Coal Dust Explosion Proof Centrifugal Fan

Overview

Pulverised coal is a combustible dust with risks from both dust clouds and dust layers. Unlike gas atmospheres, dust duty also requires dust accumulation to be designed out, because a high protection rating cannot prevent self-heating in a settled layer.

Dust Ignition Protection Configuration

Dust protection combines the type of protection with the temperature limit. The table shows a typical configuration for pulverised coal duty.

Requirement	Configuration	Basis
Motor	Dust ignition protected Ex tb IIIC T130 C Db	Per GB 12476.1 and GB 12476.5
Impeller	Aluminium alloy or stainless steel with anti-static treatment	Controls frictional sparks and static discharge
Casing	Sloping floor and anti-sediment construction	Reduces dust accumulation and self-heating
Shaft seal	Dust seal with isolated bearing housing	Keeps dust out of the bearings
Earthing	Equipotential earthing and duct bonding	Removes static build up

Anti-sediment and Operating Measures

Requirement	Configuration	Basis
Anti-sediment design	Avoid horizontal surfaces and dead corners; provide cleanout openings	Reduces accumulation by construction
Periodic cleaning	Scheduled cleaning and inspection	Prevents dust layers remaining in place
Temperature monitoring	Bearing and casing temperature monitored online	Detects abnormal temperature rise early
Inerting connection	Provision for an inert gas connection	Reduces oxygen content where required
Maintenance procedure	Shutdown, cleaning and gas freeing before work	Controls maintenance risk

Applications

Applications include pulverised coal preparation and conveying, coal mill systems, pulverised coal injection systems, coal bunkers and coal handling dust collection.

The ignition temperature of a coal dust layer varies with coal type and has to be measured; the equipment surface temperature limit should retain a safety margin against the measured value.

System level protection depends on the fan together with the ducts and the dust collector, so a compliant fan alone does not make the system compliant.

Acceptance and Verification

Standard	Title	Purpose
GB 12476.1	Electrical apparatus for use in the presence of combustible dust - Part 1: General requirements	General requirements for dust ignition protected equipment
GB 12476.5	Electrical apparatus for combustible dust atmospheres - Part 5: Protection by enclosure tD	Dust ignition protection by enclosure
GB 3836.1	Explosive atmospheres - Part 1: Equipment - General requirements	General design and marking of explosion protected equipment
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
GB/T 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing grade

Please provide coal type and dust characteristics, dust concentration, medium temperature and air flow. Size and motor power are calculated for the actual duty.

Explosion Protection Marking and Coverage

The explosion proof certificate held for our fans covers the marking Ex db IIC T4 Gb; Ex tb IIIC T130℃ Db, certificate No. SHExC26.0908. It covers both explosive gas atmospheres and combustible dust atmospheres.

Marking	Meaning	Coverage
Ex db	Flameproof, equipment protection level Gb	Zone 1 and Zone 2 gas atmospheres
IIC	Gas group	Covers group IIA, IIB and IIC
T4	Temperature class, surface limit 135℃	Covers T1 through T4
Ex tb IIIC T130℃ Db	Dust protection by enclosure, conductive dust	Zone 21 and Zone 22, covers IIIA, IIIB and IIIC

Temperature class limits are T1 450℃, T2 300℃, T3 200℃, T4 135℃, T5 100℃ and T6 85℃. A device marked T4 has a surface temperature limit of 135℃, so it covers any location that requires T1 to T4.

BT4 Marking Coverage

Required marking	Covered	Note
Ex d IIA T4, Ex d IIB T4, Ex d IIC T4	Covered	IIC is compatible with IIA and IIB; temperature class is T4
Ex db IIB T4 Gb (current designation)	Covered	Same protection type in current standard wording
Ex d IIB T1, T2, T3	Covered	T4 has a lower surface temperature limit and is more restrictive
Ex de IIB T4 (with increased safety e)	Confirm by design	Different protection type; group and temperature alone are not sufficient
Ex ia, Ex ib IIB T4 (intrinsic safety)	Not covered	Intrinsically safe equipment is required
Ex d IIB T5, T6	Not covered	Surface temperature limit is not sufficient
Zone 0 (requires Ga)	Not covered	Gb equipment is intended for Zone 1 and Zone 2

Suitable locations: pulverised coal preparation and conveying, coal mill systems, pulverised coal injection, coal bunkers and coal handling dust collection in Zone 21 and Zone 22; where combustible gas is also present in the system, the gas marking part applies as well.

Applicability of a specific model is subject to the scope of the explosion proof certificate and the equipment nameplate. Site use must also comply with the hazardous area classification and the electrical design documents.

FAQ

Why is a pulverised coal fan rated for dust rather than gas?

Pulverised coal exists as solid particles and is a combustible dust, so the GB 12476 series for combustible dust atmospheres applies rather than the GB 3836 series for gas. Where combustible gas is also present, both sets of requirements have to be assessed.

Where does the T130 C limit come from?

The surface temperature limit for dust ignition protected equipment is derived from the ignition temperature of the dust cloud and the dust layer, taking account of whether a layer can settle on the surface. The value varies with coal type and the final limit is confirmed from measured data and the explosion protection design documents.

Is inerting required?

It depends on the system risk assessment. Equipment is normally supplied with an inert gas connection, but actual use follows oxygen monitoring and process conditions and should not be modified without an assessment.

Related pages: Explosion proof fan range  Explosion proof centrifugal fan  Explosion protection classes and standards