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.
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.”
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.”