Whatever you get for the fore pump speed from Eq. (7.11), it's a good idea to use a pump with at least double that figure to accommodate pressure fluctuations and make up for line impedance within the vacuum system.

In summation, the following suggestions should be considered when selecting a mechanical pump:

1.For table-top demonstrations and/or small experiments, a direct-drive pump is excellent. Its portability, low vibration, and generally quiet operation is important.

2.For general experimentation, large manifold systems, and industrial operations, direct-drive pumps are also strongly recommended. If, however, you are pumping a dirty environment, belt-driven mechanical pumps should be considered because their greater oil chambers can better tolerate contamination (see Sec. 7.3.7). They can easily be placed in pans (to catch any accidental oil leaks) on the floor, and they can be left on for days and/or weeks of continuous operation.

3.Double-vane pumps have the least amount of vibration.

4.Piston pumps are not made with the same close tolerances as vane pumps, so they are more forgiving for particulate matter developing within the pump oil. They also exhibit greater performance than other pumps of comparable size.

The size of the pump, however, is still up to the needs and demands of the user.

7.3.4 Connection, Use, Maintenance, and Safety

At first glance, the use of a mechanical pump seems so straightforward that little needs to be said—and for the most part that's true. However, failure to observe some simple rules can result (at least) in inefficient running of the pump and (at worst) pump destruction.

Connection. Always place your pumps in some type of tray so that any leaks will be contained and will not create slick spots.* This placement is especially important if the pump is positioned on the floor. By placing the pump on the floor (and in a tray), any vibration effects from the pump on instruments sitting on lab benches will be limited.

The most efficient attachment of a pump to a vacuum system is with a tube as short in length and as large in diameter as practical. Unfortunately, it is not uncommon to see five-feet of vacuum hose between the mechanical pump and the

* This is not meant to imply that all pumps leak, and it is extremely unlikely that a new pump would leak. However, old pumps may leak, and any time the pump oil is changed, there is a chance for spillage.

350 Vacuum Systems

vacuum system coiled and doubled over on itself. This setup is not only a waste of tubing, but it may cause a decrease in pumping efficiency.

Pump accessibility is equally important. The sight gauge (see Fig. 7.15; pump belt, exhaust line, electrical connections, and oil drain should be easily accessible. If there is any chance that there could be particulate matter coming from the system, filter traps* should be placed at the pump intake.

All exhaust from a mechanical pump should be vented to a fume hood regardless of the room's ventilation quality or the type of pumped gases. Each time you bring new samples into vacuum conditions, your system is pumping at atmospheric pressured Because pump oils have low vapor pressures, and pump oils themselves are considered nontoxic, there is little concern for breathing pump oil mist. However, there may be dangers from trapped vapors within the pump oils. Regardless, there is little reason to breathe the pump oil mist if it can be avoided. Check with the manufacturer or distributor of your pump for an oil mist filter for your pump. If you use a condensate trap, be sure you position your exhaust line so that material does not drain back into the pump (see Fig. 7.14).

Use. When you first start a mechanical pump, a gurgling noise will sound. This noise is to be expected when a mechanical pump is pumping against atmospheric pressure and should subside in a few seconds to minutes depending on the size of the system. If the sound does not change, several causes may be present: There may be a stopcock (or valve) on the system open to the atmosphere, part of the system may have been broken off, or the lower portion of a cold trap may have slipped off the trap into a Dewar. Gurgling can also indicate that the pump oil is low. If this case applies, the pump should be stopped, vented, and then filled to the proper level. Finally, a somewhat different gurgling sound can be caused by an open gas ballast. Although this situation may sometimes be confused with an air leak, they are different sounds and one should listen to a gas ballast sound to familiarize themselves with that noise. A gurgling noise from a mechanical pump cannot be caused by a minor (or small) leak, so expect to look for a major reason for the noise. (For small leaks, see Sec. 7.6 on leak detection.)

Before shutting off a mechanical pump, the pump should be vented to atmospheric pressure. Although some mechanical pumps (and essentially all new pumps) have a check valve designed to prevent reverse flow, it is not a good idea to depend on such a valve. If a vacuum is held on the inlet side of a pump with the pump off, the pump oil may be drawn up and into the vacuum line. Therefore, it is best to treat all pumps as if they did not have such a valve. When turning off a mechanical pump, it is best to vent (to the atmosphere) the section of the vacuum line connected to the pump. Your pump should be separated from the rest of the line when venting so that the rest of the line is not exposed to the atmosphere. If

These traps do not significantly affect the flow rate, but without them, destructive wearing of the pump mechanism could result. Contact your pump supplier for filters to fit your specific pump.

f This pumping tends to froth the oil, and the addition of an oil mist filter is imperative.

Pump Pump

Fig. 7.14 The proper orientation of a mechanical pump's exhaust condensate trap. Reprinted from N.S. Harris, "Practical Aspects of Constructing, Operating and Maintaining Rotary Vane and Diffusion-Pumped Systems," Vacuum, Vol. 31, © 1981, p. 176, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

the vacuum system is exposed to the atmosphere, it will take longer to re-pump down because moisture from the air will have absorbed on the walls of the system.

Maintenance. Probably the most common problem with mechanical pumps in labs is inattention to the pump oil. More pumps have died because of LSETCOI (Let Someone Else Take Care Of It) than as a result of any heinous intentional act of destruction. The reason is simple: Changing pump oil is not fun or glamorous. The proper maintenance of a mechanical pump, however, is as necessary as cleaning glassware, keeping a proper lab notebook, and backing up a computer hard disk.

