Proper Operation of Vacuum Based Equipment
Source: Robert M. Rioux, Ajay Sathe & Zhifeng Chen, Pennsylvania State University, University Park, PA
Vacuum is required for a number of laboratory p…
1. Use of Personal Protective Equipment
- Safety glasses, lab coats, and face shields must be utilized when working with or near a vacuum apparatus.
- A blast shield must be utilized to prevent flying glass or debris resulting from a sudden change in pressure.
2. Use of Proper Tubing and Equipment
- Always use tubing, glassware, and other equipment that is rated for use with vacuum. Improper use can result in material failure and cause explosion/implosions.
- Check the glass and tubing regularly for defects/cuts, as these can easily crack/break under vacuum.
- The exhaust of the vacuum pump must be connected to a fume hood or a scrubbed building exhaust. This is particularly crucial if the vacuum is utilized on a system utilizing corrosive or toxic chemicals.
- Depending on the experiment and extent of vacuum involved, a guard or protective barrier between the operator and vessel under vacuum should be employed. These can be the same type of barriers used to isolate operators from high-pressure equipment.
3. Traps
- Always use a trap between the vacuum source (pump) and the apparatus that utilizes the vacuum. The trap protects the expensive vacuum source from damage in case of accidental leaks or material back flow into the vacuum line.
- The traps also help to prevent vapors/odors from being emitted into the exhaust of the pump.
- The traps are usually cryostatted using dry ice or liquid-nitrogen baths. Extreme care must be taken while utilizing such cryogenic temperatures, and proper PPE must be used to transfer the coolants in and out of the traps.
- Since the use of cryogenic fluids (i.e., liquid nitrogen) for cooling purposes can lead to the liquefaction of oxygen, vacuum systems must be under vacuum before and during operation. After achieving the desired vacuum level, the dewar containing the oil trap can be subsequently filled with liquid nitrogen. After completion of the experiment requiring vacuum, while still under vacuum, remove the Dewar flask from the liquid nitrogen filled trap, let the trap warm to room temperature, and slowly open the system to atmospheric pressure.
- The Dewar flasks themselves are under vacuum and must be handled with utmost precaution as they can instantaneously implode. Always use proper PPE when transporting or working with Dewar flasks.
4. Bleed lines
- The vacuum lines must be slowly bled before disconnecting from the traps and vacuum source. A sudden change in the pressure stresses the materials and can cause premature fracture and explosions.
5. Glassware coating
- Glassware larger than 250 mL that is utilized with vacuum equipment must be shrouded with either tape, netting, or a plastic coating to reduce the chance of flying debris, in case of an explosion. This includes traps, dewars, rotary evaporators, and any other glassware maintained under vacuum.
Vacuum pumps are employed in a wide array of laboratory procedures. Common examples include filtration, drying, degassing, evaporative coating, and mass spectrometry.
Pump equipment must be maintained and operated safely to prevent equipment failures, explosions and chemical release. This video will introduce several common pump designs, discuss common precautions to be observed when setting up vacuum equipment, and demonstrate operational safety.
Let's begin by exploring various pump designs.
In rotary-vane pumps air and other gases are drawn through an inlet by a rotor. The gases are forced via an oil-sealed exhaust, which prevents backflow, to the outlet leaving the system. Rotary vane pumps can generate vacuums of ten to the negative three Torr. These pumps are self-lubricating, but require oil changes and are vulnerable to corrosion by water vapor.
In scroll pumps air passes through an inlet between two eccentric spiral scrolls, one fixed, the other orbiting. The motion compresses the air and pushes it toward the outlet. Vacuums of ten to the negative two Torr can be achieved. Scroll pumps are "dry" mechanisms - they do not require oil or water, but the scrolls must be periodically replaced as they wear down. Scroll pumps and rotary-vane pumps are suitable for distillation, filtration, and degassing.
A water aspirator is another type of pump often found in laboratories. In this type of pump water enters through an inlet to a high-speed nozzle, and exits as a low-pressure fluid jet. The gases are drawn in through a side port and forced to the outlet. Water aspirators produce vacuums of only 10 Torr. Although they easily connect to ordinary sink faucets, they require large amounts of water. Water aspirators are frequently used for drying and extraction.
Lastly, turbomolecular pumps produce ultrahigh vacuum. Air is forced in through alternating stator and turbine blades that drive the gas molecules through the outlet connected to a roughing pump. Turbomolecular pumps can produce vacuums as low as ten to the negative ten Torr, but require another pump to first lower the pressure to 1 Torr. Turbomolecular pumps are used for electron microscopy, crystal growth, and evaporative coating.
Now that you're familiar with the designs, let's examine personal protection and safety measures that should be observed before operating these vacuum pumps.
