Overview

Housekeeping is a critical part of any plan to protect against a combustible dust incident, and NFPA requirements are designed to prevent dust layer accumulations from reaching levels which, if suspended, could yield an explosible concentration. NFPA 652 directs readers to the commodity NFPA combustible dust standards for defining the threshold housekeeping dust accumulations at which a dust deflagration or explosion hazard is deemed to exist.

This post summarizes key criteria for defining acceptable limits for combustible dust accumulations with respect to each of the commodity standards and provides detailed analysis of the NFPA 654 limits (2020 version).

Comparison of Standards

The table below summarizes the limits and methods which each of the NFPA commodity combustible dust standards identifies for evaluating the hazard posed by combustible dust accumulations. For complete information, readers should refer to each of the respective standards, but the table does capture the key elements. The Density Adjustments column refers to whether the standard explicitly permits adjusting the acceptable layer accumulations based on the material density.

Among the standards, NFPA 654 and 664 are unique because they give explicit guidance for adjusting the acceptable limits for dust layers based on the bulk density of the material. For example, according to the layer depth criterion approach, NFPA 654 provides a limit for total nonseparated dust accumulations to not exceed 1/32" over 5% of the room footprint area. This is based on a nominal dust bulk density of 75 lb_m/ft³ (1200 kg/m³), and the allowable thickness can be scaled according to the actual bulk density of the material. For a material bulk density of 37.5 lb_m/ft³ (i.e., one half the nominal amount), the allowable layer depth would double to 1/16". Similarly, if the dust accumulation was spread over 10% of the floor footprint area (i.e., double the nominal amount), the allowable accumulation would be cut in half to 1/64".

Evaluation of Layer Depths for NFPA 654

Given our ability to account for dust bulk density, we can plot the allowable levels of dust accumulation per the layer depth criterion of NFPA 654, as shown in the following figure. For a given combination of total room footprint area and total dust area (i.e., total area of nonseparated dust accumulations), a corresponding curve of allowable average dust accumulations can be read. The initial plot is generated for a bulk density of 75 lb_m/ft³, but readers can customize the plot by adjusting the input and/or slider. For scenarios where the conditions fall within the "Negligible" region, any dust accumulations must be negligible, which is identified by NFPA 654 as being 1/64" or less. Readers will observe that dust accumulation areas do not increase once the room footprint area exceeds 20,000 ft². This is a consequence of restricting dust layers thicker than the layer depth criterion from exceeding 1000 ft². For nominal dust layers which are 1/32" thick and with a bulk density of 75 lb_m/ft³, this corresponds to an actual limit of 1000 ft². However, for thicker layers, the equivalent dust mass is present for coverage areas less than 1000 ft².

Limits in Terms of Dust Mass

Accounting for variation in bulk density when setting allowable dust layer thicknesses amounts to imposing a limit on the total mass of dust within a space, which for NFPA 654 is given by:

\[ m_{\text{max}} = \left\{ \begin{array}{rcl} 0.05 A_{\text{floor}} \rho_{_{\text{B}}} L & \mbox{for} & A_{\text{floor}} \le \text{20,000 ft}^2 \\ \text{1000 ft}^2 \cdot \rho_{_{\text{B}}} L & \mbox{for} & A_{\text{floor}} \gt \text{20,000 ft}^2 \\ \end{array}\right. \]

Using the nominal bulk density (\( \rho_{_{\text{B}}} = \) 1200 kg/m³) and layer depth (\( L \) = 1/32" = 0.8 mm), this corresponds to a settled dust layer density of \( \frac{m_\text{max}}{0.05A_\text{floor}} = \rho_{_{\text{B}}} L \approx \) 1 kg/m² (~0.2 lb_m/ft²). Note that this density corresponds to the area where dust is settled, not the full footprint area of the room/space. In the event that a single nonseparated dust layer reaches the 1000 ft² limit, the total dust mass in the space would be limited to 88.5 kg (195 lb_m).

Setting a limit on dust mass is sensible because one of the metrics for measuring susceptibility to ignition is a (mass-based) minimum explosible concentration (MEC). In effect, the goal is to limit dust layers which could subsequently be dispersed and reach the MEC. However, the challenging part of this approach is to accurately estimate the airborne concentration after dispersion, which will depend upon both the entrainment fraction (i.e., how much of the dust becomes airborne) and the dilution level (i.e., to what extent the airborne dust mixes before encountering an ignition source).

The figure below parametrically evaluates the resulting airborne concentration for a dust layer present at 1 kg/m², after being dispersed. The lines correspond to the concentration which will result for any given entrainment fraction and dilution distance. We can imagine the dilution distance in a one-dimensional sense to represent the linear propagation distance for the dust layer assuming that it doesn't move laterally. The dashed line represents the median MEC for all samples reported in the CD Dashboard, and the shaded region represent the limits of the 25th and 75th percentile of all MEC values in the dashboard. Additionally, the vertical dotted line identifies the entrainment fraction of \( \eta_D = 0.25 \) which NFPA 654 recommends using with Mass Method B. For the \( \eta_D = 0.25 \) condition, the dilution distance would need to reach approximately 4 meters before the concentration would become too dilute to be ignited for 50% of the samples in the dashboard. This long propagation distance underscores the importance of limiting dust layer accumulations within a facility.

Comparison of NFPA 654 Methods

Within the multiple NFPA 654 methods for defining acceptable dust accumulations, we can observe that there is partial consistency among the methods. In fact, the reader may suspect this is the case after the prior discussion that the layer depth criterion can be interpreted as corresponding to a total dust mass limit. Consider that we previously determined that the layer depth criterion method imposes a total mass limit of:

\[ \frac{m_\text{max}}{0.05A_\text{floor}} \approx 1 \frac{\text{kg}}{\text{m}^2} \]

Or, on the basis of total mass per room footprint area:

\[ \frac{m_\text{max}}{A_\text{floor}} \approx 0.05 \frac{\text{kg}}{\text{m}^2} \]

Mass Method A provides the following equations for identifying dust flash-fire and explosion hazard areas, respectively:

\[ m_\text{max} = 0.004 \cdot A_\text{floor} \cdot H \] \[ m_\text{max} = 0.02 \cdot A_\text{floor} \]

Upon dividing these equations by \( A_\text{floor} \), we obtain the values of \( \frac{m_\text{max}}{A_\text{floor}} \) that appear in the following table. A height value of \( H = 12 \text{ m} \) was used in the calculation, which corresponds to the maximum value allowed by NFPA 654. The comparison shows that with respect to identifying an explosion hazard area, the Layer Depth Criterion method and Mass Method A are consistent. However, with respect to identifying a dust flash-fire hazard area, the Layer Depth Criterion method is much more lenient than Mass Method A when defining acceptable dust accumulations.