I am trying to design two lifting trunnions for a PV and I'm having trouble getting the shell stresses to cooporate. I'm following Procedure 5-4 from Dennis Moss' PV Design Manual 3rd Ed. but the stress values I'm getting seem to be way too high. I'm using a 6" S/XXS A106 pipe, 30kip load at 8" from shell and I'm getting stresses in excess of 300ksi! I've redone the calcs a few times and the numbers aren't getting any smaller. I can increase the pipe size up to 8" if necessary but I just have a feeling that 300ksi is just way too high and increasing to 8" won't suffice. Any help/direction would be appreciated. Thanks in advance.
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Lifting Trunnion for Pressure Vessel
- Thread starter reesecc66
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Repad is an option but not much of one. Moving it closer is not an option. The 8" is as close as I can get.
My vessel diameter is 40" at the location of the trunnion and the wall thickness is .500".
The vessel previously had two small lifting lugs (.75" x 5" base) that were damaged and removed. No shell damage was found at or around these locations.
In my mind, it makes sense that spreading out my load around the pipe would decrease the stresses. At the numbers I was getting the shell would be completely crushed.
I'm really more interested in why the stresses were so high and if there is any other source material to look at.
Thanks
My vessel diameter is 40" at the location of the trunnion and the wall thickness is .500".
The vessel previously had two small lifting lugs (.75" x 5" base) that were damaged and removed. No shell damage was found at or around these locations.
In my mind, it makes sense that spreading out my load around the pipe would decrease the stresses. At the numbers I was getting the shell would be completely crushed.
I'm really more interested in why the stresses were so high and if there is any other source material to look at.
Thanks
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- #5
The total load of the vessel, which is a heat exchanger that includes the channel, bundle, heads, and process is 24,000 lbs. I'm designing each trunnion to carry the full weight alone plus a little more, hence the 30k per trunnion.
Reesecc66:
Show us a well proportioned sketch of what you’ve got, with the important loads, dimensions and controlling conditions. I don’t know Dennis Moss from Adam, but if you can’t kinda cipher this out for yourself, without his example problem and inserting your new numbers, maybe you shouldn’t be tackling this problem. Are you looking at FEA results which appear to show 300ksi stresses at the top and bottom of the trunnions, or is this a hand calc? One solution would be to make the shell out of 400ksi steel, or you could draw a free body diagram of one of the trunnions and think a little, consider the possibilities. Start with a simple FBD, 30k at 8" vs. a shear force at the tank shell and two equal forces at the t&b of the trunnion to counter the moment. Then add the complexity, how do you get those concentrated normal loads into the shell? Why can’t you decrease the 8" or increase the dia. of the trunnion, or the height/length of a lifting lug? That’s a fairly thin tank shell, you should have no problem getting the 30k shear load into the shell. The problem is the lifting eccentricity and the moment it produces, or the reaction forces (normal to the shell) at the top and the bottom of a lifting lug or trunnion, and the trunnion doesn’t really improve that much when you look at the FEA output. Make a lifting lug longer/taller and put flanges of some arc length on the t&b to distribute the loads into the shell. Now, if you try to carry the full load on one trunnion, the tank will tip to get the C.G. under the lifting point, increasing these normal loads and you had better take a look at the end pl. on the trunnion to be sure it can take these new loads too.
Show us a well proportioned sketch of what you’ve got, with the important loads, dimensions and controlling conditions. I don’t know Dennis Moss from Adam, but if you can’t kinda cipher this out for yourself, without his example problem and inserting your new numbers, maybe you shouldn’t be tackling this problem. Are you looking at FEA results which appear to show 300ksi stresses at the top and bottom of the trunnions, or is this a hand calc? One solution would be to make the shell out of 400ksi steel, or you could draw a free body diagram of one of the trunnions and think a little, consider the possibilities. Start with a simple FBD, 30k at 8" vs. a shear force at the tank shell and two equal forces at the t&b of the trunnion to counter the moment. Then add the complexity, how do you get those concentrated normal loads into the shell? Why can’t you decrease the 8" or increase the dia. of the trunnion, or the height/length of a lifting lug? That’s a fairly thin tank shell, you should have no problem getting the 30k shear load into the shell. The problem is the lifting eccentricity and the moment it produces, or the reaction forces (normal to the shell) at the top and the bottom of a lifting lug or trunnion, and the trunnion doesn’t really improve that much when you look at the FEA output. Make a lifting lug longer/taller and put flanges of some arc length on the t&b to distribute the loads into the shell. Now, if you try to carry the full load on one trunnion, the tank will tip to get the C.G. under the lifting point, increasing these normal loads and you had better take a look at the end pl. on the trunnion to be sure it can take these new loads too.
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reesecc66, see attached WRC 107 run using approxiamtely your numbers. Note: Assumed 40 OD, CA not included. Calcs are in circumferential direction only. Trunnion is mounted on a cylinder, NOT a cone.
I get about 184,000 psi shell stress w/o pad (pgs 1-3). W/ 18 OD pad shell stresses are acceptable (pgs 4-7). Pad could likely be a little smaller.
My guess is, you are going to have to decrease your loading and add a pad. Trunnion load would often be something like 1/2 vessel weight X 1.25 or 1.5.
Hard to do more with it on available info.
Regards,
Mike
I get about 184,000 psi shell stress w/o pad (pgs 1-3). W/ 18 OD pad shell stresses are acceptable (pgs 4-7). Pad could likely be a little smaller.
My guess is, you are going to have to decrease your loading and add a pad. Trunnion load would often be something like 1/2 vessel weight X 1.25 or 1.5.
Hard to do more with it on available info.
Regards,
Mike
Intuitively, you'd think trunnions like that would give a reasonable stress. The reason I commented on the length/weight was that it looked like something might be out of the ordinary- which it is, with the extra weight and the 2x factor on the weight.
With those cone dimensions, making the cone thicker might be easier than putting a reinforcing plate on it. Upsizing the trunnion pipe would help a little bit.
Maybe a stupid idea, but can you run the trunnion pipe all the way through? If you're dropping stuff into that end, wouldn't work of course; otherwise, it would just completely eliminate the moment load. Possibly reduce the pipe size in that case.
If this is for 1-time use, can you make an attachment to bolt to the flange face? Basically a blind flange with a lifting lug in the middle?
With those cone dimensions, making the cone thicker might be easier than putting a reinforcing plate on it. Upsizing the trunnion pipe would help a little bit.
Maybe a stupid idea, but can you run the trunnion pipe all the way through? If you're dropping stuff into that end, wouldn't work of course; otherwise, it would just completely eliminate the moment load. Possibly reduce the pipe size in that case.
If this is for 1-time use, can you make an attachment to bolt to the flange face? Basically a blind flange with a lifting lug in the middle?
racookpe1978
Nuclear
When you need to lift the PV, bolt a removalable inside lifting bar between the two lugs. (Leave the top PV cover off for the lift.) You would need a simple fillet-welded reinforced plate on the inside with the (sacrificial) bolt holes or small lugs for the lifting bar if you don't need to regularly lift out the PV.
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