Explosion hazards mostly arise from flammable gases and vapors
Instead of avoiding their ignition by explosion protection measures it may be preferable to detect them before they become ignitable.
DANGER OF EXPLOSION IS LURKING EVERYWHERE
Wherever hazardous situations exist due to the presence of combustible gases and vapors e.g. in oil & gas exploration and storage, transportation and storage of flammable liquids and gases, in processes involving the use of solvents, or in the plastics processing industry, we will always encounter explosion protection measures, mostly regulated by law, to keep the personnel and plants safe.
Depending on the application different measuring principles for the detection of gases and vapors can be used: Catalytic bead sensors, point or open-path infrared sensors. When detectors are used in combination with a central controller such as Dräger Polytron or Dräger REGARD it is possible to detect flammable gases and vapors at an early stage, when concentrations are so low that a dangerous condition – a risk of explosion – can be reliably averted.
METHODOLOGY OF EXPLOSION PROTECTION
Flammable gases and vapors can only be ignited by an ignition source with sufficient high energy or sufficient high temperature if – under atmospheric conditions – they exist in a mixture with atmospheric oxygen in sufficiently high concentrations.
This mixture’s concentration is called LEL:
Lower Explosion Limit.
To have an ignition three conditions have to be met:
Vice versa this rule reads: If one of the three conditions is not met it is reliably ensured that no ignition or explosion can take place.
So, measures of explosion protection can be the following:
The safest way of concentration limiting is the removal of flammable gases and vapors from the process – this is not normally practical. Where flammable gases and vapors are used, it is normal for gas detection systems to be used to limit concentrations. When the process is closed and the level of flammable gases or vapors is allowed to exceed the LEL level, this is acceptable as long as the oxygen concentration is kept low enough to control the risk of explosion (inertisation).
If these measures however are not sufficient, all of the involved electrical devices have to be designed according to certain explosion protection standards so they will not act as a source of ignition if flammable gases or vapors are released.
Further advice concerning the methodology can be found in the harmonized standard EN 1127-1.
If the gas concentration rises a counter measure is activated when reaching the alarm range 1. If the counter measure is effective the gas concentration will decrease (blue curve). If, however, the counter measure is not effective the concentration will keep on rising (red curve). When reaching the alarm range 2 compulsory measures are activated. Properly designed gas detection systems will rarely or never reach the alarm range 2.
Preventing potentially explosive atmospheres – primary explosion protection.
BELOW THE LEL NO DANGER OF EXPLOSION
Concentration limiting (1) and inertisation (2) are also called primary measures because the formation of an ignitable concentration is averted. On the other hand when using explosion protected instruments (3) this is a secondary measure because not the formation of ignitable concentrations is averted, but only its ignition.
Concentration limiting means active dilution, e.g. by automatically ventilating fresh air into the hazardous area if concentrations have risen above the 20 %LEL threshold. If concentrations continue to rise because the counter measure is ineffective then it is necessary to automatically activate shut-down actions at 40 %LEL e.g. by switching off any non-explosion proof instrument or equipment. Gas detection systems used for this purpose must be type approved by a Notified Body in respect to their compliance with the European Standards (formerly acc. to EN 50054ff, now acc. to EN 61779 or EN 60079-29-1). This is true for the sensor and the transmitter as well as for the central controller unit.
As inertisation is also a preventive explosion protection measure, oxygen measuring instruments controlling the inertisation process at least in Europe also have to be type-approved for this purpose and shall comply with the relevant harmonized standards (e.g. EN 50 104).
Safety relevant data of flammable gases and vapors.
THE LOWER EXPLOSION LIMIT – LEL
For flammable substances there is a limit concentration necessary for ignition. Below this limit a mixture of the substance in air cannot be ignited because there is a deficiency of fuel. This limit is called the Lower Explosion Limit or LEL. The LEL cannot be calculated but is an empirical characteristic figure which is established by standardized methods. With some exceptions the LEL lies between 0.5 and 15 % by volume.
GASES AND LEL
Matter above its boiling point commonly is called a gas. Thus the pressure of a pure liquefied gas is always higher than the atmospheric pressure so that released gases can very quickly form concentrations above the LEL causing dangerous ignitable gas-/air-mixtures.
VAPOURS OF FLAMMABLE LIQUIDS AND FLASHPOINT
Matter below its boiling point is not only gaseous but exists in an equilibrium with its liquid (and also solid) state which depends on the temperature.
The gaseous component of this matter is called vapor. A vapor’s pressure is always lower than the atmospheric pressure, and, depending on the liquid’s temperature, only certain maximum vapor concentrations can form. Especially the maximum vapor pressure of a flammable liquid can be so low that the LEL concentration can only be exceeded at a certain temperature. Only above this certain temperature a flammable liquid’s vapor becomes ignitable. This empirical temperature, established by standardized methods, is called the flashpoint which is a very important safety-relevant figure to assess the hazardous nature of flammable liquids.
For example the flashpoint of pure Ethanol is 12 °C (so Ethanol is flammable at 20 °C), but for n-Butanol the flashpoint is 35 °C, so vapors of n-Butanol cannot be ignited at ambient temperatures of 20 °C, but above 35 °C they can.
And indeed: As long as the temperature of a flammable liquid is kept reliably some degrees below the flashpoint this is a primary explosion protection measure!
IGNITION TEMPERATURE AND MINIMUM IGNITION ENERGY
Sparks and arcs produced electrically (or mechanically) and hot surfaces are the best known ones of 13 different sources of ignition. To ignite mixtures of flammable gases or vapors in air the source of ignition must either have a temperature higher than the empirical ignition temperature or sparks must have an energy higher than the empirical minimum ignition energy.
Both the ignition temperature and ignition energy characteristics are established by standardized methods and are relevant when designing or selecting explosion protected instruments for a certain application.