Sciencemadness Discussion Board

Vacuum on Glassware

pyro6314 - 23-12-2008 at 19:54

I wouldn't have started a new thread but my question is kind of case specific. I have a wash bottle I just got off Ebay and I don't have any idea of how much vacuum I could safely pull on it. My question also extends to a 24/40 Kimble distillation set I got too. My massive A/C pump can pull to nearly 30 in Hg and I haven't tried my recently acquired vane pump. Can anyone speculate?

Klute - 24-12-2008 at 02:47

Honestly, if it chemical glassware and not cracked or starred, you have nothing to worry about, they will handle over 0.001 torr easily.

Sauron - 24-12-2008 at 04:29

A more precise answer is that spherical borosilicate glassware of normal or heavy wall thickness will take a variable amount of reduced pressure depending on temperature. The larger the flask the less reduced pressure it will take at any given temperature, but this is not really significant till you are talking about very large flasks in the 22 to 100 L class.

Glassware of other shapes is not particularly well suited for reduced pressure. Why would you pull much vacuum on a wash bottle?

30 in Hg is not much, that is maybe 10 torr. Pumps in normal use in a chemical lab often go do to 1 torr or 0.1 torr or 0.01 torr or less. Physicists regard all those as low (i.e., crappy) vacuum as opposed to medium or high vacuum which is a really seriously reduced pressure.

Your 24/40 distillation set, what is largest rb flask? 500 ml? 1 L? You have nothing to worry about.

Very thing wall garbage glassware made in India and often sold on eBay is unsafe for lots of purposes. Avoid it.

I am sure I have strength data published by one of the major glassware companies in a catalog and expressed in terms of flask diameter and temperature and reduced pressure limits. If I can find it I will post it.

Formula409 - 24-12-2008 at 04:55

Sauron, what is the worst that can happen if a flask is unfit for vacuum? Explosion? Small crack?

Formula409.

Ozone - 24-12-2008 at 07:46

It will *implode*, often violently. The shards of glass (and chemical) will bounce out from the center and can be dangerous. I have seen this only twice, and both times the glass was under additional stress--once with a heavy buchner on a thin-walled vacuum flask (tsk tsk) and the other whilst handling a flask under vac (the joint,which may have had a crack I could not see, actually broke under the weight of the flask, it did not implode (the hole in the flask was larger than the piece that came off), but the hose tension swung the nipple and attached glass around and I was cut).

[He did ask what the *worst* thing was that can happen]

In other words, the flask will be subject to an enormous amount of stress. Any deformation of the vessel, be it via mechanical means or heat, will increase the risk of failure. The same goes for a vessel under pressure. If you are concerned you can:

Check your glass under pressure (under water or covered with soap solution) first to spot any "invisible" cracks.

Check for bubbles or other inclusions in the glass which can lead to weak spots.

Buy good glass and know its limits.

Use the smallest size possible and avoid geometry with sharp angles (like your sparger). This not only decreases the risk of failure, but minimizes the casualty that can result during an unfortunate event (psi, kPa/cm2, etc...all contain an area derivative).

Cheers,

O3

[Edited on 24-12-2008 by Ozone]

smuv - 24-12-2008 at 11:39

Bah! I did pressure testing of various glass containers, an empty small model paint bottle (ca. 20mL), a 25mL filter flask and a 500mL kimax bottle. Every container tested took 110psi without breaking.

While I understand the forces involved with vacuum are different than with pressurization, but I think this just goes to show how strong glass containers are.

pyro6314 - 24-12-2008 at 12:15

Any glassware I have purchased that is going to be under vacuum has all been Kontes or Pyrex. I figured I would use the wash bottle as a scrubber before my pump in a vacuum distillation. I realise I wont need a rediculous amount of vacuum for most distillations, but it's more for just piece of mind in case I get stupid and want to pull as much vacuum as I can on a system. Is scratching on the inside of a RBF from magnetic stirring much to worry about or should I treat it as any other defect?

I have never really understood much beyond the measurement of "inHg" and all the reading I've done doesn't quite sink in. Can anyone simplify the "torr" in comparison?

Thanks for the insight everyone. Merry Christmas!

watson.fawkes - 24-12-2008 at 12:59

Pressure differences are what translate into forces on the glass itself, not absolute pressures. If your glassware is good to low vacuum, it's almost certainly good to the highest vacuum. For strength computations, how low the vacuum goes is lost in the rounding error. The ΔP between 760 torr (=1 atm) and 10 torr is hardly different from the ΔP between 760 torr and 0.00001 torr; in this example it's less than 1.5%. That's less that variation due to weather. Just because it's quite difficult to get past the high vacuum regime doesn't mean that glassware has the same difficulty resisting the result.

You can estimate strength under vacuum by comparing to strength under pressure. The difference is that you have to swap compressive and tensile forces within the material. Like most materials that fail through fracture, it's tensile stresses that matter more. Thus it's not only sharp corners, but corners that are concave-viewed-from-inside that are principal sources of weakness. 1 atm is less than 15 psi, so if your glass is good to 150 psi under pressure, there's a good chance (but no complete guarantee) that it will be good under vacuum to atmosphere.

I feel the need to mention that crazing, scratches, and tiny cracks, because they are all seeds for crack propagation, will significantly weaken glass. Flame polishing can mitigate or eliminate such problems

watson.fawkes - 24-12-2008 at 13:02

Quote:
Originally posted by pyro6314
I have never really understood much beyond the measurement of "inHg" and all the reading I've done doesn't quite sink in. Can anyone simplify the "torr" in comparison?
Google search has built-in units conversion. Try this: 1 in Hg in torr=.

MagicJigPipe - 25-12-2008 at 00:09

What would be the major difference between putting a flask in water deep enough to apply ~13psi to it (net pressure of course) and pulling a vacuum on that same flask down to say, 2 psi (assuming an outside pressure of 15psi)? Wouldn't the exact same amount of pressure be applied to the outside of the flask in both circumstances? If not then why?

Sorry for not converting to torrs but I'm not familiar enough with them (yet). And I'm in a hurry.

Sauron - 25-12-2008 at 06:23

The few glassware companies that offer pressure vessels at all limit them to 40-60 psig and 80 C. Parr tests their bottles to 125-150 psi but warns users not to exceed much lower pressures,

gsd - 25-12-2008 at 07:55

Quote:
Originally posted by MagicJigPipe
What would be the major difference between putting a flask in water deep enough to apply ~13psi to it (net pressure of course) and pulling a vacuum on that same flask down to say, 2 psi (assuming an outside pressure of 15psi)? Wouldn't the exact same amount of pressure be applied to the outside of the flask in both circumstances?


Yes. You are right. As watson.fawkes has pointed out, it is the "Delta P" which matters.

BTW, 1 psi is about 2.31 ft of water. So in order to apply 13 psi by dipping flask in water, you need to dip it in a well at least 30 ft deep. :)

gsd

MagicJigPipe - 26-12-2008 at 09:30

What is the rate with water under a vacuum? At 30ft, if the water had no atmosphere pressing down on it, what would the pressure be? Obviously you can't just subtract the atmospheric pressure because it would be negative. Okay, so the actual pressure at 30ft is ~28psi (with atmosphere)? And the relative/net pressure on something filled with air (1 atm) at 30ft is ~13psi, correct?

[Edited on 12-26-2008 by MagicJigPipe]