Last time we focused on the principles of operation of oil sealed rotary vane pumps including basic pump design, and pump oil. We now continue that discussion focusing on operational features and the inner workings of these pumps.
Single Stage vs Two Stage Pumps
One of the limiting factors of a rotary vane pump is the Duo Seal, which is the oil filled non-contact seal in the small 0.025 mm (0.001”) space between the rotor and the stator at the top of the pump. In a single stage rotary vane pump, the pressure difference across the seal can approach 100,000:1 (1000 mbar vs. .01 mbar). Above this, the duo seal will start to leak oil from the high-pressure side to the low-pressure side (Fig. 1). This creates backstreaming, that is, the movement of pumping oil back into the vacuum furnace chamber.
|Figure 1 | Rotary vane pump Duo Seal (courtesy of Edwards Vacuum)|
In order to generate a higher vacuum using a rotary vane pump, a two-stage pump design is used. The two-stage pump utilizes two rotary vane pumps in series (Fig. 2). The outlet of the high-vacuum stage is piped to the inlet of the low-vacuum stage. Since the inlet to the low-vacuum stage is considerably lower than atmospheric pressure, this design results in a lower pressure at the outlet of the high-vacuum stage, as opposed to the single-stage design, which experiences atmospheric pressure at the outlet. This reduces the pressure differential across the Duo Seal and the vanes in the high-vacuum stage, allowing it to operate at a higher inlet pressure. The two-stage rotary vane pump can achieve an inlet pressure of 3 x 10-3 Torr (4 x 10-3 mbar). There is no exhaust valve located between the high-vacuum and low-vacuum stages, but there is one at the outlet of the low-vacuum stage.
|Figure 2 | Two-stage rotary vane pump concept (courtesy of Edwards Vacuum)
Some two-stage rotary vane pumps are provided with the ability to operate in either high-throughput mode or high-vacuum mode. The mode is selected by turning a knob located on the pump control panel. The mode selector controls the flow of pressurized oil to the high vacuum stage of the pump, which changes the characteristics of the pump. In high-throughput mode the oil pressure (and therefore flow) is increased, and in high-vacuum mode the oil flow is decreased. This feature overcomes the problem at higher pressures of an insufficient pressure differential across the low-vacuum stage thus ensuring an adequate supply of oil into the high-vacuum stage (which is later in the lubrication circuit). When running at higher vacuum, this issue does not occur. The pressure difference is sufficient to provide adequate lubrication in the high-vacuum stage.
High-throughput mode is used to provide faster drawdown at inlet pressures greater than approximately 38 Torr (50 mbar). A typical cycle might start out in high-throughput mode to evacuate the vacuum chamber as quickly as possible, then be switched into high-vacuum mode at 38 Torr (50 mbar) to achieve ultimate vacuum. High-throughput mode is also used to pump condensable (dirty) vapor, and to decontaminate the pump oil when necessary. High-vacuum mode can only be used when the pumped gases are clean.
With the combination of mode selection and gas-ballast (see below) the pump performance can be optimized. A wide range of pumping characteristics (i.e., pressure versus flow performance) is achievable through the selection of these two modes in combination with (high, low, or no) gas ballasting (Table 1). The mode selector switch can be actuated while the pump is on or off, and some larger pumps switch between modes automatically.
|Table 1 | Effect of mode selection and gas ballast control on Edwards models RV3, RV5, RV8 and RV12 pump performance (courtesy of Edwards Vacuum)|
Isolation (Anti-Suckback) Valve
Rotary vane pumps are often equipped with an inlet isolation valve (aka anti-suckback or vacuum safety valve). As the name implies, this device closes when pumping stops, preventing gas (or air) from being sucked back into the vacuum chamber through the pump. When pumping is stopped and the valve closes, air enters the pump outlet, equalizing the pressure inside the pump with that outside the pump outlet. This prevents oil in the casing from filling the stator chambers. When the pump is turned back on, the valve does not open immediately but is delayed until the pressure in the pump has reached the approximate pressure in the vacuum chamber, thereby also preventing suckback while the pump is reaching pressure. This isolation valve (c.f., Oil Sealed Rotary Vane Pumps Part 1) is hydraulically actuated. In two-stage rotary vane pumps the isolation valve is located on the high-vacuum stage.
Moisture and vaporized contaminants (typically from dirty work introduced into the vacuum chamber) will find their way into the pump oil and interfere with efficient pump operation. As a result, it becomes difficult to reach ultimate vacuum and takes longer and longer times to do so as the oil loses its ability to provide a seal between the vanes and stator, and at the Duo Seal, resulting in reduced pumping efficiency. Also, the properties of the oil change, causing insufficient lubrication and introduce the possibility of internal corrosion. To avoid these problems, a simple yet highly effective gas ballast (aka gas ballasting) operation is used.
Gas ballasting is the injection of a non-condensable gas (e.g., nitrogen or air) into a rotary vane pump during the compression stage resulting in reduced condensation. The ballast gas is injected through a one-way (aka “gas ballast”) valve, located at the top of the pump (Fig. 3). One way to think of the use of a gas ballast is that opening the gas ballast valve deliberately destroys the efficiency of the pump, which in turn causes the pump oil to heat up and drive moisture and other volatile vapors out of the oil where they can be sent up the vent stack.
