Explosion from Accidental Hydrogen Production
A trained plant operator was blending water, sand, anhydrite, lime, cement, PFA and powdered aluminum in a mixing chamber to produce material for making building blocks. All powders are pre-weighed and dosed automatically via a computer controlled system. The mix is then discharged into a car, from which moulds are filled. During a break in production the operator took the opportunity to wash out the mixer in accordance with his work instructions. He did this by manually emptying out the water hopper into the pre-weigh batcher. However, he neglected to refill the water and when he switched the controls back to automatic, the program software did not highlight that the water container was empty (as the water hopper had been emptied manually, the digital readout still read full). As well as being an ingredient of the mix the water also helps to keep the mixture cool. At the first stage, sand and water are mixed to form a slurry. The powders are then dispensed into the mix. Finally, glycol-coated aluminum powder is added in the form of a slurry which has been mixed elsewhere and maintained in a hopper and kept cool. The aluminum is added in the last few seconds before the mix is dropped into the car. As a result of this, a small amount of hydrogen gas is evolved which disperses from the car into the surrounding ventilated area and out through roof vents.
Event Date
August 7, 2007
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Causes
Cause Comments
The incident arose because insufficient water was added to the batch: the operator manually emptied the mixer and neglected to refill the water; when he switched the controls back to automatic, the program software did not highlight that the water container was empty. As a result of this, a small amount of hydrogen gas was accidentally produced.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
Building block blender.
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
A trained Block Plant Operator, was blending water, sand, anhydrite, lime, cement, PFA and powdered aluminum in a mixing chamber to produce material for making building blocks. All powders are pre-weighed and dosed automatically via a computer controlled system. The mix is then discharged into a car, from which moulds are filled. During a break in production he took the opportunity to wash out the mixer in accordance with his work instructions. He did this by manually emptying out the water hopper into the pre-weigh batcher. However, he neglected to refill the water and when he switched the controls back to automatic, the program software did not highlight that the water container was empty (as the water hopper had been emptied manually, the digital readout still read full). As well as being an ingredient of the mix the water also helps to keep the mixture cool. At the first stage, sand and water are mixed to form a slurry. The powders are then dispensed into the mix. Finally, glycol-coated aluminum powder is added in the form of a slurry which has been mixed elsewhere and maintained in a hopper and kept cool. The aluminum is added in the last few seconds before the mix is dropped into the car. As a result of this, a small amount of hydrogen gas is evolved which disperses from the car into the surrounding ventilated area and out through roof vents.
Consequences
Number of Injured Persons
1
Currency
Lessons Learned
Lessons Learned
Company has taken a number of measures to prevent a reoccurrence including provision of intrinsically safe torches, introduction of daily checks of the vent valve, minor modification to LEV and increased venting throughout the mixing process, lab testing by the aluminum supplier to evaluate system safety with regard to hydrogen generation for all reaction conditions and quantities added (results were system deemed safe) and reprogramming/development of the software to improve both the safety of the operation and operator understanding of warning alarms.
Event Nature
Emergency Action
During the subsequent mixing batch the Block Plant Operator noticed an ``excessive mix time`` alarm on the mixer panel. This alarm ceases the process and dumps the mixer contents. However, the operator noticed an inconsistency in the mixed material discharge, which was a sludge rather than a fluid concrete. He realised that there was a blockage and followed the written procedure for dealing with this by taking manual control and introducing as much water as possible to thin and cool the mix by opening the water hopper valves. During this, a second alarm went off reading ``mix temperature invalid``, indicating that the mix temperature was not falling within the recognised range of 0 - 99 C. The operator went up to the mixer platform to investigate the reasons for the alarms and mix inconsistency. He lifted the mixer inspection hatch. He took an inspection lamp (later revealed to be faulty), the nearest one to hand, to illuminate the interior of the mixer. There was an explosion.
Release Type
Release Substance
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
Event description provided by HSE, original source confidential