Methane Explosion Proof Centrifugal Fan

Overview

Methane auto-ignites at about 537 C and belongs to gas group IIA; pipeline natural gas and biogas are also mainly methane. In coal mine gas drainage, group I equipment rules apply, so the applicability of the two rule sets has to be defined at the design stage.

Flameproof Configuration

Equipment category depends on the location: surface installations follow group II and underground mines follow group I. The temperature class is set from the autoignition temperature and the equipment surface temperature.

Requirement	Configuration	Basis
Motor	Flameproof Ex d IIA T4; group I for mines	Rated per GB 3836.1 and GB 3836.2
Impeller	Aluminium alloy or stainless steel	Controls friction sparks and resists corrosion
Clearance control	Impeller to inlet clearance per drawing	Prevents rubbing and heating
Shaft seal	Mechanical or packed seal with leak drainage	Reduces methane escape
Drive	Anti-static belt or metal coupling with earthing	Controls static build up

Safety and Monitoring Measures

Requirement	Configuration	Basis
Equipotential earthing	Casing, duct and base bonded and earthed	Removes static discharge risk
Methane monitoring	Gas detection interlocked with shutdown	Cuts operation when the limit is exceeded
Bearing monitoring	Temperature and vibration monitored online	Detects abnormal friction early
Area classification	Equipment placed per the hazardous area drawing	Defines the applicable protection level
Maintenance procedure	Shutdown and gas freeing before work	Controls risk during maintenance

Applications

Applications include gas drainage and conveying, coal bed methane, natural gas boosting and conveying, and biogas and landfill gas utilisation.

Natural gas and biogas often contain hydrogen sulphide and moisture, so impeller and casing material also has to address corrosion and the two requirements must be designed together.

When the same fan type is used in a different location, the protection level has to be re-confirmed and the original design documents cannot simply be carried over.

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
GB/T 2888	Methods of noise measurement for fans and blowers	Noise measurement and acceptance

Please provide methane concentration, location category, ambient temperature, sulphur content 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: gas drainage surface stations, natural gas boosting and conveying, and biogas or landfill gas utilisation in Zone 1 and Zone 2; underground mine duty has to be confirmed separately against group I equipment requirements.

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 does mine gas drainage require group I equipment?

Underground mine environments are managed under group I equipment rules, while surface drainage stations follow group II. Selection depends on the installation location and the hazardous area classification, and equipment rated for one cannot be substituted for the other.

How is the temperature class for a methane fan determined?

It follows from the methane autoignition temperature of about 537 C and the equipment surface temperature limit, with a safety margin applied. The actual rating is confirmed from the explosion protection design documentation and the medium conditions.

What else matters in natural gas duty?

Natural gas and biogas often contain hydrogen sulphide and moisture, so the impeller and casing face corrosion risk. Material selection has to address corrosion at the same time as explosion protection.

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