Detection, Quantification and Mitigation of Pump Oil Vapors in Vacuum Systems

Backstreaming of Pump Oil Vapors

Systems evacuated with oil-based pumps may be affected by backstreaming of pump oil vapors into the foreline, vacuum chamber, and upstream throughout the system. The act of pump oil vapors moving against the flow of molecules traveling from the vacuum chamber to the pump(s) is referred to as the backstreaming. The oil vapors are referred to as moving upstream and eventually into the vacuum chamber. Flow upstream is greatly mitigated at high pressures when the number of molecule from the chamber outnumbers the number of molecule from the pumping system. At lower operating pressures, oil vapor backstreaming is more pronounced.

This backmigration is different from pump oil vapors that may be pulled into the chamber during a power failure. Oil and vapor can move toward the vacuum chamber if proper start-up and shut down procedures are followed. Issues with the exhaust of an oil-based vacuum pump, where the pressure in the exhaust is higher than recommended by the manufacturer, will also compromise the flow of vapors out of the system and may promote them to move upstream.

Backstreaming can happen with any oil-based pump. The type of oil used in these pumps differs in terms of function but in both cases, there’s a strong possibility that oil from a rotary vane pump, when backstreamed into a diffusion pump, will be heated and reach a vapor pressure high enough to backstream. The rate of pump oil backstreaming into the vacuum system will increase because of this mix of oil types. Depending on the materials used in the vacuum system, gaseous oil vapors will condense on cooler solid surfaces and won’t be returned to the pumping system. Backstreaming is a well-known phenomenon and pumps are typically equipped with accessory devices which help mitigate upstream migration. The entrance of the pump can be augmented with water or traps.

The structure which serves as a location for oil vapors to re-condense is designed by the baffle assembly. If temperature limits are exceeded and the oil is reduced to more volatile components, the oil may break down. There is also the possibility that diffusion pump oil may break down if temperature limits are exceeded and reduce to more volatile components. There is also the possibility that oil vapors from a mechanical pump backing a diffusion pump will move through the diffusion pump and into the vacuum chamber. If the cooling fails and the surfaces get hot, oil vapors will be re-released into the system.

Backstreaming becomes critical around the cross over pressure from rough to high vacuum for mechanical, oil-sealed, rotary vane vacuum pumps. At high gas flow rates, the weight gain measured on a sample was small, at about 10 g/h. The gases rushing from the chamber aren’t enough to mitigate the backstreaming of pump oil vapors. The weight gain from pump oil vapors moving upstream was 100 times greater or 100 g/h. Two opportunities for backstreaming can be found for systems using an oil-sealed mechanical pump. During rough pumping there is a significant opportunity for backstreaming. While it may be counter intuitive, pump oil vapors can work their way through a pump spinning at 60,000 rounds per minute. There is no directionality to the flow of Molecules. In order for pump oil vapors to make their way into the vacuum chamber, they have to be able to find a clear pathway upstream.

Staying above the transitional flow region where 1 sc cm of N2 effectively stops pump oil backstreaming can be accomplished by keeping the inlet pressure of the pump. When the ultimate pressure of the pump is less than 1.33 x 10-2 hPa, some pressure controls may be required.

If the process can tolerate a high operating pressure, an upstream control system which utilizes a purge gas can be used. The purge gas flow can be managed with either a mass flow controller or manually. The purge gas can keep the pump inlet pressure high enough to allow high numbers of purge gas/vapor collisions in order to prevent upstream oil migration.

There is a chance that vacuum pump oil can migrate into a vacuum chamber. This is a serious possibility in the roughing line between a chamber and a mechanical pump and is only mitigated in the high vacuum stream. A material which doesn’t wet to vacuum pump oil, like Teflon, can be used to interrupt the surface migration of vacuum pump oil.

