EH-19 Welding Blankets: The Unexpected Hazards
ENVIRONMENT, SAFETY & HEALTH
BULLETIN
Assistant Secretary for U.S. Department of Energy
Environment, Safety, & Health Washington, D.C. 20585
DOE/EH-0039 Issue No. 19 March 1987
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Welding Blankets: The Unexpected Hazards
Some blankets lose too much overall weight to be considered heat resistant
and some release copious amounts of toxic fumes.
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Four workers were overcome by fumes from the welding blanket they were using
during a routine flame-cutting operation. The blanket had been placed around
an insulation pipe to protect it from the welder's torch as he worked nearby
but when the flames struck the blanket as he worked, it released a cloud of
fumes. Suddenly and without warning, he and the three other workmen in the
room were disabled. Although three of them recovered quickly and resumed work
the same day, one was hospitalized. The accident prompted Lawrence Livermore
National Laboratory to test more than 50 different non-asbestos welding
blankets to learn how well they resist heat and what amounts of toxic
products they generate at temperatures reached during routine cutting and
welding operations.
The first test, on a remnant of the same blanket that caused the accident,
revealed that copious amounts of hydrochloric acid (HCl), cyanide (HCN),
nitrogen oxide (NOx), and carbon monoxide (CO) were formed at a temperature
as low as 500 degrees C. By the time the blanket reached 700 degrees C, which
is several thousand degrees below the temperature of a cutting torch flame,
40% of its weight had been lost. The speed with which the workers were
overcome suggests that the toxic agents were quick-acting and disabling, such
as cyanide or carbon monoxide.
Blankets made of silica cloth are extremely resistant to high temperatures,
produce minimal amounts of toxic products, and, therefore, probably are the
best substitute for the previously used asbestos blankets.
At Least 30 Different Types of Blankets Are Sold
The researchers, Anne Lipska-Quinn and Stephen J. Priante, found that 31
different types of blankets were being sold in their part of northwestern
California alone. Although universally sold as "welding blankets," most do
not have accompanying information regarding their limitations, even though
they should be expected to suffer from intermittent periods of exposure to
cutting torch flames. Most are made of fiberglass or silica materials
ranging from felt-like composites and coated laminates to tightly woven
fabrics. The fiberglass materials are often coated with silicon rubber,
hypalon, neoprene, or acrylics; the silica-based materials are often covered
with proprietary coatings. Various manufacturers reported that both silica
and fiberglass cloths are washed in acid (which acts as an organic binder)
after they are woven. Although most of the acid is then removed with water,
residual hydrochloric acid often remains. Some manufacturers heat the
blankets to a high temperature to remove the excess binder prior to
distribution. If the excess binder is not removed, both HCN and NOx will be
formed when the fabric is subsequently subjected to high temperatures.
Some Lose Too Much Overall Weight to be Considered Heat Resistant
First, Lipska-Quinn and Priante tested their 31 samples to determine at what
temperatures degradation began, the rates of weight loss, and the total weight
losses. They used a thermal gravimetric analyzer and conducted the tests from
ambient temperature to 900 degrees C at a heating rate of 20 degrees C/min.
They found that initial degradation of samples varied between temperatures of
40 degrees and 689 degrees C. Total weight losses also varied considerably
among the samples, however the silica fabrics showed little weight change
after an initial weight reduction (probably due to water desorption) at 100
degrees C.
Some Release Substantial Amounts of Toxic Fumes
After making their thermogravimetric tests, Lipska-Quinn and Priante chose a
number of the best performers to analyze for toxic gasses. In the list above,
they eliminated the first eight samples because those samples degraded at low
temperatures, they lost too much overall weight to be considered heat
resistant, and they were coated with either neoprene or hypalon which evolve
large amounts of HCl upon degradation. Sample #21 was eliminated because it,
too, was coated with neoprene. They chose to test Termonol cloth because of
its relative resistance to heat; and Siltemp, Silica fabric #2-3700, and
Thermoglass cloth because of their low overall weight losses.
Each sample was heated to 500 degrees C and its smoke drawn with a hand pump
into a Draeger tube. Two grams of each sample were tested for each individual
gas - CO, CO2, HCN, HCl, and NOx - using a gas chromatograph and a mass
spectrometer. Concentrations could be determined by the degree and extent of
the color changes in the sensing material.