If possible, operate all vacuum equipment inside a fume hood with the sash lowered. Wear safety goggles and a face shield. These provide protection against chemicals and debris in case the glassware implodes under the vacuum.
Use glassware and equipment rated for use with the expected level of vacuum. Check the glassware and tubing for cracks or other defects. Defective or inappropriate equipment can easily implode under vacuum. Wrap glassware larger than 250 mL in tape, netting, or plastic, as a further precaution against flying debris.
If the procedure is known to generate corrosive vapors, select a pump that can withstand those vapors. Ensure the pump is clean and free from corrosion. For oil pumps, check the oil level and change the oil periodically.
Ensure the pump is level and balanced. Connect the pump outlet to the fume hood exhaust. Securely place tubing inside the hood to prevent the release of chemicals. Ensure all tubing is unrestricted, and that there are no leaks, especially near the flanges.
Now that the vacuum pump is set up, let's examine safety considerations during and after pump operation.
Connect the pump inlet to the glassware via a cold trap. A cold trap is a glass container that protects the pump by freezing volatile organics evacuated from the apparatus.
During the procedure, the cold trap is submerged in dry ice or a Dewar of liquid nitrogen. Use cryogenic protective equipment when handling these coolants.
A potential hazard is the condensation of oxygen in the cold trap to yield highly explosive liquid nitrogen. To prevent its formation, start the vacuum pump and evacuate the apparatus before submerging the cold trap in liquid nitrogen. Never allow the cold trap to contact liquid nitrogen if not under vacuum, and never open the vacuum line to air with the cold trap in place.
Check the cold trap for condensed solvents and liquid oxygen regularly. If necessary empty the cold trap to prevent solvents entering the tubing and vacuum pump. If liquid oxygen, a light blue fluid, is visible, terminate the procedure and call for assistance, but do not stop the vacuum or remove the liquid nitrogen.
Once the procedure is complete, withdraw the cold trap from the coolant and then switch off the pump. Bleed the vacuum lines slowly before disconnecting the cold trap and pump, to prevent sudden pressurization.
You've just watched JoVE's introduction to lab safety for vacuum-based equipment. You should now be familiar with different types of vacuum pumps, their potential hazards, and precautions to be observed to ensure safe operation. As always, thanks for watching!
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Q1: What are the main types of vacuum pumps used in laboratory procedures?
Common laboratory vacuum pumps include rotary-vane pumps, which generate vacuums up to 10^-3 Torr and are self-lubricating; scroll pumps, which achieve 10^-2 Torr and require no oil; water aspirators, which produce 10 Torr and connect easily to faucets; and turbomolecular pumps, which create ultrahigh vacuums of 10^-10 Torr for specialized applications like electron microscopy.
Q2: Why is a cold trap essential when operating vacuum equipment?
A cold trap is a glass container that protects the pump by freezing volatile organics evacuated from the apparatus. Submerged in dry ice or liquid nitrogen during the procedure, it prevents solvents and condensed gases from entering the tubing and damaging the vacuum pump. Regular monitoring prevents liquid oxygen formation, which is highly explosive.
Q3: What personal protective equipment should be worn when operating vacuum pumps?
Operators should wear safety goggles and a face shield to protect against chemicals and debris if glassware implodes under vacuum. Operating all vacuum equipment inside a fume hood with the sash lowered provides additional containment. Proper use of personal protective equipment is critical for preventing injuries from sudden pressure changes or chemical exposure.
Q4: How should glassware be prepared before using vacuum equipment?
Check all glassware and tubing for cracks or defects, as damaged equipment can implode under vacuum. Use glassware rated for the expected vacuum level. Wrap containers larger than 250 mL in tape, netting, or plastic to contain flying debris. Ensure all tubing is unrestricted and free from leaks, especially near flanges.
Q5: What precautions prevent liquid oxygen formation in cold traps?
Start the vacuum pump and evacuate the apparatus before submerging the cold trap in liquid nitrogen. Never allow the cold trap to contact liquid nitrogen if not under vacuum, and never open the vacuum line to air with the cold trap in place. Regularly check for light blue liquid oxygen; if visible, terminate the procedure and call for assistance without stopping the vacuum.
Q6: What maintenance is required for different pump types?
Rotary-vane pumps require regular oil changes and are vulnerable to water vapor corrosion. Scroll pumps are dry mechanisms but need periodic scroll replacement as they wear. All pumps should be kept level, balanced, and clean. For oil pumps, check oil levels regularly and ensure the pump is free from corrosion before operation.
Q7: How should vacuum equipment be safely shut down after use?
After the procedure is complete, withdraw the cold trap from the coolant first, then switch off the pump. Bleed the vacuum lines slowly before disconnecting the cold trap and pump to prevent sudden pressurization. This gradual depressurization protects equipment from damage and prevents dangerous pressure spikes.