New to vacuum; questions about rebuilding a pumping station.

[Mark Aren]

Hello all,

I am a retired electrical engineer and have recently been lucky enough to participate in the clear out of the chemistry department of a local university. Acquired equipment includes a Trivac D4A roughing pump, a Pfeiffer TPH-170 turbopump, TCP-120 + TCS-303 controller and a 1.6 litre stainless column 100mm dia x 200mm high. Also, a 531 controller + thermocouples, and a Varian 863 controller and 525 sensor.

The above claimed to be part of a complete working pumping station, but everything had to be disassembled for safe transportation. I am completely new to vacuum, and the last month has been spent slowly and cautiously working my way through this pile of gear, firstly to understand how it fits together, but more importantly avoiding advanced levels of stupidity which may damage any of it.

The roughing pump and thermocouple stuff seems quite robust, so were directly coupled, and managed to pull 1*10–2 Torr within a minute, indicating that all components are working to some extent. The D4A is specified with an ‘ultimate partial pressure’ of 3*10-4, but I have no idea what this latter phrase means, can someone please explain ‘ultimate partial pressure’ please ?

As the system is re-assembled, Dow Corning high vacuum grease is being used sparingly to coat all [cleaned] O-rings and associated centering rings. For a system sealed using O-rings, is there any significant difference in performance between Dow Corning high vacuum grease, and Apiezon ?

What is the highest vacuum that could normally be managed using a system using O-rings ?

I am very aware that that the turbo pump runs at some ridiculous speed, so I need to make sure I fully understand the rest of system before attempting to power it up. What is a typical safe vacuum when a turbo pump can be started ?

Can anyone tell me if the TCP-120 + TCS-303 controller will automatically start the turbo-pump when a suitable vacuum has been reached, or is all manual, and you have to wait for safe vacuum before pushing the go-button ?

Associated with the TCP-120 is a TSF-010 solenoid controlled vent (normally attached to the turbo pump). With this unit attached to the turbo-pump, the whole column would not attain any reasonable vacuum, and I assumed that the valve was normally closed. Is it actually normally open, or do I need to replace some seals ?

The whole pumping station was marked TSH-172, and while I have a poorly translated manual which has some rough schematics, I would really like a full set of schematics for the turbo controller. Does anyone know if they are available ?

All comments and help appreciated.

- Mark

[Maciek Szymanski]

I will recommend you to read a good high vacuum handbook. For example:

https://www.dmf.unicatt.it/~gavioli/cor ... nology.pdf

This will make you familiar with basic concepts and principles of vacuum measurement, creation, material compatibility and leak detection.

With NBR o-rings you should be able to go down to 10E-6 Tr. with Viton, very clean system, proper materials and pumpdown regime maybe to 10E-8, but that’s rather range for single use soft copper seals.

Regarding the grease - both Apiezon and Dowcorning silicone have similar performance (vapor pressure). Apiezon is hydrocarbon while Dowcorning fluorocarbon, so the chemical compatibility is different. Apiezon can be easily cleaned off with an organic solvent, silicone is not so easy to clean. But generally no grease should be used on vacuum system until it’s absolutely necessary, the connections are designed to hold while complete dry. All mating surfaces of the flanges must be clean without scratches with mirror finish. The interior of the system must be clean, as all evaporating residues may dramatically increase final pressure.

Typical procedure I’m using is washing in hot water with a detergent, rinsing in the deionized water, baking in 120°C and then wiping with ethanol.

Regarding the turbo safety - first I never vent the chamber until the rotor haven’t completely stopped (in the system without turbo isolation gate valve). And second - while mounting the turbo under the chamber looks like the obvious solution it’s in fact the worst option (I think it started by direct replacing the diffusion pumps with turbos). All loose parts will fall directly into the pump! There is usually an inlet mesh, but small chips may pass through it and kill the pump immediately. The best option is mounting the pump over the chamber. Side mounting is the second choice.

[Mark Aren]

Thank you Maciek,

The turbo mounting is the same orientation as was manufactured by Pfeiffer in the TSH 172 in the TSH 172 assembly, although the university attached a new flange to the top. This can easily be replaced with a blanking plate as the venting valve is at the rear of the photograph and mounted on the side of the column, and pointing down.

I will add a fine sieve to this side valve; your comments are well noted, and have downloaded the referenced PDF. I will make my way through the publication in due course.

If anyone else can comment on the operation of the Pfeiffer specific issues about the controller and vent-solenoid above, I will be most grateful.

Regards,

Mark

[Richard Hull]

All mechanical pump companies lie regarding lowest partial pressure. A good used pump will hit 10 microns, (~10-2 torr). A fantastic used pump will go to 2 to 4 microns at the head. Lines and a decent size chamber will typically never pump down below 8-10 microns in real life and then only if it is sealed to perfection. Fore line vacuum is important if 10e-6 torr or better is your goal. Many amateurs here do fusion with a fore line pressures of 20 microns or more and rely on the secondary pump to pull them into the 10e-4 to 10e-5 torr range which is fine for fusion in an amateur fusor.

Richard Hull

[Alan Sailer]

My (not extensive) experience with vacuum stuff si as follows;

1) I used Dow Corning grease on KF o-rings with no problems with a turbo system that was getting down to 0.1 micron.
A guy who I work with does vacuum for his job and hates Dow Corning. He does all of his stuff dry. he also has the
budget to buy brand new parts.

2) Turbo pumps are not as delicate as you might think. Opening the unit to atmosphere when it's at full speed is death.
I don't recommend it but I have stopped turbos many times by venting carefully using a valve. I just listen to the whine
of the pump and never let it get too loud.

You can start a turbo at atmospheric pressure but it will never spin up to full speed.

As Maciek noted, small debris hitting the pump rotor at almost any speed is game over.

3) Pumping anything other than non-condensable gases can mean trouble for either the turbo or the roughing pump.
Iodine will destroy a turbos bearings quickly. Pumping alcohol vapors can screw up the oil in the rough pump and then the pump.
Pumping using the air ballest can help remove solvents in your pump oil.

Foreline traps can be your friend.

Cheers.

[John Futter]

Mark to answer some of the unanswered
orings are fine down to 1 x 10^-8millibar.
Orings do not like being baked out over 70 degrees celsius
The Dow Corning grease is fine - i also use silicon stop cock grease which is cheap and appears to work fine down to 1 x 10^-7 millibar ( I use this to coat viewports with the thinest layer possible to make for easy cleaning)
If your backing pressure is below 10^-1 millibar you can start the turbo, the turbo will stall at around 2 x 10^-1 millibar
Your backing pump figure of 3 x 10 ^-4 is the calculated vacuum possible from the compression ratio of the two stage pump - with continuous running on a clean system low 10 ^-3 millibar region is possible

[Mark Aren]

Thank you everyone.

-Mark