^ An aerial view of Shell Pernis in the heart of Rotterdam’s docklands.

Article by John Butterfield

Holding a Masters’ degree in Aerospace Engineering, Mr. Woerden majored at university in aerospace materials focusing on thermoplastic materials, fiber-metal laminates, and aluminium. It all seems to be a far cry from his work at the refinery. However, with his pragmatic mentality and straight-forward way of thinking, he immediately draws a parallel between the curriculum he studied and his chosen profession today: “When all is said and done, there is little difference between a pipeline under pressure and an aircraft fuselage under pressure. The only difference is the materials used.”

Heat exchangers at Pernis

At Shell’s fully-complex and integrated refinery in Rotterdam there are more than 1500 shell & tube heat exchangers and close to 900 airfin heat exchangers. It is therefore not surprising to find that without them Shell would not be able to run its process units where they are used for process cooling and heating, and for heat integration. Certainly, working with heat exchangers is not always simple as the challenges they often pose are multiple: two of the most prominent being fouling and corrosion. The fouling issues have much to do with the changing type of feedstocks that Shell Pernis processes.

“From a feedstock perspective,” says Mr. Woerden “the crudes we process today are no longer the easy ones of the past, which we would nowadays affectionately call ‘baby feed’. Increasingly, they come from harsher and more corrosive environments and contain increasing amounts of hydrogen sulfide, sediment, acidic species, and scavenging chemicals. Moreover, to survive as a company nowadays, in an era of energy transition, as well as to stay ahead of the competition, it is imperative to be able to process every single crude the market can throw at you to maximize delivery to our bottom-line; or simply said: to make money.” In addition to the corrosion that results from processing a wide variety of crudes, the brackish water from the river Nieuwe Maas, which is used as a coolant for the heat exchangers, also leads to considerable corrosion risks due to its salt level, and the sediment and marine biology (bacteria) it contains.

“Furthermore, after cooling, we also have to be very careful that the used water we supply back to the river does not bring any pollution from internal leakages to the Rotterdam harbor,” says Mr. Woerden. “We obviously have very strict laws and regulations to comply to and any process leakage into the harbor or leakage whatsoever is unacceptable for the local government and for us as a company. We call this ‘GOAL ZERO’. Shell & tube heat exchangers are very challenging in this respect, because the pressure boundary (tubes) are usually very thin and thus have little corrosion allowance, and a tube leakage, being internally in the equipment, is not always immediately evident.

Equally important today when considering the feedstock to our units is the concern from society as a whole for a cleaner and greener environment, and also new international regulations like those of the International Maritime Organization (IMO) that limits the amount of sulfur allowed in the bunker fuel of ships. Such changes in regulations provide us with a whole new playing field in terms of thinking about both feedstock and the products we will eventually produce. Such new international regulations could lead to a significant change in crudes we process tipping a balance towards more unfavorable sulphur over naphthenic acid ratios and consequently corrosion risk changes that we had to worry much less about in the past. Handling these new different types of crudes brings a complete new range of possible corrosion and fouling issues to our horizon to deal with.

End products

At Shell Pernis a whole scala of products are made, which can be grouped into oil products and base chemicals. Amongst others Shell Pernis produces bitumen, asphalt, gasoil, naphtha, kerosene, diesel, mogas, alkylate, platformate, LPG, propane and butane, and sulfur. Other, less common products for the general public, include CO2 for the greenhouse farmers in the area. “In effect,” says Mr. Woerden “all base products that can be produced from crude oil are made in our plants at Pernis”.

Materials used in heat exchangers

When deciding which materials will be used for building heat exchangers, decisions are always made as part of a multi-disciplinary approach beginning with the needs of the operating unit that wants to either replace a unit or build a new one. Once these have been established advice will be provided as to what materials should be selected based upon lifetime expectancies. “Normally,” says Mr. Woerden “we will offer alternatives as there is not just one way to approach the question: cheaper solutions generally have less long lifetimes and more expensive ones last longer. However, cheaper solutions might need more inspection and maintenance effort over their lifetime, resulting in higher life-cycle cost. As such every solution bears a cost, which goes far beyond acquisition cost only.

Once the advice has been given, the Operational, Process Engineering, Materials/Mechanical, and the Inspections departments will sit down together and agree on the final selection of materials for the equipment.”

For heat exchangers, a whole spectrum of different materials is used depending on the operational conditions. Mr. Woerden: “The majority of the tubes used in our systems is carbon steel and low-alloy steels, and 300-series stainless steels, because high-temperature sulphidation is one of our main corrosion mechanisms in the refinery. However, in some units we experience corrosion from species like ammonium-bisulfide and ammonium-chloride, of which the latter can lead to very high corrosion rates. As such, we have upgraded some of the tube materials over the years to nickel alloys, for example. We also use duplexes and super duplexes, and have some titanium bundles in place. The duplexes and super duplexes are used in some of the low pressure sections of our hydrotreater units and in some of our brackish water heat exchangers where the temperature contrasts are on the lower side. Titanium, although scarcely, is also used in some brackish cooling water services where we work with higher process temperatures like in the cooling water bundles found in our compressors.”

