Flammable And Combustible Materials Ian Sutton Technical Books

The fabric in this article is extracted from Chapter 1 of the second version of the e book Plant Design and Operations and in the ebook 52 Process Safety Moments.

A danger this is of extremely good problem to honestly all manner centers is fireplace — maximum of the substances being processed, saved or transported are flammable and/or explosive. Therefore, when designing a process facility, the prevention and manipulate of fire are of the utmost significance. The terminology to do with flammable materials may be complicated. An review is supplied underneath.Fire Triangle

Fires require the presence of fuel, air (oxygen) and a source of ignition. These three criteria are often called the fire triangle — shown in Figure 1.

Figure 1

Fire Triangle

In order for a sustainable fireplace to broaden it ought to be stated that,

  • The gas has to be inside the form of a vapor (beverages and solids do no longer burn without delay — the fire generates flammable vapors at their surface and it’s miles the ones vapors that genuinely burn).
  • The phrase ‘oxygen’ covers now not best the oxygen present in air (21% attention) however also oxidizing substances which includes chlorine and nitrates.
  • The phrase ‘Heat’ is extremely misleading — heat, in and of itself, will now not start a fire. What is surely wanted is a supply of high temperature inclusive of a spark from an electric powered motor or the flame in a fired heater.

Figure 2 shows a fireplace triangle in the shape of a Fault Tree AND Gate.

Figure 2

Fire Triangle AND Gate

In words, IF there may be a gas leak and air (oxygen) is gift AND an ignition supply is present THEN a hearth or explosion will occur. The frequency with which such an occasion is predicted to arise is obtained with the aid of multiplying the frequency of the initiating occasion (the gas leak) by way of the chances of the contingent occasions (air present and ignition supply present). This offers a fee of 0.half yr-1. In other phrases a fire at this region is predicted to occur once every 40 years.

In a few procedures and gadget air is robotically allowed to touch hydrocarbons. Examples consist of some treating methods in refineries and many atmospheric constant roof tanks. In such situations, the capacity for a fire can be controlled through eliminating any ignition assets, maintaining a hydrocarbon/oxygen mix that is outdoor the flam­mable range, and putting in device together with flame arrestors to prevent flame propagation.Flammable Range

Not all fuel vapor/oxygen combinations will burn — the concentrations need to lie within the flammable range, that have upper and lower limits for the concentrations of the gas inside the vapor area. The flammability limits vary according to many elements, particularly the pressure and temperature of the mixture and the presence of inert components along with steam, carbon dioxide or nitrogen.

The flammable range for a gasoline is described with the aid of the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). These terms also are known as the Upper and Lower Explosive Limits. Below the Lower Flammable Limit (LFL) there may be inadequate flammable fabric for a fireplace to arise — the aggregate is ‘too lean’. It is the lowest attention of a flammable vapor in air able to producing a hearth inside the presence of an ignition source. The UFL is just like the LFL explos info except that there’s too high a concentration of flammable vapor for a fire to occur — the combination is stated to be ‘too rich’. For maximum flammable hydrocarbons the LFL is round 2 to five%. For easy alkanes which include methane and ethane the UFL is within the 10 to fifteen% variety. Some chemical compounds, inclusive of hydrogen, ethylene oxide and acetylene, have lots better values for UFL.

Values for flammable limit levels for many flammable materials are supplied by way of NFPA 704 — Standard System for the Identification of the Hazards of Materials for Emergency Response (NFPA 2017). This general additionally describes the well-known “danger/safety diamond”.Flash Point

The flash factor of a flammable fabric is described as the temperature at which a vapor this is interior its flammable range that may be ignited. An ignition supply together with a flame or spark is wanted to make the fabric without a doubt burn. It is critical to understand that an ignition source is certainly required ― the flash point isn’t similar to the car-ignition temperature. Before a flammable aggregate will burn its temperature need to be at or above the flashpoint. If the temperature is under this point then the vapor mixture will now not burn, even though a supply of ignition exists. (The word ‘flash’ is used in method flora in any other context — if a liquid at high temperature has its stress all at once decreased then it’s going to ‘flash’ or boil. This has nothing to do with it igniting.)

The flash point is decided by means of heating the liquid in check equipment and measuring the temperature at which ignition will arise when a small flame is brought within the vapor sector above the surface of the liquid.

Figure three illustrates the ideas of ignition temperatures and flashpoints and flammable limits.

Figure three

Flammability and Ignition Limits

The left line in Figure three is the flashpoint line. If the mixture has a awareness and temperature that puts it to the left of the line then it’ll no longer burn, even when a supply of ignition is present. Similarly, mixtures above and beneath the LFL/UFL values will now not burn

Even if the material is above its flashpoint, the ignition source need to be of sufficiently high temperature and must incorporate sufficient electricity to ignite the fuel. This minimal power price varies with form of gasoline and attention; for hydrocarbon vapors it is low, for high flash factor liquids, together with diesel and gasoline oil, it’s miles an awful lot better — commonly within the shape of an existing fireplace. This is why low energy flashes (inclusive of might be created by way of a cell telephone or a virtual digicam) might not ignite a flammable mixture.

If a flammable aggregate is heated to a high sufficient temperature it’s going to spontaneously ignite; an ignition source together with a flame or spark isn’t always wished. Spontaneous ignition occurs at the auto-ignition temperature (AIT), which is likewise shown in Figure three.

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