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Mitigate heat exchanger corrosion with better construction materials

12.01.2013  |  Perea, E.,  Sandvik Materials Technology , Singapore

Better materials can provide optimal equipment service life, reduce maintenance, mitigate contamination by corrosion products and minimize heat loss due to fouling.

Keywords: [steel] [stainless steel] [alloys] [heat exchanger] [construction materials]

Unseen corrosion plays a major role in the operational efficiency of process plants. In refining and petrochemical processes, greater demands are placed on construction materials for key processing equipment. The ability to withstand corrosion at elevated temperatures requires special consideration, especially for material selection throughout the plant. New guidelines improve choosing corrosion-resistant materials for heat exchangers. Better materials can provide optimal equipment service life, reduce maintenance, mitigate contamination by corrosion products and minimize heat loss due to fouling.

Harsh operating environments

In refineries and process plants, greater operating demands are required on the construction materials used in capital equipment. The ability to withstand corrosion at elevated temperatures involves special consideration for materials and metallurgy to be used throughout the plant. It is not just to the material’s suitability in the process, but, more importantly, it is the material’s ability to resist corrosion and perform efficiently and effectively. A well-considered material selection process will provide for optimal equipment service life, reduce maintenance spending and conserve energy.

Refineries now handle high-sulfur-content crude oils. The utilization rates are higher and are geared to find more yield per barrel of oil processed. The need for corrosion-resistant materials is vital to eliminate equipment failure and unit downtime. Throughout the refining process, non-hydrocarbon compounds and additives can build up within process streams and can be the root cause for extensive corrosion problems.

Buildup of such deposits, in or on the tubes of heat exchangers, can be sourced from the process side due to tenacious hydrocarbons, process slurries, or even ammonium chloride deposits in the crude unit overhead condensers. Equally important, contaminants can be sourced from cooling water, which may contain sand or sediment. Such sediments can build up due to low flowrates in horizontally mounted heat exchangers. All can have a detrimental effect on a refinery’s efficiency and heat transfer needs.

Better construction materials for exchangers

Applying corrosion-resistant materials not only eliminates unscheduled plant shutdowns, but it also reduces the risks from costly lost production and expensive emergency maintenance and repair. Attention should also be given to the formation of crevice corrosion beneath such deposits at temperatures below the critical pitting temperature (CPT) of the material.

Carbon steel (CS) is extremely vulnerable to corrosion, and austenitic stainless steel (SS), widely used in heat-exchanger tubing, can become susceptible to stress corrosion cracking (SCC), particularly in chloride-bearing environments.

Research shows that these materials are highly susceptible to corrosion at the elevated operating temperatures found in refineries. Gradually, the industry is recognizing the advantages of duplex SS. It can offer the optimum combination of corrosion resistance, mechanical properties and excellent fabrication capabilities. The cumulative benefit of such an approach is genuine cost advantages. Link this with its compatibility with other alloys during the fabrication process and duplex SS is an ideal material, not only for new equipment but also for the retubing of existing heat exchangers, replacing CS and even austenitic SS.

For example, new lean duplex SS can offer high strength with a yield strength twice that of ASTM 316L, along with low thermal expansion, very good weldability and physical properties that provide design advantages, as well as ease of fabrication and toughness.1 It can offer technical benefits and design advantages of the material, together with the cost advantages over conventional SS and CS.

Chemical composition. Due to low nickel (Ni) content, lean duplex SS has a two-phase microstructure with approximately 50% ferrite.1 A high 23% chromium (Cr), compensating for the absence of molybdenum (Mo), can provide high resistance to corrosion, as listed in Table 1. The nitrogen content further increases the material’s strength, improving weldability and pitting corrosion resistance.

 

Mechanical properties.
Direct comparisons between the mechanical properties of the material, austenitic SS and CS are summarized in Table 2. These clearly demonstrate the high yield strength of the material, along with high tensile strength and hardness properties.

 



The impact strength of the material at various temperatures, in both welded and unwelded conditions, is illustrated in Fig. 1.
Its toughness throughout the temperature range makes it far more suitable than CS, which normally has ductile to brittle transitions in the range 0°C to –80°C (32°F to –112°F).

