Performance of NBC protective filter 74 in relation to industrial pollutants
Published in :
Schweizerische Feuerwehr-Zeitung 11/91
W. Arnold,
LABOR SPIEZ,
3700 Spiez
Introduction
What is the shelf life of an air filter? This is the perennial question among chemists specialising in gas protection. Rarely are the given a satisfactory and conclusive answer because the performance of such filters cannot be determined based on filter-specific parameters; their interception of noxious gases depends heavily on the conditions in which they are used. However, we can list the parameters which are decisive to determine their performance:
- Filter
- Sorbent quantity
- Geometric configuration of the sorption membrane
- Respiratory rate
- Sorbents
- Type/grain size/impregnation
- Surface oxidation/ageing
- Pre-load with the same or other substances
- Toxic chemicals
- Type(s): chemical, physical-chemical and physical characteristics
- Number of substances to be adsorbed simultaneously
- Concentration behaviour over time in relation to the filter
- Sorbents
- Type/grain size/impregnation
- Type/grain size/impregnation
Combination filters, such as the NBC protective filter 74 which is a component of the protective masks used by the Swiss armed forces, function in two ways. They must be able to intercept both coarse and very fine dust particles and/or droplets (suspended solids-aerosols), and gaseous pollutants circulating in the air. A two-stage procedure is used to test the performance of these filters. Every NBC protective filter comprises two components: a glass-fibre filter to intercept suspended solids filter and an activated carbon membrane to intercept gases. Both tend to be used together, e.g. in screw filters or in a range of collective filters found in protective shelters. The sequence must be adhered to strictly. First, the air passes through the suspended-solids filter, where the intercepted droplet evaporates relatively quickly in the airflow. The next step involves the gaseous particles being retained in the activated carbon membrane.
The suspended solids filter offers protection against the entire spectrum of dust and aerosols which can be present following a civilian chemical incident. The sequence in which the activated carbon component intercepts the gaseous substance is primarily as follows: physical adsorption, chemisorption and catalysis. In accordance with DIN 3181, the NBC protective filter 74 is a B2P3 filter, i.e. a combination filter.
Measurements and results
The SPIEZ LABORATORY boasts state-of-the-art instruments and apparatus to measure the protective performance of filters in relation to industrial pollutants. It operates a sorption and substance database which it continually updates.
The results are based on the following measurement parameters:
| Volume of air flow: | 2 m3/h |
|---|---|
| Temperature: | 293 K (20 °C) |
| Pressure (Entry filter): | 93 kPa (930 mbar) |
| Filter conditioning: | In airflow of 2 m3/h, 48 h, 20 °C, at corresponding air humidity |
As previously mentioned, the NBC protective filter 74 corresponds to the norm B2P3 filter, because it is recommended for use against hydrocarbons and acid gases. When using a screw filter (therefore according to DIN filter category 2), the maximum concentration of a pollutant should not exceed 0.5 vol-%. As a rule of thumb, the filter should offer adequate protection if the boiling point of the organic substance is below 65°C. According to DIN 3181, a special type of filter, an “AX filter”, should be used to protect against these substances. The NBC protective filter 74 can also be used in this regard, as Table 1 shows.
| Substance | Boiling point [°C¨] | Concentration upon penetration [ppm] | Penetration time [min] |
|---|---|---|---|
| Dimethyl ether | -24.8 | 10 | 5 |
| Methyl iodid e | 42.5 | 2 (TLV) | 34 |
| 2-Aminobutanol | 63.0 | 5 (TLV) | 175 |
| Diethylamine | 56.0 | 10 (TLV) | 150 |
| Chloroform | 61.1 | 10 (TLV) | 60 |
| Acrolein | 52.0 | 0.1 (TLV) | 87 |
| 1,1-Dichlorethene | 48.0 | 10 (TLV) | 58 |
| Dichlormethane | 40.7 | 100 | 41 |
| Acetaldehyde | 20.8 | 50 (TLV) | 12 |
| Acetone | 56.2 | 100 | 74 |
| Methyl formate | 31.5 | 100 | 28 |
| Methanol | 64.7 | 100 | 32 |
| Diethyl ether | 34.6 | 100 | 65 |
| n-Pentane | 36.1 | 100 | 68 |
| Dimethoxymethane | 45.5 | 100 | 65 |
| Methyl acetate | 56.9 | 100 | 82 |
Protection against acid gases (e.g. hydrochloric acid, sulphur dioxide, hydrogen sulphide, chlorine, bromine etc.) is relatively good (cf. Fig.1). However, it performs less well when exposed to basic gases such as ammonia. The NBC protective filter provides little protection against (“nitrous gases”). According to DIN 3181, a special filter “Filter NO” should be used to protect against these gases, as well as against carbon monoxide, mercury vapour and radioactive iodine.
The aforementioned restrictions on its use mean that the NBC protective filter performs satisfactorily under realistic immission concentrations. However, it does not provide adequate on-site protection against high and highest concentrations, to which the emergency forces (fire service, chemical defence teams) are likely to be exposed at the scene of a chemical incident scene. Please note that the filter cannot provide oxygen which is absent in burning objects, septic tanks, silos etc.
We are able to provide the following answer in relation to the possible consequences of a nuclear power plant accident: The NBC protective filter cannot adequately intercept inert radioactive gases such as krypton and xenon isotopes, as their boiling points are too low (-152.3°C and 107.1 °C respectively). In terms of chemisorption, inert iodine remains as a particle in the aerosol filter or is stored in the activated carbon membrane. The protection the NBC filter offers against radioactive methyl iodide is not overwhelming, and heavily depends on the water content of the air and the carbon (cf. Tab.2). Nevertheless, the filter performs well when exposed to low concentrations expected from immission.
| Air humidity [% R.F.] |
Filter conditio- ning [% R.F.] |
Penetration time [min] |
|---|---|---|
| 0 | 0 | 65 |
| 50 | when opened | 43 |
| 50 | 50 | 34 |
| 70 | 70 | < 5 |
| 90 | 90 | < 2 |
- Fig. 1: Sorption of NBC protective filter 74 of several important gases according to concentration, and tested under a relative air humidity of 60% (conditioned filter). The concentration upon penetration is the given threshold limit value of the substance.
Conclusion
The NBC protective filter 74 (and generally the collective filters which are approved by the FOCP for use in protective structures) offers protection against industrial pollutants. The level of this protection, however, depends on the given situation in which it is used. Substances with a high boiling point do not present any problems, while for liquids their type and concentration are decisive factors in the protective performance of the filter.