Location of Gas Detectors in Explosive Environments II

Posted by Prosense 04/06/2017 0 Comment(s)

4. Selecting the appropriate hardware

In the selection of the measurement detectors for flammable gas, attention is paid to environmental factors, the characteristics of the area to be used and the intended application. The criteria to be considered are given below.

  • Gas or gases to be detected, the range of concentrations of gases that may be encountered, and therefore the required measuring range and accuracy,
  • Potentially invasive gases and their presence in the environment,
  • The purpose of the equipment; field monitoring, personnel safety, leak detection or other purposes,
  • Whether the hardware must be stationary, transportable, or portable.
  • Selection of sampling system for spreading or suction,
  • Classification of use regions or regions,
  • Environmental conditions to be encountered in use areas,
  • Materials, enclosures and their compatibility with the working environment and sensors. For example; copper components should not be exposed to the presence of acetylene due to the potential for generating explosive acetylenes.
  • Calibration features including zero controls,

The gas detection and measurement system to be installed is designed to provide the minimum reaction time required for the safe operation of the plant. The following factors are taken into account.

  • Potential spread rate of flammable gas,
  • The response time of the sensor,
  • Delay time of data transmission lines,
  • Delay time of alarm devices and switching circuits,

The lower and upper ignition limits of the flammable gas in the air vary with the temperature, pressure and oxygen concentration. Normal changes in these ambient conditions do not significantly affect the performance of the equipment. However, if larger variations of temperature, pressure or oxygen are expected, it is necessary to consult the manufacturer for proper product selection.

In the classified areas, gas detection equipment certified in the appropriate protection class is used. The certification shall include the use of the suitable gas group, IIA, IIB or IIC and temperature class equipment according to IEC 60079-0. Finally, observe the appropriate temperature class (T1 to T6) for the substances to be detected.

In general, the fixed equipment consists of sensors or sampling points located in the hazardous area and a control unit that can be placed in a hazardous or non-hazardous area. The entire system is permanently installed and takes its energy from the mains power. The point to note here is that the control unit is supplied with the battery. Battery capacity should be calculated by taking into account the total energy consumption of the devices and possible downtime. In addition, the use of the UPS system will increase the reliability of the equipment.

5. Behaviour of released gas

The rate and extent of formation of a flammable medium are influenced by the chemical and physical parameters of the flammable material and the release of gas. The effect of each parameter below assumes that other parameters remain unchanged.

5.1 The release rate of gas or vapour

As the rate of release increases, the rate and extent of the flammable media is increased. The release rate depends on the following parameters, respectively.

5.1.1 Geometric shape of the release source

This is related to the physical characteristics of the source of release: an open surface, like a leaky flow.

5.1.2 Concentration

The concentration of flammable gas or vapour in the released mixture affects the release rate.

5.1.3 Volatility of flammable liquid

This is in principle related to the vapour pressure and evaporation temperature. If the vapour pressure is not known, the boiling point and the flash point can be used as a guide. As the flashpoint decreases, the rate and extent of the flammable media are growing. Some liquids (eg, halogenated hydrocarbons) do not have a flash point, although they are capable of forming an explosive gas medium. In such cases, the equilibrium liquid temperature corresponding to the saturated concentration at the flammable lower limit should be compared with the largest liquid temperature. In such cases, liquids should be considered when their temperature (TF-x) is above K. Where TF is a flashpoint and x is the security limit. This safety limit should be increased to about 5 K for pure chemicals, but to 15 K for mixtures.

5.1.4 Liquid temperature

The vapour pressure increases with temperature, thus increasing the release rate due to evaporation.

5.2 Flammability limits

As LEL decreases as the volumetric ratio of flammable gases or vapours in the air, the extent and the rate of formation of the flammable media increases. With identical release rates, low LEL gases reach a faster ignition concentration than high LEL-rated gases.

LEL and UEL (Upper Explosion Limit) both vary with temperature and pressure, but the normal changes in these parameters do not significantly affect the limits.

5.3 Ventilation

Good ventilation generally reduces the coverage area and the rate of flammable media. Barriers to ventilation prevent the flammable environment and the rate of occurrence. On the other hand, some obstacles, such as sets, walls or ceilings, limit the rate and extent of formation of flammable media.

5.4 Relative density of released gas or vapour

The behaviour of the released gas with a negligible initial velocity is controlled by the movement caused by the density difference and depends on the relative density of the gas relative to the air. If the gas is significantly lighter than the air, it moves upwards. If the gas or steam is heavier than air, it accumulates at ground level.

5.5 Temperature and/or pressure

The temperature and/or pressure of the gas or vapour prior to release, which is substantially different from the ambient temperature and pressure, affects the absolute release density and at least the behaviour of the gas or vapour around the source.

The gas escaping to the environment at high pressure can be cooled sufficiently as it expands adiabatically. Likewise, compressed liquefied gas (LPG or ammonia) will be cooled to a boiling point well below 0 ° C.

Any thermally generated flow (such as conversion currents from hot or cold surfaces, from a plant or equipment) can affect the spread and thus dispersion of the gas/air mixture, particularly if it is adjacent to the source of a release.

5.6 Other parameters to consider

Other parameters such as climate and topographic conditions should be taken into account.

5.7 Outside areas and open structures

In the case of open spaces and open structures, the distribution of the released gas may be affected by both the wind speed and the wind direction.

More complex airflow patterns occur around buildings or other structures. In the case where gas detectors are intended to be installed in a large installation, the use of mathematical models of wind tunnel experiments for gas emission or to a certain extent enlarged or reduced is suitable during the design phase.

5.8 Buildings and indoor places

The release rate of the released gas is higher in buildings and in closed areas. This deposition depends on the rate of release of the gas, the release, the density of the gas, the ventilation and any added thermal flow. These factors should be taken into account when determining the appropriate positions of the sensors.

5.8.1 Non-ventilated buildings and enclosed places

Theoretically, in the absence of airflow and thermal effects, the release of lighter gas than the air tends to accumulate from the source of release to the ceiling. The release of heavier gas than air is prone to accumulate at the site of release from the source to the base.

This behaviour can be altered if release is in the form of a sputtering moment. If a lighter gas jet than the air is directed downstream from the release source, the accumulation layer may expand to a position below the ceiling release source. Similarly, if a gas jet heavier than air is directed upward from the release source, the deposition layer can expand to a position above the base release source.

5.8.2 Buildings with ventilation and covered places

The ventilation of buildings and enclosures is provided by ler natural means and mechanical ventilation arrangements B, or by a combination of both.

Natural ventilation is the entry and exit of the air from any opening in the structure of a building or a closed place to the building or indoor.

If a gas or steam that is heavier than the air is released in an enclosed space where the natural ventilation creates upward flow, it may expand to the top of the release source as well as downward. On the contrary, a gas or vapour that is lighter than air in an enclosed space where the natural ventilation creates downward flow may expand to the top of the release source as well as downward.

The air flow to the closed space through the fans is called mechanical ventilation. The gas concentration in a closed, mechanically ventilated enclosure should be less than the gas concentration in a naturally ventilated enclosure.

In the case of very high gas concentrations (on LEL) or in an area on a low flash point flammable liquid, increased ventilation may result in increased volume of the explosive medium.