Thermogravimetric analysis of 31 non-asbestos welding blankets
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Temperature
leading to Total weight
initial loss
weight loss
Sample identification (degrees C) (%)
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1. Duck heavy-weight hard finish 311 94.30
CFM approved
2. Weld Tex vinyl-coated fabric 276 97.60
3. Fro-Prene nylon fabric coated with 266 84.00
Hypalon
4. Armor-Pleated heavy-duty Hypalon fabric 285 92.07
5. Fro-Lon #1 fiberglass cloth with 277 30.18
Hypalon coating
6. Fro-Lon #2 fiberglass cloth with 220 19.06
Neoprene coating
7. Fro-Sil fiberglass cloth with rubber coating 220 7.96
8. Jaxcolite glass cloth 289 40.36
9. Thermonol cloth 569 71.26
10. Weld flex 205 41.07
11. Weld-O-Glass #2300 (fiberglass) 289 44.60
12. Fro-Flex special fiber blend 291 43.20
13. Fromelt, silica fabric 328 19.64
(9.81)(a)
14. Siltemp. silica fabric 182 15.33
(9.92)(a)
15. Silica fabric #2-3700 182 18.29
(9.89)(a)
16. Thermoglass cloth 40 5.25
17. LLNL Stores stock welding blanket, 307 3.46
Sample #1
18. LLNL Stores stock welding blanket, 218 17.95
Sample #2
19. LLNL Stores stock welding blanket, 207 10.38
Sample #3
20. Weld-O-Glass #2-1900 (same as Silica 182 18.29
#2-3700) (9.89)(a)
21. Weld-O-Glass #3000R 452 13.22
22. Weld-O-Glass #2000G 274 21.75
23. Nor-Fab material 689 61.05
24. Thermo-Sil Style #1550 299 22.61
25. West Mac Co. #HR100-96 324 22.03
26. Weld-O-Glass #900FC 346 2.85
27. Weld-O-Glass #1000FC 380 2.54
28. Weld-O-Glass #2650 381 0.82
29. Weld-O-Glass #1850 503 0.63
30. Zetex 1100 245 3.42
31. Zetex 1200 313 2.39
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(a)Percent moisture loss.
Silica Cloth Found to be Best
Although they caution that care must be taken in predicting the specific
performance of any given blanket based on their small-scale laboratory tests,
Lipska-Quinn and Priante have concluded that unless welding blankets are
composed of silica cloth, they will lose a considerable amount of weight when
exposed to temperatures as low as 300 degrees C and will form substantial
amounts of toxic products. Blankets made of silica cloth are extremely
resistant to high temperatures, produce minimal amounts of toxic products,
and, therefore, are probably the best substitutes for the previously used
asbestos blankets.
Suggested Actions
Based on the findings of the researchers, LLNL changed its purchasing policy
and now buys only those welding blankets that demonstrate acceptable levels
of out-gasses when exposed to high heat. Testing is relatively quick and easy
using measuring devices that are designed for individual gases, such as
Draeger tubes. For best results, the indicator tube should be placed directly
in the smoke stream.
Note: There was a ventilation fan in the area where the men were working but
they did not use it. (Since the accident, LLNL management has strengthened
procedures to assure that workers use ventilating fans while they are welding
and flame-cutting inside poorly ventilated areas.)
In addition, the workers had no cooling water or fire extinguishers within
easy reach. Such equipment might have been useful to cool the work and
quench the production of decomposition products.
Finally, they mistakenly assumed the blanket could effectively endure direct
exposure to the cutting-flame. No welding blanket, even if made of asbestos,
can prevent heat transfer and eventual degradation if it is subjected to the
flame directly.
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Toxic-product formation from welding blanket
samples heated in air at 500 degrees C
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Toxic products and amounts
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CO CO HCN HCl NOx
Sample identification (%) (%) (ppm) (ppm) (ppm)
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1. Thermonol 0.5 5 150 50 N.A.(a)
2. Siltemp 0 0 2 20 1
3. Silica fabric #2-3700 0.25 0 9 20 6
4. Thermoglass 0 0 10 0 3
5. LLNL Stores stock blanket sample 0.5 5.0 30 50 10
6. Weld-O-Glass #900FC 0.5 2.5 75 10 7
7. Weld-O-Glass #1000FC 0.5 2.0 75 100 0
8. Weld-O-Glass #2650 0.1 0 15 10 7
9. Weld-O-Glass #1850 0.1 0 30 10 15
10. Zetex-1100 0.5 trace 50 0 trace
11. Zetex-1200 0.4 trace 25 10(b) 0.5
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Threshold Limit Value 50 ppm 5000 ppm 3 ppm 5 ppm 5 ppm
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(a)Too much interference by other compounds; cannot interpret results
(b)Color change is due more to Cl2 than to HCl.
The above materials were not coated with a special organic finish, thus the
HCN and NOx were degradation products from an organic binder used during the
fiber-manufacturing process. For comparison, the bottom row of the table
lists the allowed threshold limit value (TLV) of these agents as defined by
OSHA. The blanket which caused the accident was sample #5.
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Information for this Bulletin was provided by Norman Alvarez, Hazards Control
Department, at DOE's Lawrence Livermore National Laboratory.
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Bulletin is published so that DOE program managers and contractors can share
information about potential health and safety problems relevant to DOE
operations. For additional copies, contact Nona Shepard, Editor, Office of
Operational Safety, Assistant Secretary for Environment, Safety & Health,
U.S. Department of Energy, Washington, DC 20545. Telephone: FTS 233-2958;
Commercial 301-353-2958.