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Use of high vacuum pumps with cold traps - hazard alert

A recent incident in the School of Chemistry has highlighted the need for vigilance in adherence to safe operating procedures when using high vacuum pumps with cold traps. Following the investigation of the causes of the incident, the following recommendations have been proposed. It is recognised that, in many areas of the School, these procedures may already be in use. However, this incident has clearly demonstrated that strict observance of these recommendations is necessary to avoid serious injury.

Recommendations

  1. Under no circumstances should a vacuum line with a cold trap be opened to air while the Dewar containing liquid nitrogen is in place as there is a potential for the formation of liquid oxygen in nitrogen-cooled vessels
  2. Safe operating procedure
    • The safe operating procedure outlined on the following page must be adopted by all areas using high vacuum pumps with cold traps
    • All personnel who use high vacuum pumps with cold traps must receive and read a copy of these procedures
    • The safe operating procedure must be readily available in all laboratories that use high vacuum pumps with cold traps
  3. If it is suspected that liquid oxygen has condensed in a cold trap the operator must:
    • Notify all laboratory personnel
    • Wear a face shield, fastened laboratory coat and leather gloves and carefully place an empty Dewar box and safety screen around the cold trap in situ. Do not attempt to remove
    • Allow system to stand overnight vented to air to slowly vaporize liquid oxygen
  4. Training in the use of high vacuum pumps with cold traps. It is recognised that training in the use of high vacuum pumps with cold traps occurs:
    • During third year practical classes, and
    • During safety induction training of honours students

It is, however, essential that the supervisor of students commencing postgraduate studies reinforces training in the use of high vacuum pumps with cold traps. The student must demonstrate their competence to their supervisor in the use of high vacuum pumps with cold traps before undertaking experiments without supervision.

  1. A label will be provided by the Safety Officer to affix to Dewar holders as a visual reminder of correct safety procedures. All laboratory supervisors must ensure that the label is present on all Dewar holders in the laboratories that they supervise

Safe operating procedure - Use of high vacuum pumps with cold traps

To turn on

  1. Check that the cold trap is clean and dry before attaching to the vacuum line
  2. With the Dewar lowered and the system closed to air, turn the vacuum pump on and ensure vacuum performance is leak free (check gauge display)
  3. Half-fill the Dewar with liquid nitrogen and carefully raise into position. Never immerse a cold trap in liquid nitrogen unless under vacuum as liquid oxygen (blue in colour) may condense. The result of this can be a violent explosion caused by re-vaporisation or by oxidation of organic solvents etc
  4. Attach the apparatus to the manifold via thick walled rubber tubing. Evacuate the apparatus

To clean trap or shut system down

  1. Carefully lower the Dewar and turn the pump off
  2. Vent the system to air and wait for any frozen liquid, which may prevent easy removal of the trap, to thaw
  3. Once the cold trap can be removed, put on a face shield and carefully place the cold trap into an empty Dewar holder to help contain any possible explosion. (Remember that liquid oxygen can be shock sensitive)
  4. Carefully transport the cold trap in the Dewar to a fume cupboard and allow to stand overnight behind a safety screen
  5. If the cold trap is likely to have condensed toxic or corrosive vapours, the entire assembly must be placed in the fume cupboard behind a safety screen
  6. Replace with a second clean, dry trap and continue

Under no circumstances should the system be left open to air with the Dewar containing liquid nitrogen in place.

If liquid oxygen is suspected to have been condensed

  1. Notify all laboratory personnel of possible danger
  2. Wear a face shield, fastened laboratory coat and leather gloves
  3. Make sure that the cold trap is no longer in a sealed system. Verify that its contents are either open to the atmosphere or connected to the atmosphere by a pressure relief valve
  4. Carefully place an empty Dewar box and safety screen around the trap in situ. Do not attempt to remove
  5. Leave cold trap to stand overnight vented to air and then carefully re-inspect to check that all suspect material has evaporated

Further information

Please contact OH&S on: