Plasma Cutting Machine Explosion
EDT was asked to investigate a computerized numerical control/analysis (CNC) plasma cutting machine which had been involved in an explosion. The purpose of the EDT investigation was to determine the cause of the explosion.
EDT Analysis and Findings:
According to reports, moments after a CNC plasma cutting machine was turned on, an explosion took place. The machine was used to cut intricate shapes out of sheet metal of varying thickness and operated in an automatic manner, requiring minimal input from an employee.
The machine consisted of a table base, approximated to be 25-feet long by 10-feet wide. The base contained a sump, which held water. The table design was such that material to be cut would rest on a cage, submerged below the surface of the water. During operation, as the material is cut, the ejected material would collect in the sump. In addition, compressed air is supplied to the sump, which raises or lowers the water level, providing a level of control depending on the material to be cut. The water would also cool the material, increasing the speed at which the material could be cut.
Similar to using a magnifying glass to focus sunlight into a small beam, plasma cutting constricts ionized gases into a focused stream. Ionized gases, in terms of plasma cutting operations, are heated gases which carry the cutting energy to pass through the material. The ionized gases, after passing through and cutting the material, also serve to eject the cut [removed] material, which results in increased cutting speeds. In most cases, for cutting mild steel, oxygen gas is used as the source gas to be ionized.
Explosions, by definition, include production and release of gas(es) under pressure, or an over-pressurization. Explosions, in general, are defined as being chemical or mechanical in nature. With mechanical explosions, the over-pressurization is not due to a chemical reaction, but rather a physical change (a tank rupture, for example). By contrast, chemical explosions are the result of exothermic reactions in which the chemical composition of the fuel is changed. By definition, exothermic reactions are chemical reactions in which energy is released. A chemical explosion, therefore, contains an exothermic reaction which propagates away from a point of initiation.
In terms of the CNC plasma cutting machine, a few key observations were made...
An oxygen supply line, which provided the oxygen gas to be ionized (part of the plasma cutting process) was found to contain multiple fittings that had been displaced prior to the explosion. Upon further investigation, the staff of the facility noted higher-than-expected oxygen gas usage in the weeks leading up to the explosion.
The pressure front and point of initiation of the explosion were observed to have been contained within the sump area of the table. Upon further examination, large quantities of debris were observed within the sump.
No fire was reported with the machine. However, it was reported that the explosion took place moments after the machine had been turned on at the start of the work shift. The explosion sent debris, including large sections of the machine itself, through the roof of the facility, which was around 20-feet in height. There were no reported injuries, however, the machine was damaged beyond repair.
A combustion explosion is one form of a chemical explosion. Combustion, as defined by the National Fire Protection Association, standard NFPA 921, is a chemical process of oxidation that occurs at a rate fast enough to produce heat and (in most cases) light, in the form of either a glow or flame. Typical combustion explosions are characterized by the presence of a fuel with air as an oxidizer. Given that a leaking oxygen supply was noted, the concentration of oxygen gas within the air and within the sump would have been elevated.
As debris accumulates within the sump area, the debris (which consists of ejected metal material) begins oxidation as it is submerged within water. One of the products of the oxidation process, when in the presence of water and oxygen, is hydrogen gas. While the quantity of hydrogen gas produced is small, if the oxidation process is allowed to continue, the hydrogen gas will accumulate. As a consequence of the build-up of debris (from lack of maintenance), the hydrogen gas was unable to vent and accumulated within the sump of the table.
Under standard atmospheric conditions, the minimum concentration of hydrogen gas (percent by volume) that is capable for a fire or explosion to take place is 4.0 percent. In other words, if the concentration of a given quantity of air contains a minimum of 4.0 percent hydrogen gas, that quantity of air will combust in the presence of a competent ignition source. When combined with the elevated quantities of oxygen gas in the area, the minimum concentration of hydrogen gas required for a combustion explosion to take place would have been reduced. A competent ignition source was identified to be molten material, which was ejected as the machine was turned on and began to cut.
The explosion was identified to be a combustion explosion which took place due to a build-up of hydrogen gas, combined with elevated quantities of oxygen gas within the area. The build-up of hydrogen gas took place as a result of improper maintenance with the machine.
About the Author
Michael W. Dickenson, P.E., CFEI, CVFI is a Consulting Engineer in our Nashville, Tennessee Office. Mr. Dickenson provides technical consultation and analysis on commercial, industrial, and residential incidents involving mechanical and heavy equipment. His services include origin and cause, failure analysis, damage assessment, interpretation of codes and standards, and evaluation of fire and explosion origin and cause.. You may contact him for your forensic engineering needs at firstname.lastname@example.org or (615) 915-5255.