Heating a Sealed Container Results in Over-Pressurization

Key Learnings

Consider the pressures associated with heating sealed containers (e.g. autoclave vials) relative to the pressure ratings of container assemblies and safety release mechanisms.  

The potential for vessel rupture should be anticipated and additional layers of mitigation should be adopted such as heating the containers in a fume hood with the sash closed. 

Conduct a hazard analysis prior to engaging in a new process or non-standard activity.  Changes to an experimental plan require a new look at potential hazards and mitigation methods.

Published research papers usually do not address the health and safety aspects of experiments.  Research project-specific hazard analyses are required.

Effects of Incident

A small sealed vial containing a cellulose sample in 3 mL water popped open during unattended heating.  Some of the liquid content had sprayed onto the underside of a shelf and a nearby dry bath heater.  There were no personnel in the immediate vicinity at the time of the incident, and no one was injured.

Description

A researcher placed 3 mL of water (adjusted to a pH of 10) into an autoclave glass vial (volume 10 mL) containing a non-hazardous material (cellulose).  The researcher sealed the vial with a butyl rubber stopper and foil crimp.  The vial was inserted into a heating block set to 15000 C.  Some minutes after the researcher had left the room, the lab supervisor discovered that the vial’s cap had popped out.  Some of the liquid content had sprayed onto the underside of a shelf and a nearby dry bath heater.  See Figures 1-3 below.  There were no personnel in the immediate vicinity at the time of the incident, and no one was injured.

The experiment was intended to reproduce the results of a published experiment.  However the published paper did not address sample containment during heating or any other safety information. 

The initial research plan was to heat the samples in an autoclave (steam sterilizer).  The experimental plan was changed to a dry bath heater, and a job hazard analysis was conducted addressing the hazards of the new heating method.   The resulting safe job procedure specified that the work be done in a fume hood using a pressure-rated vial. 

Subsequently the researcher moved the experimental activity to a different laboratory.  The experimental work was not discussed with the laboratory supervisor.  Additionally the researcher did not conduct a job hazard analysis for the work planned in the new lab and did not utilize a fume hood.  The research policy for that laboratory dictates that researchers are responsible for analyzing the hazards of their work, but a written job hazard analysis authorized by the lab supervisor was not completed.

Causation

The direct cause for the stopper ejection was pressure build-up in a closed, sealed vial.  The selected vial (with rubber stopper and crimp seal) was not designed for use outside of an autoclave for heating liquids that could boil and vaporize. 

The indirect cause of the incident was failure to conduct a prospective hazard analysis for the laboratory where the work was conducted.  The laboratory supervisor was not consulted.  The changes made during experiment planning were not adequately analyzed for safety.  As a result, the experiment was performed in an inappropriate location and proper hazard mitigation was not incorporated. 

Additionally the risks of heating a liquid in a closed vial were not addressed in the published paper, and consequently not considered by the researcher during planning.

Corrective Actions to Prevent Reoccurrence

  • Conduct hazard analyses prior to engaging in a new process or non-standard activity.  Changes in experiment plan require a new look at potential hazards and mitigation methods.

  • Consult with supervisors to ensure that the proposed work is within the scope of the laboratory and research project.

  • Prior to heating a sealed container, estimate the maximum pressure and compare the estimated value to the pressure rating of the vessel and its safety release mechanism.  This analysis should be part of the job hazard analysis. 

  • Consider alternate heating equipment or heating in a fume hood.  Close the fume hood sash during heating thereby providing enclosure of flying liquids and debris in the event of an over-pressurization. 

  • Depending on the level of the hazard, it may be advisable to place a safety shield in front of the apparatus to protect the operator when it is necessary to open the sash (adding or removing samples, etc.) during the procedure.

Heating devices should also be labeled as "hot" when in use.