7.3.7 Mechanical Pump Oils

In this section on mechanical pumps, constant attention is placed on the protection and/or maintenance of pump oils. This is because of the incredible demands placed on these oils. Ideally, all mechanical pump oils should:

1.Be thermally stable

2.Be chemically inert

3.Exhibit a low vapor pressure over a wide temperature range

4.Lubricate

5.Maintain the same lubricity and viscosity over a wide temperature range

6.Maintain a vacuum seal between sections

7.Cool heated sections of the pump

8.Trap vapors and paniculate matter from the vacuum system

9.Be compatible (and not interfere) with the environment and provide protection from the environment

10.Be nontoxic

In addition, mechanical pump oils can be specialized for use in specific environments such as those with high-oxygen contents. Some are blended for use in specific types of pumps such as direct-drive pumps, belt-driven pumps, and rotarypiston pumps. As with most things, no single product fits the bill for all circumstances. Thus, there are many varieties, grades, and types of mechanical pump oils.

Various manufacturing processes produce oils with different characteristics. Each of these characteristics varies the oil's tolerance of what it can withstand and still provide safety and protection to the pump. Among the properties that can be altered and enhanced are:

1.Molecular weight: It is better to have a highly refined fluid composed of a narrow range than to have a wide range of molecular weight.

2.Vapor Pressure: Although the vapor pressure of an oil itself is seldom a limiting factor of what the vapor pressure in use will be, it still provides a benchmark level to compare various oils.*

3.Pour point: This property is critical if the pump will be used in cold environments. An oil with too high a pour point could prevent a pump from starting and require the pump to be artificially heated to begin

Regardless of the pump oil used, the vapor pressure of a pump oil (off the shelf) is seldom the limiting factor in the potential vacuum a mechanical pump can achieve. For example, when diffusion pump oils are used in mechanical pumps, they tend to exhibit a decreased ability to pull a vacuum due to the partial pressure of the dissolved gases within the pump oil. Thus, due to the heat of the pump and contamination in the oil, the vapor pressure of the oil can be greater than its stated pressure by a factor of 10 to 100.

operation. On the other hand, an oil with too low a pour point may burn off if used in too hot an environment.

4.Viscosity: Pumps with tight tolerances, such as vane pumps, require a low viscosity oil, whereas pumps with low tolerances, such as piston pumps, require a higher viscosity oil.

5.Fire point: Any pumping of pure oxygen (or a high percentage oxygen mixture) can cause most oils to explode or degrade rapidly. Therefore, pumping these materials requires a very high (or nonexistent) fire point.

With all the varieties of mechanical pump oils available, there is one further complication: Pump manufacturers are selective about the oil that goes into their pumps. Most pump manufacturers want to sell you their brand of pump oil(s). This tactic is not so much greedy as it is an attempt to ensure that (what they feel is) a high-quality oil goes into (what they believe is) a high-quality product. It is the best way they have to prevent you from destroying your pump. If there are any questions about the selection of pump oils, check with your pump's manufacturer. The manufacturer usually would rather help you for free than have one of his or her pumps look bad—even if it is your (the user's) fault! However, be warned: If you place a non-approved oil into a pump, a manufacturer can void any warranty associated with the pump! If you have been advised to use a pump oil on a new pump that is different than what the operating instructions recommend, be sure to find out (in writing) what effects this oil may have on the warranty.

There should be no problem when changing pump oils to remove the old, and simply replace it with a fresh amount of the same oil. However, it is not always possible to simply pour one type out and pour a different one in. Some mechanical pump oils are specifically designed for specific pump designs, while others are designed for specific gas types and conditions. Likewise, you may need to change pump oil types when changing a pump's service to a different type of service (e.g., standard vacuum pumping to oxygen pumping). Depending on the intended service, it may be necessary for a pump to be returned to the manufacturer and rebuilt. For example, if you need to pump pure oxygen, a mechanical pump must be altered at a factory. This alteration will include changing the lubricating oils in the bearings and seals; if this is not done, an explosion could occure. In addition, because some oils are more viscous than others, the pump may need alteration to increase or decrease clearances between parts.

Just like auto engine oil, mechanical pump oil breaks down and needs to be replaced. And, similar to synthetic auto engine oils, mechanical pump oils should be replaced over a period of time, even if they have not broken down. Although the quality of an oil may not have diminished, replacing old oil also removes particulate matter and collected chemicals. Paniculate matter, if soft, can turn pump oil into a gel. Because the viscosity of this gel will be greater than the original oil, the pump will be operating at a higher temperature and is more likely to fail. Hard paniculate matter can scratch the insides of a pump, causing leaks and decreasing performance potential. Chemicals typically get trapped in a pump by entering as

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vapors that were not stopped by a trap. Vapors in the vacuum state will condense out during the compression stage within a pump (using a gas ballast can limit the effects of this occurring). Once condensed out, a condensed chemical's vapor pressure can be sufficiently high to lower the pump's efficiency. Condensed vapors can also react with pump oil to form gum deposits or simply degrade the oil. In addition, although pump oil may be impervious to acids, the pump containing the oil is not likely to be as resistant. Thus, if your work produces acid fumes, periodically test the oil's pH.

Direct-drive pumps can successfully run with less oil a they are designed to be used with. This is because their internal oil pumps successfully force oil into operating parts that would otherwise be insufficiently fed by gravity in a belt-driven pump. However, it is not wise to depend on this approach to pump maintenance because, as discussed in the beginning of this section, pump oil does more than just the lubrication of parts and maintenance of the vacuum seal between moving parts. Pump oil is also used to cool pump sections and trap vapors and particulate material. Therefore, it is certainly not wise to run a direct-drive pump with less than manufacturer's recommended oil volumes on an actively used vacuum system. If your vacuum system has particularly large concentrations of vapors, it may be better to use a belt-driven pump because of their greater oil capacity. A given amount of contamination in the larger oil tank of a direct-drive pump results in a smaller percentage of contamination.

7.3.8 The Various Mechanical Pump Oils

There are five primary types of mechanical pump oil.* The advantages and disadvantages of these various materials are explained below and in Table 7.7.

The Hydrocarbon Oils. These oils are the most common and least expensive. They are composed of paraffinic, naphthenic, and aromatic hydrocarbons and provide excellent sealing qualities. When these oils are properly distilled, they can exhibit low vapor pressures (10~4 to 10~6 torr). However, they oxidize quickly when heated, have a tendency to form sludge and tars, and are likely to foam if pumping against atmospheric pressure (as opposed to a vacuum).

By fractionating (or vacuum-distilling and double vacuum-distilling) the oils,* finer grades have been developed that exhibit widely varying properties. Furthermore, oils from different geographic locations have different (both good and bad) properties. Blends of these oils can create new oils of wildly varying compositions and qualities.

Many of the negative properties of hydrocarbon oils can be overcome by the use of additives which "inhibit oxidation, reduce foaming, disperse contamination,

Despite their high success in diffusion pumps, the silicone oils do not have the lubricity characteristics that are required for use within mechanical pumps. Some mechanical pump oils with a silicon base have been formulated, but they have not exhibited any improvements over other oils currently in use.

* Vacuum-distilled grades are often called mineral oils.