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Laminar Flame Speeds Data Collection

Regarding vessels and tubes containing combustible gases or dusts, it is important to acquire knowledge on the conditions under which a fuel and oxidizer could undergo explosive reactions. These conditions are strongly dependent on the pressure and temperature. Given a premixed fuel-oxidizer system at room temperature and ambient pressure, the mixture is essentially unreactive. However, if an ignition source is applied locally and the composition of the mixture is within certain limits (the so-called flammability limits), a region of explosive reaction can propagate through the gaseous mixture due to mainly two phenomena: (1) Temperature rises substantially, (2) High concentration of radicals to form. Characterizing potential explosive reactions is one of the main objectives of hazard assessment. Safeguards to be implemented in process equipment, best process conditions, appropriate prevention and/or mitigation measures, are some of the key purposes to be clarified when handling flammable mixtures. This characterization requires knowledge of several parameters that directly influence on the explosive reaction behavior. One of these parameters is the Laminar Flame Speed, which is one of the key factors that define the kinetics of the reaction. The present paper addresses how to characterize fuel-oxidizer explosive reactions, and highlights the importance of the laminar flame speed concept. The main purpose of this study is to provide reliable data regarding laminar flame speeds with the aim to ensure accurate calculations for hazard assessment purposes. Read more

Making Sense of Combustible Dust PHAs

Process hazard analyses (PHAs) have been conducted for decades in many industries. First conceived at ICI in the 1960s (Kletz, 2009), they have been refined and adapted for various applications, now finding their way into combustible dust hazard management. No matter the industry, the premise is the same, identify hazards, understand their causes and consequences, implement safeguards, and risks will be managed. The CCPS Guidelines for Hazard Evaluation Procedures, Third Edition, states: “A hazard evaluation is an organized effort to identify and analyze the significance of hazardous situations associated with a process or activity.” (Center for Chemical Process Safety, 2008) Keeping these in mind, a simple inclusive approach can be developed and applied. Several NFPA standards on combustible dust contain provisions for conducting process hazard analyses. The newest standard, NFPA 652, Standard on the Fundamentals of Combustible Dust, 2016 Edition, (NFPA, 2016) became effective on September 7, 2015. It requires that dust hazards analyses (DHAs) be completed on existing facilities and large modifications. The legacy standard, NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, 2013 Edition, (NFPA, 2013) contains requirements for process hazard analysis that includes hazard assessments. If the facility falls under an industry- or commodity-specific (dust specific) NFPA standard (e.g., metals, agricultural and food, wood processing and woodworking, and sulfur) different hazard analysis requirements may apply. All of these competing recommendations and requirements can make it difficult to know where to start and what approach to use. This article will summarize the specific requirements in the standards and present some guidance to meet them. The result is a basic, easy to apply approach that will guide implementation of this critical technique. Read more

Predicting Dust Deflagration Behavior Using A Burn Rate Model

NFPA 68 (2013) provides simple calculation methods for sizing vents for combustible dust deflagrations. These methods have limited applicability to process operations. For systems, outside of the applicability limits of NFPA 68, Murphy and Melhem (2016) presented a methodology to use a burn rate model for deflagration vent sizing. It involves estimating a reference laminar burning velocity and then using this velocity to simulate behavior at different operating conditions. The Murphy and Melhem paper introduced the methodology and derived a laminar burning velocity for Niacin from measurements in an explosion severity test in a 20-liter sphere. Laminar burning velocity is a fundamental parameter of the dust. According to Eckhoff (1997), the laminar rate at which a laminar combustion wave or reaction zone propagates relative to the unburnt gas is called the fundamental or laminar burning velocity. Read more

Pressure Relief Design for Reactive Systems

Pressure relief design for reactive systems requires a different way of thinking than standard non-reactive designs. Reactive systems are in a constant state of flux: compositions, mixture properties, temperatures, and pressures are all changing throughout the relief event. Being able to safely vent a reactive system requires careful consideration of the mixture properties before reaction, the pressure and temperature rates during the reaction, and the mixture properties after reaction. To understand how these system properties effect the sizing of pressure relief systems requires careful experimentation, analysis and scale-up application. This presentation will explain a model-based approach to reactive pressure relief system design. Read more

Quickly Develop Chemical Interaction Matrices with SuperChems™

The development of accurate chemical interaction matrices can provide valuable information for the management of potential chemical reactivity hazards. SuperChems™, a component of Process Safety Office®, provides intuitive and easy to use utilities for the rapid development of chemical interaction matrices. These utilities were developed based on known heuristics and rules for the interaction of certain chemical groupings. SuperChems™ also provides additional utilities for the calculation of energy release and stoichiometry of one or more chemical reactions using detailed multiphase chemical equilibrium algorithms and reacting flow dynamics. In addition to thermo-physical and transport properties databanks, SuperChems™ provides hazards databanks where chemical groupings and other reactivity and toxicity data are available for approximately three thousand chemicals. Of particular interest is version 8.5 of the hazards databanks, released in March of 2018. Read more

Reactive Chemical Storage

It is a common practice to insulate storage tanks containing reactive chemicals to protect against fire exposure. While this mitigation technique is appropriate for vessels handling non-reactive chemicals, reactive chemicals storage represents a special challenge and must be examined on a case-by-case basis. For certain classes of reactive chemicals, given a sufficiently long hold time, the insulation will always lead to a runaway reaction. If insulation is to be used, special handling is required in order to insure that after the fire is extinguished, the vessel contents do not reach a temperature that causes a runaway within 48 hours. The 48 hours time limit is selected arbitrarily and should be long enough for most installations to empty the tank contents, inject and circulate additional inhibitor into the tank, cool the tank contents, and/or use the vessel contents in the process. Read more