Author Dipl. Ing. Jörg Liebetruth
In the every day life of the fire brigade, the danger of acetylene is equivalent to the danger of other flammable gases and vapors and can create one of the most explosive situations. Acetylene has been the cause of many fires and disastrous accidents in the industry, and fire brigades must be aware of the danger an acetylene leak can cause.
Two kinds of danger can arise from acetylene gas bottles:
First of there is the danger of pressure when overheating occurs due to fire, causing the gas bottle to explode. The second danger arises from an unsupervised acetylene leak; e.g., gas bottles which are not air-tight. Under certain circumstances, acetylene can form a dangerously explosive gas-air mixture with the ambient air.
With acetylene there is the danger of ignition followed by an explosion in a wide range of the gas-air ratio. In other words, acetylene ignites at almost every gas-air ratio.
The explosive range of any explosive gas or vapor is limited by the lower and upper explosion limits (LEL and UEL), which are measured in a ratio of the % volume of flammable material to the total volume of the mixture. Below the LEL, the mixture is too sparse and above the UEL the content is too concentrated. The explosive range for acetylene is between 2.3 and 78.0% volume, so any ignition source must be avoided.
For
the fire brigade, this means that their personal protective clothing
and electrical equipment, such as lights, radio equipment, and gas detectors
must be explosion proof. However, explosion approvals differ as explained
below:
Explosion proof electrical
instruments are divided into different categories, which are specified
in the approval description. Consequently, it is possible that fire
fighters can expose themselves to danger despite "explosion proof instruments,"
and that their instruments do not have approval for use in areas where
certain flammable gases occur.
In explosive atmospheres
only appropriately certified instruments, which are marked as such,
may be used. The details on the explosion proof instrument’s label
must include the manufacturer's name, the device designation, and the
product number. The label must be visible, legible, and long-lasting.
As an example, the G750 label is shown below:
|
EEx
|
ib
d
|
II
C
|
T5
|
||
|
EEx
|
Electrical
equipment for explosion hazardous areas The instrument complies with valid European (EN) standards (certified by a recognized European testing authority). |
||||
|
Ex
|
The instrument complies with valid German (DIN) standards (certified by a recognized testing authority). | ||||
|
d
|
Types
of ignition protection Flameproof enclosure |
||||
|
e
|
Increased safety | ||||
|
ia
|
Intrinsic
safety: |
Category ia | |||
|
ib
|
Category ib | ||||
|
m
|
Molding | ||||
|
p
|
Pressurized apparatus | ||||
|
o
|
Oil immersion | ||||
|
q
|
Powder filling | ||||
|
I
|
Explosion
groups Electrical equipment for mining |
||||
|
II
|
Electrical equipment for all other hazardous areas | ||||
|
II
A
|
Limit
of slot width: |
> 0.9mm (flameproof enclosure) | |||
|
II
B
|
0.5mm to 0.9mm (flameproof enclosure) | ||||
|
II
C
|
< 0.5mm (flameproof enclosure) | ||||
|
T1
to T6
|
Temperature classes | ||||
Table
1 Identification of electrical equipment
for explosion hazardous areas (EN 50014)
The type of ignition protection describes (according to the manufacturing requirements of the EN 50014 standards) the technical details of the ignition protection of explosion proof electrical instruments. In assessment it plays only a secondary roll, as from a safety point of view all standardized types of ignition protection are regarded as similar. The application depends in essence on the type and function of the instrument. The type of ignition protection for gas warning devices is in most cases "intrinsic safety" or "flameproof enclosure."
Fire brigade members should pay special attention to EXPLOSION GROUPS and TEMPERATURE CLASSES:
Flammable gases have specific material characteristics that must be considered when assessing ignition danger. Some gases are easy to ignite, while others are difficult. Instruments that are used in these atmospheres must take these characteristics into account and must be manufactured accordingly.
The easiest method would be to manufacture all explosion proof electrical instruments in accordance with the maximum requirements, independent from their respective applications. Since this is economically impossible, all explosion proof material is categorized by the qualities of the specific components of the explosive atmosphere for which it is intended.
The explosion groups classify the ability of an explosive gas-air mixture to ignite, whereas the temperature classes subdivide the gas-air mixtures according to their ignition temperature.
The danger of the gas increases from explosion groups IIA to IIC and also from temperature class T1 to T6. Therefore, the requirements for electrical instruments increase within these explosion groups and temperature classes. Instruments that are used by fire brigades are all group II instruments.
| Explosion groups |
Temperature Classes (maximum temperature) | |||||
| T1 (450°C) | T2 (300°C) | T3 (200°C) | T4 (135°C) | T5 (100°C) | T6 (85°C) | |
| IIA | Ammonia 630°C Carbon monxide 605°C Methane 595°C Benzene 555°C Acetone 540°C Toluene 535°C Ethane 515°C Propane 470°C Ethyl acetate 460°C Methanol 455°C |
Ethyl
acohol 425°C Trichlor ethylene 410°C n-Propyl alcohol 375°C n-Butane 365°C n-Butyl alcohol 340°C |
Benzine 220°C - 330°C Diesel fuel 220°C - 330°C Heating oils 220°C - 330°C n-Hexane 240°C |
Acetaldehyde 140°C |
||
| IIB | Town
gas 560°C |
Ethylene
oxide 440°C Ethylene 425° |
Hydrogen
sulphide 270°C Ethyl glycol 235°C |
Ethyl
ether 180°C |
||
| IIC | Hydrogen 560°C |
Acetylene 305°C |
Carbon
disulphide 95°C |
|||
For
the incalculable work of fire brigade forces, explosion proof material
must be categorized according to maximum requirements. Where a fireman
has to deal with any kind of dangerous gas, his instruments should have
the marking IIC (explosion group), since these instruments are
classified for use with all lower explosion groups, as well.
