Explosion protection lower explosive limit

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

For combustible dusts, the explosibility limits do not have the same meaning as with flammable gases and flammable vapors, owing to the interaction between dust layers and suspended dust. This protective measure can, for example, be used when dust deposits are avoided in operating areas or in the air stream of clean air lines after filter installations WTiere in normal operation the lower explosibility limit is not reached. However, dust deposits must be anticipated with time. When these dust deposits are whirled up in the air, an explosion hazard can arise. Such a hazard can be avoided by regular cleaning. The dust can be extracted directly at its point of origin by suitable ventilation measures. 

A number of accidents have occurred as a result of the discharge of static electricity. The latter can be generated even by the flow of fluids. To provide additional protection against explosion or fire due to the discharge of static electricity, determine what temperature an acetone drum filling operation would have to maintain so that flammable mixtures would not be produced by vapor-air mixtures in equilibriurr with the liquids. The lower flammable limit (LFL) for acetone vapor in air is 2.55 percent by volume. 

There can also be a flammability limit associated with dust clouds. The flammability limits of combustible dusts are reported as the minimum explosion concentrations. The upper explosion limits for dust clouds have not been determined due to experimental difficulties. In the fourteenth edition of the Fire Protection Handbook [National Fire Protection Association (NFPA), Boston, MA, 1975], numerous results from the U.S. Bureau of Mines reports are listed. These results were obtained with dusts 74 fim or smaller. It should be noted that variations in minimum explosive concentrations will occur with change in particle diameter, i.e., the minimum explosive concentration is lowered as the diameter of the particle decreases. Other conditions which affect this limit are sample purity, oxygen concentration, strength of ignition source, turbulence, and uniformity of the dispersion. The NFPA tabulation is most extensive and includes data for dusts from agricultural materials, carbonaceous matter, chemicals, drugs, dyes, metals, pesticides, and various plastic resins and molding compounds. Except for metal... 

IDLH Immediately Dangerous to Life or Health levels indicate that exposure to the listed concentrations of airborne contaminants is likely to cause death, immediate or delayed permanent adverse health effects, or prevent escape from the contaminated environment. IDLH levels are established to ensure that the worker can escape from the contaminated environment in the event of a failure of respiratory protection. An indication of "10% LEL" indicates that, for safety considerations, the IDLH was based on an atmospheric concentration of ten percent of the lower explosive limit even though toxicological impacts might not appear until higher concentrations are reached. 

National Fire Protection Association. System gives a qualitative rating for health, flammability, and spontaneity. These values range from 0 to 4, with 4 being the most hazardous. These can be used for an initial screen to sensitize engineers to the potential hazard. Quantitative values are published elsewhere for each separate issue health (TLV, STEL, IDLFl) flammability explosivity (upper and lower explosive limits). 

An example of an application global constant would be the high combustible gas alarm limit used in fire and gas protection systems, for example, 20 % LEL (Lower Explosion Limit). 

Primary explosion protections means also that in case of an unintentimial escape of gas this event is detected in due time and the gas is removed as quickly and as safely as possible, for example through self-opening windows or fans, before the mixture concentration exceeds the lower explosion limit. The necessary air intake must be calculated on the basis of the amount of gas which can escape during incidents or accidents like leaks or pipe ruptures. Diffuse leaks should also be taken into account. For such leaks and also for gas trapped during plant shut down ventilation should be provided which is appropriate to handle the gas volumes which could escape. 

At places where hydrogen is handled commercially and an escape carmot be excluded, the facilities are usually protected by stationary gas alarm devices. They frequently give a pre-alarm at about 10 % of the lower explosimi limit which is 0.4 % H2 in air, and a general alarm at 25 % of the lower explosion limit, which is 1 % H2 in air (these limits are not prescribed and are at the discretion of the operator). So there is enough time for countermeasures or evacuation of the facility. 

An instrument designed to detect the presence or concentration of combustible gases or vapors in the atmosphere. It is usually calibrated to indicate the concentration of a gas as a percentage of its Lower Explosive Limit (LEL) so that a reading of 100 percent indicates that the LEL limit has been reached. They use either a solid-state circuit, infrared (IR) beam electrochemical or duel catalytic bead for the detection of gas in an area. Portable monitors are used for personnel protection, and fixed installations are provided for property protection. The Instrument Society of America (ISA) has provided a guideline for combustible gas detector utilization, ISA TR 12.13.03, Guide for Combustible Gas Detection as a Method of Protection. 

So the reason no heat is added — to avoid spark sources which might ignite the flammable solvent and no solvent concentration is permitted above 25% of the lower explosive limit, so as to have no ignitable mixtures — is to be doubly protective against the serious hazard of ignition of flammable solvents. 

The intention of Regulation 20 is to ensure that when a coneentration of firedamp in excess of 1.25% by volume is detected, the electricity supply, at the location, is cut off to equipment which is not explosion-protected before the concentration reaches the lower explosive limit and becomes a hazard. 

An example of gas detector protection is found in unattended cold stores and cooling plants which use ammonia as the refrigerant. There is practically no risk in attended plants because a few parts per million of ammonia in air is easily detectable by the pungent smell, so the attendant is aware that there is a leak and can take remedial action before the concentration becomes dangerous. The lower explosive limit is comparatively high at 16% this concentration is only likely under abnormal conditions, such as a blown compressor head gasket, and is intolerable for the eyes and respiratory system. 

When dealing with issues of flammability, let s say that OSHA uses a limit of 10 percent LEL (lower explosive limit) as a safe level. In this case, a more progressive company may believe that the OSHA level will not provide employees (or the company) with adequate protection and chooses to adopt a level of 5 percent LEL or lower. 

Accidents in which fire and/or explosion of ammonia-air mixtures have been reported to cause the death or injury of personnel are known [77, 78], but are not very frequent. Such accidents have occurred in cold storage warehouses in which ammonia was released in an enclosed space and ignition was caused accidentally. One reason that explosions and lire accidents have not been frequent is that upon release of ammonia from atmospheric or pressurized tanks, rapid evaporation of ammonia cools down the liquid and thereby reduces the vapor pressure of ammonia over the liquid. As noted above, even momentary fatal concentrations of ammonia are much lower than the lower explosion limit in air. Also the fairly high ignition temperature and the low normal flame velocity of an ammonia/air mixture (0.07 to 0.08 m/s) are important [12]. For protection against fire and explosion in ammonia storage or transportation vessels see [13]. 

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