Thoughts on this type of support 1

[VPbris]

Hi,

First of all , I'm a fresh mechanical engineer so forgive me any "unacceptable" mistakes. Also english is not my first language, but I'll try to be as clear as possible.

Recently I came across with a semi trailer tank for the transport of bulk liquid oxygen (ie operating temperature ~ -183°C , operating pressure up to 17bar). In my country there is a legislation that tells that the design os these tanks should be made according to ASME BPVC VIII-1, and consider some additional loads such as aceleration/decelerations and some surges (algo giving values on that).

For people that are not aware of these kind of equipment, it consist of a double-walled vessel, where the inner vesel is normally from austenitic steel and is design to withstand the pressure loading, and the outer vessel is designed to withstand vacuum. The inner space between them is normally filled with perlite, and then evacuated to increase heat loss efficiency.

What really triggered me was the design of inner supports. I had never seen ones like those before and would like some inputs on its design. I'm attaching some pictures to help understanding.

1st thing to notice, thermal conductivity shall be minimum, hence the area of direct contact between the inner vessel and the outer vessel should be minimum.

1 -So here is my doubts about the inner vessel support. First is the presence of stiffners rings on the inside vesse at the points of support (Figures 1 and 2), what is the use of stiffners on the support points of these vessels? (If anyone could also indicate some bibliography on this I'd appreciate a lot)

2- Now the tricky part (at least for me). As you can see on figs 3 and 4, the support at these section looks like some kind of "piston-cylinder" I've never saw it before, any inputs on that? And what to consider on its design?

Figure 1 -http://files.engineering.com/getfile.aspx?folder=b8991a39-4a21-4580-8cbf-aac1f970e748&file=fig1.jpg
Figure 2 - http://files.engineering.com/getfile.aspx?folder=595886a6-8d90-4f1a-bb9a-0564c0c523ae&file=fig2.jpg
Figure 3 - http://files.engineering.com/getfile.aspx?folder=d06460fb-9658-4ec8-943d-4c830a77efaa&file=fig3.jpg
Figure 4 - http://files.engineering.com/getfile.aspx?folder=b7604ef5-6630-4fbd-9bd2-e6b4e804380d&file=fig4.jpg

Thank you so much for any inputs!

Kind Regards

 

[mk3223]

The liquid oxygen tank is a cryogenic double wall vessel. The support rings per Fig.1 & 2 are to keep the proper space between the walls for the perlite insulation.

Fig.3&4 - Verify the material of the "cylinder". It could be the temperature isolator to separate the metals, i.s. inner SS wall and outer CS wall. So, it won't sweat at the surface of the CS wall.

 

[VPbris]

Actually mk3223 it can't be. The ring is located on the inside of the inner vessel, i'm almost sure it is a stiffening ring. On figures 3 and 4 the supports are in yellow, pink is the inner vessel and blue is the outer vessel.

My question is concerning some guidance on the design of these kind of support, ie. how to design supports for this kind of application.

But I'll check on the material of the cylinder, may give some hints. Thank you for your reply!

 

[SnTMan]

VPbris, no disrespect, but your pretty pictures are indecipherable.

Regards,

Mike

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

 

[EdStainless]

VP, the standoffs must be compliant to allow motion, the change in size of the inner vessel is considerable and you can't make this rigid.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[VPbris]

Hello,

Thank you for your replies. I was able to gather more information. The support is as follows: on the inner vessel on the very bottom part there is a small height tube welded to it. Inside this tube there is a cylindrical insulation material, that is fixed on the other end the same way (inside a tube welded on the outter vessel). There are two of these supports which are fixed. The other "supports" are not fixed to the inner vessel, only to the outside vessel trough these same tubes, and they are used to impose a force on the vessel to keep it from moving, hence the stiffening ring on the inside, to support these forces.

Any thoughts on design considerations of these supports?

