The Importance of Inert Gas Blanketing Safety Systems on LNG Cruise Ships

The essential role of dew-point monitoring in marine gas inerting systems

All new marine vessels with a deadweight exceeding 8,000 dwt must have on-board inerting systems. That's according to the updated International Convention of Safety of Life at Sea (SOLAS), developed by the International Maritime Organization (IMO). The Convention states that the gas inerting systems are specific to the transportation of flammable cargo. In 2022, proposals were also made to adopt this measure for smaller oil and gas tankers, carrying volatile cargoes with a flashpoint of 60 degrees or lower.

Inert gas systems are designed to reduce oxygen concentration in the air or gas filling empty space in a container to less than 8 %. This level is considered safe because it is insufficient to support combustion.

Inert gas systems draw flue gases from the main or auxiliary boiler, or from burning diesel oil. The flue gas is then passed through a specially designed cooling and gas scrubbing unit to produce an inert gas comprised primarily of nitrogen and carbon dioxide.

This type of plant is generally found on larger vessels. Smaller gas and chemical tankers often use on-board nitrogen generation systems which apply technology similar to that found in many industrial applications. On these smaller vessels, compressed atmospheric air is forced through a suitable air preparation and drying unit and then into hollow fiber semi-permeable membrane filters. These separate the nitrogen and oxygen, with the former subsequently being used as an inert blanketing or purging gas.

How to ensure the safety of your inert gas system using dew-point measurement

It is essential to monitor oxygen concentration levels in the gas being used to blanket or purge ships’ tanks and pipework. In many instances, however, the addition of specialized sensors or transmitters to measure the dew point of inert gases can be equally important, as a means of optimizing operational efficiency.

This is especially true for gas inerting systems that incorporate desiccant adsorption media to dry the process gases. Here, close control of the moisture content plays a significant role in helping to reduce energy consumption, improve process gas quality and potentially extend the life of the adsorption media. Modern nitrogen generation systems normally include a suitable dew-point sensor. For older systems, a simple retrofit is an option, whereas for all systems with extended pipework the introduction of a sensor further downstream can guarantee the quality of the processed gas.

It is worth noting that the cost of a dew-point sensor or transmitter is modest, compared with the overall cost of a gas purging or blanketing system, and can make a significant contribution to operational cost reductions.

What other marine applications are dew-point sensors used in?

Although monitoring the quality of inert gas systems is clearly an important application for dew-point sensors, they are also used in marine engine systems, especially on large passenger and car ferries. Configured to monitor the humidity in charge-air systems, they can help to minimize the risk of corrosion, improve fuel economy and reduce CO₂ and NOx emissions. Other applications include ultra-high pressure compressed air systems, often installed in military vessels and submarines, where completely dry air is essential to maintain optimal operating conditions in ballast tanks, for example.

Understanding the conundrum of marine approvals

One of the challenges for component and equipment manufacturers – and for specifiers and purchasers – is the wide range of commercial marine standards and guidelines for ships carrying hazardous or flammable cargoes.

The IMO SOLAS convention is clearly the most important international treaty governing the safety of merchant ships. In addition, national and international criteria can affect the choice of on-board equipment. The approval bodies that oversee these criteria include:

For many suppliers to the commercial marine sector, dealing with such a wide range of bodies can be challenging. However, there are often common parameters for device specifications: for example, the need for components to meet flameproof, explosion-proof or intrinsically safe (I.S.) standards.

This is where our range of I.S. and explosion-proof moisture sensors (for applications from -100 to +20 °C dew point) can play a crucial role in helping to improve the safety, efficiency and performance of a variety of marine systems. For example, our latest Easidew I.S. Dew Point Transmitter is certified for trace moisture measurement in flammable or explosive gases and has global hazardous area certification from IECEx, cQPSus, ATEX and UKCA.

Moisture measurement is acknowledged as essential for a variety of industrial and process applications. Although the marine sector has not recognized its importance, this is gradually changing with the growing need to improve safety while at the same time reducing operating costs.

Nitrogen is commonly used as an inert gas for displacement of oxygen. Nitrogen generation PSA (pressure swing adsorption) membrane technology is used for onboard nitrogen generation on marine vessels. The Senz-TX Zirconia Sensor with marine approval has been designed specifically to meet this need, i.e. to verify the oxygen concentration in the generation of nitrogen supply.

You can view the marine approval certificate here.

Learn more about our latest dew-point sensors and transmitters.

With almost 50 years’ experience in the development of innovative precision sensors, we are the application experts in dew-point and moisture measurements for all marine applications. If you would like to discuss your requirements, please contact our team today.