Suspended Particulate Matter Definition - SPM and RSPM

Practical Determination And Location Of Windblown Dust Sources And How To Establish A Good Monitoring Program - Publication in Chemical Technology December 2003 Edition

Publication in Martin Creamer's Engineering News
magazine, November 2002

Paper written on different monitoring solutions available

Research:  How does vegetation affect fall-out dust?

Information:  How far do dust particles travel?

Information on First Aid for Snake Bites in the Cape Peninsula

Dust Particles and Lower Explosive Limits (LEL) - Instruments

Health and Safety Induction Training Material - www.safetyhealthtraining.com

The effect of heavy & sustained rains on fallout dust monitoring

What not to do with your equipment

Reactions on Atmospheric Dust Particles

Practical Determination and Location of Windblown Dust Sources and how to Establish a Good Monitoring Program

New Dust Monitoring Technology Developed

Airborne Pollutants - Current and Future Issues in Environmental Monitoring



How does vegetation affect fall-out dust?

 

How far do dust Particles travel?

Note that the information provided here is purely to be used for estimation purposes.  If accurate calculations are required, then please look at the original sources.

It is often required to estimate how far dust particles will travel once they are liberated into the air by moving cars, wind, or fire.

The factors that affect this are:

particle size
wind velocity
height at which the dust is liberated.

Particle size is usually the most important factor because the terminal settling velocity is highly dependent on this particle size.

The information below is taken from the text book "Environmental Engineering in South African Mines" published in 1989 in association with the Mine Ventilation Society of South Africa.  Chapter 12. 1

Sizes of Dust Particles

The geometric diameters of air-borne particles may vary between 0.001 µm and 100 µm.  The figure below indicates the size range for a few common particles.

From the diagram it can be seen that dust particles are seldom larger than 100 µm.

Terminal Settling Velocity - Stokes' Law

The gravitational force acting downward on a free falling sphere is:

Where
d = the geometric diameter of the sphere (m)
Ws = the density of the sphere (kg/m3)
Wa = the density of the air (kg/m3)
g = acceleration due to gravity (m/s2)

The drag forces acting in resistance to the fall are:

= the Velocity of the particle (m/s)
= viscosity of the fluid (kg/(m*s)

If the motion of the fluid around the particle is symmetrical, the terminal velocity of the sphere is reached if  G = F.  Equating these two equations yields:

This is known as Stokes' law.  It applies to spheres of size below that at which their own velocity creates turbulent flow and (NReynolds greater than 1) or in other words spheres approximately less than 250 µm.

Click here to be able to determine your own settling velocities.

A unit density quartz sphere of 1 µm would require almost 13 hours to drop from a height of 1.6 metres (theoretically).  When particles are very small (less than 1 µm) the actual settling could take much longer because of the bombardment by air molecules, which cause random Brownian motion.  In fact particles having terminal settling velocities of the same order as the displacement caused by the Brownian motion will remain permanently suspended, even in still air.

Air pressure and moisture content will affect the terminal settling velocity to some extent, basically because of the effect these parameters have on the density of air.

For particles with a geometric mean diameter of 0.1 µm the Brownian displacement is about 15 times that of the settling velocity.  For particles of 0.01 µm it is almost 900 times.  This may also be of consequence in gold mines, as it has been found that nearly 80% of the dust particles in mine air are smaller than 1 µm.  These particles may thus penetrate deeply into the alveolar region of a miner's lung.  Admittedly they may deposit in the respiratory tract by impingement or aggregate due to electrostatic charges and cohesion forces to form larger aggregates which will then settle at a finite terminal velocity.

While most of our focus is on dust that is generated at ground level, larger dust (greater than 250µ

Height of stack 100 metres
Size of particles, 250 µm
Terminal settling velocity, 2.5 m/s
Wind speed, 10 m/s or 36 kph or 22.37 mph
Assumption:  acceleration time is not
included in this estimation.

As the particle size increases, so Stokes' Law is no longer applicable and other formulae need to be used.  As an example the settling velocity of a 0.2 cm or 2 mm particle is about 73 m/s.  This particle may be blown by the wind, but it will not blow very far unless the winds are constantly above about 100 kph or 62 mph.

As an aside, the following table is interesting taken from Perry's Chemical Engineers' Handbook, sixth edition, Robert H. Perry and Don Green, pg 5-64.

Particle Reynolds Number ** Orientation
0.1 - 5.5 All orientations are stable when there are three or more perpendicular axes of symmetry
5.5 - 200 Stable in position of maximum drag
200 - 500 Unpredictable.  Disks and plates tend to wobble, while fuller bluff bodies tend to rotate.
500 - 200000 Rotation about axis of least inertia, frequently coupled with spiral translation.

Free-Fall Orientation of Particles

** Based on diameter of a sphere having the same surface area as the particle.

Irregular particles on falling, will not take up a preferred orientation as would particles having an axis o symmetry and they may fall edgewise, for example.  The shape and surface of a free falling particle will thus influence its rate of fall in a sense that the particle will always attains a velocity smaller than that of a smooth, regular sphere of equal radius.

Dust Particles and Lower Explosive Limits (LEL) - Instruments

Dust particles have a minimum or lower explosive limit to almost no upper limit. Here are examples of minimum explosive limits (oz/ft3): Polystyrene (0.02) Cornstarch (0.04), Coal (0.055), Iron (0.12).

It is important to note an explosimeter gives a reading as a percentage. The reading is based on the percentage of the LEL and not the full concentration of the vapour or gas in the mixture. For example a 50% explosimeter reading of a gasoline/air mixture really translates into 0.7% gasoline (50% of 1.4%, the LEL). It is common practice to consider a 10% explosimeter reading or 10% of the LEL, as a safe working area. When taking explosimeter readings, consider the possibility that the vapour or gas may have accumulated in recessed areas (top or bottom of tank depending on the density of the gas or vapour).

Quick Contact

Chris Loans

Tel : 021 789 0847

Fax : 086 6181 421

Cell : 082 875 0209

Email : chris@dustwatch.com

Dustwatch CC Disclaimer

All information pertains to the South African context, but in many cases equivalent legislation is available in other countries. The information contained in this website is our interpretation of the acts and guidelines available.  While we may have ventured our opinion on certain issues, our opinion is based on our experience in the field of Fall-out Dust Monitoring.

We hope that the information contained in our website will give you some direction on issues related to fall-out dust monitoring.

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