Flammable Vapor Deflagration Unraveled in Industrial Oven

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An incident occurred in an oven that was drying a product that contained a flammable vapor. The deflagration was controlled by explosion vents on the oven and there were not injuries.

An incident occurred in an oven that was drying a product that contained a flammable vapor. The deflagration was controlled by explosion vents on the oven and there were not injuries. This event is instructive, however, in that the cause was not straight forward. The oven was an indirect heated unit with a steam heat exchanger mounted inside the oven chamber. The steam pressure at the oven was approximately 100 psig. All of the flammable vapors that are evaporated in this oven had been tested for Minimum Auto Ignition Temperature and the values were found to be above 3380 F, the saturation temperature for 100 psig steam. In normal operation the concentration of flammable vapors is constantly monitored and there are automatic interlocks to prevent exceeding 25% of the Lower Flammable Limit of the vapor.

The deflagration occurred after a power outage that resulted in a loss of airflow through the oven. Air flow is the critical control element for limiting flammable vapor concentration. At the time of the event, several minutes after the power loss, temperature and the LFL were still within the normal range for the process. There was no movement of product, nor of the conveyor in the oven, so static electrical energy did not seem a likely ignition source.

Bill Stevenson, General Manager of CV Technology, a leading explosion protection and consulting company, states, “Clearly, the identification of an ignition source is the key to unraveling this mystery.” The hot spot in the oven is the steam heat exchanger, but the temperature did not seem sufficient to ignite the vapors as the MAIT was well below the point for ignition. Or were they? As Stevenson explains, “Well, it turns out that steam in this plant is generated at 2500 F, which has a temperature of over 4000 F, very close to enough to ignite the vapor. The pressure is reduced through a pressure reducing valve. There is a constant enthalpy across a PRV and so even though the pressure was reduced, the temperature of the steam was not.”

CV Technology helps companies identify their risk for explosions by careful calculation of materials involved and scenarios unique to each given application. As Stevenson points out; another key to the mystery can be found by considering the nature of ignition. “It is well known that as temperature goes up the rate of chemical reactions follows. In the mixture of vapors in the stagnant oven leading up to the event it is very possible that intermediate products such as radicals and active particles were forming and increasing in concentration. The formation of these intermediate products changes the ignition process with the result that ignition temperature can be well below those determined in standardized ignition temperature tests.”

The combination of the high temperature of the steam heat exchanger and the loss of air flow in the oven resulted a relatively prolonged contact time of the flammable mixture with a surface at an elevated temperature. In this situation the standard ignition temperature for the vapors should not have been relied upon. Rather, it would be necessary to desuperheat the steam to reduce the temperature at the steam heat exchanger and to ensure that there is adequate airflow even in the event of a power outage. It should also be appreciated that this series of events was very unusual and would have been very difficult to predict in advance of the incident.

CV Technology combines a legacy of experienced explosion consulting with revolutionary and completely unique explosion prevention and explosion protection technologies to specialize in the prevention, protection, and elimination of dust explosion hazards in all industries which process powders and dry bulk materials.

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CV TECHNOLOGY, INC

2580 Metrocentre Boulevard

Suite 1

West Palm Beach, FL 33407

USA

Phone: (561) 683 - 1200

Web: http://www.cvtechnology.com

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