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Overcoming inertia

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LNG Industry,


Innovation is a subject that the maritime industry spends a significant amount of time talking about. With the multiple challenges of regulatory compliance, operational efficiency and crew competence, the adoption of new technology and systems is often promoted as the answer to the industry’s problems.

That may be true in part – and certainly when considering topics such as navigation, communications and operational efficiency – but when the subject is safety, the industry seems less able to take into account the reality of innovation.

This is despite fundamental changes that encourage it, such as the International Maritime Organization’s (IMO) goal-based standards, as well as the adoption of risk-based, probabilistic design approaches by classification societies. However, as the entire industry is judged by its safety record and environmental performance, it is necessary to embrace new technologies.

Why inert gas systems?

IMO regulations mean that vessels carrying LNG or LPG – as well as crude and refined products – require a system that generates inert gas to eliminate the potential risks of vapour explosion. Build up of vapour can occur both on the voyage and during cargo operations. Either way, the outcome can be devastating.

Dirty cargoes, including crude oil and crude oil products, normally require inert gas to be produced by burning diesel oil in a dedicated inert gas generator, or using the boiler exhaust as the source.

For LNG and LPG tankers, a dry inert gas generator is used to produce the gas needed to inert and dry the cargo tanks, cargo piping and machinery and purge the tanks. The inert gas has an oxygen content of less than 1%. These systems need an additional cooler and dryer, compared with systems on traditional oil tankers, which can use humid gas.

In addition, classification societies require that all tank inspections are carried out with a safe atmosphere where the oxygen level is increased to 21% before the inspection. This is achieved by operating the inert gas generator in fresh air mode to aerate cargo tanks. This can also be used for drying and aeration of the hold spaces.


Assembly of inert gas cooler and dryer at WTS Kristiansand assembly line.

 

Dual-bed drawbacks

Traditional dry inert gas systems are generally dual-bed systems. Dual-bed dryers consist of two beds, or towers, filled with activated alumina adsorbent beads of between 4 and 8 mm dia., as well as process and regeneration heaters, cooler, blowers and valves to re-direct the airflow.

While one bed of desiccant supplies dry inert gas to flow through the drying vessel to the consumers, the other bed – with saturated desiccant – is regenerated by forcing hot air through it.

When the regeneration is complete, the two beds run in parallel for a short time before the process is reversed and the regenerated bed becomes the one supplying the dry inert gas and the first bed goes into the regeneration mode.

Many modern LNG/LPG vessels are still fitted with this type of dual-bed inert gas system, despite their many disadvantages.

These include long start-up time and high energy usage, because the system needs to regenerate the bed, which is not in use. This requires a large volume of gas and also has a long cycle time, which allows the heat of adsorption to dissipate from the desiccant material, wasting energy.

Spikes and deviations in temperature and dew point can occur during the bed changeover process, with additional risks of dust carry over. Dual-bed systems have a large footprint, meaning that they take up a large amount of space in the engine room. Their complexity means they have relatively high lifecycle maintenance costs and a relatively short life of between six and eight years.

Assessing alternatives

In the last two decades, several major advances have been made to the drying technology used in inert gas generators, and several types of dryers have been developed that overcome the disadvantages of dual-bed dryers.

This is not to say that traditional technologies do not work – they do and have done for many years – but that shipyards and ship owners continue to specify the old dual-bed technology suggests that they may not be aware of the new technologies available.

Both ship owners and shipyards have preferred supplier and vendor lists. Ship owners will often look for a significant number of installation references before specifying new equipment. For some shipyards, as long as the suggested systems meets the required specifications and target price, then these are the systems that they will specify.

In considering how to improve inert gas generator performance, increase operational efficiency and lower costs, Wilhelmsen Technical Solutions looked at other, well proven, alternatives to assess technology that had not been previously considered in maritime applications.

The result is a system – marketed under the well-known Maritime Protection brand – that replaces the traditional dual-bed adsorption dryer with a compact, rotating adsorption system. Dryers of this kind have been used for many years in other parts of industry for drying low pressure gases with great success, but Maritime Protection inert gas systems are the first to have successfully adapted these for inerting purposes.


Preparation of inert gas system at the Kristiansand assembly line.

New approaches

The key element in the system is the rotation adsorber, which results in a compact adsorption dryer with a constant dew point and inert gas temperature, with no variations during the inerting process.

Adsorption rotor dryers, which feature desiccant rotor dehumidifiers, are far more efficient, more compact and require less maintenance than any previous dual-bed desiccant dryer.

The rotor matrix is manufactured from alternate layers of corrugated sheets of silica gel and metal silicates, chemically bonded into a tissue of inorganic fibres in a honeycomb structure. As a result, the rotor has a small external surface compared with a conventional system, but also has a large internal surface.

This, combined with the special microstructure of the silica gel material, ensures maximum contact area to give the rotor a high capacity for absorbing water vapour. The rotor is driven by a gear-motor with a timing belt transmission. A belt tension device prevents belt slipping and overloading the rotor’s motor.

The process of creating dry, clean and soot-free inert gas begins with the combustion of gas oil supplied by the fuel oil pump, with air provided by blowers. The gas produced by the inert gas generator is led into the cooler and dryer skid through an inlet box designed to produce a uniform bulk flow through the cooler.

The cooler reduces the humidity to a dew point of +5°C before the inert gas is led into the rotating dryer. The cooling effect is provided through a glycol/water-based system from a separate refrigeration plant. In the dryer, the dew point of the inert gas is lowered to the required specifications below -45°C with oxygen content less than 1%.

The system comprises of a series of compact modules, providing important savings in space and cost for the shipyard. It is also increasingly preferred by ship owners too.

Installation is easier and less costly, as the Maritime Protection DIGG system is 50% lighter and smaller in terms of engine room footprint compared with dual-bed systems

The benefits in context

Maritime Protection inert gas systems are built in compliance with the IMO SOLAS Convention, including its latest amendments, and meet all class and IMO guidelines in the demanding conditions of shipboard operation.

Ultimately, Wilhelmsen Technical Solutions approaches inert gas production by focusing on overall production quality by improving the performance of critical components – in this case, the dryer units.

As a result, its inert gas units are efficient and easy to install, operate and maintain. Maintenance is affected through a hinged burner front door, allowing easy access. Filters, dryer rotors and other major components can be easily checked through inspection hatches.

With a short cycle time of approximately 8 min., it takes only 15 – 20 min. from start-up to delivery of inert gas to the tanks. This is more responsive than dual-bed systems, which may need 4 – 6 hr to start up, saving operating costs and consuming less fuel oil and energy.

The system is fully automatic, meaning that it can operate unattended with no manual adjustments required. An intuitive operator panel displays all process parameters and allows different modes of operation to be selected.

The unique design of the combustion chamber means that there is no need for a drain to the bilge, as any oil spill from the burner nozzle remains in the combustion chamber.


Written by Frode Lauritzen, Wilhelmsen Technical Solutions, Norway. Edited by

 

Read the article online at: https://www.lngindustry.com/special-reports/20052016/overcoming-inertia-2456/


 

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