 

[LittleInch]

Your fig 1 and 2 look ok to me - simply internal bracing rings taking into account the point loads on the outer shell caused by the supports

Figs 3 and 4 look to me like a sliding support due to the issue of the inner shell contracting and expanding as it gets colder and hotter respectively. The bottom supports have the vertical weight / load and the top ones add tot he support for axial and sideways loading caused by movement of the road tanker.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[VPbris]

Hello,

Sorry for the long time to reply. I managed to get a better picture of the type of support, as you can see attached.

My questions lies on the design of them. Here are some thoughts I had.

From what has been stated before I think I can assume the upper blocks are used to keep the internal vessel in place and allow for thermal contraction, where the bottom supports are the ones actualy carrying the weight. Is this correct?

If the above is true, I'm having trouble on figuring out how to check the imposed longitudinal and hoop stresses on the plate. If it was just on block on each side then would be just a matter of designing the two support reactions (which will be half the weight as they are symetrically placed), and checking on the stresses imposed by the moments, which in this case would be only longitudinal moments, just like a simple beam problem. However we need to check on the hoop stresses too, and the presence of the other angled bottom supports will definetly affect them and I`m having trouble on how to assess these.

Any ideas?

 

[dhengr]

Vpbris:
My first paragraph was written after viewing you first sketches, and much of it still applies to your problem. Then, I got busy with something else and never posted it. Your latest sketch probably shows things more realistically, but could still use some improvements. By all means see what you can find out about how other tank/trailer designer/builders handle these problems. There is no need to reinvent the wheel here. Over-the-road trainer builder’s associations, Highway Depts., and Transportion Safety Depts. will have some stds. for loading of trailers; longitudinal, lateral and vert. as a function of the tank/trailer weight/mass, including impacts and overall stability of the trailer. The people who use these type tank trailers might have some ideas of knowledge of the topic.

Where did you find this support scheme for a double walled cryogenic vessel for bulk liquid oxygen transport, particularly one which is to be mated with an over the highway semi trailer underframe? It seems pretty complicated and confused. Your vessel cross sectional view actually shows something I would expect to see, and that is a thicker shell quadrant at the bottom of the tank. I would/might expect to see this thicker bottom plate to be mated with the two longitudinal center sill beams on the semi trailer in some way. Then, there are two relatively hard support points on this center sill/tank support system. They are at the king pin on the front of the trailer and over the rear axle sets at the rear of the trailer, and I’d expect to see some form of a tank saddle at these points. These support saddles must be located longitudinally on the tank so as to cause appropriate axle loadings for over the road transport, and fairly near the heads of the PV so as not to hit the tractor cab, or overhang the rear of the trailer underframe. Then, the tank must be designed around these conditions. I would wonder why you might not use a strong foam insulation btwn. the two tanks in the bottom half, bottom third, so as to make them act together in the load transfer area. There would likely be some hard steel structure too, with thermal break details, for concentrated loads. There are various ceramic, rubber, plastic, fiberglass, or wooden standoffs which could be installed btwn. the tanks for the foaming process. The top half could be filled with any insul. with an appropriate insulating value. But, some special structural detail and thermal break detail will have to be developed at the two main load saddles. This might be an inner saddle on the main vessel, a thermal break layer, and a lower saddle through the outer tank and fixed to the trainer structure.

Your two inner tank support rings should located as mentioned above, at the king pin and over the rear tandem axles, the two primary cross trailer structural axes. While a full inner ring stiffener would be o.k., we design external support saddles for tanks all the time which only support approx. the lower 90̊ (or so) quadrant of the tank. Why wouldn’t this work for your problem too, if you used slightly thicker shell pl. in some specific areas of the inner tank? This should simplify the tank design/building, and lower the C.G. Then, your yellow blocks in most of the interstitial space would be good thermal spacers btwn. the tanks, during construction and the insulation process. And, those in the lower 90̊ quadrant of the tank, at the two primary support points and those along the trailer backbone (center sill beams) would transfer the concentrated loads from the pressure vessel to the trailer structure.