Health, Safety and Environment | Sigma HSE

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Health, Safety and Environment

• Frequently Asked Questions from our Dust
  Explosion Seminars

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What is the significance of MIE (with inductance)
and MIE (without inductance). Which one of them
is lower?

• MIE (without inductance) simulates a purely
  capacitive electrostatic discharge such as from
  isolated conductors in an industrial situation.
  MIE (with inductance) simulates longer duration
  discharges as the introduction of the inductor
  into the circuit, delays the spark discharge to
  earth, hence this corresponds to MECHANICAL SPARK
  SENSITIVITY.
• As the spark duration is shorter for pure
  static electric discharges so the probability of
  ignition is less than for the MIE (with
  inductance) and therefore, in many cases the MIE
  (with inductance) produces a lower value of MIE.
   A review of your facility will determine whether
  electrostatic spark discharges or mechanical
  sparks need to be assessed and the corresponding
  MIE test performed.

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How do you establish the maximum allowable
temperature for equipment using powder properties?

• The maximum surface temperature of enclosure for
  equipment located in hazardous zones is based on
  Minimum Ignition Temperature (MIT of a dust
  cloud) and Layer Ignition Temperature (LIT of
  powder layer). After establishing these values,
  then the lowest value of the two is selected to
  give the maximum surface temperature after
  applying a factor of safety as follows
• For MIT data, Max. Allowable Surface Temperature
  2/3 (MIT)
• For LIT data, Max. Allowable Surface Temperature
  LIT 75o
• Methods of protection along with Ingress
  Protection are equally important along with the
  maximum allowable temperature of equipment.

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What is the significance of particle size for
dust explosions. What is the smallest particle
size where a dust explosion could not occur?

• As the particle size distribution decreases so
  the explosion properties of the powder changes,
  sometimes dramatically. Explosion severity will
  increase, for example aluminium with a particle
  size of 150µm will have a Pmax of 8 barg and a
  Kst value of 150 bar.m.s-1 whereas at lt 10µm the
  Pmax rises to 17 barg and the Kst to gt 650
  bar.m.s-1. The same applies to explosion
  sensitivity where the material becomes more
  sensitive to ignition especially with MIE.  Using
  the same example of aluminium then the MIE value
  drops from 100 mJ to lt10 mJ as particle size
  distribution decreases. Usually it is accepted
  that when the particle size distribution is gt
  500µm then ignition of a dust cloud cannot occur.
  However, even where a powder has a large particle
  size distribution, attrition can occur which can
  result in fines being present. Due to their
  nature these fines will remain in suspension in
  air for a considerable period of time and should
  be taken into consideration when performing a
  risk assessment.

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What is the difference between St1, 2, 3 and St0
H, 1H, 2H?

• Explosion severity is quantified in terms of the
  Maximum Explosion Pressure and the Max. Rate of
  Pressure Rise (which in-turn relates to the Kst
  using Cubic Law, i.e. (dP/dt)max V1/3 Dust
  Constant or Kst). As can be seen Kst is a
  measurement that is used as it is a constant and
  does not change with vessel volume. The St Class
  is based on Kst value, i.e. St 1 (Kstlt200
  bar.m.s-1), St 2 (200bar.m.s-1 lt Kstlt 300
  bar.m.s-1) and St 3 (Kst gt 300 bar.m.s-1). The
  complete explosion severity test is normally
  carried out in a 20 Litre Sphere, although the
  1m3 vessel can also be used.
• Sometimes, for screening purposes (St 0H, 1H, 2H
  determination), you can use the Modified Hartmann
  tube apparatus (1.2 L), which can indicate
  Explosibility and dust explosion class. The MIE
  tube is fitted with a hinged lid and a constant
  arc ignition source. When an ignition occurs, the
  lid opens to various angles depending upon the
  severity of the explosion. Therefore, the results
  are not accurate but do provide an indication of
  explosion violence.
• St 0H No explosion St 0
• St 1H indicates an explosion which indicates a
  St 1 powder.
• St 2H indicates an explosion which indicates a
  St 2 powder.
• Note there is no ST 3 indication and design of
  Explosion Protection Measures cannot be defined
  based on Modified Hartmann tube apparatus
  results. This can only be achieved with full 20L
  Sphere testing.

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What explosion protection measures should be
selected, such as Explosion Pressure Relief
Venting, Explosion Suppression and Explosion
Containment?

• All Explosion Protection Measures serve the same
  purpose and can be used depending on feasibility
  with the process. Explosion Venting is the most
  common example that is being used in industry.
  The important aspect is that all three measures
  must be designed using Explosion Severity data of
  the material being handled. Another important
  aspect is that explosion isolation needs to be
  provided along with explosion protection measures
  to prevent explosion propagation to unprotected
  systems. Also, it is necessary to assess the
  specific requirements for any operation.
  Explosion venting needs to be designed and
  installed correctly and it may not be suitable
  for materials of a toxic nature, explosion
  suppression is expensive to install and maintain
  but can be used in places where venting is not an
  option. Explosion containment needs to be
  maintained over the longevity of the process and
  maintenance engineers must understand the
  complexities of the system.  As stated
  previously, all three methods of protection will
  require explosion severity data (20 litre sphere)
  to ensure safe design.

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