The battle against corrosion in sugar industry costs a lot of money and resources. Therefore, it is tempting to select the least expensive solution to solve a corrosion problem. You save the day and your project budget – at least temporarily.
Carbon steel is less expensive than stainless steel (price per kilo) – no doubt about it. Furthermore, ferritic stainless steel (e.g. S40977) is less expensive than austenitic stainless steel of type 304, and lean duplex stainless steel (e.g. S32101) is more expensive than type 304. If we consider the current price level in Thailand, ferritic stainless steel is about 2-3 times more expensive than carbon steel, and lean duplex is about 40-50% more expensive than type 304 according to Table 1.
So, why even consider using duplex stainless steel for process equipment in sugar industry?
To answer this question, we need to take a broader perspective. Total material cost is not only dictated by the cost to procure the material. There are many other aspects to consider as well: design aspects and design rules, which are derived based on material performance, e.g. mechanical strength, wear and corrosion resistance. The proof strength of Forta LDX 2101 is approximately twice as high as type 304 or 316 and therefore, the thickness of structure can potentially be reduced by half, which would result in a weight saving of 50%. However, for most applications this saving cannot be fully utilized due to stiffness and buckling constraints dictated by applicable design rules. Nevertheless, there are other aspects to consider as well, the corrosion resistance of a carbon steel is much lower than a stainless steel and therefore a protective coating must be applied to the steel surface, as well as corrosion allowance. The meaning of adding a corrosion allowance is to use a thicker material than required from a structural point of view to “allow” for some corrosion if the coating for instance is damaged without jeopardizing the structural integrity. Stainless steel can corrode as well, therefore it is important to select a grade with adequate corrosion resistance for the environment. Austenitic type 304 has higher corrosion resistance than ferritic stainless steel, but for some sugar process equipment also this grade is insufficient e.g. for juice production and downstream application, and therefore, higher alloyed type 316 or duplex stainless steel must be considered. Similarly, for carbon steel, more advanced (expensive!) coatings must be used in harsher environment.
Figure 1. Required shell thickness of storage tank, H=6; D=6, according to tank standard EN 14015.
Let us consider a basic application, a process or storage tank, 6 m high (H) and 6 m in diameter (D) operating at ambient temperature and pressure, density of content 1 g/cm2, tank design according to the European standard EN 14015. Calculations are carried out by using the tank design tool accessible from www.outokumpu.com > Stainless Steel Finder. The result of the calculation is depicted in Figure 1. Minimum thickness according to standard is 3 mm for stainless steel and 5 mm for carbon steel. Hence, the weight saving of the cylindrical shell is approximately 40%. However, the tank is too small to utilize the higher strength of duplex stainless steel, to achieve that we must consider larger tanks. The next tank case depicted in Figure 2 is significantly larger, height 20 m and diameter 15 m. This calculation is based on the American tank design standard API 650 which perhaps is more commonly used in Thailand than the European counterpart. Austenitic type 316 is compared to the duplex grade Forta LDX 2101. The potential material saving of the cylindrical shell by using the duplex grade is approximately 20% which indicates also a cost saving of 20% considering their similar price per unit of weight.
Figure 2. Required shell thickness of storage tank, H=20; D=15, according to tank standard API 650.
A carbon steel also needs a protective coating for most environment. Let us go back to tank case in Figure 1 and consider a coating on both tank inside and outside. The cost for application is estimated to 20 USD/ m2. The total material cost for the H6D6 tank is shown in Figure 3 (ferritic stainless steel excluded). Note the huge impact of the coating cost on the total material cost for the carbon steel tank. The material cost for type 304 is now 5% lower than the coated carbon steel.
Figure 3. Total material cost for, H=6; D=6, tank including coating cost.
With an advanced coating system, a carbon steel can withstand a harsh environment as long as the coating is undamaged. However, the durability and service life of a coating is normally much shorter than the service life for stainless steel (on condition that that an appropriate grade for specific environment is selected,) and regular maintenance is mostly required. How do your company handle your maintenance and repair cost for materials and coatings? Do you have separate budgets for investments and maintenance work or are you looking at the total cost generated by materials and coatings throughout its lifecycle including maintenance and replacement costs? If you think your maintenance expenses are growing due to increasing repair and replacement costs caused by corrosion or wear we strongly recommend you practice the total life cycle cost approach. If you still are not convinced, let us have a look at the first tank case again and include a complete recoating of the carbon steel tank after 10 and 20 years of service for a total lifespan of 25 years using 4% real discount rate. The result of the calculation is illustrated in Figure 4 and shows the cost impact of the maintenance cost. The total life cycle cost for the type 304 is 43% lower, and the duplex solution is 18% lower, than the carbon steel solution. If environment is very corrosive a higher alloyed grade must be selected: 304 < LDX 2101 < 316L.
Figure 4. Total life cycle cost for, H=6; D=6, tank shell materials including coatings and repair.
Lining with stainless steel is another solution to protect carbon steel than using a coating. A liner is typically more durable than a coating by utilizing the benefits of a stainless steel surface. However, selecting an austenitic liner of type 304 or 316 also has its disadvantage. The thermal expansion coefficient of austenite is higher than carbon steel (ferrite), with consequence of that a 2 meter long austenitic sheet will expand approximately 1 mm more if the temperature is increased from 40 to 150°C, and if this is part of a cyclic process this cyclic strain induces cyclic stresses, and eventually the stainless steel lining could fail due to thermal fatigue. The solution to this problem could be to use duplex stainless steel instead of an austenitic. The duplex stainless steel has a dual microstructure of ferrite and austenite and its thermal expansion behavior is more like the carbon steel. The strain induced cyclic stresses will therefore be lower. Additionally, with its higher mechanical strength, duplex stainless steel also has higher fatigue strength than austenitic stainless steel, and its lifetime as liner could easily be several times longer and therefore be a more cost effective solution from a life cycle cost perspective than using e.g. type 304. The graph illustrated in Figure 5 shows a life cycle cost case applying a 2 mm lining of type 304 or Forta LDX 2101 of the H=6, D=6 tank assuming that the 304 lining has to be replaced every 10 year due to fatigue failure during a lifespan of 25 years (4% real discount rate is used). No fatigue failure of the duplex solution. Under these conditions, the life cycle cost of LDX 2101 becomes 33% lower than type 304.
Figure 5. Total life cycle cost for 2mm lining of H=6; D=6 tank shell.
In addition to corrosion, wear is a critical factor to consider for sugar process equipment, and which shorten the service life of the equipment, with costly maintenance and repair work as consequence. The abrasive wear resistance is typically related to the hardness of the material. Outokumpu R&D has carried out abrasive wear test of several steel grades in accordance with ASTM G 65. If we consider the wear index from the test (lower value = higher wear resistance) multiplied with the relative cost for the different materials, we can illustrate the relative cost for wear resistance in Figure 6. The result indicates that the cost for resisting wear is similar for using Forta LDX 2101 and type 304, whereas the ferritic 12Cr stainless steel grade clearly provides the most cost effective solution. Worthwhile noting is that only abrasive wear is considered here, other benefits by using duplex in terms of utilizing its higher strength for weight savings, higher corrosion and fatigue resistance, are not considered, which can make the duplex solution more cost effective depending on critical design criteria for intended application.
Figure 6. Wear index and relative cost to resist abrasive wear.
Hence, by changing mind set and adapt to life cycle cost philosophy rather than striving for lowest initial investment cost, the potential of using stainless steel becomes evident. Duplex stainless steel, if fully utilized, can enable the ultimate solution in terms of cost effectiveness for process equipment in sugar industry.