Friday, December 20, 2019

As Sulphur Cap Comes Into Force How Long is Lifetime of a Gas Scrubber

A Look at Potential Corrosion Issues
Shipping News Feature

US – NORWAY – WORLDWIDE – Over the past few months we have written about the benefits and drawbacks of the two types of scrubber systems being installed in vessels globally to clean harmful gaseous emissions and ensure compliance with the mandatory IMO Sulphur cap, coming into force in just a few days.

Whether open or closed loop, both scrubber types are intended to last for the anticipated lifetime of any vessel in which they are installed, either as a new build or if retrofitted. However any machinery is of course only as reliable as its weakest component and it seems the quality of build materials could have a dramatic effect on the life of a scrubber.

We are talking here of equipment which not only operates in an environment where salt water is literally everywhere, but which is subject to constant mechanical vibration and thermal expansion-contraction cycles. A study performed by the American specialist metal group ATI, titled ‘Evaluation of Alloys for Marine Exhaust Scrubbers - Effect of Welding and a Crevice’, confirms the importance of correct material choice for scrubbers.

In the study the weight loss and corrosion rate of five corrosion resistant alloys commonly used in marine scrubbers were tested in a simulated scrubber environment. The results revealed that for a severe corrosion case, the alloy 254SMO had a crevice corrosion rate of approx. 0.7 mm per year, while a corrosion rate close to 0 was found for Alloy59.

This of course highlights the necessity of knowing exactly what is going into the making of a chosen scrubber. In theory the choice of a scrubber which contains the more susceptible metal could lead to corrosion depths up to 7 millimetres over a decade, thicker than many scrubber walls!

This matter was brought to our attention by Norwegian headquartered scrubber supplier Yara Marine Technologies in a Q&A session with Yara’s Anders M. Sørheim, M.Sc. Chem. Eng. Asked what were the most important factors when looking to choose a corrosion-resistant scrubber, in addition to alloy quality, he replied:

”Welding, welding and welding. Even if a scrubber was built in pure nickel, it could be susceptible to corrosion damages if the welds were not properly performed by approved welders, following an approved welding procedure using approved welding materials. A good rule of thumb is that the welding material should be of a higher grade than the material being welded together, also known as ‘overmatching the filler material’.

”In addition, the amount of heat applied during welding should be kept to a minimum to avoid hot cracking caused by the formation of carbides and intermetallic compounds in the weld. Furthermore, the welds should be properly pickled and cleaned, and finally subject to 100% NDT testing to ensure the absence of crevices caused by poor welding, including visual surface examination and ultrasonic testing. A high focus on quality is as important in the scrubber material selection as in the post-manufacturing inspections.”

Yara claims to know the weak spots in scrubber performance and for that reason, after initially choosing the material Hastelloy for its preliminary run of scrubbers due to its excellent corrosion-resistant properties, it switched to the aforementioned Alloy 59 for areas where conditions are worst, especially in the bottom part of the scrubber and AL6XN or similar in other areas where the temperature is lower. This, although not as resilient as Alloy 59, these have great corrosion-resistant properties matching the corrosive environment in the upper part of the scrubber tower.

Sørheim points out that the rules set by the class societies such as DNV GL cover the structural strength, hull integrity and the safety and availability of the main functions in order to maintain essential services on the ship. Unfortunately, the scrubber system is not considered a main function, and therefore has less stringent rules for manufacturing.

When it comes to corrosion protection, DNV GL amended their rules for scrubber systems in 2017 to include the text: ‘The exhaust gas cleaning unit and exhaust piping exposed to the cleaning water or treated exhaust shall be suitable for the corrosive properties of the two medias.’ How well this rule is interpreted and enforced, however, is a different question. As we have seen, not all corrosion resistant alloys are sufficiently resistant.

Many ship owners will of course look to the initial and service costs rather than the life expectancy of such technology, particularly if their own fleet rotation plans mean the vessels involved will have been passed on to a new owner by the time problems arise. When asked what the future holds for the scrubber market, Sørheim commented:

”Over the last few years we have seen a shift in our competitor’s material choice towards more corrosion-resistant alloys, although not to the extent we would have hoped. We have seen players in the market experimenting with ceramic materials for the scrubber, which is an interesting approach, as ceramics do not corrode, but they are unfortunately susceptible to other types of damages due to their brittle nature.

”Various kinds of plastic composites such as GRE has been tested but are not a real option due to their low melting point. Some manufacturers have even tried different coatings inside the scrubber, but unfortunately the coating does not last long in the extreme environment. The next developments we see will be about the continued optimizing of the scrubber tower design, as long as it does not increase the risk for corrosion.”