Vacuum cylindrical vessel 10

[edison123]

I need a cylindrical vessel about 700 mm dia and 1200 mm long that will withstand full vacuum. I am planning on making one out of 20 mm thick plate as per the attached drawing.

Will this design withstand full vacuum ?

Muthu

http://www.edison.co.in

 

[edison123]

If this is not the right forum, could you please point me to right one ? Thanks.

Muthu

http://www.edison.co.in

 

[desertfox]

hi edison123

A 20mm thick vessel in terms of the hoop stress will be fine, but really you need to design it to a pressure vessel code ie ASME or PD5500.
Also what apart from a vacuum is the vessel going to be used for by that I mean is it nuclear, offshore, water industry etc as that will also have a bearing on what code you should use.
Finally your biggest problem might be buckling of the vessel and bending on the flat cover plates, although it looks quite short and stout with reference to buckling, but again the code you use should cover this.
Finally your welds on your diagram appear to be fillet, these really would be better if the were full penetration butt welds.
Another important factor is whether the pressure will fluctuate inside the vessel, if so fatigue might need to be considered.

desertfox

 

[jte]

desertfox says it well. One further consideration is what is the consequence of failure? If failure can possibly cause injury to people, release of hazardous materials, or substantial economic harm... well, "I won't pretend to be an electrical engineer if you won't pretend to be a mechanical engineer." Simply put, there's more to vessel design than you are aware of - you don't know what you don't know. Please get competent help for this design.

jt

 

[edison123]

Thanks desertfox and jte. To be clear, I'm not playing to be a mechanical engineer. I just posted that drawing to show my requirements. The vessel is being made by my supplier, who has supplied such things before to nuclear plants, thermal plants etc.

I agree the flat ends are the weak points of buckling. Will adding two right angled stiffeners inside of these plates will help ?

Also, what would be the minimum recommended thickness of the plate (cylinder as well as the end plates) for vacuum bottle of this size ?

The application is for vacuum impregnation of transformer windings where after the vacuum is pulled, the resin is allowed to enter the chamber from the bottom to impregnate the windings placed inside the chamber. The whole process will take about an hour at about 80 deg C.


Muthu

http://www.edison.co.in

 

[prex]

The shell with 20 mm thickness and all those stiffeners is well beyond your needs, perhaps 10 mm without stiffeners (gut feeling) could suffice. On the contrary the flat covers with 20 mm thickness are not sufficient.
However you shouldn't really ask such things here, no one here will take any responsibility for what you do, and you'll take a big responsibility, the fact that such a vessel is not to be code stamped doesn't change anything.
If your supplier can really do such things, OK (but if that drawing comes from them I have doubts), otherwise please ask for help to someone close to you (a consultant?) that will help you go through the whole process: choice of material, material testing and certificates, weld processes and control, design calculations, gasket, bolting, etc.
Do you have an idea of the force that will longitudinally squeeze your shell when it's under vacuum? It's close to 4 tons.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[corus]

I believe in the design codes there is a paragraph that says that if the design is known to have worked in the past under the same or worse design/operating conditions then there's no need to assess it, or words to that effect.

ex-corus (semi-detached)

 

[desertfox]

Hi edison123

Will the vessel just be used once or will it be used on other jobs?
I am vaguely familiar with the resin impregnation technique you describe.
Yes adding stiffeners across the flat covers will help stop them bending, but they need to be sized in relation to the forces they are under and the stresses they are expected to under go,alternatively you could have dished ends on your vessel which would save some welding, ie no stiffeners required, but then you would probably need legs to support the vessel.
The million dollar question is who is responsible for the design? does your supplier merely take your drawing and re-design it to a working pressure vessel with your overall sizes, or does he work entirely from your drawing and use the material sizes and sections you have specified, in the latter case then, you or your company are responsible for the design.
I suppose the fact that you show no details of bolting on the top cover leads me to believe your supplier will do that for you,in which case it would appear he was responsible for the design but its not 100% clear from your post.
The vessel wall could probably be a lot thinner, if I consider only the Hoop stress in the vessel an approximation of wall thickness may be given by:-

