pipe support bkt clip 1

[picasa]

I am using a pressure vessel program to check pipe support bkt clips. I have the following questions.

1. I was reading thru WRC-107 bulletin. Section 4.2.2.3 tells you how to calculate beta for rectangular attachment. It only considers radial load P, Longitudinal moment ML, Circumferential moment Mc. My question is why it does not consider longitudinal load VL, Circumferential load Vc and torsional moment Mt? Longitudinal moment VL is certainly important for pipe support bkt clips. Why is it that WRC-107 does not consider it?

2. What is the benefit of welding the bkt clip on a poison pad? I guess it distributes the load somewhat, but how big of a difference does it make? The pressure vessel program is suggesting to put a pad for the clip. I thought a gusset will be a better solution with the gusset welded perpendicular to the clip.

 

[JoeTank]

This answer depends on how the loads are reported (ie; at the vessel surface or the flange face). If at the flange face, the Vl and Vc components could be convert to bending loads at the surface plus a surface shears. Shear loads at the surface usually are not of great consequence, perhaps that is why they are mnot addressed in WRC-107. The surface shears and Mt create in-plane shear forces that should be checked, but I think your will find that the shear stresses are likely to be low. Hope this helps

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[jte]

picasa-

Steve's right. The two shear loads and torsional load will produce neglegable stresses in the vast majority of cases. Bending stresses will be what kills the design, and the radial load and long and circ moments cause bending stresses. The shear stresses are simple to calc (force/area); run the numbers a few times to prove this to yourself. You don't need a WRC-107 program to do this for you.

A repad (poison pad?) helps to distribute the loads, as you perceived. You'll have to check the local loadings at the clip to repad joint using the shell + repad thickness. The presumption is that the shell and repad thicknesses are close enough to being equal and that the perimeter of the repad is small enough so that the repad and shell behave as a single chunk of steel in bending. If that works, then you check the shell at the perimeter of the pad for the same loads. The other approach which you presented is also valid. It distributes the load into the shell, thereby reducing the stresses. Either approach is valid and which one is preferred is a matter of the details of the particular equipment being designed.

jt

 

[TGS4]

Stev and jte hit the nail on the head w.r.t. the loads, so I won't say anything more about that. I do want to take amoment and talk about gussets.

In my 10+ years of engineering, the times that I have seen welds crack are at the ends of gussets that "brace" small and medium-sized nozzles/brackets. The reason for the problem is that there are usually significant thermal gradients caused by these gussets (or cooling fins?). With variations in environmental conditions (outside temperature from day to night), this can accumulate a large number of fatigue cycles.

Include in this assessment that most gusstes are welded with fillet welds, which perform poorly in fatigue service.

So, in conclusion:
- gussets - BAD
- repads - less bad to sorta good
- integral reinforcement - GOOD

 

[picasa]

Can you please explain integral reinforcement further?

 

[jte]

picasa-

Integral reinforcement in this case would be an "insert plate" which is simply a thicker plate at the location in question. Instead of say a 12" x 18" x 1" repad on a 1" thick shell you'd cut out a 12" x 18" hole and replace it with a 2" thick piece with properly (3:1 taper) contoured edges. The full pen butt welds along the edges are readily RT'd and are not subject to stress concentration factors. Expensive, but without a doubt the most robust solution to this kind of issue. For the purposes of WRC-107 type of analysis, you'd still have to run the stresses at the clip to insert plate junction and at the edge of the insert plate.

jt

 

[picasa]

Ok.
One more question: When the software program suggests putting a repad, how should that repad be welded? For nozzles, we make a hole in the repad and weld the repad around the nozzle. For clips we just weld the repad onto the vessel and weld the clip on top of it. Why dont we make a hole in the clip repad and weld it around the clip instead of welding the clip onto the pad?

 

[TGS4]

There's certainly nothing to stop you from doing that. However, it would be substantially more welding , and I think that the clip-to-pad weld would be difficult (not impossible) to NDE.

 

[codeeng]

I'm inclined to disagree with TGS4 regarding gussets (flanges). I've done a lot of FEA on the two types and the gussets generally work better for reducing stress at the tips of bracket. Also, with repads there's still some unknowns about how these should be attached i.e. brackets set-on or set-through.

As for WRC107 remember that the bracket height is limited to 4 x thickness. So the analysis for say a 3/8" clip will likely fail every time. PV Design Manual by Moss gives an equivalent rectangle for thin clips, so using this approach would be more practical. But as usual I'd stay away for WRC and use FEA, like NozzlePro.

 

[jte]

codeeng-

I've also done a lot of FEA's of these, and agree, the gussets do a swell job of reducing the stress on a clip which are induced by mechanical loadings. Imagine that! A line load reduces stresses compared to a point load! On the other hand, TGS4's observation was with regard to thermal stresses at the gussets. I don't recall any of my FEA's in theses situations including a thermal stress component, but his commentary makes sense qualitatively. Of course, for reasonably cold vessels, the thermal stress concern is reduced. Have you ever included thermal stress analysis in your clip FEA's? How did it turn out? Sounds like a good paper topic!

jt

 

[picasa]

While we are on this topic, let me also try to get an answer to another of my questions. The question pertains to stress concentration. How should we account for stress concentration for a pipe support bkt clip? I know for a nozzle, Kn and Kb can be calculated as function of r which is radius used for nozzle to shell interface and T which is the shell thickness. Can the same formula be used for for rectangular attachment?

BTW, how common is it to calculate Kn and Kb for each case? Is it true that more commonly Kn and Kb are assumed to equal 1?

 

[picasa]

So from your experience, what are some typical values of loads that you see on pipe support bkt clips on tall towers? Is it surprising to find that there is no moment loading and only force loading on some clips? Assuming that all the loads are reported at the vessel surface, from this thread, I would guess typically I should see high values for ML, Mc and radial load P. Am I correct?

 

[codeeng]

jte
I've never come accross a thermal problem with single clips with small gussets on top and bottom. Although I've seen cracking occur on continuous rings around the shell probably due to the cooling fin effect as TGS4 described. In any case, for hot vessels, the gussets would be buried in the insulation for the most part.

picasa
I would use 1 for Kn and Kb

 

[TGS4]

It's good to see people disagree with me! I really like a healthy discussion like this one.

Although this discussion mostly has been talking about pipe support clips, I think that my comments regarding gussets should be restricted to nozzles. I'm not a big fan of the angled gussets at 0°, 90°, 180°, and 270° around nozzles.

I must confess that most of the vessels that I was referring to were rather hot (600°F and above). Unfortunately, the designs that I have seen stuck out of the insulation. I know - it would have been better if everything would have been buried in insulation. That said, the insulation constractor did a very professional job of coping the insulation around the gusset, and then coping the cladding finally and sealing it. It's one of those situations where, if someone is going to mess something up, it will be this one.

Finally, to back-track a little - I too have used "gussets" on pipe support / L&P clips. Essentially, I use a W-section (I-beam) as the support clip. Don't have a problem with that, and theyy usually are better than just a knife-edge.

Guess I'll have to be a little clearer next time.

 

[codeeng]

picasa
If these are cantilever platform clips or pipe support clips then you would expect to see only Ml. If they have knee bracing then only + and - P. If these are pipe guide clips then you would expect to see either Ml or Mc and P depending on the guide geometry.