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This is the first in a series of articles that G&I is publishing on gas fundamentals. Most of our readers utilize gases, but their expertise may lie in other fields. We hope that they will find these articles and fact charts helpful.
There are three main types of gas hazard: Flammable; Toxic, and Asphyxiant. There is only a limited band of gas/air concentration which will produce a combustible mixture. This band is specific for each gas and vapor and is bounded by an upper level, known as the Upper Explosive Limit (or the UEL) and a lower level, called the Lower Explosive Limit (LEL). At levels below the LEL, there is insufficient gas to produce an explosion (i.e. the mixture is too “lean”), whilst above the UEL, the mixture has insufficient oxygen (i.e. the mixture is too “rich”). The flammable range therefore falls between the limits of the LEL and UEL for each individual gas or mixture of gases. Outside these limits, the mixture is not capable of combustion. An increase in pressure, temperature or oxygen content will generally broaden the flammability range.
In the average industrial plant, there would normally be no gases leaking into the surrounding area or, at worst, only a low background level of gas present. The detecting and early warning system will only be required to detect levels from zero percent of gas up to the lower explosive limit. By the time this concentration is reached, shutdown procedures or site clearance should have been put into operation. In fact this will typically take place at a concentration of less than 50 percent of the LEL value, so that an adequate safety margin is provided. However, it should always be remembered that in enclosed or unventilated areas, a concentration in excess of the UEL can sometimes occur. At times of inspection, therefore, special care needs to be taken when operating hatches or doors, since the ingress of air from outside can dilute the gases to a hazardous, combustible mixture.
Flammable gases also have a temperature where ignition will take place, even without an external ignition source such as a spark or flame. This temperature is called the Ignition Temperature. Apparatus for use in a hazardous area must not have a surface temperature that exceeds the ignition temperature. Apparatus is therefore marked with a maximum surface temperature or T rating.
The flash point of a flammable liquid is the lowest temperature at which the surface of the liquid emits sufficient vapor to be ignited by a small flame. Don’t confuse with ignition temperature as the two can be very different.
Vapor density is the density of a gas / vapor compared with air when air = 1.0
If vapor density < 1.0, gas will rise
If vapor density > 1.0, gas will fall
Vapor density will help determine sensor placement
Toxic Gas Hazards
Some gases are poisonous and can be dangerous to life at very low concentrations. Some toxic gases have strong smells like the distinctive ‘rotten eggs’ smell of H2S. The measurements most often used for the concentration of toxic gases are parts per million (ppm) and parts per billion (ppb). For example 1ppm would be equivalent to a room filled with a total of 1 million balls and 1 of those balls being red. The red ball would represent 1ppm.
More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas. (It should be noted that there is a large group of gases which are both combustible and toxic, so that even detectors of toxic gases sometimes have to carry hazardous area approval). The main reason for treating flammable and toxic gases separately is that the hazards and regulations involved and the types of sensor required are different.
With toxic substances, (apart from the obvious environmental problems) the main concern is the effect on workers of exposure to even very low concentrations, which could be inhaled, ingested, or absorbed through the skin. Since adverse effects can often result from additive, long-term exposure, it is important not only to measure the concentration of gas, but also the total time of exposure. There are even some known cases of synergism, where substances can interact and produce a far worse effect when together than the separate effect of each on its own. Concern about concentrations of toxic substances in the workplace focus on both organic and inorganic compounds, including the effects they could have on the health and safety of employees, the possible contamination of a manufactured end-product (or equipment used in its manufacture) and also the subsequent disruption of normal working activities.
The term “hygiene monitoring” is generally used to cover the area of industrial health monitoring associated with the exposure of employees to hazardous conditions of gases, dust, noise etc. In other words, the aim is to ensure that levels in the workplace are below the statutory limits.
This subject covers both area surveys (profiling of potential exposures) and personal monitoring, where instruments are worn by a worker and sampling is carried out as near to the breathing zone as possible. This ensures that the measured level of contamination is truly representative of that inhaled by the worker.
It should be emphasized that both personal monitoring and monitoring of the workplace should be considered as important parts of an overall, integrated safety plan. They are only intended to provide the necessary information about conditions as they exist in the atmosphere. This then allows the necessary action to be taken to comply with the relevant industrial regulations and safety requirements.
Whatever method is decided upon, it is important to take into account the nature of the toxicity of any of the gases involved. For instance, any instrument which measures only a time-weighted average, or an instrument which draws a sample for subsequent laboratory analysis, would not protect a worker against a short exposure to a lethal dose of a highly toxic substance. On the other hand, it may be quite normal to briefly exceed the average, long-term (LTEL) levels in some areas of a plant, and it need not be indicated as an alarm situation. Therefore, the optimum instrument system should be capable of monitoring both short and long term exposure levels as well as instantaneous alarm levels.
Our next installment of Gases 101 will discuss European and US occupational exposure limits.
The information for these series of articles is taken from the Honeywell Analytics Gas Book. Used with permission. For more information call 800-538-0363;or write to: email@example.com
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