Article by David Sear
According to Mrs. Musch herself she does not really receive that many outside guests at the Porvoo refinery. And that’s something of a surprise for whenever SSW visits we are always treated to a friendly welcome, a hot cup of coffee and the opportunity to listen to an honest, in-depth review of materials.
And make no mistake about it, Mrs. Musch can talk with authority about materials from various angles: properties, selection, failure analysis, welding, etc. For she has continually looked for opportunities to expand and improve her knowledge of as wide a range of materials as possible. This is exactly why she joined the Neste refinery back in 2012, eager to work with stainless steels and some of the more exotic CRAs. She has certainly had her wish, having been involved in plenty of interesting cases within the refinery gates. For example, she has focused attention on the refinery’s water quality which was affected following a leakage of resin balls from the ion exchange system.
Now when safely contained inside the ion exchange unit those balls do an excellent job in removing calcium and magnesium from the boiler feed water. However, when allowed to escape from the ion exchanger, to circulate and to subsequently become exposed to higher temperatures, these self-same balls can degrade, causing corrosion problems elsewhere.
As Mrs. Musch explains, that’s because the resin balls are fully saturated with positively charged sodium ions (Na+) and contain a lot of sulphur in the form of sulphonic acid. The resin starts to decompose at elevated temperatures where evaporation takes place. The decomposition process is fairly complex but results in the formation of both sulphuric acid and sodium hydroxide. Comments Mrs. Musch: “So what we have is both an acid and a base. In some instances these two compounds will neutralise each other. Indeed, pH readings in some areas may show that the water is perfectly all right. However, these readings can create a false sense of security as, for example, sodium hydroxide can accumulate under surface fouling which leads to severe pitting corrosion or stress corrosion cracking.”
The ideal solution from the scientific perspective, she indicates, would be to shut down the refinery, drain the feed water and condensate tanks and clean out the balls. However, there are clearly good economic reasons for waiting until the next scheduled outage, as closing even parts of a refinery can lead to an immediate loss of revenue. “We therefore have to accept that the resin balls will continue to be a concern for items such as heat exchangers, boilers, etc. For example, the tube bundles need replacing more frequently than normal. Fortunately now that we are aware of this particular corrosion mode we can take all the necessary steps well in advance. However, we are currently looking to hire a manager to be responsible for the water and steam systems so I am confident that he or she will be able to push ahead with the actual implementation of the cleanup project. This is an indication that the top management is seeing the need for a strong ownership for the boiler and the water quality issues.”
At this juncture SSW decide to ask Mrs. Musch for an update on some of the other topics she raised two years previously. Such as, for example, her concern that rising sulphur levels in the crude oil feedstock might necessitate using chromium alloys as an alternative to carbon steels in certain areas. In reply, Mrs. Musch notes that colleagues from local engineering company Neste Jacobs had in fact just concluded a material upgrade project in the crude oil distillation systems. “They reviewed the suitability of existing piping and equipment for increasing sulphur levels. This evaluation highlighted the need to upgrade from carbon steel to 5Cr or even to 9Cr/12Cr steels. However, if sulphur levels continue to rise than perhaps a further step up to stainless steels will be the next choice.”
Another research project instigated by Mrs. Musch in 2015 was to address fouling in a tank fabricated from 321 stainless steel clad onto a carbon steel base layer. To that end she had coupons installed inside the tanks. With the coupons now having been removed and examined, Mrs. Musch says she needs to free up time to properly analyse the results. “Fouling can accelerate corrosion, so is something that I take very seriously,” she notes.
A more recent area of attention for Mrs. Musch is a hydrogen-rich process unit, where cracks have been found in a duplex vessel. Commenting, she says: “the investigation is still ongoing but there is some evidence to suggest that improper manufacturing or welding practices may be part of the problem. The preliminary results revealed some microstructural anomalies which could have made thematerial susceptible to cracking.”
“It is important for everyone involved to remember that duplex is absolutely not the same as carbon steel,” she continues. “For example there are clearly defined welding procedures which must be followed. This type of information should therefore really be included in the purchase orders so that everyone involved is properly informed in advance. Only certified welders should be used but even then the procedures should be discussed so that they really appreciate what is needed and why. The cooling speed for one is a key parameter. But it is equally important for managers to consider what it is like for a welder to work inside a vessel installed in the field. In other words: make sure the welder has easy access to the work area and does not have to strain to reach the weld.”
Mrs. Musch goes on to say that the ability to distil complex engineering issues into easily understandable language is an important skill for materials engineers. “We have developed a rich vocabulary that enables us to precisely define materials phenomena. That is ideal when looking to explain complex ideas to our immediate peers. But we also need to be able to discuss materials issues with specialists in other fields. For example, that could be with a refinery owner about the need to change materials. Or with a welder, to explain why it is important to follow specific procedures for welding exotic materials.”
This is why Mrs. Musch avoids using technical jargon during her regular presentations to Porvoo staff. “Start discussing microstructures and you can see people quickly becoming disengaged. Therefore I try to use a straightforward language to ensure that everyone understands and can contribute to discussions. My presentation shows materials failures that can happen in a refinery and how they can be avoided. For example, I recently explained to process operators why they need to be really careful with the operating window. If for whatever reason they raise the process temperature so that the material temperature is raised by just ten degrees that can reduce the furnace tube life by fifty per cent! That is a simple message but one which everyone can immediately understand.”