Threaded fittings to join a threaded pipe of different material 2

[mechiit]

Hi,

I am planning to Use alloy 800HT pipe (NPS is 1" and Sch. 160) for a high temp process application (35 bar pre and 650 C temp). However, the pipe threaded fittings (union, tee, elbow etc) that I am planning to use for joining different pipe lengths are of SS316L material. Will it be thermally compatible ? What factors should I take into consideration before I decide it to be safe to use SS316L fittings with pipe of alloy 800HT. Has it got to do something with the coefficients of thermal expansion. Pls let me know. Thanks in advance

 

[moltenmetal]

Thermal expansion be damned: what the heck are you planning to use as a pipethread sealant at that temperature? NPT threads are NOT designed to seal without a pipe thread sealant, and you're beyond the temperature of X-Pando which is the sealant of last resort around here...

 

[mechiit]

Hi,

Silver goop is a thread sealant that can work up to 815 C. I guess that should take care of that. Pls let me know the compatibility of SS316L fittings with Alloy 800HT pipe. Thanks in advance

 

[rustbuster]

Depending on the environment, you could have issues with carburization of the 316L, 304H would offer better resistance.

 

[EdStainless]

What about the very low strength of 316L at those temps?
Yes, you joints will loosen slightly at high temp., but that isn't my biggest concern.

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
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or edstainless@earthlink.net

 

[moltenmetal]

"Silver Goop" is an anti-galling compound, not a thread sealant. It permits the parts to be disassembled after they've been heated. Any thread sealant character the material may have had at room temperature will be long gone after the oil substrate of the "goop" has pyrolyzed away. On closely machined threads at small sizes (1/8 and 1/4 NPT) such as what Swagelok sells that stuff for, you may be able to get some thread sealing using silver Goop- but at 1" on a dissimilar metal threaded joint I don't think you've got a hope in hell. Given your temperatures, maybe you should consider silver brazing the joint after assembly. That would still permit you to disassemble it later, albeit with the aid of a torch. If later disassembly is not required, I'm pretty sure you can weld 800H to 316 using the appropriate filler metal (probably Inconel 82 (ER CRNi-3)- but I'm not sure).

The traditional way to do a transition between materials in piping is by means of flanges. Lap-joint flanges will allow you to use a cheaper high-temperature material than 800H for the backing flanges. And there are several gasket options for the temperatures you're thinking of.

Another option is to use a straight thread joint design like ISO parallel with a metal gasket. Then your silver Goop can do what it's designed to do and the gasket will do what it's designed to do. But you should expect to need to re-tighten the joint after a number of thermal cycles.

 

[mechiit]

Hi,

Thanks everyone for sharing your valuable comments.

Since I don't need to disassemble, I guess welding would be the best option. Also as the working fluid is He gas and is a very small molecule, probably I should avoid flanges in order to have a leakproof joint.

The reason for choosing SS316L fittings is that they are cheap in comparison to Alloy 800HT fittings(around $500 each). As one of you pointed out that SS316L has lower strength at temps of 650 C, can anyone pls suggest me a much better and cheaper option than SS316L. I will appreciate that. Thanks

 

[moltenmetal]

At those temperatures you should not be using 316L fittings, period, though I presume what you have is 316/L dual grade material- straight 316L grade is seldom stocked any more at the small sizes. You need the higher temperature strength of the straight 316 grade at minimum, and in fact 316H is more appropriate. 321 is another option. In any case, you need to determine if the material you're using has adequate strength by doing the appropriate code calculations using the correct stress values for the grade you've selected.

If your service is helium, forget about threaded joints. You're best off welding. Ring-type joint (RTJ) flanges are also an option, as they work well for high pressure/temperature hydrogen services.

 

[EdStainless]

Why not use plain end pipe and make full penetration butt welds? He will be a challenge.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net