A recent near-miss at a National Laboratory involving a large Dewar has highlighted the hazards of transporting large Dewars. 

What happened: A full 230 Liter (L) round base liquid nitrogen (LN2) Dewar was being transported through a building. This particular Dewar was outfitted with 5 casters located directly underneath the vessel (all in working condition) and handles approximately halfway up the side (see Image 1). As the user was pulling the Dewar across the threshold of a door, one of the casters stuck against the threshold bringing the Dewar to an abrupt halt. The more than 500-pound Dewar tipped over, knocking the user to the ground. The user was temporarily pinned beneath the Dewar, but thankfully did not sustain serious injury. There was no visible damage to the Dewar. 

Incident Analysis: Four contributing factors were identified for this incident: (1) the high center of gravity of the Dewar, (2) the positioning of the casters under this type of Dewar, (3) the total weight of the Dewar, and (4) the location of the casters in relationship to the handles and force exerted when pulling the Dewar (Image 1 and 2). When pulling the Dewar via the handle in Image 1, the handle is located directly above a single caster: if the forward motion of the caster is stopped, a vertical pivot point is created whereby the force applied may cause the Dewar to become unbalanced. In Image 2, there are two casters located between the handle at the bottom of the Dewar: in this case, if one caster is stopped the force applied would cause the rest of the casters to pivot sharply around the obstruction and keep rolling.

Corrective Actions: Users were made aware that these types of Dewars should be pulled via the handle in Image 2 or pushed when possible. This type of Dewar will be removed from service. 

Click here to read the full account of this near-miss. 

Application to UC Davis researchers: Researchers transporting large LN2 Dewars should be aware of the proper handling to ensure the Dewar remains stable and upright during transport. If your Dewar is similar to the one highlighted in Image 1, identify the most suitable handle for transport and label the handles accordingly. Dewars transported either on a square caster base (Image 3) or using a cryogen tank cart (Image 4) offer more stability during transport. Pushing the Dewar/cart is preferred to pulling unless the manufacturer instructions state otherwise. It is also recommended that researchers walk the intended route prior to moving the Dewar and note any floor cracks, gaps, or raised transition in doorways. Researchers should ask for additional help to get these objects over or around trouble areas.