I love threaded galvanized pipe for air lines above grade, but even I wouldn't do this for buried pipe. The cost of repairing a leak is prohibitive relative to the cost of a few welds in the buried portion.
our EPC contractor has chosen galvanized threaded pipes for above and underground.
The underground will be HBE (High Build Epoxy) coated.
My problem is that these lines belongs to Instrument Air system.
How to avoid leaks?
By seal welding or there's other ways by using resins or other?
Thanks for helping me.
If the concern is leaks at the threaded connections, then thread sealants are an option. Several available from Henkel/Loctite, 3M, Precote and ND Industries.
Is there any limitation of using thread sealants as the lines will be buried (underground)?
I mean is it technically acceptable to use threads sealants or it is better to have a seal weld.
A good thread sealant system is essential any time you use threads, but what concerns me is the inability to readily access the buried line to repair a leak should one develop. Leaking air wastes significant amounts of energy. Above ground, you can detect and repair those leaks, but below grade you're just going to lose and lose.
You can't seal weld galvanized pipe. The fittings are typically malleable iron and are not weldable.
To bring up yet another concern with this plan: Coating galvanized material is an iffy proposition. Unless it is coated in the first 24 hours or so, the surface develops a rather unfriendly, unstable blend of mostly zinc oxides and hydroxides which make achieving a good coating bond difficult. Think of it like painting over rusted steel - except instead of being red, the zinc "rust" is clear or white, and difficult to see with the naked eye.
After more time* the surface largely transitions to stable zinc carbonates, and is much more friendly to coating.
I suggest having a good look at ASTM D 6386 if you insist on coating galvanized material, it describes some of the common surface preparation methods to deal with that interim surface - most people don't want to wait 2 years before painting.
Why not just use FBE-coated lengths of carbon steel, with joints welded and coated in the field? Might also look at a CP system (or integrating with the existing system) for longer-term protection.
* In the range of 6 months to 2 years, typically. Depends on storage conditions.
Pipe threads are designed with a leak path, it is a truncated thread profile.
If you use threaded lines you are entirely at the mercy of your thread sealant to prevent leaks.
We have gone to either welded lines or swagged (like Victaulic pressfit) for sir lines.
And then there is external corrosion.
I would rather see good quality coated plain steel that trying to coat galv.
Dear TomDot/EdStainless,
thank you for your suggestions regarding changing material and/or painting system but it's too late for that.
My EPC contractor already has the material at site and ready to work.
My question is "How to prevent the buried portion of threaded lines to leak as the fluid is "Instrument Air".
The coating system is HBE (High Build Epoxy) on previously blasted pipes and already approved.
As seal welding will spoil the galvanization, beside health problems to welders, I was thinking to use a Loctite sealant such as "Loctite 577". If any one have already use this method is welcome to advise, thanks.
YOU CAN"T WELD GALVANIZED THREADED PIPE- forget about the zinc fumes or damage to the zinc coating, the fittings ARE NOT WELDABLE! If you ask for advice, please at least read it when it is given to you!
Again, you're using the WRONG METHOD for this piping. Numerous people have told you that. Who cares if they're on site with the wrong materials! Unless this is a very short and very unimportant line, haul them off the site and use either welded pipe with your epoxy coating (and anodes if required) or stainless Victaulic Pressfit (or one of the many similar systems out there- fittings that are crimped on and seal with a rubber o-ring).
If you MUST use threaded pipe, the thread sealant system we use ABOVE GROUND is heavy (1.0 s.g. or higher) Teflon tape top-dressed with an anaerobic pipe thread sealant paste- we use Loktite 567 or 592. This is a time-proven combination that works reliably and LASTS. Neither system (tape or paste) works as reliably alone as both perform together- the tape fills the large voids and provides anti-galling lubrication during tightening, while the paste provides flexibility/memory against thermal cycling, vibration etc. We did a job recently where a client specified paste only for threaded lines and it was a total mess- literally and figuratively.
I think your real problem is in the phrase "our EPC contractor has chosen galvanized threaded pipes for above and underground.". Was he allowed to under the pipe specification issued to him?? If yes then you will need to issue a change order and if you've missed it on the detail drawings and dcouments that he has presumably submitted ( line list, isometric, pipe spec etc) then it is also now your fault. If not then you point out to him the clause in the spec that says no threaded fittings underground and tell him to do it properly.
Trying to make this good to avoid delay, claims, etc I can understand, but when he hydro's the air line and it starts leaking, you'll find out pretty fast that that was not the right way to do it and will take a lot longer and cost you more than just getting in a load of carbon steel pipe. I can't believe it will take that long to find small bore air piping.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Unfortunately engineering approved and materials have been purchased accordingly and are at site.
There's no way to change things, only to find a way to assure no leakage will occur in the underground (buried) section of the line.
There is a way and that is to instruct a change and pay the difference. I think you have enough evidence above to tell you that sometimes you just need to admit a mistake was made, fix it and move on and not leave a residual issue which will end up costing your client much more in future costs.
Screwed galvanised buried pipe is just the wrong material. Write an explanation, give the issues and provide costs for replacement pipe and then get it approved. If someone higher up vetoes this you have done you're best.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
