Published on 16 May 2019 by Arun Patel
Following is the list of the types of hazards and hazardous events that should be addressed in searching for the best chemistry. Some key factors to consider relative to process hazards include:
|TOXICITY RELATED HAZARDS||
- Important flammability characteristics are the lower and upper flammability limits, the flash point, the minimum ignition energy, the minimum oxygen concentration, and the autoignition temperature. Values of some of these properties are published for many compounds. These numbers have typically been developed under standardized test conditions. Process conditions may influence their values.
- The fireball resulting from ignition of a cloud of flammable vapor may be relatively long lasting (2-5 seconds), and represents a thermal radiation hazard to those close to the cloud).
- Vapor cloud explosions can cause damaging overpressures.
- A flammable vapor explosion in a vessel initially at atmospheric pressure can create a pressure of 10 atmospheres (NFPA 69, Section 5-3.3.1). A weak process vessel requires substantial vent area for vapor-air explosion relief.
- A flame in a pipeline containing a flammable mixture can transition to a detonation.
- Pool fires are of long duration and the radiation intensity near the pool is high. Storage vessels exposed to a pool fire may explode. Exposure of a storage vessel to a fire is one cause of a BLEVE (boiling liquid expanding vapor explosion). This can occur if the pressure above a mass of liquid at a high temperature and high vapor pressure suddenly drops to ambient by the catastrophic failure of the storage vessel. The shock wave produced by the flashing of the superheated liquid is destructive. If the liquid is flammable, there will likely be a subsequent fireball and an unconfined vapor cloud explosion.
- An unvented runaway reaction in a vessel or a physical over-pressurization of a vessel can cause it to lose its structural integrity. There are numerous methods to calculate the energy of the explosion of such a vessel. The reaction stability is a complex function of temperature, concentration, impurities, and degree of confinement. Knowledge of the reaction onset temperature, the rate of reaction as a function of temperature, and heat of reaction is necessary for analysis of a runaway reaction. Minimum exothermic runaway temperature and runaway reaction consequences can be characterized by thermal stability screening. CCPS presents additional information on testing techniques and apparatus for the reactivity of chemicals.
- Toxicological effects are expressed in terms of the population affected, whether the effect is acute or chronic, the route of entry, dosage, and extent and type of injury.
Tools for Hazards Identification
The identification of hazards includes both a search for those hazards reduced or eliminated by inherently safer design, and a search for hazards controlled by instrumentation and administrative procedures.
There are a number of tools designed to identify and evaluate hazards. Several of these "identification tools" are described below.
Molecular Structure and Compounds
Certain molecular groupings are likely to introduce hazards into a process. Engineer should identify groupings and molecular structures that may introduce these hazards. A search of the open literature will assist in identifying which types of compounds are likely to create potential hazards.
The hazards of new compounds may not be known but the hazards of analogous compounds, or of those with the same or similar molecular groupings may be known. Testing may be necessary to determine hazardous characteristics.
Reactivity of Types of Compounds
Many additional hazards result from the hazardous reactivity of combinations of chemicals. The open literature contains numerous lists of the reactivity of different types of chemical combinations.
The Interaction Matrix
The chemical interaction or reaction matrix is a recognized useful hazard identification tool. Necessary utilities, such as inerting nitrogen, and the materials of construction should be listed as components in a matrix, as should the operator and other populations impacted by the process. Tests or calculations may be appropriate if the effect of an interaction identified in a matrix is unknown. The CHETAH software program developed by ASTM may also assist in this determination. A complete review of the topic of reactivity is also found in CCPS.