Home Heat risks

Thermal hazards - heat and / or fire

Sources of heat risks in industry are technological processes with high operating temperatures and the spreading of the heat by radiation, convection or conduction. Extremely radiant heat as well as contact heat, open flame, hot melt spray, ardent or burning particles often threaten the security of people in difficult industrial environments and it is necessary to be free of them. SAFEPRO is specialized exclusively on heat protection and offers solutions for professional security of workplaces endangered by thermal risks.

The source of heat is electricity or combustion - intense oxidation reaction that forms the flame (column of burning gaseous substances) and releases heat and light. The presence of flammable matter and an oxidizing agent in the ignitable ratio is essential for combustion. Oxygen in the air (in about 21% concentration) is the most common oxidizing agent (fire-promoting substance).

The concentration of oxygen is crucial for determination of the degree of flammability of fabrics (resp. their "fire resistance") and is determined by the index LOI (Limiting Oxygen Index). LOI indicates the minimum concentration of oxygen in mixture with nitrogen at which the sample still burns. Methodology for the LOI setting determines standards ISO 4589 and ASTM  D286.

Oxygen content in the atmosphere is less than 21%, and this value is therefore crucial for the determination of flammability. Materials that have the LOI value higher than 21 are considered to be non-flammable (fire resistant) and materials that have LOI values lower than 21 are included among flammable. The lower is the value, the more flammable is the fabric and vice versa, the higher value of LOI represents more resistance to ignition and combustion.

For example, cotton has LOI 18-20%, polypropylene, about 17.5%. Flame resistant Para-aramid (Series K, Series 400) 30-31%, PBI ™ (Polybenzimidzol, part of the material Series 800) 38%, Preox (part of the material Series 900) 55-60%, and 95% has PTFE.

Limited flame spread

Requirements for protective clothing resolves the standard EN ISO 11612 by methodology in accordance with ISO 15025 - exposure to flame at the distance of 20 mm at 90° or more precisely 30° for 10 s, by two test methods:
A1: ignition and burning of the surface and A2: ignition and burning at the edge.
The material must not burn at the surface / edge, does not create therein blown holes, does not melt, and must not create drops (melt), standard amount of after-flame and after-glow time must be less than 2 seconds.

The requirement of flame resistance for gloves defines the EN 407 standard in section 6.3 using the methodology according to EN ISO 6941. The glove material is exposed to a gas flame, the flame is situated at a distance of 20 mm for 15 seconds. Following the withdrawal of the burner, the after-flame time and after-glow time are measured in seconds.  There are four levels of protection:
1: ≤ 20 after-flame, glow without requirements
2: after-flame ≤ 10, after-glow ≤ 120,
3: after-flame ≤ 3, after-glow ≤ 25,
4: ≤ 2 after-flame, after-glow ≤ 5.

Flame threats are rare and they are usually related to other types of heat risks in industrial environments. Flame and burning based risks are related to fire emergency departments. Technical standards and requirements for the firefighters equipment are therefore separated and apart from the requirements for industrial PPE (Personal Protective Equipment).

TEST: Limited flame spread, methodology ISO 1502

Contact heat

Contact heat is baseline risk at hand protection and must be distinguished from convection and radiation heat.

Request to the protection against contact heat for clothing resolves standard EN ISO 11612 in contact heat section. The method of testing specifies standard ISO 12127 – fabric specimen (in horizontal orientation, with sample diameter 80 mm) is subjected to a contact temperature of 250 °C (by heating cylinder with contact surface diameter 25 mm), then the time required to reach the pain threshold (damage to the PVC film) is measured. The result represents code F in three different levels: F1: 5 <10 s, F2: 10 <15 s, F3: over 15 s.

Standard EN 407 specifies  gloves protection by the methodology of test according to EN 702. The principle follows this procedure: Material (resp. material combination from which gloves are made) is subjected to contact temperatures + 100°C (Level 1), 250°C (Level 2), +350°C (Level 3) and 500°C (Level 4) under strictly defined conditions (area and contact pressure). Time to 10°C temperature gradient (relative increase of temperature) inside the glove is measured. The value of about 25°C is determined as the starting level and the minimal acceptable value of measured time to meet correspondent requirements is 15 s.

We are often encountered with the requirement to "glove up to 1000°C" in our longstanding practice. Whist analysing this demand, we usually discover that although operating temperature is really around 1000°C (technological process in the closed area of the furnace), but the handling (contact) temperature is at about 250-300°C level, accompanied by strong radiation and heavy spraying melt risks. Our "basic" Series 400 gloves with aluminized back can solve this problem. Generally Series 400 perfectly suits for more than 90% of applications.

TEST: Contact heat, methodology ISO 12127, EN 702

Convection

Convection is another one of the basic forms of heat transfer. From the physical point of view, convection is of the biggest importance in liquids and gases, where loose particles are trans-located due to their different thermal energy.

Convection is based on the fact that the density of matter varies with temperature (for gases considerably) and through the downward force consequently occurs a natural convection of loose particles, their mixing and temperature equalization. Convection can be classified as loose (effect of gravity - heat naturally rises) and forced (fans, mixing liquids...).

Standard EN ISO 11612 (for protective clothing) distinguishes three levels of protection performance against convection heat by methodology ISO 9151. The Heat Transfer Index (HTI), which is measured for the material or for material combinations respectively, determines them. Specimens with dimensions 14 x 14 cm are placed in a horizontal position above the gas burner with the power of 80 kW/m2 and the heat passing through the test sample is measured by copper calorimeter subsequently. Test result represents average value of three measurements therefore determining the time necessary for temperature to increase (on the other specimen side) in 12°C or 24°C. Index HTI12 correlated with pain threshold index HTI24 is above the threshold of pain.

