Can leaving HiVac on with no foreline pressure cause damage?

[Chem_Life06]

Hello,

I have a hypothetical question to ask the experts because I am curious the explanation (if any).

In our lab we have >50 MSDs (Agilent 5973's/ 75's and 77's) in which some have turbo pumps and others have diffusion pumps (not sure if this really matters for the question I am asking-- but I figured I'd throw this info in).

Anyway, not all of our rough pumps are connected to our MSD's (some are plugged into an outlet) so after a system is vented (or pumped down), the rough pump needs to be shut off (or on) separately from the MSD (HiVac) being turned off (on).

My question is, after pumping down a system, if the MSD/HiVac was on and the rough pump was accidentally not plugged back in (and remained off) could this cause damage to your system?

Is there any way for your system to be contaminated by say... the oil in the rough pump if you are creating a vacuum in the analyzer from the HiVac but not having your rough pump on? Or is this generally not a concern at all?

Thanks

[James_Ball]

The oil diffusion pumps should have a sensor so that if the rough vacuum is not high enough the heater will not turn on, or if lost it will turn off. For those if rough vacuum is lost during a run and the diffusion pump is still hot you can get back streaming of the diffusion pump oil into the vacuum chamber. Not a killer but can be difficult to remove. So turning it on without rough pump is ok since it will never heat up, hopefully if you lose roughing vacuum the anti backflow valve will hold long enough for the diffusion pump to cool before any back streaming can occur.

If you try to pump down a turbo pump model with no rough pump, the controller should sense too much load on the turbo pump and shut it down after a minute or two because it can not reach full speed, it is a safety for just such a problem. If you lose rough vacuum while it is running the same thing will happen. Usually only if you were to disconnect the rough pump and have the sudden reverse flow of atmospheric pressure air washing back into the turbo pump would it cause damage. So either way I think you would be ok.

We run larger rough pumps on most of our 5973/5975 instruments so they are all connected to mains power instead of the instrument itself to prevent overloading the power boards.

[MSCHemist]

For an oil diffusion pump if it is turned on not under a decent vacuum then it will heat up the oil in the presence of oxygen and oxidize and form deposits on the baffle and other parts that are a nightmare to remove. Also Santovac is very expensive.

[benhutcherson]

To add on a bit-Agilent diffusion pump instruments have what's called a convectron gauge, which is used to measure moderate vacuum levels, between the diffusion pump outlet and the foreline hose attachment.

As James alluded to, the convectron is tied into the circuit that controls the diffusion pump. Specifically, the diffusion pump heater won't turn on until the gauge reports 300mTorr or lower, and will also shut down the heater if the foreline pressure goes above that.

I have never used an Agilent turbo instrument, but my Varian 300 will kill the turbo if it doesn't reach 90% of its rated speed in 3 minutes. That instrument has a large glass plate covering the vac manifold, and I've had it happen a few times that I didn't get the cover seated exactly correctly and had a smallish leak. Even with manifold pressures in the ~100 torr range(which is not a hard vacuum, but well below atmospheric pressure) the turbo will typically only hit ~70% and makes a terrible sound doing so. That's a vacuum system that will hit 10^-8 torr with no column flow(low 10^-7 with typical column flows) and can hold 10^-6 with pretty stout flows of methane through the source and argon through the collision cell. With no roughing pump, I can't see it spinning up very fast at all. At least on the Varian, I can monitor not just the pump speed but also the current and drive frequency-it takes almost no power to keep it spinning, but a whole lot when spinning up and even more with a leak.