If you are supplied with a used mechanical pump and do not know (or trust) its history, check the oil, both for level and quality. On the side of any mechanical pump is a sight gauge (see Fig. 7.15). The oil level should be between the "high" and "low" sight lines. If it is too high, some oil should be drained from the pump. If it is too low, some oil should be added.

When examining pump oil quality, it can be beneficial to compare the used oil with unused oil. To do this comparison, keep a small sample of pump oil in a closed jar for inspection. The oil in the pump should be discarded if the oil appears cloudy with a whitish, emulsified appearance, is discolored, or (especially) has tar or particulate flecks.

If you have the facilities to check the viscosity of pump oil, the oil should be about 300 SSU at 100°F (SAE 20 W), + 100 SSU.15 If the oil's viscosity is beyond that range, it should be changed.

V J

Fig. 7.15 Check the gauge window on mechanical pumps to see that the oil level

is adequatearlpnnatp.

Fig. 7.16 A suggested log book layout for routine mechanical pump maintenance in the laboratory.

If you cannot check the viscosity of the pump oil, you should be able to crosscheck the oil by the current pressure obtained by the pump vs. its fresh (new) oil pressure. If there is a 100-mtorr, or greater, increase in vacuum using fresh oil as opposed to the current vacuum, change the oil. You can maintain a log book (see Fig. 7.16) of your lab's pump history to have a record of pressure changes over time.

There can be no rule for how often pump oil should be changed. It cannot be done on an "every two to three thousand mile" basis, or even after 20 to 30 hours of operation. It may be necessary to change the oil as often as every day or as seldom as once every several years. It all depends on how often you use your system, what types of chemicals your system is being exposed to, how effective your trapping systems are, and how effectively you are using your gas ballast (if any) (see Sec. 7.3.5). If your system requires an oil change every six months, it does not mean that you only have to check it every six months. Monthly, or even weekly, oil quality checks are advised for any system. A log book, kept in the lab, with sections that note important data (as shown in Fig. 7.16) will help in lab maintenance. (This log book will also be of value when looking for leaks.)

As ironic as it may seem, a pump that is run continuously may require fewer oil changes than a pump that is constantly cycling between atmospheric and vacuum pressures. This condition may be true even if the second pump is run the same number of hours because the cycling pump will be exposed to a greater amount of condensable vapors, which are more likely to severely affect the pump's oil. By continuously running a pump against a load (vacuum) or even better yet, a dry nitrogen leak, the percentage of condensable gases in the pump's oil can be significantly decreased. By removing the condensable gases (especially those that may be particularly corrosive or reactive to the pumps insides) there are less opportunities for internal pump damage.

When changing a mechanical pump's oil, first run the pump for a short time to warm the oil. Warm oil will drain more efficiently from the pump. Open the side valve and let the oil pour directly into a container (use a funnel if necessary). If it is physically possible, tipping the pump may speed oil draining, but it should not be required. The pump may retain small pockets of pump oil within small sections of the pump. These pockets can be emptied out by partially* closing the exhaust

Completely closing the exhaust port can cause the oil to come out in forceful spurts.

Parallel, but not in same plane

Not parallel

Parallel and in same plane

Fig. 7.17 Alignment of mechanical pump belt pulleys.

port and turning the pump by hand. If you start up the pump with a fast click-on and click-off to remove this last bit of oil (as opposed to rotating the pump by hand), the oil may spurt out with surprising force and create a mess.

Return the old oil to where ever used oil is to be returned. Although mechanical pump oil by itself is not toxic, during use it becomes a repository for condensable vapors that may have passed through it. While changing the oil, wear rubber gloves and safety glasses. Never pour used mechanical pump oil down the sink or throw containers of it in the trash. Because the oil could be carrying toxic materials, it should not be sent to a trash dump or landfill. If there are any questions, check with your pump oil supplier, your safety coordinator, and/or your city's Department of Waste Management.

Typically, the new pump oil is poured into the exhaust port, but check with your manual to verify this for your pump. The amount of oil required is dependent on the individual pump and should be stated in the pump manual. If you cannot find the manual and have to "wing it," pour slowly and allow ample time for the oil to show up in the observation port.

If there are any spills when emptying or refilling a pump, clean them up immediately. Pump oils are not toxic, but such spills become slick and slippery and are considered accidents just waiting for someone to walk by.

Emptying the old oil from a pump and refilling the pump with new oil is generally sufficient when changing a pump's oil. However, if pump oil is particularly filthy, empty the old pump oil and refill the pump with a flushing fluid.* Let the pump run for 10 minutes with the flushing fluid against a load, then drain and refill with the correct oil. A flushing fluid can only be used with hydrocarbon pump oils. Alternately, fill the pump halfway and do not let it run for a minute, but no longer (never partially fill a pump, and let the pump run for any extended period of time). A final alternative to filthy pump oil is to fill the pump up with pump oil and let it run. By alternately plugging and unplugging the inlet for some five minutes, the oil will agitate and aid the flushing of any undesirable materials and/or old oils within the pump. If you use this technique, refill and empty the

A flushing fluid is a special hydrocarbon liquid designed specifically for "flushing out" mechanical pumps. It can be obtained from most mechanical pump, and mechanical pump oil manufacturers and suppliers. Do not use a hydrocarbon solvent, such as acetone because you will destroy your pump.