Fitting Weld Resting on Free Pipe Support 6

[amats42]

I searched through the forums as well as the codes and other online forums and I don't see a definitive answer on this.

Is there a code that prohibits a fitting weld (circumferential weld) from resting on a free pipe support? We have 6" and 12" coated carbon steel pipes that rest on free supports as part of an expansion loop. There are 1/4" teflon pads on these free supports between the support and pipe.

Pipe is low pressure fluid (<150 PSIG).

I can't find any code restrictions, but it doesn't sound like good practice. Anyone have experience with this setup? Pipe is outdoors so it will experience thermal expansion/contraction over time, and I'm wondering if there is concern with the weld moving on/off the support or even just sliding around on the support.

 

[MJCronin]

This is one of those questions "Is there any code anywhere that permits/prohibits such-and-such ?" ...... Or, even better.... It's first cousin: ... "Show me in the code book where it says that I can't do so-and-so !!!

The question is only posed by the field engineer when an installation is done, complete, and the welds are all cooling

Yes, indeed.... We misfits that try to answer some of the crazy questions are of course deeply familiar with ALL domestic and international mechanical codes and standards.

Here are my questions:

1. What is the Code of Record for your system ?

2. What is the design temperature for your system ?

3. What do you mean by "free support" ? Can the pipe support be easily moved ? (Most can be)

MJCronin
Sr. Process Engineer

 

[amats42]

Apologies--details float in my head and I forget the information needs to be stated.

[ol 1]
[li]ASME B31.3[/li]
[li]Ambient temperature, assumed 50°F to 110°F[/li]
[li]The support is just a v-stop--a steal beam anchored to a deep concrete foundation with a 1/4" teflon spacer on it. There are no shoes, saddles, u-bolts, etc.[/li]
[/ol]

The pipe support foundation is already poured so I cannot recommend relocating the pipe support to clear the eventual weld. It is possible to extend the piping so that the fitting is off of the weld, but then I would be increasing the distance from the elbow to the support and I'm not the EOR/pipe stress engineer. As such, unless it is unsafe (better yet, code prohibited), I do not want to push for a redesign.

 

[Dhurjati Sen]

Dear amats42,

Not codes but best practices and standards do prohibit this as this is a very poor practice.

Why? Think and answer me back. (No need for you to be a piping engineer)

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[DekDee]

I would be interested in learning why it is considered "poor practice".
If you are worried about thermal expansion moving the weld around on the teflon pad ( pipe fluid is ambient so you must have some pretty hot sun) just lift the pipe and grind the weld flush on the bottom.

 

[amats42]

@Dhurjati,

I was trying to think of reasons why it would be against best practices (and my gut says that it is), and I could only come up with possible weld damage if it slips on and off the support or if the support now had a point load on the weld (and the pipe would be slightly higher here than in areas where it is not resting on a weld). Though not ideal, I can't see this causing a failure or leak.

Seeing as you have NACE in your signature I did see on some other forums mention of corrosion, but in all of those cases it was pipe on steel. Here we have coated pipe on 1/4" of teflon and I just don't see any real concern for corrosion. You might lose some visual inspect of the weld, but it should still be possible to recoat during normal maintenance schedules and we shouldn't have dissimilar metal contact.

And I agree with DekDee, I would like to hear a great explanation why it's poor practice. I've seen it stated on quite a few boards, but never a justification as to why. The usual response is it's against best practice/industry standard and here's how we'll fix it. Thanks in advance!

 

[XL83NL]

I was trying to think of reasons why it would be against best practices (and my gut says that it is), and I could only come up with possible weld damage if it slips on and off the support or if the support now had a point load on the weld (and the pipe would be slightly higher here than in areas where it is not resting on a weld). Though not ideal, I can't see this causing a failure or leak.

With the provided service conditions I dont see much reason for issues. The only thing I can think of is damage to the PTFE pad, provided the weld has a lot of external protrusion, and would wear into the pad (due to thermal cycle movement).

So, I dont agree with this:

Not codes but best practices and standards do prohibit this as this is a very poor practice.

