SCHOOL OF CHEMISTRY PROTOCOL
PROTOCOL FOR THE OPERATION OF VACUUM LINES
Hazards:
(1) Chemical hazards associated with the gases on the lines
(2) Explosion hazard from too high a pressure of gas for line
(3) Implosion hazard from collapse of vacuum line or bulbs
(4) Cuts from flying glass associated with (2) and (3)
Ensure COSHH and Risk assessment for the gases already on the vacuum line, or to be put onto the vacuum line, have been performed. Ensure that COSHH and Risk assessment has been performed for the operation of any vacuum pumps and associated oil, and any mercury manometers (if used). Wear safety spectacles at all times. Be careful not to over pressurise vacuum line. Use the vacuum line according to the following protocol.
General:
1. Check vacuum line for the presence of cracks (if made of Pyrex, glass or similar material).
2. Check that any greased taps are easy to move, if not, remove, clean, and re-grease (bringing appropriate part of vacuum line up to atmosphere first)
3. For the vacuum pumps, check the level of oil and that it is clean, that their exhausts are connected to the extraction system, and that water connections to rotary (if there are any) and diffusion pumps are sound. Diffusion pumps should be fitted with flow-switches with fail-safe electrical shut-off. Check that PAT testing of pumps is up to date.
4. In order to prolong the lifetime of the pump oil it is suggested that a trap surrounded by a Dewar of liquid nitrogen (or other coolant) should be positioned between pump and vacuum line to catch noxious materials. The trap should be easily detachable and stored in a fume cupboard overnight.
5. Vacuum line should be in a well ventilated area.
6. The use of mercury diffusion pumps is not recommended. Replace with oil diffusion pumps.
Turning on pumps, and getting ready to use vacuum line:
1. Make sure you are wearing safety spectacles.
2. Check valve or tap between vacuum pump and vacuum line (via trap if used) is closed.
3. Switch on rotary pump, and close gas ballast.
4. If a diffusion pump is also used, switch on cooling water and electrical power, and evacuate diffusion pump with rotary pump. (or follow recommended procedure for particular pump used, for example turbomolecular pump, Roots blower or ion pumps). Damage can be caused to these pumps if they are not first evacuated before switched on).
5. Wait the required time for diffusion pump (or other) to warm up.
6. Take trap from fume cupboard and mount between pumps and vacuum line.
7. Using rotary pump only, evacuate trap.
8. Surround trap by Dewar of liquid nitrogen (or other coolant). DO NOT place liquid nitrogen around a trap that is not being evacuated due to risk of forming liquid oxygen. Check protocol for use of liquid nitrogen. Begin with partly filled Dewar, and once in position, fill Dewar slowly, to prevent spitting. Insulate top of Dewar.
9. Check that pressure gauges (capacitance manometers, transducers, Pirani or Penning gauges) are switched on and in working order.
10. Once pressure in trap region is below ~ 50 mTorr (use suitable pressure gauge, for example Pirani gauge), evacuate using diffusion pump (if fitted).
11. Ensure that pump is able to reach base pressure (value dependent upon whether single or two stage).
12. Remainder of protocol is dependent upon experiment to be performed with vacuum line.
Preparing gas mixtures into Pyrex bulbs:
1. Make sure you are wearing safety glasses.
2. Ensure that bulbs are protected using masking tape, plastic or wire mesh against risk of implosion.
3. If bulb is stored separately, mount onto vacuum line, handle with extreme care (as may be evacuated).
4. Do the Gas Check list (another protocol), to ensure that the cylinder, regulators, tubing and connections are safe and appropriate. If gas is highly toxic or corrosive then cylinder must be stored in vented cupboard or fume cupboard.
5. Ensure that a two stage regulator is used. ON NO ACCOUNT SHOULD A HIGH PRESSURE CYLINDER BE DIRECTLY COUPLED TO THE VACUUM LINE WITHOUT A REGULATOR. The only exception to this rule is for a lecture bottle containing a gas at or close to atmospheric pressure, that is equipped with a main shut off valve and a metering valve.
6. Evacuate section of vacuum line to be used, first with rotary pump, and then with diffusion pump. Check for adequate base pressure (ensure that pressure gauge is mounted) and that there are no leaks. As a rule of thumb, if valve to pump is closed, leak rate should be less that 1 mTorr per 10 seconds, or better. Base pressure with single stage rotary pump typically 50 mTorr, (with two-stage 1 mTorr), if used in conjunction with diffusion pump pressure should be below 1 mTorr. If base pressure too high, or there is a leak, get pumps checked or find leaks before proceeding.
7. Evacuate Pyrex bulb. Use rotary pump only until pressure reached low enough pressure, then with diffusion pump (diffusion pumps can be damaged if exposed to atmosphere pressure when switched on).
8. Isolate bulb using tap.
9. Attach suitable fitting between tubing from cylinder regulator and vacuum line. Make sure there is a metering value (needle valve would be suitable) between the regulator and the vacuum line.
10. Evacuate up to the cylinder head.
11. With main valve to cylinder closed, evacuate the regulator
12. Close all regulator valves.
13. Isolate pump
14. Open tap to bulb. DO THIS FIRST SO THAT ANY PRESSURE SURGE HAS A LARGE VOLUME TO FILL.
15. Adjust regulator valve so that low pressure side of regulator is approximately 1 Bar.
16. Open the metering valve slowly, watching the pressure gauge on the vacuum line, and fill up the bulb to the required pressure. The maximum pressure in the bulb should not exceed about 2 atmospheres (1500 Torr ), and if possible only a pressure up to 1 atmosphere should be used.
17. Close the tap to the bulb.
18. Isolate the cylinder by closing main valve, and evacuate right up to the main valve. THIS IS IMPORTANT, RISK ASSOCIATED WITH TOXIC GAS REMAINING IN REGULATOR IF METERING TAP OPENED INADVERTENTLY, OR GAS REMAINING IN TUBING WHEN DISCONNECTED.
19. Close all regulator valves, disconnect regulator from tubing and return cylinder to correct storage.
20. Depending upon the requirement, a mixture of gases may be required. If this is the case, it is important that none of the first gas is lost when adding the second. Normally gas X is contained in a dilute mixture with Y. Once X is admitted to the bulb to the required pressure using the above procedure, then gas Y must be added using a different procedure. With the bulb valve closed, a region of the vacuum line adjacent to the bulb (but less than the volume of the bulb), is filled with Y using a similar procedure to above. Once the cylinder is isolated, the tap to the bulb is opened quickly and gas rushes into the bulb, the pressure is noted quickly, and the tap bulb is closed. The adjacent section of the vacuum line is filled with gas Y again and the procedure repeated until the total pressure in the bulb is as required.
21. Freeze-thaw cycles or other purification methods can be applied to the gas in the bulb.
Shutting down the vacuum line:
22. Evacuate the vacuum line to base pressure.
23. Isolate the trap from the vacuum line.
24. Isolate the pump from the trap.
25. Carefully break the vacuum seal with the trap, and transfer the trap, STILL IN THE DEWAR, to the fume cupboard
26. Remove the trap from the Dewar and allow to warm up in the fume cupboard. DO NOT LEAVE TRAP IN DEWAR
27. Switch off the diffusion pump (if applicable), and allow to cool for until cool (at least 15 minutes).
28. Switch off the cooling water to the diffusion pump
29. Isolate the diffusion pump
30. Open the gas ballast (to avoid oil suck back) and switch off the rotary pump.