Fume Hood Fire Involving A Combination Hotplate And Stirrer

Key Learnings

Having dual purpose equipment, such as a combination hotplate and stirrer, provides advantages. However, in some situations dual purpose equipment may contribute to an "error likely situation" when two identical dials turn on two different functions.  In this case, a liquid in a plastic container was being stirred and the heater was turned on, inadvertently. This caused the plastic container to melt allowing the combustible liquid to contact a hot surface resulting in a fire. To prevent similar events when stirring liquids in plastic containers, use a stirrer that does not have a hotplate capability. 

Effects of incident

The incident caused damage to the laboratory fume hood and its contents.  See photo. There were no injuries.  

Description

At approximately 8:00 a.m. on October 22, 2012, a researcher and a co-op student used a combination hotplate/ stirrer and sonicator to suspend ultrafine copper particles in approximately 900 mL of a synthetic heat transfer fluid. The sonication process required monitoring at fifteen-minute intervals.  At the conclusion of the sonication, the researcher asked the co-op student to remove the sonicator from the plastic container and continue stirring the fluid.  At approximately 11:00 a.m. the co-op student removed the sonicator, covered the plastic fluid container, removed it from the hotplate/ stirrer, cleaned the sonicator tip, and placed the sonicator in the rear of the hood.  Next, the co-op student placed the plastic container back on the hotplate/ stirrer, checked to make sure the liquid was being stirred, and left the laboratory immediately thereafter.  

The bottle melted and its contents, a combustible liquid, ignited on the hotplate/ stirrer.  Then at approximately 12:00 noon, two employees passing the lab noticed the fire, and used a hand-held extinguisher to put-out the fire.  The fire was limited to the hotplate platform and initially the smoke was contained within the lab hood.  However the discharge of the extinguisher’s dry chemical forced some of the smoke out of the lab hood, and this smoke activated the room’s fire alarm system.   

 

The photo taken a short time after the fire was extinguished indicated that the hotplate and the stirrer adjustment dials were in the "on" position. The heater control knob was found to be turned to near its maximum heat setting.  

During the sonication activity the hotplate was in the "off" position.  Although it is not known how the hotplate dial was placed in the "on" position, the incident investigation team identified two likely scenarios that could not be substantiated.  Scenario #1 - the hotplate dial was bumped into the "on" position when either the sonicator was removed from the plastic container or during equipment movement within the lab hood; or,

Scenario #2 - the co-op student inadvertently placed the hotplate dial in the "on" position when checking to make sure the liquid was being stirred. The co-op student did not recall checking to make sure that the hotplate dial was in the "off" position prior to leaving the room. Also, the researcher did not check to ensure the hotplate/stirrer was in the proper configuration, i.e., the hotplate was "off" and the stirrer was "on."  

Causation 

Having dual purpose equipment, such as a hotplate and a stirrer combination, provides advantages in some applications. However, in this case it may have contributed to an "error likely situation" because two identical dials (one for each function) turn on the heater and the stirrer. Specifically, a plastic container with combustible liquid inside was placed on the hotplate/ stirrer combination for stirring. Both the stirring and heating function were turned on, inadvertently. Heating caused the plastic container to melt. The melted plastic container released the combustible liquid to the hot surface resulting in a fire on the hotplate platform.

Causal mapping was performed and identified three factors:

  • A plastic container was placed on a hotplate/stirrer to stir a combustible liquid.

  • The hotplate was in the on position.

  • A check of the hotplate/ stirrer configuration was not performed by either the co-op student or the researcher.

Corrective Actions to Prevent Reoccurrence

To address causal factors 1 & 2:


  • Require stirrers without heating capability be used when stirring plastic containers.

  • In areas where hotplate/ stirrers are present, post signage stating that plastic containers shall not be used on hotplate/stirrers.

If the continue the use of the dual purpose hotplate/ stirrer is necessary, then periodic checks of the hotplate stirrer settings is necessary by laboratory personnel.  However, requiring the use of stirrers without the hotplate function is a more acceptable engineering control.  

Additionally the design of the hotplate dial can be improved by having a locking feature, e.g., locking the dial in the "off" position and requiring a lifting of the dial and turning to place the hotplate dial in the "on" position.​