10-27-03 23:04
No 467100
swim's been having problems with a vacuum pump (2 stage rotary vane, SV-140) as of late and is on the border of replacing it. if the pump can be repaired, swim will try, but a suggestion like "the pump is fucked" will prompt swim to obtain another pump. the pump troubles are due entirely to swim, as poor judgement was exercised when stripping solvents (DCM in particular).
the pump had pulled about 600 mL of DCM (total) through it, and was not pulling anywhere near the maximum vacuum of .5 mmHg achieved earlier, it was pulling ~1.6 mmHg. this was unacceptable for a vacuum distillation that had product coming over at 90°C and .2 mmHg (swim's hotplate can't generate enough heat, and i'm not using an acetylene torch), so swim thought it would be a good idea to disassemble the pump and clean it out (oil that emerged was brown and contaminated).
disassembly went easily enough, but upon reassembly there were clicking sounds when the pump was turned on and a successive oil draining showed the oil to be brown and very much contaminated again. re-disassembly demonstrated that the vanes were inserted improperly (swim's bad) a.k.a. backwards (curved edge inward, towards shaft). once the vanes were properly seated, the pump ran OK for a few hours, enough to get through another distill (no solvent pulling this time). it was noted that the maximum vacuum attained was ~5 mmHg, which is, again, unacceptable.
a third disassembly was done and everything reassembled after degreasing and a wipe-down with acetone-soaked paper towels (cleaning was done in a more limited scope on earlier disassemblies). the vanes were inserted properly and it had the clicking problem again. this is where things stand, and unless swim can diagnose the vacuum pump problem, another must be purchased.
another option is to send it back to the manufacturer to service it, but this option must be investigated. the oil that comes out of the thing is always brownish and i suspect it has something to do with the carbon (i think they're carbon...) vanes. it is also to be noted that swim didn't have a torque wrench available to restore all original bolts to a specified torque, but is this really that important for a small vacuum pump? also, should swim try replacing the vanes and gaskets?
for the record, swim already read the definitive post on vacuum pump overhauls (starts w/ post No 244556) and swim followed nearly all the suggestions contained therein. the only thing swim didn't try was replacement of the vanes, and that could be just what is responsible for the clicking, especially after the initially improper installation.
thx for reading
exit stage left bereft of all but a bass clef, i manifest zest
(Chef d'Equippe)
10-29-03 06:07
No 467444
Probably new vanes are needed. They kinda wear to their own slot and changing their location can cause issues. A backwards vane with the sguared edge out instead, may have changed the vanes shape enough to decrease its seal within the rotary slot it fits in. Vaccum parts are generally precision machined and the vanes are usually a softer metal(brass) than the rotary cylinder and its important to keep the parts orientation consistant or a diminished performance is most likely to occur. Same concept as an auto engine's cylinder & piston parts, worn to fit.
GODISNOWHERE
Shoot Narcs, Not Drugs
(Stranger)
10-29-03 16:13
No 467542
Before I knew any better I also used to use a rotary 2-vane style pump for stripping solvents but this greatly decreases the life span of the pump. These pumps really should only be used for the critical points ewhere you need a high vacuum for temperature sensitive compounds when distilling. For solvent stripping you really should be using aa diaphragm pump. These will last much longer and are relatively easy to fix when they do crap out. It will also allow your rotary 2-vane pump to last for years. Solvent fume are hell on the inside of the rotary vane pumps. You should also replace the oil after EVERY use of these pumps. They will last much much longer if you do that. Besides, that's what the manufacturers recommend I believe and they are usually talking about non-corrosive refrigeration gases which is what most of these pumps are made for. (At least the one's that I've been using).
In the diaphragm pumps when the thin metallic reed valves wear out or corrode (yes they have little reed valves in them) I made temporary replacements with little cut-outs from plastic yogurt containers. They'll work and last for a while depending of course on what solvents you're stripping. But they're much easier to replace. When the rubber diaphram craps out replacements can be made using a sheet of neoprene rubber which I managed to find at a local home-depot/ construction supply store which came with a cheap chimney flashing used in roofing. Costs less than $10 and each one can make several diaphragms when you cut them using the old one as a template.