downstream). The seawater is normally pressurized to 2-6 barg in the stripping process. The seawater would then be saturated with nitrogen at this pressure, but as there would be pressure drops before suction side of injection pumps downstream, nitrogen would "vaporize" on the way.

Only done some rough calculations, but came up with a possible worst case of ~7-10% entrained nitrogen if seawater pressure was brought down from ~6 barg to ~1 barg at pumps suction. (Only assumed that the difference between nitrogen saturation at 6 barg and 1 barg, would be volume entrained at pump suction side)

Looked at

http://www.spirotherm.com/

as well, but was hoping for something "simpler".

I have attached a paper that goes even deeper into the details if anyone is interested.

many thanks,

 

[Compositepro]

Interesting process. I just don't understand why nitrogen stripping would be preferable to vacuum stripping, where water vapor would strip both nitrogen and oxygen from the water. Under vacuum the water would boil and rapidly carry out and dissolved gasses. Nitrogen stripping does not involve boiling and would be rate limited by diffusion of oxygen out of water droplets. Perhaps energy savings?