Doc B
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Leaking stopcocks
Dear Sirs.
I'm an aging man, as are we all, therefore the pun white elephant need be addressed before we inadvertently start relating how many times a night our
leaking stopcock cause as to wake and become melancholic of our youth. 
*sorry for my poor taste and sense of humour, if I've offended*
Sirs, allow me to iterate that I post this topic in regards to the inexplicable leaking of stopcock used by the madscientst. In particular those
attached to separatory funnels.
Is it just me or do they all leak?
Granted it's only a slow leak but it's irrespective of condition, material glass v. PTFE, well vented/depressurised content, tension upon the thread,
even of capped or uncapped stoppers & sealed or open at the dripping tube end. I tinker this way and that but can't seem to resolve the issue.
Maybe I've been heavy handed on the thread, like every bloody thing else I touch, from first blush and have somehow deformed it? Maybe there's
something I can do that works for you dry madscientist or that has closed the flood gates for you in the past?
I'm not sure but before I've got two issue with leaking stopcocks keeping me up at night, can anyone help or share their experiences?
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JJay
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Umm.. I find that putting vacuum grease on a stopcock will keep it from leaking. It's usually not necessary on PTFE stopcocks, but I have one that
tends to leak and bind, and vacuum grease fixes both problems.
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Sulaiman
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I've run an experiment for 72 hours with no sign of leakage around a pe-funnel stopcock,
and I've had burettes holding liquids for days,
both using glass in glass.
I use relatively cheap silicone grease, a 20g tube has lasted over two years and only 1/2 used so far.
e.g. http://www.ebay.co.uk/itm/Silicone-Grease-Multi-Purpose-Grea...
Most of my glassware is cheap Chinese, with a red ST symbol,
many of the joints are slightly leaky with positive or negative pressure applied but the grease seems to make a good seal,
but I still get tiny leaks, as evidenced by corrosion of ss Keck clips (and the grease) by chlorine etc.
My quickfit glassware joints seal and feel much better, joints self-seal reasonably well without grease, but I use grease for most setups, to reduce
the chances of frozen joints.
... but the difference in price means that I will mostly buy Chinese glassware in future.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Doc B
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I've silicone greased the Teflon stopper in my litre Shott Duran separatory funnel, still drips like its got the clap.
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Chemetix
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Really, the only way to stop leaking stopcocks is use a rotary type valve.
If you can find some 300 grit silicon carbide abrasive...valve grind paste at an auto store ....something like that. You might be able to make the key
and barrel somewhat more acquainted with each other and the problem will resolve. The concept of interchangeable becomes less relevant, but who cares
when it's one sep. funnel in a drawer?
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Doc B
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Quote: Originally posted by Chemetix  | Really, the only way to stop leaking stopcocks is use a rotary type valve.
If you can find some 300 grit silicon carbide abrasive...valve grind paste at an auto store ....something like that. You might be able to make the key
and barrel somewhat more acquainted with each other and the problem will resolve. The concept of interchangeable becomes less relevant, but who cares
when it's one sep. funnel in a drawer? |
You know what, that's something I'd not thought of. I dismissed the use of rotary valves after I promptly broke the first couple I owned so soon
after their acquisition, that I forgot if they were leaky or not. I'm vaguely recalling some misgivings about my prohibition of them on these very
grounds though. I might have to lift the ban, because you're also on to one of the two as yet unverified corrective actions at my current disposal.
One is to re-hone the stopcock into its housing, just as you've described. Though the dilemma I had over which abrasive to use, promoted the method
currently in testing. That being the application of light teflon tape (the white coloured one) to a glass stopcock that seems rouge to its native
25ml funnel. There is the additional complication of fastening it appropriately, as thers only an equaly effective wayward folded metal type key, the
grove which houses it is greatly reduced and unable to accommodate even one of the clips halves. However some creative wrapping patterning pre and
post setting and some very slight gauge copper wire along with the good old elastic band are looking promising.
Be that as it may, honing is a simpler and idealic approach. Especially if the 300 grit silicon carbide abrasive is one of the cutting compounds on my
shelf already and your experienced or confident enough that it a suitable candidate?
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PirateDocBrown
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What do you use to provide the pressure to keep the stopcock in its housing?
Could you show a photo of the problem?
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Chemetix
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The abrasive honing solution works, just go for the finer stuff. I said #300 grit , but on reflection I think that's the coarse stuff.
Honing teflon will definitely need a fine powder, which I have honestly never tried; glass on glass is the technique I can say will work. Just keep it
wet and replace the abrasive when you feel the grinding action disappear. I much finer grit to polish is advised.
A risky solution for teflon keys is heat the barrel up to just on the melting point of teflon and jam the key into the hot barrel and then use
compressed air to rapidly cool of the barrel. It's risky, you could melt the key too badly or crack the barrel but if you are skilled in the art you
can sometimes reform the key to the exact dimensions of the barrel.
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JJay
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My grease is the $20/oz expensive fluoridated Dow-Corning stuff. Not sure if that makes a difference
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DJF90
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Check that you have the right tap in the stopcock for starters... If its a polished barrel it should be a teflon tap; a ground barrel requires a glass
tap. Not getting this right (possible if bought second hand) will give problems from the get-go.
Glass stopcocks should be adequately (but not excessively) greased.