 
  Fig. 1.  strength Charpy-V for duplex SS
  and CS. Specimen size 10 mm x 10 mm
  (0.40 in. x 0.40 in.).



Low thermal expansion. It is the low thermal expansion of duplex SS that offers significant design advantages. As shown in Fig. 2, it is much lower than austenitic SS and very close to that of CS. When used in tubular heat exchangers, whether a new build or to replace existing CS tubes, the low thermal expansion of the duplex SS is a favorable option to use with other alloys.

 
  Fig. 2.  Thermal expansions, mean values.



Corrosion resistance. Even in acid solutions, the lean duplex SS has better resistance to corrosion than ASTM 304L, owing to its very high Cr content. In fact, it is even better than ASTM 316L in most acid environments. This is demonstrated by the isocorrosion curves, as shown in Fig. 3. This figure illustrates the corrosion rate in formic acid of just 0.1 mm/yr (4 mpy).

 
  Fig. 3.  Isocorrosion diagram showing
  materials in formic acid.



Again, the high Cr content of the material also offers good resistance to general corrosion, pitting and crevice corrosion, exceeding that of austenitic grades, as shown in Fig. 3. Duplex SS offers excellent resistance to SCC in aqueous solutions. The lean duplex SS is suitable for use in temperatures around 140°C (284°F) without risk of SCC. By comparison, ASTM 304L and ASTM 316L should only be used in operating temperatures below 60°C (140°F), as shown in Fig. 4. 

 
  Fig. 4.  Critical pitting temperature (CPT)
  in neutral chloride solutions (potentiostatic
  determination at 300 mV, SCE).


Cost advantages

For many projects, cost is the primary priority. However, the ability of a material to fully meet the application requirements is likewise a major consideration for plant efficiency.

Strength of the material is a significant factor. For example, selecting a duplex grade, such as a lean duplex SS, despite a higher price per kg, can prove to be the most economical solution.1 This is because the wall thickness of the tubes subjected to internal pressure or tensile loads is directly related to the material strength. As thinner wall tubes can be specified, the cost of the duplex material can be around 35% lower. This should be compared to the cost of tubes of other material grades, which would require a thicker wall to achieve the same strength, as summarized in Table 3. There are also associated savings to be achieved on transport, installation, welding, etc., when specifying the lighter thinner walled duplex grade tubes. For many applications, the material offers economical solutions because of the high corrosion resistance, especially to SCC. The use of computerized life cycle cost calculations, using all relevant data, including maintenance, investment costs, service life, inflation, etc., can clearly demonstrate the savings that are possible.

 



Better design can save money

Within refineries, along with hydrocarbons, there are chlorides, ammonia compounds, hydrogen sulfides, carbon dioxide, various acids and water. Such mixtures can result in the premature failure of many construction materials, such as copper-based alloys to steels, as well as different types of austenitic SS, due to corrosion. In such environments, duplex SS can provide excellent resistance to corrosion attack as recorded in extensive laboratory testing as well as in successful documented installations in process plants and refineries worldwide. The ease of fabrication, cost savings and the durability of duplex SS can offer significant advantages not only for new equipment, but also when retubing existing heat exchangers. HP

 
  Fig. 5.  SCC resistance in oxygen bearing
  (~8 ppm) neutral chloride (Cl) solutions.
  Test time of 1,000 hr. Load ≥ yield strength
  at testing temperature.


 
  Fig. 6.  Cut-away of horizontal heat
  exchanger, illustrating flow direction.


NOTES

1 Lean duplex SS, such as Sandvik SAF 2304, offers high strength with a yield strength twice that of ASTM 316L. Sandvik and Sandvik SAF 2304 are trademarks owned by Sandvik Intellectual Property AB.

The author
Eduardo Perea is the global business developer for tube chemicals for Sandvik Materials Technology, and is based in Singapore. He is a metallurgist engineer and graduated from Faculdade de Engenharia Industrial in Brazil. In 2004, Mr. Perea joined the company as a trainee and was later promoted to technical marketing and sales engineer. In 2008, he relocated to Sweden and joined the global technical marketing, high-temperature products group. In 2011, he was promoted to regional sales manager for tube before assuming his present role in 2012.


 



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pranab basu
12.20.2013

Very useful.

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