Placement Of Gas Detectors In Explosive Environments I

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

Selection, Installation, Use and Maintenance of Gas Detectors in Explosive Gas Environments

Özkan Karataş



In industrial plants, gases and vapours produced as raw material or production output can lead to great destruction for death and property for man if they reach the lower explosion boundaries. Detection and control of gases and vapours in time can prevent major industrial accidents. Gas detection systems, which have recorded technologically large stages, are one of the most important factors for ensuring explosion safety in facilities. Although explosion detection systems have been installed; This shows that these systems are not used correctly and efficiently. False detection, faulty system and technology selection, installation, use and maintenance in the absence of errors and errors despite the main cause of the explosions.


1. Introduction

The air, fire source and explosive gas mixture that make up the explosion triangle often cause accidents in industrial processes. The existence of fire source in the air and industrial processes for life has brought the necessity of continuously monitoring and controlling the explosive gas.

Gas detection systems have been needed for the first time in mines. With the industrial revolution and the evolving industrial processes, there has been a huge increase in coal demand and explosions have increased in the mine quarries that respond to this demand. As a result of these explosions, large human losses occurred.

Gas detection is being made by the bravest miners who land on the coal mine with a long wick on the head and a wet blanket on their shoulders. The explosion caused by a burning wick in the presence of gas causes only one miner to die; the rest of the team continued to work. This method has been used for a long time as large deaths cause production disruption.

The nervous system that controls the respiratory system is the most similar to the human, and the canaries which sound very loudly are the second method used for gas detection in mines. The canaries, which have left their voices in the increase of methane or the lack of oxygen, have been the precursors of explosion or poisoning.

Together with the point where the technology came from, great progress was made in gas detectors and solutions were developed for almost all needs. In spite of developing systems, there are major accidents which cause deaths due to wrong detector selection, improper installation, usage errors and negligence.

In order to make the right choice in applications and minimize the risk of accidents, the characteristics and behaviour of the vapour and gases should be examined well, the appropriate measurement principle should be decided, the correct equipment should be selected, the behaviour of the released gas should be calculated and the design and installation of fixed gas detection systems should be ensured.


2. Properties of steam and gases

False readings lead to poor readings of the properties of gases and vapours even during the selection, installation, commissioning, training, operation and maintenance of even the simplest of equipment. These cause false alarms or incorrect operation, incorrect actions due to these faults, or fault alarms. This situation poses an unnecessary danger to human life and property.

All gases and vapours are mixed together by diffusion or by mixing. In addition, some gases and vapours react chemically with one another in the mixing process.

The density of pure gases and the effective intensity of the vapours are proportional to their molecular mass. The equivalent molecular mass corresponding to the relative density of air 1 is approximately 29. The gases with a molecular mass less than 29 have a relative density of less than 1, and therefore these gases are lighter than air. The gases with a relative density greater than 1 are heavier than air and these gases tend to accumulate in the pits into the channels or to the ground point.

If the diffusion source and the surrounding air are warmer than the ambient air, the relative intensity of the mixture will initially increase even if it is greater than 1. As a rule of thumb, the effect of the temperature increase of 30 K will be greater than the calculation resulting in a relative density of 10% greater than air. The opposite is applied when the spread is colder than the ambient temperature.

Due to temperature variations in the spread and normal turbulence, gases and mixtures with relative densities between 0.8 and 1.2 are generally considered to have similar relative density to air, and therefore such gases are capable of spreading in all directions.

Vapours behave differently than gases and these behaviours are more complex than gases, so the detection of vapours is more difficult than gases.

When liquid is present, the evaporation rate increases with temperature. As a generally accepted rule, the maximum volumetric ratio of any vapour at constant pressure increases by a factor of 1.5 to 2.0 for each 10 K increase in liquid temperature. Similarly, for every 10K decrease, it decreases by a factor from 1.5 to 2.0.

Only water vapour is lighter than air, there are four vapours with a similar intensity to air (methanol, hydroxylamine, hydrazine and hydrogen). All other vapours are heavier than air, most of which are heavier than air.

The flammable liquid with a high flash point may not be detectable if the ambient temperature is well below the flash point. For example, if the ambient temperature is below 60 K of the flashpoint, it can be estimated that the steam can reach up to 1% to 8% LEL (Lower Explosion Limit) and then slowly close to the liquid if the vapour does not blow up. The steam concentration increases up to 8 times for a 30 K increase in ambient temperature.


3. Measurement technologies

In order to make the gas and steam measurements in the relevant environment analytically accurate, the most important consideration is to choose the right measurement technology. Measurement technology also directly affects the cost of the system to be installed. The most widely used measurement technologies are given below.

  • Catalytic sensors
  • Thermal conductive sensors
  • Infrared sensors
  • Semiconductor sensors
  • Electrochemical sensors
  • Flame Ionizing Detector (FID)
  • Flame temperature analyzers (FTA)
  • Photo-ionizing detector (PID)
  • Paramagnetic oxygen detector