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Posts Tagged ‘seals’

When Things Suck: It’s Good?

Posted by Tom Benedict on 30/10/2010

In this instance they are, anyway:

Among the many systems we maintain at work, one of the most critical is our collection of vacuum systems.  Every instrument we use relies on vacuum to some extent, some more than others.  One of the easiest approaches for reducing noise on a CCD or CMOS detector is to cool them well below ambient temperatures.  But once a device is cooled below the ambient dew point, it will condense water onto it.  Water and electronics rarely mix well, and water and optics almost never do.  So you have to take away all the air around the device before it is cooled: vacuum.

The systems we run fall into the mid-range.  With low vacuum you can rely on mechanical pumps like roughing pumps or scroll pumps.  Achieving higher vacuum relies on more complicated systems: diffusion pumps, turbomolecular pumps, sublimation pumps, cryogenic pumps, you name it.  Higher vacuums also require a higher level of attention paid to things like dust, oil, water, and fingerprints.  Yep, fingerprints.  The out-gassing from a single fingerprint can outpace a fairly large pumping system at 10^-10 torr.  But I’m getting ahead of myself.  Our systems typically peak out at 10^-8 torr.  That’s good enough for me.

Pumps create a pressure differential.  Different pumps are suited for different absolute ranges of pressure, and can create only so much differential.  For example, a good dry scroll pump will get you into the 10^-3 range, but won’t draw you much beyond that.  A good turbomolecular pump will get you into the 10^-8 range, but it can’t do that if there’s atmosphere on the other side.  For mid and high level vacuum, pumps are stacked in series.  On our coating chamber we use a diffusion pump to pull our high vacuum, backed by a Roots blower, backed by a large roughing pump: three stages.  Our cryostat pumps use turbomolecular pumps backed by dry scroll pumps: two stages.

But no matter what kind of vacuum system you build, you will rely on pumps to get you there.  And you can only get there if they work.

A few months ago we had a test cryostat in our lab, and had it hooked up to our headquarters pump station.  Everything was fine, but after a while the vacuum gauge just wasn’t reading as low as it should’ve.  We’ve had issues with our gauges, so that’s where we checked first.  Low.  We compared that reading to the readings on the pump station’s gauges, and found they agreed.  The gauges were good.  The pump just wasn’t pulling vacuum.  Eventually we traced it to the dry scroll pump.  It had been in service for close to ten years with intermittent use, and it looked like it had finally been run too long.  When we uncoupled it from the system, it was clear why things had stopped working: half an inch of seal material was sticking out of the exhaust port.

Vacuum pumps come in two coarse varieties: those that use oil and those that don’t.  Our coating chamber system uses all oil pumps.  Our cryostat systems use all oil-free pumps.  The reason is simple:  If we have our primary mirror in the coating chamber and the vacuum system chokes, we just deposited oil on the glass.  The fix is to re-strip and clean the glass, scrub out the chamber, and do it again.  That’s a cost of roughly $15-20k.  To get an oil-free system that large is several hundreds of thousands of dollars.  Not going to happen.  But if we have a cryostat pump system on a camera and the vacuum system chokes, we could deposit oil onto several million dollars of detectors.  The cost of an oil-free system to pump our cryostats is several orders of magnitude cheaper.  That’s a no-brainer.

Dry scroll pumps like the ones we use on our detectors work because of a noxiously complex set of seals.  Blow the seals, the pump won’t suck air.  This pump had one horribly blown seal.  Once we opened it up and looked inside, we saw it wasn’t the only seal that had gone.  It had simply reached end of life.

Some years ago several of our engineers were trained in how to rebuild the dry scroll pumps we use.  They came home with rebuild kits, a field service tool kit, books, and knowledge.  Now we rebuild our own.  This was my first time rebuilding a pump, so I worked with one of the engineers who’d been trained.  The rebuild was fairly straightforward, but there were a number of tricks he showed me to make the job easier.  When a dry scroll pump is rebuilt, all the seals are replaced, but so are all the bearings and o-rings.  The bearings are all pressed in, so getting them out is a bit of a trick.  The manual calls for an oven that can swallow all the parts and go up to 350F.  The manual called for us to heat parts, remove bearings, cool parts, heat parts, install bearings, cool parts, etc.  The real trick, I was told, was to line up as much of the work as possible, heat them all at once, remove the bearings and wipe them clean, then install the new bearings before the parts were ever cooled.  One cycle, all done.  In this way the two-day job of rebuilding the pump can be shortened to a single day.

Of course things never work out the way you expect.  In the middle of the rebuild we had a failure on one of our cooling systems, and the guy who knew what he was doing was called away to deal with it.  >gulp<

So the two-day job of rebuilding the pump was shortened to one, then stretched back out into two.  This afternoon I installed the new seals, put in the new cam shafts, closed everything up, and crossed my fingers.  After a rebuild the pump needs to run for 24 hours to run the new seals in, so I put a thermocouple gauge on the input port, closed it up, and turned it on.  I was overjoyed to hear the air getting sucked out of the small volume of the gauge, and then to see things start to drop.  I won’t hit 10 millitorr until tomorrow, but by the time I walked out of the room it was down to 20 millitorr.  Not a bad start.

Sometimes it’s good when things suck.

– Tom

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