Flash fire protection

As indicated above, the most significant hazard requiring protective clothing in the oil and gas sectors has been the potential for flash fires. Thus, in these sectors, protection from flame engulfment is more important than protection from purely radiant exposure. To provide protection from flash fires, a garment system, and thus any material comprising the system, must have the ability to:

• resist ignition and self-extinguish when ignition source is removed

• limit heat transmission during a short-term exposure to high heat flux

• not melt or shrink upon exposure

• maintain its structural integrity and flexibility upon exposure.

As well, the material ideally should not generate much smoke or toxic combustion product when exposed to flame.

There are many bench-scale tests that address flame resistance and heat transmission. Such tests are covered in detail by Horrocks in Chapter 15. One factor that is important in considering flash fire protection, but upon which little agreement exists, is whether materials should be tested in contact with the sensor or with air gaps, especially when testing single-layer materials. Torvi et al. (1999) used numerical modelling and flow visualization to study the effects of such air gaps, and unlike others who had speculated various optimum gaps, concluded that there is no optimum value.

Dale et al. (2003) discussed several limitations of bench scale tests and issues regarding their use. While flame resistance tests may take into account shrinkage and structural integrity after exposure, most heat transmission tests do not do so adequately. Dale et al. (2000) described the effect of changing the test geometry from planar to cylindrical on the thermal protective properties of single-layer fabrics. If shrinkage occurred during exposure it shortened the time to the end point, indicating poorer fabric performance. Some materials showed over 35% shorter times than when tested in the planar configuration. In addition, Crown et al. (2002) demonstrated a better correlation with full-scale manikin test results and tests of single-layer fabrics when using the cylindrical, rather than a planar, test.

Researchers generally agree that only a complete garment test can come close to reality when testing thermal protection (Dale et al., 1992; Prezant et al., 2000; Zimmerli, 2000). Such full-scale tests of garments or, more realistically, layered garment systems on an instrumented manikin exposed to a simulated flash fire not only take into account shrinkage and structural integrity better than do bench scale tests, but also account for the effects of air gaps or, more generally, of garment style and fit. Facilities for such tests exist in several countries (Zimmerli, 2000). The design and construction of one such facility built primarily for testing protective clothing for the oil and gas sectors are described in detail in Crown et al. (1989) and Dale et al. (1992). A recent upgrade of this facility providing capability of extending the test duration up to 20 seconds for multi-layered garment systems is described by Ackerman et al. (2004).

Several test variables affect the results of instrumented manikin testing of garments and garment systems typically used in the oil and gas sectors. These include, among others, the preconditioning of the specimens, the shape, size and stance of the manikin, the duration and the heat flux distribution of the simulated flash fire exposure, the type, distribution and calibration of sensors used to measure heat transmission, the skin burn injury model and the data analysis routines used. In an attempt to standardize these and other exposure variables a standard test method was developed by ASTM (2000) and a revision of this method is under consideration as an ISO test method. Although procedures are standardized, exposure conditions may vary but must be reported. The ASTM standard includes specification of a standard garment for evaluating material properties and allows for comparisons of different garment designs with the same material, as well as testing actual end-use garment specimens. End-use garment testing is generally of most interest both to protective apparel producers and their clients in the oil and gas sectors.

Material evaluation

When evaluating garment systems for the oil and gas industries on an instrumented manikin, a 3 or 4 s exposure at 80-84 kW/m2 is normally considered appropriate because such conditions simulate real-life flash fires. Garments typically used by the oil and gas sectors include those fabricated from meta-aramid fabrics, flame-retardant (FR) treated cottons and viscose, FR viscose/aramid blends and para-aramid/pbi blends. Although several fabric and garment system design variables affect results, it has generally been found that when tested with or without underwear at 3 s exposures, typical FR cotton and viscose fabrics and viscose/aramid blends of approximately 300 g/m2 provide somewhat better protection than do the lighter (approximately 200 g/m2) aramid and aramid/pbi fabrics, due primarily to their greater thickness and mass. At 4 s exposures, however, the FR treated fabrics may disintegrate and the lighter aramid and aramid/pbi tend to offer as good or better protection. The FR treated materials also generate more combustion products. (It should be noted here that some laboratories will not test modacrylic garments on their manikin systems due to the possible generation of acidic, toxic combustion products.) At both 3 and 4 s exposures, many meta-aramid garments shrink and, depending on the fit of the garment, may lose protective quality when air gaps are decreased due to shrinkage.

Garment system design

The design and fit of the garment system are as important as the material used to fabricate the garments. Oil and gas sector employees work both in extremely hot environments (outdoor climate or indoor plant conditions) and extremely cold outdoor environments. For the former, many workers may choose to wear nothing but light underwear under the lightest possible single-layer protective garment. For these workers, choice of underwear is important for both comfort and protection. If wearing light-weight outer protective garments, especially if the material may shrink and come into closer contact with the skin, undergarments can provide a needed extra layer of protection for vulnerable areas of the body. It is important that such undergarments do not melt when exposed to the heat that may be transmitted through the outer garment during a flash fire exposure. For that reason, synthetic thermoplastic materials such as polyester, nylon, polyethylene and PVDC are not recommended for undergarments. Although it has not been demonstrated that close-fitting underwear needs to be flame resistant, Crown et al. (1993) demonstrated that aramid underwear offers more protection than does similar cotton underwear.

Those who work in cold environments need to wear multi-layer protective ensembles such as parkas or insulated coveralls. Likewise, many workers need to wear water- or chemical-resistant garments over their thermal protective garments, to protect against drilling mud or processing chemicals. In the early days of wearing protective clothing, it was the practice of many oil sector workers to wear flammable winter garments, rain gear, or disposable coveralls over an FR protective coverall. It has been demonstrated very clearly, however, that the outermost layer of protective clothing systems for the oil and gas sectors must be flame resistant (Crown et al., 1993). This research also demonstrated the important benefits of garment layering in general. Protection provided by multi-layered garment systems was significantly greater than would be expected from the additive effects of the layers used singly. This result is due mainly to the added insulating effect of air between the fabric layers. Thus, for vulnerable areas of the body such as shoulders where there is no room for air between the garment and the body, at least two layers of fabric are recommended, even in what are otherwise considered single-layer garments. The potential of multi-

layered garments to store and later release energy is recognized and has been studied recently by Song (see Chapter 11).

Research on military flightsuits has demonstrated the benefits of incorporating air gaps through controlled fullness in garment design. The protective value of air gaps is lessened, however, in garments fabricated of materials that shrink upon exposure (Crown et al., 1998; see also Chapter 24). More recently, the effects of different widths of air gaps within garment systems have been studied through the use of body scanning technology to quantify the gaps (Lee et al., 2002; Kim et al., 2002; Song et al., 2004). These studies confirm the benefit of incorporating air gaps through garment design.

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