Hydrogen compressor snubber vessel nozzle welding issues

[qualitypro]

We are seeing repeated failures near the toe of the SS nozzle weld in Stainless snubber vessels in hydrogen service lately. The vessel shell is SS 316H while the small bore nozzle material is 1" SS 316L schedule 40S as in the data report. The nozzle is only about 6-8" long. The recent crack is at the toe of the weld in the pipe segment closer to vessel. This failure occurred within a few hours of starting up. Roughly a week prior to this event, we had another failure on this sch 40 nozzle but in that case the failure was near the toe of the fillet weld closer to the Class 1500 socket weld flange side. There are lot of vibration issues with the compressor as well which we are trying to mitigate. To strengthen the nozzle we have now used sch 160 piping and replaced with weld neck flange to bring the critical compressor back up again. What could be the possible cause for this failure to recur? The OEM is troubleshooting his design but we want to get to the root cause and not just have a temporary fix.

 

[metengr]

If the cracks are occurring along the toe of the fillet welds this could be from mechanical fatigue crack propagation. I would make sure that the weld toes are also blend ground smooth with no undercut. You need to reduce the vibration stimulus to below the threshold for fatigue crack initiation.

 

[r6155]

Add two supports flat bar, 90º each, welded between pipe and shell.

Regards
r6155

 

[TGS4]

What's the temperature? If it's low (<300F) then r6155's advise may assist, but definitely follow metengr's advice. Otherwise, r6155's advice could exacerbate the problem.

 

[r6155]

Please, can you send us pictures of failure?

Regards
r6155

 

[SnTMan]

qualitypro, are we to assume it is a full-pen detail, or no?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

If you are certain the fracture mechanism is fatigue, then you have two basic routes to mitigate the problem: (i) reduce the source of the vibration; or (ii) introduce design changes to reduce the effect on the part (i.e., amplitude), which is acting as a cantilever.

Option (i) is more difficult, requiring isolation of the compressor (expensive and difficult), or changing to a unit that causes less vibration (ditto).
Option (ii) has several possibilities that are fairly easy and inexpensive to implement (best, good, not so much):
(1) Attach brackets to stop the bending vibration that is driving the crack;
(2) Blend the weld toe. I'll wager the crack runs from the toe into the pipe base metal, so increasing the weld size is useless and in fact could make things worse;
(3) Increase the pipe wall thickness. This has limited effectiveness as you have already learned. This kind of fatigue is strain-controlled.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

And don't overlook thermal fatigue as part of it. How hot is the gas? This material has poor thermal conductivity and high thermal expansion (bad combination).
Be careful, gong heavier will significantly increase the stress on the welds. IT may not help at all.

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P.E. Metallurgy, Plymouth Tube