LPG, LNG & Methanol Relief Gas Treatment

The Increasing Importance of Relief Gas Treatment

Modern marine fuels require modern safety systems. As the industry shifts from traditional heavy fuels to alternative low-carbon options such as LPG, LNG and methanol, ships must deal with a new category of emissions: relief gases. These gases originate from pressure relief valves, tank warm-up cycles, vent mast discharges, purge operations and boil-off handling.

Unlike conventional exhaust streams, relief gases are unburned, raw vapours with high concentrations of hydrocarbons, methane, methanol and aldehydes. If released untreated, they contribute to greenhouse gas emissions, safety risks and odour issues. Ports and regulators are increasingly tightening emission rules for these streams, especially on gas carriers and methanol-fueled vessels.

As a result, shipowners and shipyards are turning to engineered scrubber solutions to treat relief gases safely and compliantly — and Ravebo is at the forefront of this transition.

What Are Relief Gases?

Relief gases are vapour streams released during normal operation of LPG, LNG and methanol systems. They may originate from:

• Pressure Relief Valves (PRVs)
• Boil-off gas (BOG) management
• Purge operations during fuel changeover
• Tank cool-down / warm-up cycles
• Inerting or de-inerting routines
• Fuel Valve Train venting
• Emergency vent mast discharges

While these events are routine, the gas released often contains compounds that are hazardous or harmful to the environment.

Methane is a potent greenhouse gas. Propane and butane create flammable atmospheres. Methanol vapour is toxic and forms formaldehyde during oxidation. Together, these factors make relief gas treatment a critical part of modern marine engineering.

Why Treat LPG, LNG & Methanol Relief Gases?

Relief gas treatment is no longer optional on many newbuilds. The combination of environmental regulation, port requirements and operator safety standards has made controlled treatment a necessity.

For methanol-fueled vessels, relief gas streams can contain high amounts of methanol vapour. These must be scrubbed effectively to protect crew, port workers and the surrounding environment.

For LNG vessels, methane slip from relief vents is increasingly scrutinized. Methane has a global warming potential many times higher than CO₂, making it essential to capture or neutralize these emissions.

Typical Relief Gas Treatment Flow

Relief gas handling depends on vessel type and fuel system design, but the core process is generally the same. Below is a simplified process diagram showing how LPG, LNG or methanol relief gas flows through a treatment system.

The scrubber acts as the primary cleaning unit, removing hydrocarbons, methanol, formaldehyde and other components before the gas is discharged safely.

Scrubber Technologies for LPG, LNG & Methanol Relief

1. LNG Relief Gas Scrubbing (Methane, Ethane)

LNG relief gas streams primarily contain methane, with possible traces of ethane or nitrogen. Methane is only slightly soluble in water, which means conventional scrubbing alone is ineffective. Instead, Ravebo integrates:

• Oxidative scrubbing for aldehydes
• Ultra-low-pressure-drop packed beds
• Cryogenic condensation interfaces (optional)
• High-efficiency demisters to remove entrained droplets

The scrubber removes reactive and odorous components, ensuring safe venting of non-condensable gases under controlled conditions.

2. LPG Relief Gas Scrubbing (Propane, Butane, Olefins)

LPG vapours contain propane, butane and sometimes olefins. These are more soluble than methane, but still require engineered scrubber design. Ravebo systems use:

• Tailored liquid chemistry to enhance hydrocarbon solubility
• Multi-stage packed bed absorption
• Pressure and temperature control for safe operation
• Corrosion-resistant materials suited for LPG vapours

This ensures the vapour stream is cleaned effectively, reducing emissions and improving operational safety.

3. Methanol Relief Gas Scrubbing

Methanol vapours are highly soluble in water, making wet scrubbing particularly effective. However, relief streams may contain formaldehyde, which requires oxidation. Ravebo integrates:

• Primary water absorption
• Hydrogen peroxide injection for aldehyde oxidation
• Instrumentation for methanol concentration monitoring
• Advanced demister stages

This ensures near-complete removal of methanol and aldehydes before discharge.

Benefits for Shipowners and Shipyards

For shipyards, integrating relief gas treatment from the design phase ensures smoother class approval, faster commissioning and fewer late-stage engineering changes. For operators, it provides long-term environmental and operational security.

Why Choose Ravebo for Relief Gas Scrubbers?

Ravebo has decades of experience in hazardous gas scrubbing, odour control, VOC removal, methanol scrubbing and BTEX handling. Our engineering teams develop purpose-built solutions tailored to the specific chemistry and operational environment of LPG, LNG and methanol vessels.

Every system is designed for compactness, marine durability, low maintenance and high operational reliability — essential characteristics for shipboard equipment.