I had a welding inspector tell me that root pass reinforcement could lead to internal erosion next to the root due to turbulence caused by the reinforcement. He said it was desirable to keep reinforcement to a minimum which in my book means walking the line between reinforcement and suck back. The method I use for root passes (very common here in Oz) does tend to give reinforcement and nobody I know of has heard of Jody's forward and back method so I thought I would try it out and share my results.
OK so I tried 3" sched 40 butt joint in the 6G position. 30* bevel on each pipe, knife edge, tight 2.4mm gap with four tacks. Pipe shiny inside and out for 25mm. Wire used is 2.4mm ER70S-4. Root pass done with 105amps fixed (no pedal). Started from the bottom to top in one pass consuming tacks, used both hands for both runs.
P1.jpg (31.61 KiB) Viewed 784 times
P2.jpg (32.86 KiB) Viewed 784 times
I am certainly pleased with two results, root pass is certainly neater than my usual free hand method without excessive reinforcement, no flat sections or suck back either. These were done in the shop on the bench. Next time it will be in the field to see how repeatable it is on difficult positional prep and welding. Looks like Jody's method is a winner.
Some welding inspector has too much time on his hands.
There are reasons one may wish to limit internal reinforcement in a pipe root, including turbulance, but if the product stream is erosive to the pipe in turbulent flow, the material chosen is inadequate. No pipe ever made maintains laminar flow throughout; there is turbulance at the tiniest imperfection, every joint, every elbow.
That said, if he's the inspector you have to satisfy to get your check, then you're pretty well stuck with "his way".
Glad Jody's method seems to work for you (looks great in the pics), and I think you'll find it effective in any position, provided you can prop comfortably and control the torch and rod.
I agree. I have cut into many aged systems for modifications, etc. Never seen erosion inside pipes in my working life. That is to say it may or may not happen in other industries, but not mine.
Still if a welding inspector (known to be good and competent) imparts some knowledge, I am all ears. If nothing else I have learned a knew root pass method which will make my beer and shrimp taste that much better on Friday night's barbie.
AK: NDT will have to wait for field work compliance, will let you know when that happens. I have no intention of x-raying the dumpster snot I used for coupons and prepped up. You just know from experience whether a butt would pass (or fail) an x-ray and I have no reservations about these two butts. Anyway there's probably not enough meat left on the coupons now they are cut short.
In certain parts of the process in the plant I'm running right now there happens to be the exact thing the inspector is talking about. The reinforcement on the weld causes an eddy that traps the low PH moisture on the leading edge of the weld and it actually attacks the weld more so than the pipe. Repairs make it even worse.
We replaced a large section of this piping completely with 321 stainless but it still attacks the weld (347 rod If I remember right). We try to limit the reinforcement as much as possible and still be sound, it helps.
Some welding codes will tolerate zero internal reinforcement or even some minor suck-back. So long as the weld deposit thickness, including the root and cap reinforcement is equal to or greater than the thickness of the pipe.
I've seen root erosion on seawater piping. Sometimes it's because of cavitation behind a heavily reinforced root pass. Sometimes it's microbially induced corrosion. little bacteria that eat iron and sulfur....that starts pitting corrosion and the seawater takes it from there....
A_DAB_will_do wrote:Some welding codes will tolerate zero internal reinforcement or even some minor suck-back. So long as the weld deposit thickness, including the root and cap reinforcement is equal to or greater than the thickness of the pipe.
I've seen root erosion on seawater piping. Sometimes it's because of cavitation behind a heavily reinforced root pass. Sometimes it's microbially induced corrosion. little bacteria that eat iron and sulfur....that starts pitting corrosion and the seawater takes it from there....
Interestingly enough, the processes at Braehill's plant are the processes my certification is for. On reflection, my cert test had almost no internal reinforcement. I was told the coupon would be x-rayed, and I welded for that, but they did a six-bend instead, and I still passed.
In my repairs, I've never seen signs of cavitation-erosion in sch. 5 304SS in 35-year service conditions, but, unlike Braehill's conditions, my pipes are in intermittent service, where his are nearly continuous...
The piping in the plant sees temps from 1800* to -424* F and everywhere in between, and the piping that sees the worst corrosion is the piping that is around ambient temps. Reason being is the moisture is being carried in these transient temperatures that has had most of it's Hydrogen stripped out and is being moved back and forth in a pressure swing to strip out the rest. This makes it very low Ph and it etches the welds where it has a place to stand as in one of these eddies.
Granted most of the pipes that need replaced are upwards of 30 years old and are Schedule 80 or thicker and are either low pressure or on the vacuum side. They just produce nuisance leaks for the most part.
I've put 3000# iron fittings and XXS pipes in a drain that collects this knockout moisture and it was reduced to paper thin in 29 days of service and filled with pinholes, yikes. I didn't have all the stainless socket weld fittings on hand at that moment to do it right and had to keep the plant online until we had a scheduled shutdown.
My point is if your a pipefitter and you go to a plant that's shut down and are asked to weld in some piping and you don't know the process you might want to consider there may be a reason that the inspector is asking for a certain weld procedure to be followed that's foreign to what you're used to. It may be critical to the person who's walking around that plant. Pressure isn't the only thing that's encountered in piping failures.
Just a different perspective on things, not beating you over the head.
That's an excellent point. Despite the similarites in our work, the process pipe I work with sees 99.999% purity, according to the analysis tags left in the trailers. H2 is Ph neutral, so I won't see erosion.
BTW, I've been meaning to take a picture to share with you. You identified a LOX Equipment atmospheric trailer from a single close-up picture of a torn doubler on a driveline. I'm wondering if you'll recognize this LHY trailer by it's manufacturer.
Steve,
I've had limited exposure to LHY trailers as most of my career has been around Atmos. and Compressed Hydrogen trailers but I'm up for a game of Name That Trailer.
Trailers in liquid cryogenic service are seldom if ever exposed to moisture as it can't exist at those temps. Bulk gas trailers are evacuated to a vacuum and then filled with gas that's usually around -115 or lower Dew Point which translates to around .01 PPM of moisture, that's dryer than a popcorn fart.
As welders and fab guys we go to s/s when we want it to stand up against corrosion and most times we are good, but not all s/s and most s/s welds are not impervious to all corrosion. Since it doesn't rust and pit this corrosion isn't always evident until a failure happens.