Time 4 <10 s corresponds to level B1, time 10 <20 s represents level B2, and 20 s and more corresponds to B3 for index HTI24.

Standard EN 407 (for gloves) determines the level of performance of protection against convective heat (using EN 367 methodology) by threshold time which is necessary for the relative temperature to increase inside the glove (through material combination, sample 140 x 140 mm) in 24°C. The heat generated by the gas burner with an output power of 80 kW/m2 is used for test purposes.

There are 4 levels of protection: 1: 4<7 s, 2: 7<10 s, 3: 10<18, 4: 18 and over are distinguished for gloves.

TEST: Convection, methodology ISO 9151, EN 367

Radiation

Radiant heat is a basic form of heat transfer and also the only one where the material environment is not necessary. The heat is emitted and spread across the environment (air) nearly without loss unless it encounters a solid obstacle. This partly reflects, partially absorbs it and temperature of affected obstacles increases. Different materials absorb radiant heat differently - dark and harsh elements warm up more and faster compared to shiny and smooth.

Textiles with “aluminized” surface are most conveniently used for protection against radiation. Glossy coating reflects major part of the radiation (depending on surface quality) and the material warms up slowly. Current technologies allow several different types of "aluminized" coating and final quality depends on several factors such as:
- The thickness and structure of the fabric,
-
Al pigment carrier material (Mylar, polyester, Al-foil...) and its thickness,
- Used
adhesives (silicone, polyurethane...),
- Lamination technology and it’s precision.

Measurement methodology is defined by the standard EN 6942, which uses the Radiant Heat Transfer Indexes – (RHTI) for results evaluation. Precisely specified density of radiation, where lower intensity can be corrected by a longer exposure and vice versa, is used for testing samples with dimensions 230 mm x 70 mm. Test methodology distinguishes two testing methods:
Method A describes visual changes of material exposed to radiant heat.
Method B measures time in seconds, required for the temperature to increase in 12 or more precisely 24°C. Temperature gradient is measured by calorimeter on the reverse fabric side - RHTI12, RHTI24 respectively, radiation at a density of 20 kW/m2.

There are 4 levels are distinguished for protective clothing (C) and 4 levels for protective gloves by index HTI24:
Protective clothing: C1: 7<20 s, C2: 20<50 s, C3: 50<95 s and C4: 95 s or more,
Protective gloves: 1: 7<20 s, 2: 20<50 s, 3: 50<95 s and 4: 95 s or more.

TEST: Radiation, methodology EN 6942

Resistance to molten metal splash

The assessment of textiles resistance against large molten metal splashes handles the EN ISO 9185 standard, which specifies testing method for determination of the resistance to heat transmission through fabric at this risk.

The molten liquid metal (aluminum at 780°C, iron at 1400°C) is poured from a height of 225 mm on the fabric sample (260 mm x 100 mm) at the angle of 60° (aluminum) or 75° (iron) in the warp direction. Behind the sample is placed a PVC film which simulates human skin. Changes (damages) of the PVC film evaluate the test. According to the result - the test is repeated with lower or higher amount of metal up to the determination of critical mass which damages the film.

Protective garments have three performance levels for aluminium (D) and three levels for iron (E):
D1: 100<200 g, D2: 200<350 g
, D3: 350 g or more,
E1: 60<120 g, E2: 120<200 g, E3: 200 g or more.

Gloves distinguish four levels of protection related to the molten iron:
1: 30<60 g, 2: 60<120 g, 3: 120<200 g, 4: 200 g
or more.

Test method for evaluation of materials used for protection against small spray of molten metal is specified in EN 348 (Protective clothing. Test method: Determination of behavior of materials during the impact of small splashes of molten metal).

The test is used for gloves and it involves determining total number of molten metal drops required for the temperature to increase in 40°C on the inside of the glove. There are four levels of protection are distinguished using molten iron drops:
1: ≥5, 2: ≥15, 3: ≥525, 4: ≥35
.

TEST: Resistance to molten metal splash, methodology EN ISO 9185

Consequences (burns) and first aid

A burn (Combustion) is a type of injury to skin, or other tissues, caused by heat or hot substances in solid, liquid or gaseous state.  They can cause tissue damage at various degrees, depending on the type, intensity and duration of the harmful effect. Injuries are classified to four burns levels – from first to fourth degree.

First aid for burns:

- Protect the burned person from further harm,
- Cool the burn area to help soothe the pain – but do not open the injury, for approx. 20 min.
- After that cover the area of the burn
. Use sterile gauze if possible, smooth and cool bandage or a clean cloth.- Remove jewelry, watch, rings or other tight items from the burned area. Try to do this quickly and gently, before the area swells.
- Remove
charred parts of clothing that is loose, but don't remove burned clothing stuck to the skin,
- Do not apply
powder or ointment to the wound,
- Ensure anti-shock treatment:
Silence – Adequate temperature – Fluids – Pain Management – Transport (for professional treatment).

Materials:

Safepro, s.r. o.

 
Language  SK 
SAFEPRO s.r.o. Pekna 10, 040 01 Kosice, SK
Phone: +421 905 651 814
Company seat: Lesna 6, 040 01 Kosice, SK
e-mail: safepro@safepro.sk