Storage tank venting

[Andresondo]

Hi Everyone! Thank you for taking your time to read and answer my question.

I am a young Mechanical Engineering graduate and I have been tasked to come up with a solution to a corrosion issue on a sea water storage tank vent piping. The tank nozzle is connected to two 90-degree elbows which are welded to two plates with a bug screen in between them. The two plates, the bug screen and a very small section of the elbow have experienced extensive corrosion-as can be seen from the attached file. This tank was put in operation in 2007 (ten years ago). All the fittings are made of carbon steel A105.

The solutions I am thinking of are:
1. Replacement in kind. Replacement of all the corroded fittings by new ones. The only problem is that the elbows are of 8 in. nominal size and schedule XS. But at the moment the only elbows we have of this size are of schedule 160.
2. Remove the corroded plates, cut out the corroded section of the elbow and then weld the bug screen inside the elbow (at approximately 5 mm from the edge of the elbow). No plates will be required for this arrangement.

What option would you suggest me to pursue? Or does anyone has a better idea?

Thank you in advance. [pre][/pre]

 

[MJCronin]

Some things to consider:

- Uncoated carbon steel is a bad choice, no matter what the thickness.... corrodes rapidly

- Internal and external coatings for carbon steel elbows will be difficult to apply and inspect

- Premium stainless steels are available suitable for sea water service, but are expensive as elbows.

- Your vent undergoes virtually no pressure loadings, therefore a thicker material is not necessary.

- Glass reinforced fiberglass is a suitable material for seawater exposure.


Suggest that you contact a glass/GRP vendor with your problem and ask for pricing of a special layup suitable for your purposes.

A vendor that supplies special/unique configurations for pollution control equipment would be best....

Good luck ....

Let us know what your final decision is ....

He will also

MJCronin
Sr. Process Engineer

 

[nickypaliwal]

I agree with MJcronin.

However can you please tell the purpose of elbow since I believe a straight pipe with mesh open to atmosphere probably can also be used for sea water.

Why would you collect seawater ? Is it for disposal?

 

[bimr]

The simple solution is to procure a flanged FRP replacement piece and then bolt the flanged replacement to the tank nozzle.

 

[JStephen]

If the vent looked like that, I'd be concerned about the condition of the tank itself.
Anyway, looks like the existing vent can be unbolted, a new similar piece bolted on. I'd say, confirm bolt pattern, then fab and line/coat the new vent before bolting it on. Carbon steel with proper lining, or other materials as desired. It is more common to put screens at the vent exit where you can see them. Avoid fine-mesh screens unless there is a specific need to keep insects out.

 

[LittleInch]

The pictures seem to show that the "bug screen" and nearby metal of the elbow has severely corroded.

If this was simple steel mesh then I'm not surprised it has corroded if it has had sea water vapour blown through it.

I would have a look at the elbows and maybe a UT inspection to check metal loss, but I would simply replace the last 10 degrees of the elbow and then fit some sort of plastic / fibre glass mesh.

I would also have a good look at the velocity you're getting through that vent as it might be that you need to enlarge the opening to reduce velocity through the screen. If it is a s corroded as it looks, the restriction on flow might be enough to over pressure or pull in a vacuum. The design might be considering this as an open vent. check with the screen vendor what the differential pressure is at different air flows.

something like this

http://www.pro-marinc.com/products-services/inverted-tank-vent-valves/

http://www.tomcatconsultants.com/2002/01/13/roof-vents-and-stacks/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Andresondo]

Thank you MJCronin, nickypaliwal, bimr, JStephen, and LittleInch for your valuable insights. I have learned a lot from your answers.

It turned out the plates, and bug screen were never coated. Since we have a very limited budget this year (and cannot afford purchasing FRP/GRP fittings), we have decided to just cut out the section of the elbow that is corroded and then clamp a new coated bug screen directly to the elbow.

Thank you to you all!

Andres