TURBO/FORELINE PUMP TECH NOTES

Turbo pumps, like all high vacuum system components, do not like sudden changes (such as opening a vent valve when the turbo is at high revs). Make any significant pressure/flow changes slowly, and if you can vent it with the turbo spun down to <10%, it will smile at you (you won’t be able to see it, but it is!).

The normal direction of flow of gas through your mass spectrometer is called the forward direction. Gas flows from the source area, through the vacuum chamber and into the high vacuum pump (turbo or diffusion pump), through the vacuum hose and through the foreline pump to atmosphere. Any movement in the opposite direction is called a backstream, and generally bad things happen with this. When you vent a system, you want to follow the same normal forward direction of flow as you bring your system up to atmospheric pressure after the normal venting procedure. Opening the manual vent valve on the front of the manifold, or removing the capillary column in the GC oven will allow atmosphere to satisfy the vacuum in the forward direction through a restrictive means that will take ~ 30 seconds or so to complete. If you were to release the vacuum by opening the vacuum hose connection at the foreline pump then you would send any oil vapor remaining in that hose up through the high vacuum pump and into the manifold. This is a backstream, and you will likely have a noisy baseline on the lower end of your spectral range at least.

Don’t put an inexpensive or marginal foreline pump on your high vacuum system! If they begin to lose their pumping capacity, they will directly affect the overall pumping capacity of your total high vacuum system, resulting in more ion transmission loss (lower signal) and higher background (even lower signal/noise ratio). If they backstream when you shut them down, they will cause minor to extreme contamination of your instrument. Always do an anti-suckback valve test when you change your foreline pump oil (use Inland 45 oil and change at least annually). See the note below for the 5–minute test description. If the foreline pump begins to show any significant signs of difficulty, save your turbo pump and shut down the system. Significant pulses of vacuum from a failing foreline pump can excessively load and crash the turbo pump. Fitting an expensive $60k - $100k GC/MSD system with a general purpose vacuum pump to save some money seems like a big mistake in retrospect, if there has been resultant poor performance and possibly a very expensive repair resulting from a pump backstream or failure. We provide a selection of new Edwards foreline pumps here.

Make sure your table or bench for your “bench-top” MSD is unmovable. If the bench is movable, or (gasp!) on casters, find a way to lock it in place so that it cannot be moved by someone leaning or bumping into the table. Turbo pumps have mechanical and magnetic bearings that end up absorbing tremendous loads when the pumps are spun up and the turbo is moved. The gyroscopic normal of the rotor wants to stay in place while your bench with your MSD sitting on top of it are forced to move. Something will take the grief from this event, and if the bearings are overstressed, the spinning rotor blades will meet up with the non-spinning stator blades, resulting in a rotor crash. All the kinetic energy that took minutes for the pump to develop as it spun up will be dumped into shredding the innards of the turbo in less than a second. It is a remarkably destructive event, sometimes bursting through the protective screen and sending shards of conductive metal up into your quadrupole area. If this happens, please co\nsider having a qualified person clean out and inspect the analyzer section, as any metal fragments can cause serious damage to the quad and electronics. We offer on site services here.

If your turbo pump bearings are starting to make significant noise when spinning up or down, then begin thinking about changing your turbo pump before the bearings fail to keep the rotor spinning safely. If, while the turbo is spun up, you occasionally hear bursts of noise from the pump (bearing noise), think more quickly about changing your turbo pump. If the pump can be exchanged in a non-locked rotor condition (after removal, you can hold the turbo pump body and rotate it around the rotor shaft axis, and the rotor itself can stay still by its own inertia instead of following the pump body – this is called a non-locked rotor repair) it will save significant cost for the rebuild process. A locked rotor condition likely has significant bearing damage, heat related damage and often rotor/stator damage, even though it never completely crashed. In many cases, there is little to be salvaged from the pump because the core components (the expensive parts!) have been significantly damaged.

The foreline pump (sometimes called a “rough” or “roughing” pump) is intended to back up a high vacuum pump (either a turbo pump or a diffusion pump) and provide it with approximately a 10 x E-2 Torr pressure on the high vacuum pumps outlet (in comparison, our Human bodies enjoy 1 atmosphere or 760 Torr of pressure). When the vacuum system is shut down (whether with an intentional vent procedure or more unfortunately with a power failure), the vacuum system as a whole will be relying on an anti-suckback valve in the foreline pump to stop atmosphere from rushing backwards through the foreline pump, taking the warm oil vapor that sits in the first foot or so of vacuum hose above the foreline pump and satisfying the liters of vacuum in the manifold on the other side of the high vacuum pump. The movement backwards through the high vacuum system is called a backstream, and it is a very bad word! Contamination (mild to extreme), and in the worst cases a crashed turbo or a backstreamed diffusion pump (where its normally-contained vapor is distributed liberally throughout the MSD analyzer section) are all memorable events that most people never want to revisit. 

How to perform an Foreline Pump Anti-Suckback Evaluation:

After changing the oil in the foreline pump, cap off the inlet flange fitting on the foreline pump where the vacuum hose normally is mounted, and clamp the cap and an o-ring seal in place. Connect the pump directly to power and turn it on for 30 seconds, then turn it off and then wait a minimum of 5-minutes. You will now open up this cap and look for evidence that it has retained a vacuum by listening for a pop or hiss as atmosphere rushes in. If, after removing the clamp (and lifting the cap carefully) you don’t get any evidence of residual vacuum, make arrangements to change out the foreline pump. It is now a hungry bear on your vacuum system’s landscape and it is not friendly! The next power outage might take you into an expensive stormy situation. We have caps, o-rings, and clamps available on our pump parts page for a minimal cost.

Edwards manufactures turbo, diffusion and foreline pumps. In my nearly 3 decades of experience with HP and Agilent Analytical equipment, I have learned that there is value with investing in proven vacuum equipment. The converse is unfortunately very true as well. For my GC/MSD service contract Customers, I only put in new Edwards foreline pumps, and I do not put in the small, half-sized pumps that initially were touted as offering a “total bench-top” MSD solution, where the small foreline pump could be slid into the back area of the MSD. As a former Agilent Service Engineer, we would shake our heads at the thought that many Customers would forget that there is a pump tucked away back there, let alone keep an eye on the foreline pump oil level. Edwards turbo pump manuals for the EXT250/255 turbo pumps specify the RV3 as the foreline pump solution, and these are a better long-term solution than the double-speed (wonder why all the noise?) small pumps provide. I have wrestled with challenges brought on by rebuilt 2-stage foreline pumps, no matter who rebuilt them, so I have come to the conclusion that the GC/MSD foreline pumps need to be replaced with new ones for better long-term value. No one is really ahead in value if the pump is not properly pulling its weight, and especially if it is developing a serious issue. Turbo pumps cost 2-3 times what the foreline pump does, but saving money on the cheaper element is an expensive gamble. On the other hand, moving up to a full-sized pump that easily backs up the high vacuum pump and only needs oil changes and free breathing of open room air for cooling, is an easy choice for a long-term solution. A $2500 RV3 is less than 5% of the cost of your MSD system and will likely take it through the next decade of life in great form. 

Much of this kind of information and more comes with my “Care & Feeding” courses that I offer for Agilent GC, LC, & GC/MS Customers at their own lab sites. If you have an interest in learning how to live with your analytical instrument in a happier, more effective and productive way, give me a call!