Inert Gas Compressor (Tanker)

What sets the inert gas compressor apart

An inert gas compressor does not produce compressed air; it moves flue gas, or gas from a dedicated inert gas generator, from close to atmospheric pressure up to around 1 bar into the cargo tank distribution main. It runs on dirty, oxygen-depleted, sulphurous gas rather than clean air, which drives every design choice that separates it from a starting air or service air compressor: corrosion-resistant wetted parts, water-flushed casings and seals designed to tolerate soot and moisture instead of clean, filtered intake air.

Main components

Compressor casing and impeller or rotors

Centrifugal single-stage machines are common on larger tankers; positive displacement rotary types appear on smaller ships. Wetted parts are typically bronze, stainless steel or coated to resist sulphuric acid condensation from the flue gas.

Water seal and scrubber tower

Gas is scrubbed and cooled by seawater spray before it reaches the compressor, removing soot and sulphur oxides and dropping gas temperature to protect downstream rubber-lined pipework. A water seal on the compressor discharge prevents any flammable cargo vapour from tracking back toward the boiler or generator.

Deck water seal

Positioned just before the gas enters the cargo tank main, this non-return water barrier is the last line of defence against a flashback reaching the machinery space if the inert gas plant trips or a tank overflows gas back down the line.

Oxygen and pressure monitoring

Continuous oxygen analysers and pressure transmitters at the compressor discharge and in the cargo tank main trigger automatic shutdown if oxygen content rises above the safe limit for the cargo being carried.

Selection and sizing

• Capacity matched to the maximum simultaneous cargo discharge rate, since tanks must be topped up with inert gas at least as fast as cargo is pumped out
• Discharge pressure sufficient to overcome deck main and tank valve losses while keeping tank pressure within the design range, commonly a few hundred millibar above atmospheric
• Materials matched to the sulphur content of the fuel burned, since flue gas from high-sulphur fuel is more corrosive than gas from a dedicated inert gas generator
• Standby capacity — most tankers carry either a second compressor or a generator-based backup, since loss of inert gas during cargo operations stops the operation

Regulations and class

SOLAS Chapter II-2 requires inert gas systems on crude oil tankers, product carriers above a certain size and most chemical tankers, maintaining tank atmosphere below the oxygen level that supports combustion, generally under 8% by volume, with a positive tank pressure at all times during cargo operations. Class rules require the deck water seal, oxygen analysers and pressure alarms to be tested at defined intervals, and the whole system is checked at each Enhanced Survey Programme special survey along with confirmation the compressor can reach and hold rated capacity.

Typical faults

• Scrubber water flow interrupted — hot, wet, corrosive gas reaches the compressor bearings and seals, causing early corrosion failure
• Deck water seal running dry through a blocked drain — the one safety barrier against flashback is lost without an obvious alarm
• Oxygen analyser drift uncalibrated — false low readings let genuinely unsafe tank atmosphere pass unnoticed during discharge
• Non-return valve on the compressor discharge sticking open — cargo vapour can migrate back toward the scrubber and compressor room

What to look for in a supplier

• Wetted-part material specification matched to the fuel sulphur content the vessel actually burns, not a generic offering
• Type approval covering the specific inert gas system configuration, flue gas or dedicated generator
• Spare seal and bearing kits suited to service on corrosive, moisture-laden gas rather than standard air compressor spares
• Documented capacity test at rated discharge pressure, since underrated compressors only reveal themselves during a fast discharge

Never accept a running compressor as proof the tanks are inert — trust the oxygen analyser and tank pressure reading, since a tripped scrubber water supply can leave the compressor turning while it delivers unscrubbed, oxygen-rich gas.