The maximum demand, namely temperature group T6, is excessive because technically it requires a lot of research and development time, energy, and money. The only gas which falls in group T6 (carbon disulphide) is found very rarely, and moreover the toxic effect of this gas is more dangerous than the explosion danger. Therefore, T6 is rarely found in portable instruments. Groups IIC T4 or T5 are sufficient, as they cover more then 99.9% of all gases.
Sometimes instruments are approved for a specific explosion group, but individual gases from a higher explosion group are also tested (e.g. instruments with the marking EEx ib d IIB+H2 T4). These instruments can be used in atmospheres which contain IIB gases and hydrogen, but in an atmosphere which contains acetylene there is the danger of ignition and therefore they are not recommended for fire brigade use.
Multicomponent Gas Detectors
The G750 gas measuring and warning unit from GfG Instrumentation can simultaneously measure and monitor up to six different gases. With interchangeable, plug-in "smart sensors," the user is able to adjust the compact, hand-held instrument to comply with new or changing requirements. Every sensor has a memory chip with pre-set data that is automatically transferred to the instrument once it is plugged in.
The G750 meets the highest demands of measuring technique suitability and approval for operation in explosive atmospheres. The suitability of its measuring technique is certified by the PFG function test. The G750 can be used for IIC, the most dangerous explosion group. In addition, it is the only multi-gas detector that is equipped with an IR-CO2 sensor, and which is allowed to be used in a dangerously explosive acetylene atmosphere. All measured gas concentrations are simultaneously displayed on a rich in contrast graphic display, which offers additional display illumination and a zoom function for single gas concentration readouts. In normal operation, the instrument can be operated with only two buttons ("ON" and "OFF"). In addition, there are also special buttons for the available user functions.
In service mode the instrument can be configured with a menu, and the most important parameters are protected by safety codes. With a rechargeable battery the G750 can operate for more than 8 hours on one charge in diffusion mode.
For every measured gas, three alarm thresholds are possible. Different alarm frequencies indicate which threshold has activated the alarm, and the display shows which alarm was caused by which gas. Monitoring is performed continuously through diffusion or with the help of a built-in pump. If needed, the G750 can also perform a dosimeter function to control short and long term exposure levels.
The alarm signals must be clearly recognizable in every situation. Therefore, the instrument provides 4 different warning systems:
1. A penetrating audible signal with a sound level of 80 dB.
2. An earphone alarm for very loud environments.
3. A flashing alarm light with 6 LEDs.
4. A built-in vibrating alarm (optional).
An optional built-in data logger can save peak, average, and current values together with the date, time, and location when the measurements of dangerous goods are required to be documented, according to regulations. The data can be transmitted to a computer by means of professionally developed DOS or Windows programs.
The maximum demand, namely temperature group T6, is excessive because technically it requires a lot of research and development time, energy, and money. The only gas which falls in group T6 (carbon disulphide) is found very rarely, and moreover the toxic effect of this gas is more dangerous than the explosion danger. Therefore, T6 is rarely found in portable instruments. Groups IIC T4 or T5 are sufficient, as they cover more then 99.9% of all gases.
Sometimes instruments are approved for a specific explosion group, but individual gases from a higher explosion group are also tested (e.g. instruments with the marking EEx ib d IIB+H2 T4). These instruments can be used in atmospheres which contain IIB gases and hydrogen, but in an atmosphere which contains acetylene there is the danger of ignition and therefore they are not recommended for fire brigade use.
Multicomponent Gas Detectors
The G750 gas measuring and warning unit from GfG Instrumentation can simultaneously measure and monitor up to six different gases. With interchangeable, plug-in "smart sensors," the user is able to adjust the compact, hand-held instrument to comply with new or changing requirements. Every sensor has a memory chip with pre-set data that is automatically transferred to the instrument once it is plugged in.
The G750 meets the highest demands of measuring technique suitability and approval for operation in explosive atmospheres. The suitability of its measuring technique is certified by the PFG function test. The G750 can be used for IIC, the most dangerous explosion group. In addition, it is the only multi-gas detector that is equipped with an IR-CO2 sensor, and which is allowed to be used in a dangerously explosive acetylene atmosphere. All measured gas concentrations are simultaneously displayed on a rich in contrast graphic display, which offers additional display illumination and a zoom function for single gas concentration readouts. In normal operation, the instrument can be operated with only two buttons ("ON" and "OFF"). In addition, there are also special buttons for the available user functions.
In service mode the instrument can be configured with a menu, and the most important parameters are protected by safety codes. With a rechargeable battery the G750 can operate for more than 8 hours on one charge in diffusion mode.
For every measured gas, three alarm thresholds are possible. Different alarm frequencies indicate which threshold has activated the alarm, and the display shows which alarm was caused by which gas. Monitoring is performed continuously through diffusion or with the help of a built-in pump. If needed, the G750 can also perform a dosimeter function to control short and long term exposure levels.
The alarm signals must be clearly recognizable in every situation. Therefore, the instrument provides 4 different warning systems:
1. A penetrating audible signal with a sound level of 80 dB.
2. An earphone alarm for very loud environments.
3. A flashing alarm light with 6 LEDs.
4. A built-in vibrating alarm (optional).
An optional built-in data logger can save peak, average, and current values together with the date, time, and location when the measurements of dangerous goods are required to be documented, according to regulations. The data can be transmitted to a computer by means of professionally developed DOS or Windows programs.