An anti-suck-back valve is a fail-safe device for most modern vane pumps. In the event of an interruption in the normal operation of the pump, this device is designed to close immediately, so that oil and oil vapors aren’t allowed to move through the plumbing and into the chamber. Older, belt-driven pumps don’t have this feature. The vintage designs need to be shut down immediately in order to prevent the backflow that will occur because of the differential pressure between the pump oil case and the foreline.

The anti-suck-back valve can be an effective protection mechanism against up reverse flows of liquid and vaporous oils if it is clean, pristine, and free of particulates from the vacuum system or solidified oil deposits. In the event of a power failure, the valve needs to be properly monitored and maintained. The valve closes after a power-off event. The rubber seal of the anti-suck-back valve will degrade if the pump isn’t properly vented. As an emergency device, it shouldn’t be used as a valve. A pump that isn’t in service should be thrown into the atmosphere.

Installation of a gate valve between the pumping system and the vacuum chamber is a preventative measure. The vacuum chamber can be isolated from the pumping system with a gate valve. Vent gas should be introduced close to the gate valve so that the direction of flow is away from the chamber. Vent gas near the pumps encourages flow in the direction of the vacuum chamber.

Tests for Backstreaming

There are several ways to test a system. In terms of complexity, equipment required, and level of detail, the techniques vary. The following paragraphs detail a few methods.

The coupon method is a highly quantitative method. In this method, clean coupons of optically flat materials, like polished Silicon wafers, are placed around a vacuum system where backstreamed oil, liquid or vapors are likely to condense. The system is evacuated at intervals of 1, 2, 3 or more hours. At each interval, coupons can be collected and analyzed. The use of an ellipsometer which is sensitive to the growth of thin films is one of the analysis options. A scanning electron microscope can be used to look at the condensation of pump oil vapors. The Space Power Facility is a large vacuum system operated by NASA.

Researchers were able to quantify the height of the islands of pump oil after 72 hours using methods and an evaluation model adapted to consider the formation of islands. The coverage area is 70%. The backstreaming rate is an average of several runs. The rate of diffusion pump oil vapor escaping the pumping system increases over time with shorter runs. Deposits of 150 angstroms were found near the inlet of the pump, compared to 34 angstroms at locations distant from the pumps. An effective system can be developed with this knowledge.

The mass gain method can be used to measure pump oil backstreaming. The technique can be used to look at the weight change for an entire chamber or the mass of a coupon. The pump oil backstreaming has been reported at rates on the order of 10 cm x 10 cm x 0.2 cm and has a volume of 30 cm3 and a weight of 236.1 grams. After 100 hours of pumping, the sample will have gained more than 200 cm2 x 6,000 minutes.

The weight gain is on the order of the sample weight. Even with the best available, a highly accurate weight scale will be required to make this measurement.

It is possible to look at how water behaves with a sample. The wettability method looks at the behavior of water on the surface of metal to determine if the sample is clean or contaminated. This won’t provide a quantitative measure, but it does serve as a go/no-go test for backstreaming vacuum pump oils.

Water loving metal surfaces, such as steel, aluminum and brass, are hydraphilic, meaning that water will spread across the surface. The image on the left of the fig shows water sheeting off the clean steel. 9. Water will bead up on a contaminated sample. The surfaces are considered clean for our purposes.

Backstreaming Can Be Mitigated

The forelines are used to provide resistance to the counterflow of pump oil vapors. The back-flow of gases and vapors will be provided by the planned low conductance section. The performance metrics such as ultimate pressure and time-to-pressure must be maintained because of a low conductance section.

• The use of traps between oil sealed pumps and vacuum systems to stop the flow of oil vapors.
• Use inserts of materials, like Teflon, that don’t wet to pump oil and will interrupt pathways for oil migration. Dry pumps are used between chambers and wet pumps.
• Fomblin has shown a 30 % reduction in backstreaming by using specialized pump oils.
• Dry pumps can be used in place of wet pumps. The degradation of tip seals may cause dry scrolls to need a particulate filter.

This article hopes to answer some of your questions, but you might still have more. You can always reach out to the experts at High Vac Depot.