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The limiting oxygen concentration (LOC), [1] also known as the minimum oxygen concentration (MOC), [2] is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. It is expressed in units of volume percent of oxygen. The LOC varies with pressure and temperature.
Consider the first triangular diagram below, which shows all possible mixtures of methane, oxygen and nitrogen. Air is a mixture of about 21 volume percent oxygen, and 79 volume percent inerts (nitrogen). Any mixture of methane and air will therefore lie on the straight line between pure methane and pure air – this is shown as the blue air-line.
The system can be flushed with an inert gas to reduce the concentration of oxygen so that when the flammable gas is admitted, an ignitable mixture cannot form. In NFPA 56, [1] this is known as purge-into-service. In combustion engineering terms, the admission of inert gas dilutes the oxygen below the limiting oxygen concentration.
Higher temperature or pressure, as well as higher concentration of the oxidizer (primarily oxygen in air), results in lower LFL and higher UFL, hence the gas mixture will be easier to explode. Usually atmospheric air supplies the oxygen for combustion, and limits assume the normal concentration of oxygen in air.
Oxygen sensors are installed in the protected volumes to continuously monitor the oxygen concentration. The exact oxygen level to retain in the protected volumes is determined after a careful assessment of materials, configurations, and hazards. [3] Tables list ignition-limiting oxygen thresholds for some materials. Alternatively, the ignition ...
Oxygen concentration may refer to: What oxygen concentrators do - increase the fraction of oxygen in a gas mixture; Oxygen saturation, the fraction of oxygen dissolved in or carried by a fluid; Limiting oxygen concentration, the concentration below which combustion can not take place
In combustion engineering terms, the admission of inert gas can be said to dilute the oxygen below the limiting oxygen concentration. Inerting differs from purging. Purging, by definition, ensures that an ignitable mixture never forms. Inerting makes an ignitable mixture safe by introduction of an inert gas.
In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. [1] In an ideal mixture, the microscopic interactions between each pair of chemical species are the same (or macroscopically equivalent, the enthalpy change of solution and volume variation in mixing is zero) and, as a result, properties of the mixtures ...