An exothermic reaction may lead to electrostatic charge generation with certain chemicals if the reaction causes excessive turbulence or formation of fine liquid droplets.

NFPA 664, Paragraphs A.8.6.2.2 states: “Provide a spark detection and extinguishing system on the main airflow duct between the dryer drum and cyclone. The spark extinguishing system should activate every time a single spark is detected [emphasis added]. It will reset after a few seconds (if no additional sparks have been detected), and the dryer can continue to operate. The spark counting features available in some approved spark extinguishing systems can be used to shut down dryers when an excessive number of sparks are encountered, but they should never be used as a measure of when to actuate the extinguishing spray.”
There is no discussion concerning “black-body radiation” in NFPA 664, and the only discussion of “black body” is in NFPA 1971 and concerns flammability of clothing. However, there is good discussion of detection methods – including a mention of “Planck’s Law” [regarding black-body radiation] – in NFPA 72, in Sections 3.3, 5.8, A.5.8, and B.5.1.4.
If small, low-energy sparks occur so frequently that spark extinguishment interferes with production AND a prolonged historical record indicates no significant explosion hazard from such sparks, then a “cut-off” based on thermal [black-body] radiation might be appropriate. However, for infrequent sparking, a large low-temperature spark or firebrand might have a temperature below an established black-body temperature criterion but might have more energy – to ignite a dust cloud – than a small particle having a temperature far above the black-body temperature criterion.
Thus, the “bottom-line” response to this question would be a suggestion to consider the above-quoted guidance from NFPA 664, and detect and extinguish every single spark.
Based on the results of a recent explosion investigation by Chilworth, it also would be prudent to count all sparks and document the counts, rather than document the number of extinguishment actions. An increasing number or frequency of sparks could indicate serious problems with upstream equipment.

The burning of ibuprofen and sulfamethoxane powders in your vibratory sifter could be attributed to electrostatic “spark” discharges if the following conditions were simultaneously present in the sifter:
• A conductive (metal) section of the sifter that comes into contact with the powder such as the sieves/screens or the inlet or outlet conveying chutes is electrically isolated from ground
• The electrical capacitance of the ungrounded section of the sifter is sufficiently large to accumulate enough electrostatic energy to ignite/burn the dust particles, when released as a spark
• The movement (flow/vibration) of the powder particles over the ungrounded metal section gives rise to a sufficiently high level of electrostatic charge buildup on that section
• The isolated metal section is close (2-4 mm) to a grounded part of the sifter so that the spark can jump from the charged section to the grounded/uncharged section
• Dust particles are present within the spark gap
To prevent “spark” discharges I suggest that all the metal sections of the vibratory sifter be electrically bonded together and connected to ground so that the resistance to ground is less than 10 ohm. Please note that I have assumed that the vibratory sifter does not comprise of any plastic sections such as plastic pipes.
I also suggest that other sources of ignition such as frictional heating, frictional sparks, electrical arcs (from electric motors, etc), and self-heating be investigated and eliminated/ruled out.

Occasional fires in iron dust can be attributed to a low rate of heat transfer [resulting from iron-particle oxidation] to the external environment, resulting from occasional thick layers of dust and prolonged infiltration of air into the “center” of the accumulated dust. To prevent fires in iron dust, one may consider avoiding the formation of thick accumulations of dust by frequent [or continuous] removal of dust from trays, hoppers, or drums, or discharge iron dust into an “excess” of water [since "moist" iron dust could generate hydrogen gas].

Generally speaking the measures that may be considered in order to reduce the risk of a flash fire/explosion during the charging of powders from bags into a dump station include:
• Ensure that all the metal sections of the bag dump station are electrically bonded together and grounded
• If the Minimum Ignition Energy of the dust cloud is less than 30 mJ, ensure that the operator(s) are grounded (resistance to ground less than 100 M ohm)
• Ensure that the dust exhaust system is preventing the spread of the dust cloud into the room
• Avoid vigorous dumping of powder from bags into the dump station
• Prevent the accumulation of any dust outside of the bag dump station by implementing an effective housekeeping procedure
• Seek expert advice (consider conducting an onsite dust explosion hazard assessment to identify and control all potential ignition sources and make provisions to protect people and the plant against the consequences of a flash fire or explosion)