Im open for discussion to learn more, but ... if something isnt the best practice, doesnt per se mean it's considered bad practice.

 

[MJCronin]

How about posting pictures of the offending support ???


MJCronin
Sr. Process Engineer

 

[EdStainless]

I would be concerned because the support would restrict your ability to inspect the weld fully.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear amats42,

Sir Ed has the answer.

People like him separate the men from the boys.

FYI, I am just a boy.

Just to add a few lines to his one liner.....how do you carry out various NDTs like RT, PT, MT, UT for that particular weld joint?

If there is a very minor leak from that weld joint, say little sweating, how quickly you detect it?

Also, your pipes are 6" & 12" (not small bore pipes, say, below 2") so paint damage at the support location is expected if you have lot of rains / fog / mist / salty climate etc. and wow, under the paint there is the weld joint.

Regards.



DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[racookpe1978]

Several reasons not to deliberately place a support under a pipe weld.

Yes, if the support " has to be there" it is not illegal, immoral, unethical or fattening to do that, it is just not good practice.

The weld is a discontinuity in the pipe metal, and so (despite all the best welding practices and efforts), it is stress riser in the otherwise smooth stress in a pressurized pipeline. Don't add trouble to a trouble spot. Even a perfect weld, perfectly inspected by X-ray, is a stress riser.

The weld needs to be inspected, it needs to be accessible all-around to weld itself. So the support MUST be moved (or not installed yet) when the pipeline is being fabricated and lifted into position.
Using temporary supports while welding makes fabrication more expensive, more difficult, more time-consuming.
Moving the support back to position after welding and inspection takes more time, requires moving the pipe or specially fabricating the support after the pipe. Again, more expensive than making a good support once, putting in place, then putting the pipe in the support.

The weld is a surface "flaw" so any support touching that weld is point-supported: As pointed out above, this means a "sliding, smooth surface" will dig into the plastic or liner, will not slide, and will tear up the hanger or distort the pipe flex calculations. This MUST be minimized by surface grinding the weld reinforcements smooth before mounting the pipe support - but - again! - more time, more money, more work in the field that is otherwise unneeded if the support were someplace else than under the weld.

Surface grinding the weld flat for a support to slide or press uniformly on the pipe also removes the "reinforcement" of the weld (obviously), and so the weld strength is reduced from the "normal" full-penetrant weld butt weld.

One place where a pipe support under a weld is "less intolerable" than other conditions: Assume the pipe is already welded, and that weld has passed NDE. Then, if a welded skid plate is attached to the pipe, the fillet welds attaching the skid plate (an insulation offset bracket under under a pipe) can be welded to the pipe outer wall without grinding the pipe butt weld too far below the surface of the pipe. It is the bottom of the skid plate that is skidding on the metal below, not the weld itself. The fillet welds attaching the skid plate, and the skid plate tabs themselves, can be selectively ground or prepped to and selectively welded properly with the proper preheat to protect the pipe butt weld.

 

[amats42]

Thank you all for the detailed replies! Definitely more than I found on the net and in my references.

Inspection is a really great point and while the foundations are installed, the pipe supports are not. I will speak with the contractor to see if installation on a temporary support is possible--thank you for the idea. Cost is a huge deal, but with this particular job engineering changes (such as lengthing the pipe/changing the engineer's stress analysis) are even more challenging.

This is the first time I have run in to a weld on a support and so this is definitely something I will look out for moving forward.

I will also note the potential for teflon slip/tearing.

 

[LittleInch]

I can understand all this angst in something like a nuclear power plant, but I think a few people sure are being a bit "precious" about the pipe.

Sure it's not a great plan as it makes construction and testing more difficult (how do you actually weld it if there's a support in the way?) but at this sort of pressure and temperature grinding a bit of the weld down flat where it touches the support isn't going to be an issue (IMHO). Welds are supposed to be stronger than the parent metal.

Corrosion of the steel under "simple" supports like this are a far bigger issue long term.

Some sort of photo or sketch would help a lot to let us all see what you're looking at, otherwise we all tend to imagine what the situation is.

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