XIX. Translate the first paragraph of the text in writing

XX. Read the text and give its short summary.

Odorants

The gas industry has long recognized the inherent danger of a gas leak. To provide means of early leak detection, the industry adds odorants to their products for identification of a leak.

Neither natural gas nor the liquid propane (LP) family of gases has a natural odour. It is necessary to add an odorant to these gases so that the consumer can detect their presence. According to the NFPA Standard 58, the odorant should be characterized by a distinctive odour so that presence of gas can be detected at less than one fifth of the lower limit of flammability of the gas.

An odorant should be chemically stable with a high odour intensity under all conditions. It should have a distinctive odour and be nontoxic and nonirritating in combination with the gas or with the products of combustion. It should be easily introduced into the system at a low cost per gallon. Commercially available odorants are generally a compound containing sulphur, which provides the distinctive odour.

The effectiveness of the odorant is measured by its ability to be detected by the average person under normal conditions. The dosage is such that the presence of the gas can be detected long before the lower flammability limit of the gas is reached.

This does not automatically eliminate any danger of an explosion, but it does allow the consumer early detection of small leaks before sufficient gas has accumulated to cause a major explosion.

XXI. Translate the microtext:

Identification of the Source of Ignition

After the origin of the fire has been determined and a leak in the gas system or control has been found, the next step is to identify the source of ignition.

A flammable mixture, or raw gas, will not start a fire by itself; an ignition source is needed. The minimum energy requirement for ignition is 0.25 millijoules. It takes very little energy to ignite a flammable mixture. The spark from a light switch can supply sufficient energy to perform the act.

In every gas fire or explosion, it is obvious that ignition occurred. However, the source of ignition should be determined with a high degree of probability. Each possible source should be proved by determining that the appliances were operating and were the most probable source of ignition.

The source of ignition does not necessarily need to be located in the area where the explosion occurred. For example, when a valve malfunctions gas will pour from the main burner and will eventually migrate upstairs as a result of convective currents until a source of ignition is found. The ignition will light a trail of gas, which will flash back immediately to the main source in the basement, causing an explosion and fire, die to the overrich condition.

TEXT 2

Gas Fires

A gas fire is usually preceded by an explosion. The explosion may be so minor that it does not leave evidence that an explosion occurred. Any gas that will burn in the normal concentrations of oxygen in air is a flammable gas. As with other gases or vapours flammable gas will burn only when its concentration in air is within its combustible range and the mixture is heated to its ignition temperature.

The hazards presented by a gas that is confined in its container are different from those presented when the gas escapes from its container. We shall discuss them separately, even though these hazards may be present simultaneously, in a single incident.

When a confined gas is heated, its pressure increases. If enough heat is applied, the pressure can increase sufficiently to cause a gas leak or a container failure. In addition, contact with flames can reduce the strength of the container material, possibly resulting in container failure. To prevent explosions of compressed gases, pressure relief valves are installed in tanks and cylinders.

Another cause of explosion is a very rapid buildup of pressure in a container. The pressure cannot be released through the safety valve. Tanks and cylinders are also subject to explosion when flame impinges on their surface causing the metal to lose its strength. Flame impingement above the liquid level is more dangerous than that on the container surface area that is in contact with the liquid. Heat from flames above the liquid is absorbed by the metal itself; below the liquid line most of the heat is absorbed by the liquid. This is not a safe condition because absorption of heat by the liquid also causes a dangerous, although less rapid, pressure increase. Spraying the surface of the container with water can help keep the pressure from explosive force. Cooling with water is not a guarantee that an explosion can be averted, especially when flame impingement is occurring.

Compressed or liquefied gas represents a great deal of energy kept in check¹ by its container. When the container fails, this energy is released - often very rapidly and violently. This type of failure is called Boiling Liquid Expanding Vapour explosion, or BLEVE². Most BLEVEs commonly occur when LP gas³ container is exposed to excessive heat. The heat flux must be greater than the capacity of the relief valve. As a result, pressure within the tank rises intill the metal fails. This sudden release of internal pressure of the vaporized gas has a reaction similar to that of a rocket. The container ends may be propelled for considerable distances as the pressure is released.

Ignition of the vapour is immediate. Liquefied petroleum gas may fall to the ground as a cold spray, due to the refrigeration effect of the expanding gas.

The effect of a BLEVE on tank cars, which carry up to 30,000 gallons⁴ of propane, are spectacular since the long end of the tank car resembles a rocket in size, shape and weight.

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