Carbon Monoxide Explosion Proof Centrifugal Fan

Overview

Carbon monoxide is both flammable and toxic, with an autoignition temperature of about 605 C. The protection design is therefore twofold: the equipment is configured by gas group and temperature class, and leakage control is a design requirement because escaping gas directly threatens personnel.

Flameproof Configuration

Equipment group and temperature class are confirmed from the medium composition and ambient temperature. The table shows a typical configuration, subject to the confirmed process conditions.

Requirement	Configuration	Basis
Motor	Flameproof Ex d IIB T4	Rated per GB 3836.2 and the gas group
Terminal box and entry	Flameproof box with sealed cable entries	Per GB 3836.1 general requirements
Impeller	Aluminium alloy or stainless steel, spark controlled	Avoids hard impact between dissimilar metals
Clearance	Impeller to inlet clearance controlled to drawing	Prevents rubbing and frictional heating
Shaft seal	Double mechanical seal or nitrogen purge	Prevents toxic flammable gas escaping

Safety and Monitoring Measures

Requirement	Configuration	Basis
Equipotential earthing	Casing, impeller and duct bonded and earthed	Controls static build up
Bearing monitoring	Temperature and vibration monitored online	Detects abnormal friction early
Static bonding	Bonding jumpers across flanges and ducts	Eliminates potential differences
Leak protection	Gas detection and forced ventilation on site	Protects maintenance personnel
Flame arrest and relief	Flame arrestor or relief device where required	Limits the consequences of an event

Applications

Applications include gas cleaning and conveying, syngas plants, metallurgical gas and carbon monoxide bearing tail gas recovery.

Where the medium is a mixed gas such as coke oven gas, the equipment rating is set by the component with the higher group requirement rather than by a single constituent.

Site safety measures and equipment protection level operate on different levels: a correctly rated fan does not replace ventilation, gas detection and safe working procedures.

Acceptance and Verification

Standard	Title	Purpose
GB 3836.1	Explosive atmospheres - Part 1: Equipment - General requirements	General design and marking of explosion protected equipment
GB 3836.2	Explosive atmospheres - Part 2: Equipment protected by flameproof enclosure d	Flameproof motor and terminal box
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
JB/T 10563	General purpose centrifugal fans - Technical specifications	General technical conditions and works acceptance

Please provide medium composition and group, ambient temperature, CO concentration, air flow and pressure. 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: gas cleaning and conveying, syngas plants and metallurgical gas duty in Zone 1 and Zone 2, where the medium is carbon monoxide with a gas group up to IIC and an autoignition temperature above 135 C; Zone 21 and Zone 22 with conductive dust can also be covered.

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

How is the temperature class for carbon monoxide determined?

The temperature class follows the autoignition temperature of the medium with a safety margin applied. Carbon monoxide auto-ignites at about 605 C, but the equipment surface limit and ambient temperature still have to be reviewed, and the result confirmed in the explosion protection design documentation.

Why is carbon steel not used for the impeller?

Rubbing between a carbon steel impeller and the inlet can generate frictional sparks, and carbon steel also corrodes faster in sulphur bearing gas. Aluminium alloy or stainless steel is used with controlled clearances to address both friction sparks and corrosion.

Is a nitrogen purge necessary?

Where the medium is toxic and the allowable leakage is very low, the shaft seal is often a double mechanical seal with nitrogen purge, which both prevents escape and protects the seal faces. Whether it is required depends on the allowable leakage level of the process.

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