There is not enough information provided in order for us to provide specific replies to the above questions. However, the following general comments are made to help push the discussion forward:
1. “Granulated material of wood chips with plenty of dust, ambient or high temperatures and 40 to 100 psig inside silos”
Space inside silos should be classified as Class II, Division 1, Group D hazardous location. If silos are located indoors, depending on: (a) amount of dust being released from silos to surrounding area during normal and abnormal operating conditions, and (b) housekeeping practices, the area around silos’ dust release points should be also classified as Class II, Division 1 (or 2), Group D hazardous location using NFPA 499 guidelines.
2. “The above material transported via screw conveyors”
Space inside screw conveyor enclosures should be classified as Class II, Division 1, Group D hazardous location. If the conveyors are located indoors, depending on: (a) amount of dust being released from conveyors to surrounding area during normal and abnormal operating conditions, and (b) housekeeping practices, area around conveyors dust release point should be also classified as Class II, Division 1 (or 2), Group D hazardous location using NFPA 499 guidelines.
3. “Electrical Motors that can be exposed to the granulated material in case of upset condition”
If granulated material contains sufficient amount of fine dust it would be prudent to classify area as Class II, Division 1 or Division 2 (depending on presence of combustible cloud and/or combustible dust layers) hazardous location and motor should be rated and installed according to NEC requirements for such an area.
However, if granulated material does not contain fine particles less than 420 micron in combustible concentration and hazardous area classification is not required per NEC, there is still the possibility of a fire hazard. Upset conditions involving granulated materials quite often involve significant spills that can partially or completely cover the motor. This could create a condition that: (a) affects dissipation of heat associated with the motor thermal loses. This may lead to the motor overheating resulting in motor fire, and/or exothermic decomposition (spontaneous combustion) of granulated material; and (b) material can penetrate the motor enclosure and jam the rotor.
Motors that are subject to such upset conditions should be: (a) equipped with an internal high temperature sensor having contact connected with the motor control circuit in such a manner that the motor is stopped when internal temperature exceeds the temperature rating of the motor internal insulation; and (b) should have effective ingress protection to prevent the powder entering the motor enclosure (motor should be at least TEFC or TENV type).
4. “Grounding system that could fail during upset condition”
Grounding system should be designed with redundancy that ensures equipment grounding even during “the worst case scenario” upset conditions.
As I mentioned above, these comments are based on the information that was provided and is therefore not intended to provide specific answers.

It is likely that an exothermic reaction was the ignition source for the fuel in the dust collector. Of the two types of fuel – steel dust, and oil – it is more likely that the oil was the fuel and also [indirectly] the ignition source. It is well-known that “partial oxidation” of hydrocarbons is exothermic – leading to an increase in temperature – and can lead to the formation of aldehydes which have low auto-ignition temperatures. For acetaldehyde, the AIT is 175 °C, and a thick layer of oily dust can prevent escape of the heat that results from the “partial oxidation”. To prevent recurrence of fire, you may consider either: (1) discharge the accumulated dust into water, or (2) minimize accumulation of oily dust by frequent removal from the dust collector and deposit the dust in water prior to storage, transport, and/or disposal.

The following factors should be considered while handling carbon black dust:
In my experience, fine carbon black dust is an explosible/combustible material. If you are unsure whether the grade of carbon black dust that you are handling/processing is explosive or not, consider conducting an explosibility screening test (Go/No Go test) on a representative sample of the carbon black.
If the dust is found to be explosive, minimize the spread of the dust cloud by the provision of an effective local exhaust ventilation, prevent the accumulation of dust on the outside surface of the equipment and in the room by adapting an effective housekeeping procedures, and control potential ignition sources.
Ways to control potential ignition sources include: ensuring that all metal sections of the bag dump station are bonded together and grounded; conducting a hazardous area classification to ensure the use of appropriate electrical equipment; preventing foreign objects from entering the screw conveyor; maintaining the mechanical (moving) parts of the system to prevent their failure. Additionally, if the Minimum Ignition Energy (MIE) of the dust cloud is less than about 30mJ, the operator(s) handling the dust must be grounded. Also, protect processing equipment such as receiving vessel(s) and dust collector(s) against the consequences of an explosion by the provision of appropriate protection systems (e.g. explosion relief vents).
Also, seek expert advice on identification, assessment, and control of potential dust cloud explosion hazards.

The following factors should be considered while conducting the cleaning of combustible paper dust:
Wrapping the outside surfaces of the standard plastic hose (generally a highly insulting material) with a copper wire would not prevent the buildup of electrostatic charges on the inside surfaces of the hose. Indeed, very energetic electrostatic discharges could result that are capable of igniting most combustible dust cloud atmospheres.
The use of water spray may prevent the accumulation of electrostatic charges on the inside surfaces of the hose if all the surfaces are adequately wetted and the wet surfaces are in contact with electrical ground at all times. An adequate water spray may also render the paper dust particles non-combustible. However, one may end up with electrostatically charged isolated wet surfaces (patches) that could produce energetic sparks if the water spray is inadequate or intermittent.
I suggest that you consider using an electrically grounded conductive or static dissipative hose or performing a dust explosion hazard assessment on the operation with the objective of identifying, assessing, and controlling dust cloud explosion hazards throughout the operation.

The question lacks some details about the design. Our response below assumes that the PVC pipe is going through the coal silo carrying drain water.
The PVC pipe may be used for this application provided that all of the following conditions are met:
• The only material that goes through the PVC pipe is water,
• The water flow through the PVC pipe is slow and does not result in the formation of droplets within the pipe,
• The mechanical integrity of the pipe is maintained and no water can leak into the coal silo,
• There are no electrically isolated conductive (metal) objects such as metal brackets or flange coupling on the outside of the PVC pipe, and
• There is no flammable vapor/gas atmosphere present in the coal silo at any time.
Generally speaking, it is suggested that an electrically grounded conductive pipe is considered instead of the proposed PVC pipe. Alternatively, consider routing the pipe from the outside of the silo.