Hello fellow stressers!!

Have anyone have experience regarding creep-fatigue interaction for a thermal mixing tee. How do you normally approach this?

Or there is a standard design for thermal sleeve, for a certain thermal gradient between injecting stream & receiving stream.

Kindly share your experience.

Warm Regards

We are all in the same page in this case...

Cheers,

Comments:

1. I can't really speak to cold liquid injection into hot gas, because I've never had access to software that could analyze this. Below comments are for liquid/liquid interaction. Also assume that I refer to cold injection into hot below, so reverse hot/cold as necessary.

2. The best solution to fatigue issues is to try to avoid them to begin with. Ensure injection velocity is in the same neighborhood as the main run velocity. Too little injection, and you'll end up with cold fluid running down the length of the branch side. Too much injection, and you'll end up with cold fluid running down the opposite side. While in the "sweet spot" your fluid will seem to cool down at a uniform rate (with respect to radius) until you accomplish the new thermal uniformity.

3. Don't place a bend too close downstream to the injection point. Even if you do manage to maintain that sweet spot, when the fluid turns the bend, you can end up with a cold spot in and around the centerline of the "pre-bend" line projected onto the extrados of the bend.

4. The operator's / process engineer's desire might be to "thermal shock" the fluid as much as possible to knock off buildup. It's dangerous to thermal shock the pipe itself. Thus, they'll want to ensure that there's sufficient build-up so that you crack the buildup and not the pipe.

Thanks Michael for practical explanation. Good design practice is really the key in lieu of complicated calculations.

Any other experience is greatly appreciated.

Cheers!!

To add a little more here:

The software I had access to at the time of the requested analysis couldn't handle a true liquid being injected into gas stream. Thus, the best I could offer would be to demonstrate what would happen if a cold (250F) slug of liquid contacted a hot (650F) metal.

The results were quite unsurprisingly an impressive disaster, with somewhere between 200-400F difference between inside and outside of the pipe.

I'm not completely convinced that even if the software I had could depict a gas/liquid temperature and scattering effect that its survivability would increase drastically.

Hi Michael,

Thanks for sharing your experience. This is really a subject matter that is complicated. I will try to see if I can dig deeper into CFD world and maybe understand more the cold liquid being injected hot gas stream phenomena.

And this is exactly same scenario I experience 2 years ago, a 6" cold liquid stream (130 deg C) & 12" hot gas receiving stream (400 deg. C). The system experiences cracks near the tee at the main pipe after 2 years operation. Anyway, I think the licensor did some detailed analysis which I haven't seen and they are the one who recommended to use thermal sleeve.

My task is just to update the stress analysis, and I was surprised why the original design have installed a diagonal bracing on the vertical tee connection (at the mixing point itself). And when I check the CII model, the diagonal bracing was modelled as rigid with temperature of 400 deg C. Obviously you know what's next if you correct the temperature of the bracing. And I think, everybody will agree to un-install this diagonal bracing. Just correct me if there are some reason for this bracing (vibration?). I'm not sure if this is what they call elastic follow-up, where the system is highly constrained locally. Or maybe this a combination of creep & low cycle fatigue which is both time dependent phenomenon.

Another one I observed is the tee is near the elbow which may also add to the effect on higher thermal gradients.

Just correct any wrong statement I've made.

Any other experience is greatly appreciated.

Cheers!!

I'm not sure what exactly the vertical tee with a diagonal brace on it looks like, but I'm sure I wouldn't like it based on those words alone.

A couple thoughts:

1) It was probably added to fix an existing problem not considered in the basic analysis. You'd need to find out what problem was fixed before you determine which bad design is still the better design.

2) Maybe they're thrusting open the liquid valve abruptly and there's an impulse / thrust component they're considering - correctly or incorrectly. Or they're treating it as a hammer situation.

3) Looking at Danb's links, it appears the analysis is mostly about temperature, thus we can infer some similarities: too little liquid flow and lots of gas flow, and it rides one wall of the pipe. Too much liquid flow and too little gas flow and it'll ride the opposite wall of the pipe. Turbulence enhances mixing and temperature transfer, but more turbulence means more speed also means more pipe affected.

We can't really conclude anything about vibrations from the link since I don't think any of these diagrams are a gas/liquid isosurface (just TEMPERATURE isosurfaces). But I would expect that you could at least do a superficial flow regime / EIG check.

Thanks Michael for additional information. Actually, this system is already finished two years back & so far no concern as of the moment.

I'm just preparing myself in case I encounter the same case again in the future. My current projects are pipeline & mostly large diameter low temperature with some above ground piping connected to tanks & pumps.

I will inform as I go thru my studies if I found something confusing.

Cheers!!!