t = p * r/(allowable stress)

t = wall thickness

p = pressure (0.101 N/mm^2 in this case)

r = radius of vessel

if allowable stress is 150N/mm^2 (depends on material)

t = 0.101 * 350mm/ (150) = 0.23566mm thickness

That doesn't take into account stresses induced due to the flat covers or crushing / buckling load, but does show that from the pressure point of view, your pretty safe with your 20mm thickness.
The covers as previously stated would be the weakest link here and they will drive what thickness of plate you will need, but thats not a quick simple calculation unlike the example above, it is also very unlikely that the covers and vessel will end up the same section thickness as it would be uneconomical to do so and not to mention manufacturing issues with reference to welding etc.
As previously stated do not take the design responsibility yourself, engage someone familiar with pressure vessel design otherwise it could end in tears.

desertfox

 

[LSThill]

Hi edison123

Add (2) stay brace stiffner on the top flat cover per RE desertfox (Mechanical) comments.



L S THILL

 

[edison123]

Thanks everyone. I understand that top and bottom plates may be weak cogs, so I have asked the supplier to make it 30 mm thick and with two 90 deg stiffeners (20 mm thick and 50 mm deep) welded on the outsides. As I said, I only give in my drawing the general requirement and the supplier should take care of the materials, welding etc.

He says he will supply it in next 3 days. Then I plan to do a gradual vacuum test on the cylinder with continuous multiple monitoring of the inward bulge along the axis and the across the top and bottom plates. How much inward movement should I allow before I stop the test ?

Muthu

http://www.edison.co.in

 

[desertfox]

hi edison123

How do you intend to monitor the deflection on the vessel?
To me there should be no visual distortion during pulling of the vacuum, deflections should only be detected with strain gauges etc.
Can your supplier not do calculations to verify vessel integrity? Judging by your last post it doesn't sound as though they can, which is worrying to say the least if there doing anything for the nuclear industry, unless of course the are just fabricators and the design responsibility lies with the purchaser.
So in answer to my earlier question I assume you are taking design responsibility.

desertfox

 

[hakkik]

you will not able to see any deflection untill it collaps. Check this out and see how vacuum(or external pressure) works on vessels

 

[Crompto29]

This is a joke, right?

Why are people halping this amature that has no idea what he is doing.

edison,

Get someone who knows what they are doing to design it. Not a group of some random online strangers.

 

[edison123]

Thanks hakkik and Crompto for making your points rather 'forcefully'.

I will meet up with the supplier today and ask him to provide the design details.

Muthu

http://www.edison.co.in

 

[edison123]

Met with the supplier today and asked for his calculations. After 5 hours, he says the end plates have to be dished ends of 15 mm thick. Now he will take 10 days to make the tank. I have put the burden of vacuum testing in his hands.

Muthu

http://www.edison.co.in

 

[desertfox]

hi edison123

I think you made the right decison on giving them the design responsibility.
Dished ends are much stronger then flat covers thats for sure.

goodluck

desertfox

 

[edison123]

Thanks desertfox. Keeping my fingers crossed.

Muthu

http://www.edison.co.in

 

[LSThill]

edison123 (Electrical), / desertfox (Mechanical)

Other option is to add stiffener cross on the flat head.

REFERENCE: low pressure heat exchanger 50 psi. FLAT HEAD PLATE with stiffener cross on the flat head.



Regards

Leonard Stephen Thill

 

[desertfox]

Hi LSThill

Thanks for the option, its already been discussed in the post of the 4th of march by edison123.

Thanks again

desertfox

 

[edison123]

An update

Finally got the vessel yesterday with flat top with stiffeners (the supplier "reworked" his design and said dished ends were not needed !).

Tested today up to 720 mm HG vacuum (pump couldn't hack it beyond that). Top plate deflection was 0.05 mm. Side walls zero deflection. Tank hloding the vacuum steady for over 3 hours till now. Planning to leave it under vacuum overnight and see what happens tomorrow morning.

Photos attached (650 kB). Thanks everyone for your tips.

Muthu
www.edison.co.in