Purchasing equipment

Where Shell procures its heat exchangers or its parts depends largely upon whether they are for new or replacement projects. For large-scale projects, Shell puts out tenders and receives offers from all around the world. Small-scale replacements are usually carried out by Dutch based or European manufacturers to be found on Shell’s approved supplier list.

When purchasing equipment either for refurbishments or for new builds, the process requirements obviously have to be taken into account. This is the work of the company’s operational and process engineering staff. The next most important factors to be considered are corrosion resistance and the selection of materials so that corrosion phenomena can be at least calculated if not entirely ruled out. Increasingly, therefore, lifecycle costs are taken into account. “Effectively,” says Mr. Woerden “we look to determine what provides us with the cheapest solution for the longest and safest (no leaks) equipment lifetime. However, a lot can be done to extend lifetimes by maintaining equipment or replacing it in a higher grade material so that inspections are needed less frequently. Apart from corrosion resistance and lifetime costs, when we look towards energy transition, two very important factors are energy efficiency and maintainability. Looking at maintainability, for example, we might delve into the possibility of either using a normal shell & tube heat exchanger or a plate-heat- or a spiral-heat-exchanger. The latter two are probably more efficient. However, incorporating them into a production process might bring with it the risk of having a less-maintainable system because shell & tube technology is better understood and generally easier to maintain. You have to consider: ‘Can I always maintain it onsite or do I have to ship it off to the original equipment manufacturer and have repairs done there.’

Refurbishments and inspections

Most refurbishments start up from Shell’s engineering department, which carries out risk-based analyses on the entire refinery’s equipment, including heat exchangers. As a result, staff has a clear indication of what the corrosion and degradation mode for each piece of equipment is since this can be extracted from collected process data. In this way, corrosion rates can be predicted and decisions made as to whether this is age related. The end-of-life of equipment can thus be accurately gauged and replacements can be timely planned.

Not everything is easy to predict, however, as not all degradation mechanisms are age related, like stress corrosion cracking. Such forms are harder to predict and plan on. In these cases, staff works to preventively replace equipment and, at the same time, upgrade materials to solve possible problems ahead of time. Updates therefore take place throughout the year whether these are planned or occur as a result of a situation suddenly developing that could not be predicted.

Repairs/updates are carried out within Shell’s production units partly by own staff and partly by contractors that come in to assist. Within Mr. Woerden’s units, the team largely takes on the responsibility for equipment inspection, planning, risk-based analyses, and predictions. The actual refurbishment of equipment, retubing on- and offsite, and manufacturing new equipment, is always done by approved contractors.

Inspections at Shell are a combination of both remote and visual inspection – the latter carried out by staff walking around the facilities. Additionally, a lot of ‘health checks’ take place based on the analysis of process data and through inspecting integrity operating windows, which provide information as to whether equipment will live up to predictions. There are naturally also walk-around inspections and intrusive inspections, which are stipulated by law.

Staffing and training

At the facility there is a lot of on-the-job training. “We try to ensure that staff who has worked in the industry for a long time mentor the new, bright people who come to us directly from university. This provides a very interesting mix between staff with a lot of experience of our processes and new staff who often bring fresh and innovative ideas to us,” says Mr. Woerden.

To improve skills and knowledge (including that about heat exchangers: how they work, material selection, operational challenges, corrosion issues, etc.), Shell also offers a number of in-depth courses in-house. If a course is not given internally then opportunities are provided to follow them through organizations like API and NACE, for example. However, the best way to learn is through on-the-job training in the field – to roll up your sleeves and get your hands dirty. Another possibility to learn is provided by attending conferences and expos. Finally, there is a Shell Projects & Technologies organization, over-reaching all production sites, and working to ensure that all engineering staff is continually updated with regard to new techniques, new technical requirements, and making sure knowledge is going back-and-forth between all Shell sites.”

Summing up

Summing up Mr. Woerden had this to say: “What I always find particularly enjoyable about my job is that even though I am employed to be proactive and to ensure that we avoid downtime with our assets, I always enjoy the surprise that every workday brings. I never know exactly what I will be doing. As such my job continues to challenge me on a daily basis. You have to be able to get up to speed very fast, particularly when you get a phone call from a unit to help them solve an issue. My job therefore combines daily problem-solving with the longer term goal to make all our production units work better, with minimum downtime, and with a very-low-to-non-existent incident rate. In Pernis we have units over fifty years old and we want to keep them running for many years to come. Moreover, in the current times of energy transition, where the general public is logically vociferous in scrutinizing and criticizing, amongst others, oil majors on their impact on the climate and calling out louder for a cleaner and greener future, I feel I live in exciting times to work in this industry. We are committed towards a cleaner and brighter future and actually can show the public that Shell is also working fervently towards these shared goals. Even better, as an engineer I can actually have a big impact on choices that make our units more energy efficient or enable us to process more and more bio-based feedstock! This impact might already start as small as in heat exchanger selection and heat exchanger material selection”.

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