Do tubing minimum wall thickness recommendations for gas service apply to welded tubing?

[springwaterbr]

Hello,

Many tubing manufacturers recommend a minimum wall thickness for tubing used in gas service. The reasoning behind that is that a heavier wall has a greater capacity to resist ferrule actions and coin out defects that might enable a leak path for gas molecules (for example, Swagelok's catalog:

https://www.swagelok.com/downloads/WebCatalogs/en/MS-01-107.PDF)

My application will use welded tubing and tubing fittings. I haven't however seen an explicit exemption from this recommendation anywhere for this case. If there is no reason for a heavier wall other than resisting ferrule action, it is my understanding that once ferrules aren't employed, the lighter wall could be used, correct? Or is there is still some other motive that cautions against using lighter wall tubing for gas service, even if welded?

Thanks.

 

[1503-44]

You must also choose wall thickness to resist mechanical forces and pressure stress.

 

[LittleInch]

Welding small bore really thin tubing ia=s far from easy and with stainless you need some sort of inert gas system.

The risk of burning through is high and just plain adding too much heat and deforming the tubing.

Or are you socket welding this?

Why?

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

 

[springwaterbr]

@1503-44, absolutely. Stress-wise the lighter gauges are more than enough for my low-pressure gas service application. I'm asking specifically about the recommended minimum wall thickness for gas service, which is another consideration.

 

[springwaterbr]

@LittleInch, the client requested welded tubing and tubing fittings with gas service. They had a bad previous experience with leaky compression fittings and decided on welded tubing, to minimize the chance of in-service leakage.

 

[EdStainless]

Yes and no. That table is based on specific rules. We supplied tubing for high performance systems (that were He leak tested) that were lighter wall than in that table. But they were for lower pressures of course.
With double ferrule fittings the biggest issue is hardness and a round smooth OD (no longitudinal lines).
And they must always be installed using gauges, not by 'feel'.

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P.E. Metallurgy, consulting work welcomed

 

[Tuckabag]

Probably easier to use the specific orbital welders designed for light gauge tube if you're planning on buttwelds.
They can be hired if need be.
Welding does make tube work a right pain in the backside to work on if things need to be serviced/changed/etc.


I'd agree with the above sentiments re compression fittings, we very rarely find issues with them leaking unless they have not been installed correctly. Over/under-tightening of the ferrules, or use on tube that is too hard, out of round or welded tube.

The only time we have had a client insist on welded fittings was for ppb moisture analysis sample conditioning system. They believed that the compression fittings allowed H20 to permeate the sample system, which struck us as odd since they insisted on other metal/metal seals such as VCR fittings in the same system.

 

[EdStainless]

The ASME BPE has a lot of information on orbital welding for high purity systems so it might be a good ref for welding.
For both welding and fittings welded tubing often works much better than seamless because the dimensions are so uniform.
With compression fittings the biggest problems with welded tube is if it is under annealed the welds can be a lot harder than the body (both with CS and SS) or if they leave a line/grove along the OD of the weld. These are both easily avoidable by careful sourcing.
All of the sanitary/hygienic service tube is welded because they care about surface finishes inside and out and they want the clean welds that you get with uniform wall thickness.
Welding seamless with huge wall variations can leave very uneven ID surfaces.


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P.E. Metallurgy, consulting work welcomed

 

[moltenmetal]

I had to read that post several times to understand the question properly.

Yes, the MINIMUM thicknesses listed by Swagelok in their tables for each size are for gas-tight sealing when using their 2-ferrule OD compression fittings. While you MAY have success using very good quality tubing, properly annealed, properly handled without OD scratches, of wall thicknesses lower than that minimum, you MAY encounter leaks more frequently that cannot be addressed by merely tightening the fitting a little more. Compression fittings for low leakage risk services (i.e. hydraulic oil etc.) can be successfully used on tubing considerably thinner than the minimum required for gas tight sealing per Swagelok's table.

Tubing of lighter wall thickness can be used in any service where another method of attachment is being used, i.e. tubes which are welded to fittings, or tubes roller expanded into tubesheets in heat exchangers etc.

B31.3 requires you to calculate a mininmum wall thickness by a very specific method. The material safe allowable stress value at temperature, the OD and wall thickness tolerance must be included in the calculation (i.e. minimum wall, maximum OD per the spec). Note also that if tubing is welded seam type rather than seamless, you need to further de-rate the tubing with the joint efficiency of the long seam weld- 0.85 or 0.8 depending on the nature of that weld.

Tubing being very thin, you will find that in absolute terms, you have very little excess wall thickness left over to serve as a corrosion or damage allowance. That's something which comes with the use of small bore lines, period.

I have had FAR more problems over the years with seamless than with welded seam tubing. Welded seam tubing has more consistent dimensions. Bought from reputable manufacturers, the quality is absolutely excellent and causes no problems for use with OD compression fittings. I use seamless only when clients insist on paying more for what is, in my experience, no meaningful quality benefit- or where I can't get anything else in a particular unusual material of construction when it's needed. Occasions when there's a metallurgical benefit with respect to corrosion resistance not handled by annealing do exist but are rare in my experience.

Heavy wall welded seam PIPE is another matter. Light wall stainless steel pipe (sch 10S, sch40S) I have had excellent experience with. But when you go to heavier wall thicknesses, defects in the long seam not detected during manufacture become more prevalent- and of course riskier too.