flat bottomed flask+vaccum=implosions!??????
Little Steve
I have a question about the glassware used to evaporate solvents under
reduced pressure (though you probably could have guessed that from the
title).
I know you are not supposed to use glass flasks with flat bottoms
(even though they stand without a cork ring!) as the corners where the
sides curve to the bottom ar weaker and the flask more likely to
implode. What I was wondering is exactly why this is the case? Is it
something to do with the glass on the inside edge of the corner beign
more compressed than that on the outside thus not too strong?
And why then are the corners where the neck joins the flask not also
an issue?
Just curious as the boss made me waste a days work by not allowing me
to use a flat bottomed flask (we have little glassware and chances of
a clean or uncracked round one are slim to nothing). Ive used them
before though I admit I knew of this slight side effect.
Just curious what the actual structural reasoning is behind them being
weaker (save the fact that spheres are almost crtainly th best shape
to cope with compression).
Cheers
Little Steve
Uncle Al
Glass fails in tension. Flat is the worst possible shape,
concentrating forces at a defect.
I've seen somebody try to degas a huge erlenmeyer of aqueous
biological buffer with a water aspirator, a length of red rubber
vacuum tubing, a short length of glass tubing, and a cored black
rubber stopper. I receded out the door as she turned on the faucet.
There followed the sound of a large door closing, then a splash.
It hit the ceiling.
--
Uncle Al
http://www.mazepath.com/uncleal/
http://www.ultra.net.au/~wisby/uncleal/
http://www.guyy.demon.co.uk/uncleal/
(Toxic URLs! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
Larry Smith
The flat bottom flasks do not withstand pressure as safely as round ones,
all other properties being equal.
This is why chickens don't lay flat bottomed eggs.
Depending upon the amount of vacuum you apply, you might get away with it
though. "Mights" are frightening
to management in light of the litigious value of accidents in America.
Little Steve <Dr.L...@btinternet.com> wrote in message
news:381dde17...@news.btinternet.com...
>
> I have a question about the glassware used to evaporate solvents under
> reduced pressure (though you probably could have guessed that from the
> title).
>
> I know you are not supposed to use glass flasks with flat bottoms
> (even though they stand without a cork ring!) as the corners where the
> sides curve to the bottom ar weaker and the flask more likely to
> implode. What I was wondering is exactly why this is the case? Is it
> something to do with the glass on the inside edge of the corner beign
> more compressed than that on the outside thus not too strong?
>
> And why then are the corners where the neck joins the flask not also
> an issue?
>
> Just curious as the boss made me waste a days work by not allowing me
> to use a flat bottomed flask (we have little glassware and chances of
> a clean or uncracked round one are slim to nothing). Ive used them
> before though I admit I knew of this slight side effect.
>
> Just curious what the actual structural reasoning is behind them being
> weaker (save the fact that spheres are almost crtainly th best shape
> to cope with compression).
>
> Cheers
>
> Little Steve
John Vinson
Steve) wrote:
>
>I have a question about the glassware used to evaporate solvents under
>reduced pressure (though you probably could have guessed that from the
>title).
>
>I know you are not supposed to use glass flasks with flat bottoms
>(even though they stand without a cork ring!) as the corners where the
>sides curve to the bottom ar weaker and the flask more likely to
>implode.
As Al says, glass fails in tension. A sphere with a vacuum inside is
under *compression* all over its surface, so it's basically getting
stronger for being evacuated. A flat surface like the bottom of an
Erlenmeyer will bow in (just a tiny bit of course), putting the
exterior under tension. If there is *any* trace of a crack or scratch
- likely - or any ripple or irregularity - absolutely certain - it can
nucleate a crack that causes the flask to fail catastrophically.
Steve
wrote:
>
>
>Little Steve wrote:
>>
>> I have a question about the glassware used to evaporate solvents under
>> reduced pressure (though you probably could have guessed that from the
>> title).
>>
I can speak from experience - erlenmeyers collapse in vaccuum!
Vaccuum = about 14 psi at sea level....that's 14 pounds ---- let's say
you have a 500 ml erlenmeyer - that's , what, 30 square inches on the
flask? 30 X 14 -420 pounds or so.....
This is obviously a rough guesstimate & an extreme simplification, but
just a little pressure over a large area can cause devastating
effects, for instance houses exploding near tornadoes.
Round flasks compress under stress just like eggs do............
CRP/DRP
talking here?
John Vinson wrote:
> On Mon, 01 Nov 1999 18:47:03 GMT, Dr.L...@btinternet.com (Little
> Steve) wrote:
>
> >
> >I have a question about the glassware used to evaporate solvents under
> >reduced pressure (though you probably could have guessed that from the
> >title).
> >
SJN8888
After I dropped and broke an HPLC mobile phase filtering-flask, I tried to
attach the top part (funnel, filter, side-arm vacuum connection) to an ordinary
erlenmeyer flask with a rubber ring and some para-film. It imploded nicely as I
was filtering my mobile phase....
Uncle Al
CRP/DRP wrote:
>
> Never seen a flat-bottomed filter flask implode. How big a flask are we
> talking here?
[snip]
1) Filter flasks are thick-walled, champgane bottle vs wine
bottle. Drop one of each onto concrete.
2) Filter flasks do NOT have flat bottoms. Look. They are concave
upward to evenly spread the strain.
Michael Press
>On Mon, 01 Nov 1999 18:47:03 GMT, Dr.L...@btinternet.com (Little
>Steve) wrote:
>
>>
>>I have a question about the glassware used to evaporate solvents under
>>reduced pressure (though you probably could have guessed that from the
>>title).
>>
>>I know you are not supposed to use glass flasks with flat bottoms
>>(even though they stand without a cork ring!) as the corners where the
>>sides curve to the bottom ar weaker and the flask more likely to
>>implode.
>
>As Al says, glass fails in tension. A sphere with a vacuum inside is
>under *compression* all over its surface, so it's basically getting
>stronger for being evacuated. A flat surface like the bottom of an
>Erlenmeyer will bow in (just a tiny bit of course), putting the
>exterior under tension. If there is *any* trace of a crack or scratch
^^^^^^^^
interior
>- likely - or any ripple or irregularity - absolutely certain - it can
>nucleate a crack that causes the flask to fail catastrophically.
--
Michael Press
pre...@apple.com