Teflon taps should not be greased. A new teflon tap should be "bedded in" to the barrel by applying light yet firm pressure whilst rotating in the
barrel. Also bear in mind that teflon suffers elongation under tension - for this reason make sure to slack off the tension and loosen the tap when
not in use (just make sure retighten before trying to use).
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JJay
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I have read that teflon stopcocks shouldn't be greased, but I couldn't find any other way to get this one to work without leaking and binding (my
stopcock is rather large). I usually keep it out of the funnel when it is not in use so that I don't have to deal with the nightmare of removing a
seized teflon stopcock from an expensive piece of equipment that is already at room temperature.
I have not had any problems with greasing it, despite the fact that this is not recommended.
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DJF90
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The "bedding in" process essentially makes the teflon tap fit perfectly in the barrel by plastic deformation. I had issues with the taps on brand new
glassware until I read about it and gave it a go.
A binding tap in my experience is an indiction of one of two things - the tension on the plug is too great, or, if coincident with leaks, a non
uniform contact of the plug with the barrel (tighter in some places than others). This can sometimes be observed as a white line perpendicular to the
length of the tap (where the teflon contacts the glass barrel). I'd recommend cleaning off the grease and trying the "bedding in" process. Best to fix
the problem rather than just covering it up.
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JJay
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I will have to try bedding in....
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Doc B
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Sulaiman- did you do anything to get that level of reliability from your funnels and buttered?
JJay- I've tried that same grease, as well as others and haven't had any lasting success. Do let us know how you go if you try bedding in, please.
Chemetix- thanks, you've added weight to the honing and polish method I've been considering. I'll be sure to document and report the results if I
proceed with such an undertaking. I'm reluctant to perform thermal refitting of Teflon stopcocks, due to my skills in the art of heating barrels only
ever resulting in a painful and frustrating gradual destruction of the barrel. Although it's still worth considering as it only needs to be hot enough
to increase the plastic deformation of the stopcock enough to improve its fit. Much like the bedding in method DJF90 has kindly mentioned as being
successful but under mild heating.
The rotary variant is of high priority to reevaluate, I don't actually see them offered for sale widely though, any suggestions for acquisition in the
same country as you it seems?
DJF90- I can confirm the appropriate size and type of stopcock used in each barrel. Furthermore even on brand new and high quality equipment as well
as in repair of used equipment, I've had some success with bedding in, however fleeting it was. The unfortunate aspect of it was that after the period
of grace that bedding in allowed, a noticeable increase in the rate of fluid loss was observed, when compare to the rate of loss from prerepaired
equipment. I may have overtightened it and being so difficult to turn, inadvertently and stupidly deformed it in such a manner as to decrease it's
seal or after concern for damage reduced the tension and thus the seal that way. However if I recall correctly, when I've reduced tension on the
thread of a tightly bedded stopcock to be just a little more than loose, there is again a brief period of seal.
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Sulaiman
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My Jaytec burette has lost its glass tap so my remaining burettes are; a Jaytec with an all glass stopcock that I've lost the rotating taper part
somewhere,
an E-Mil with a rotoflow valve (left), two all-ptfe stopcocks (centre) and my PE-funnel is GG17 (right)
(not my photo's ... copied from the 'net)
My burettes are all well used second-hand, with unknown history, but do not leak.
I do not use grease on ptfe as that defeats the benefit of ptfe - no contamination.
Before posting this reply I put about 200 ml water in my 250 ml PE funnel last night, this morning there are no signs of leakage at all.
My PE funnel came without a clip to secure the taper so I tried an external circlip - that was too tight and jammed the glass taper
- quite a lot of swearing was required to un-freeze the joint - maybe that helped ?
I also have an all-glass quickfit D3/32 addition funnel that I remember using as a vessel for electrolysis and that was running in my bedroom for over
a week without leakage,
just an occasional tapping of the product ( colloidal silver )
A sep.funnel has very little pressure difference across the stopcock so it must be a really poor fit to leak.
..........................................................................................
Although I may be lucky with stopcocks, I have not much luck with NS24 joints, especially the cheap hollow glass stoppers.
I'm sure that it is no surprise - I feel that Quickfit joints are ground to a much better profile than the cheap Chinese glassware, especially the
joints with a red ST mark.
Quickfit joints rotate smoothly, the cheap ones definitely are not perfectly circular, but the price difference means that if I can't find used
quickfit cheaply, I buy Chinese.
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Sulaiman
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Update
Stopped the test after 10 days ... Pressure Equalising funnel has not let one drop of water through.
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Chemetix
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"The rotary variant is of high priority to reevaluate, I don't actually see them offered for sale widely though, any suggestions for acquisition in
the same country as you it seems?"
The best bet is I make one... I have the taps and can make a funnel you request with the joints you want...
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JJay
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Oh, that reminds me... I cleaned the grease off of my addition funnel and tried the bedding in procedure suggested by DJF90, and it seems to have
greatly reduced the frequency of stopcock jams compared to no grease without bedding in.
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DJF90
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| Quote: Originally posted by JJay | | Oh, that reminds me... I cleaned the grease off of my addition funnel and tried the bedding in procedure suggested by DJF90, and it seems to have
greatly reduced the frequency of stopcock jams compared to no grease without bedding in. |
Glad to hear you've had some success with it. Did you have a chance to test the potential for leaks yet?
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JJay
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Not yet, but I'll let you know when I do.
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