The theory behind this is that the injected gas dilutes the vapor in the pumped gas so that the partial pressure of the vapor never reaches saturation during compression. Injection starts at the beginning of the compression cycle. After it starts, the pump rotor continues to rotate, increasing the pressure generated in the pump, which forces the one way ballast valve closed, but not until sufficient dilution has occurred. As the rotor continues to turn, the pump
discharge valve is forced open and discharges the mixture of pumped gas, ballast gas and vapor.
|Figure 3 | Principle of gas ballasting (courtesy of Edwards Vacuum)
In addition to diluting the condensable vapors, the gas ballast raises the temperature of the process gas 10 – 20°C (18 – 36°F), which further inhibits condensation. In addition to use during normal operation to prevent vapor
condensation, gas ballast is also used to decontaminate pump oil that has already been contaminated with condensed vapor. This can take several hours with badly contaminated pumps.
It is recommended that a vacuum pump be ballasted at least once a day, typically on startup of the equipment and before the first load is run. This should be done for a minimum of 30 minutes. In some critical applications or where dirty work is being run and significant outgassing is anticipated, it is good practice to ballast the pump after each cycle for 20 to 30 minutes between runs. This helps decontaminate the oil after each operating cycle.
The choice of air or nitrogen as the ballast gas is dependent on the characteristics of the process gas being pumped from the vacuum chamber. As an inert gas, nitrogen is used when moisture, oxygen or hydrogen contained in the air would react with the process gases. In most other cases, air is the preferred ballast gas.
The primary disadvantage of gas ballasting is that while in use it reduces the ultimate vacuum of the pump (Fig. 4). It also increases the rate of oil discharged from the pump. The volume of gas created by ballasting is selectable on most pumps with a low flow and a high flow feature available. The negative effect of ballast on ultimate vacuum and oil loss is less in the low flow mode than during high flow.
|Figure 4 | Effect of gas ballast on pumping speed (courtesy of Edwards Vacuum)
In addition to gas ballast, another approach to pumping gases containing condensed vapors or moisture is to remove them prior to entering the pump. This is done via a cold trap (aka inlet condenser) located at the pump inlet.
A condenser (Fig. 5) works by cooling the pumped gas below the condensation temperature of the vapors (moisture and others) carried in the gas. The vapors turn into a liquid and collect on the interior surfaces of the heat exchanger inside the condenser, preventing them from entering the pump. The resulting condensate is collected and removed. Inlet condensers can be water cooled using a shell and tube heat exchanger, or cooled with refrigerant or cryogens such as liquid nitrogen.
The condenser also helps minimize backstreaming of oil vapors from the pump into the vacuum chamber. Even with an inlet condenser, a rotary pump can still accumulate condensed contaminants in the oil. Therefore, often both an inlet condenser and a gas ballast are used, for maximum vapor handling capability with minimum sacrifice of pumping capacity.
|Figure 5 | Condenser located at the pump inlet (courtesy of Edwards Vacuum)
In any vacuum system with a pressure lower than 0.75 Torr (10-1 mbar) there is a potential for backstreaming, which is the migration of oil vapors against the flow of pumped gas, and back into the vacuum furnace chamber (Fig. 6). Backstreaming (c.f. Oil Sealed Rotary Vane Pumps Part 1) is a result of vaporization of the oil under low pressure. It causes contamination, as the oil deposits as a film on interior furnace surfaces and can interfere with the process being performed.
|Figure 6 | Back migration of oil vapors from the rotary vane pump (courtesy of Edwards Vacuum)|
One way to prevent backstreaming is the use of a foreline trap (Fig. 7), which is a molecular sieve installed on the inlet of the pump. It is filled with activated alumina (also referred to as sorbent), which traps and collects the oil vapors. The alumina media is replaceable and must be changed at the same interval as the pump oil, typically every 6 months although this depends on usage frequency. The foreline trap will stop 99% of the oil vapors.
|Figure 7 | Foreline trap (courtesy of Edwards Vacuum)
The alumina also will remove moisture from the foreline and collect it as liquid water. Over time, this will slow pumpdown as the alumina becomes clogged with water. For this reason, when moisture is present in the pumped gas it is recommended that an inlet condenser be used with the foreline trap.
When a foreline trap is used, it is necessary to bypass the trap (Fig. 8) during roughing, which is the period of high flow initial pumpdown at higher pressures. Only after roughing is complete and higher vacuum is achieved is backstreaming a concern. At this time the gas is then routed through the foreline trap. This bypass arrangement prevents the alumina from quickly and unnecessarily becoming clogged during the high flow of gas and vapors pumped during roughing.
|Figure 8 | Foreline trap bypass arrangement (courtesy of Edwards Vacuum)
Although foreline traps are common, the first defense against backstreaming is to use a pump oil with a low vapor pressure, which is less prone to vaporization and therefore less likely to backstream.
In addition to the foreline trap, other accessories are used on the pump inlet side to capture moisture, vapors and solid contaminants. Among these are the desiccant trap, zeolite trap, catalytic trap, catch pot and dust trap. The selection of trap(s) is based on the specific application and the constituents of the pumped gas.
More could and perhaps should be said on the subject of oil sealed rotary vane pumps but the key is to recognize their importance to the overall performance of your vacuum furnace. Know how they operate and how to use them properly. Change the pump oil every month (300 hours) and perform the other steps necessary to take care of them and you will be rewarded with years of trouble-free pump operation.
- Herring, Daniel H., Vacuum Heat Treatment, Volume I, BNP Media, 2012.
- Mr. David Sobiegray, Edwards Vacuum, technical contributions and private correspondence.