Find the flaw on this design 7

[vonz33]

Hi there

I'm building a few pressure vessels and having some issues with one of them, here is a video of a failed hydrostatic test

https://goo.gl/photos/d3mvpcHzmzNXnp1V6

Here are the drawings, hope it makes sense

https://drive.google.com/open?id=0By1l7HueUo__SXJyYlB2QnZkVzQ

Here is the sequence of the test assembly in the video

EndCap(10mm)>Gasket(3mm)>Spacer(25mm)>Gasket>Plate(0.7mm)>Gasket>Spacer>EndCap

Note that I have not recorded the torque on the studs, all I can state is that my DW059 was maxed out with a full battery here are the specs

http://www.dewalt.com/products/powe...m-18v-cordless-xrp-impact-wrench-kit/dw059k-2

I think the issue is from the plate, this was not my design just tried to save costs on engineering... I have no clue why there are slits instead of holes for the fasteners, if anyone have an idea why the designer of this plate would have made it that way, please let me know... It canot be to ease the assemblies because if its a hole (fully surrounded by plate metal) assembly is not really an issue even for a big stack.

Cheers

 

[vonz33]

SnTMan (Mechanical)18 May 17 21:53
vonz33, changing to a cylindrical design will not, of itself, prevent problems.


So then why does gas suppliers are is still using the old cylindrical type tanks to store their gas when they can make them square it would be a lot easier to transport! (joke)

But yes I know what you mean.

 

[LittleInch]

Looking at this again it seems to be that one of your issues is simply a variable pressure on the gasket caused by an irregualar bolt pattern.

It looks to me like you could have at least 6 more bolts in this arrangement to have a much more even coverage, but currently there is a concentration at each corner.

I don't think this thing will ever seal though at 300 psi or even 150, it's just too weedy.

Of course you then need to tighten then up as noted above, by finger tight, 10%, 25%, 50, 75, 100 in a cross over figure of eight pattern,

This is a simple sequence taken from a cylinder head, which is quite similar

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[vonz33]

LittleInch (Petroleum)19 May 17 14:34

[highlight #8AE234]I don't think this thing will ever seal though at 300 psi or even 150, it's just too weedy.[/highlight]

Exactly... I mean, an assembly that fails at 20psi when it's supposed to withhold 300 that's a huge gap to cover up, honestly I don't see that working no matter how much glue or bolting techniques combined. These tricks would cover smaller gaps let's say if we fail at 250 and we have to reach 300 then yes I would consider it but that's just too high to jump no matter the technique of the athlete, the bar is just too high still.

The unit I'm drawing right now will have a much stronger structure from the start by being cylindrical, and also it will allow us to use compliant parts that will make the accreditation a lot easier in the end.

Thanks for playing with us that was good fun, will come back with MkIII design in a couple of months.

 

[LittleInch]

Fair enough.

Have a look at the dimensions etc of some flanges of a similar pressure rating and square area to give you an idea of number and size of bolts. Many other flange codes use more smaller bolts than asme so might be a better fit for you.

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

 

[vonz33]

I'm considering the local AS2129 table F as it is rated for 300psi however would that be for max pressure or working pressure, just double checking as my usual boiler maker said the standard rating is for max pressure. We will be working under 150psi (setup of the pressure switch) but the unit will be tested for 300psi as we like to over engineer our gear for many different reasons, so right now I'm looking at working with table F since they are the ones for 300psi, however if there is any avenue the can allow me to jump back down to table E (rated for 200psi) I would actually don't mind... Any thoughts welcome..

 

[LittleInch]

Normally flanges are rated for max working / design pressure, but are good for testing to 150% of rated pressure.

you need to check the code, but that is a pretty standard clause.

So if design is 200psi you should be able to test at 300psi without issue normally.

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

 

[vonz33]

Thanks for this precision. So let's say we have:

- Table D rated at 100psi
- Table E rated at 200psi
- Table F rated at 300psi

Working pressure of unit is 150psi, therefore I will choose Table E to work at 150psi and run hydrostatic test at 300psi (150% of 200) correct?

Will read AS2129 when I get some time but it should not differ too much from ANSI

 

[LittleInch]

Yes


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