Cargo Tank Fan

What makes a cargo tank fan explosion-proof by design

A cargo tank ventilation fan moves potentially flammable vapour-laden air in and out of tank spaces during gas-freeing, tank entry preparation and purging, so it has to be built so it cannot itself become the ignition source for the atmosphere it is moving. That means non-sparking impeller and casing material combinations, or a design where the impeller cannot contact the casing even under bearing failure, certified for use in the hazardous zone the tank represents. A domestic or engine-room exhaust fan has none of this and must never be substituted, even temporarily.

Main components

Impeller and casing pairing

Aluminium impeller in a steel casing, or fully non-metallic composite impellers, chosen specifically so that a rubbing contact under wear or misalignment cannot produce an incendive spark.

Bearing arrangement

Bearings mounted outside the airstream where practical, with the shaft passing through a gas-tight or purged seal so a failed bearing cannot introduce a spark source into the vapour path.

Portable vs. fixed types

Fixed fans serve tank ventilation trunks permanently; portable explosion-proof fans, often air- or water-driven rather than electric, are rigged temporarily over a tank hatch for gas-freeing and confined space entry preparation and carry their own certification independent of the ship's fixed system.

Ducting and flame screens

Ducting routes vapour to a safe discharge height above deck; flame screens at the outlet reduce the chance of an external ignition source flashing back into the tank.

Selection and sizing

Sizing is based on the tank volume and the number of air changes required within the gas-freeing time the operation calls for, plus the static pressure loss of the ducting run. Certification for the correct gas group and temperature class for the cargoes carried is not optional - a fan rated for one hazardous zone classification cannot simply be assumed adequate for a different cargo's vapour.

Regulations and class

SOLAS Ch. II-2 and the associated hazardous area classification for tankers set out where ventilation of this type is required and to what standard, and class rules require fixed and portable cargo tank fans to hold recognised explosion-proof certification for the zone they work in, checked at survey alongside the vessel's gas detection and inert gas arrangements.

Typical faults

• Impeller-to-casing rub - bearing wear or a bent shaft lets the impeller contact the casing, defeating the non-sparking design even though the fan still appears to run normally.
• Flame screen fouled or removed - corrosion clogs the screen and it gets pulled off to improve flow, removing a barrier against flashback.
• Portable fan certification lapsed or mismatched - a borrowed or spare portable fan used without checking it is certified for the cargo's actual gas group.
• Static discharge from ducting - poor earthing continuity across duct sections can build a static charge in a high-velocity vapour stream.

What to look for in a supplier

• Certification documentation matching the specific gas group and temperature class, not a generic explosion-proof label.
• Clear statement of bearing and seal arrangement, since this is where most non-sparking designs actually fail in service.
• Support for both fixed installation and portable rental or purchase units carrying independent certification.

Never assume a portable fan pulled from stores is rated for the cargo on board - check its certification plate against the tank's actual gas group before rigging it over an open hatch.