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.