Before choosing a shell and tube heat exchanger for your operation, it's crucial to know the strengths and weaknesses of the many alloy options available to you. Different applications will have different requirements in terms of resistance to corrosion, heat or pressure. Learning about these traits is imperative to choosing an alloy that will best meet your needs.
Alloys are created when specific metals are combined to create a new material. Each element has particular properties that change the characteristics of the resulting alloy. For example, nickel is typically used to increase strength and the alloy's ability to harden, but without sacrificing ductility, Manufacturers' Monthly explained. Nickel-based alloys are optimal for operations that require equipment that has high resistance to stress corrosion cracking.
There is a whole rainbow of nickel-based alloy options that are durable and corrosion resistant, Machine Design pointed out. One popular one is Hastelloy C-276, composed of:
- Molybdenum (which reduces brittleness).
- Chromium (which improves ductility and wear resistance).
- Tungsten (which offers additional corrosion resistance).
How it's made
When choosing an alloy for a shell and tube heat exchanger, it's not enough just to choose a material that meets your needs. Making sure it meets government regulations and industry standards is also critical. These guidelines are set by a wide variety of entities. One such organization is ISO, which creates international standards for industries ranging from agriculture to technology.
ISO 15156 is a standard that corresponds to the petroleum and natural gas industries, and includes recommendations and requirements for the materials that help create equipment used in highly corrosive environments. This standard lists five nickel-based alloys that are ideal for use in hydrogen sulfide-rich environments, Manufacturers' Monthly reported. The five alloys are categorized on their chemical composition as well as the method in which they were formed. Two ways to create alloys are through solution annealing and cold-working.
According to ISO, annealing is the process of heating the material to a particular temperature and holding it at that temperature until the metal becomes a solid solution. Once cooled, it becomes easier to cut and work with, and isn't as hard.
Cold working refers to the manipulation of the material at a temperature below the recrystallization point, according to Total Materia. This can improve strength, though it can make the metal harder to work with. To offset this effect, cold-worked metals are often intermittently annealed.
Reducing the hardness is important in some applications, Manufacturers' Monthly pointed out. Cold-worked nickel alloys can make great tubular structures as a part of larger pieces of equipment, as long as the hardness is lower than 40 on the Rockwell hardness scale.
Standing up to hydrogen sulfide
ISO 15156 also offers recommendations on how to address areas where hydrogen sulfide is abundant. This chemical compound is commonly found in natural gas and crude oil, and tends to collect in spaces with little air flow, according to the Occupational Safety and Health Administration. As such, it's highly abundant in offshore drilling applications.
"Hydrogen sulfide can wreak havoc on the wrong material."
Additionally, hydrogen sulfide can be especially abundant in sour reservoirs, or those where abiotic and biotic reactions begin to occur. Manufacturers' Monthly reported that sour reservoirs in the northern Caspian Sea can have hydrogen sulfide contents as high as 20 percent.
Hydrogen sulfide can wreak havoc on the wrong material, and as ISO 15156 points out, equipment failure due to chemical corrosion can pose great health and safety risks to those in the vicinity of the operation, as well as the environment. Noting how resistant to hydrogen sulfide corrosion a material is should be a key priority to manufacturers working in these industries.
Hastelloys 825, 625, and C-276 that are solution-annealed can work efficiently in high-hydrogen sulfide environments, Manufacturers' Monthly pointed out. Hastelloy C-276 is particularly useful and able to withstand higher pressures of hydrogen sulfide than the 825 and 625 alloys.
In addition to hydrogen sulfide, plenty of chemical compounds can induce corrosion or other weakening or undesirable reactions. Acidic environments are particularly damaging to many alloys. Hastelloy C-276, for example, doesn't hold up well to nitric acid, Corrosion Materials noted. However, it has shown considerable resistance to negative effects caused by other common acids and compounds, including:
- Hydrochloric acid.
- Sulfuric acid.
- Acid chlorides.
- Phosphoric acid.
- Acetic and formic acids.
- Wet chlorine gas.
- Acetic anhydride.
Given the ability to withstand a host of potentially dangerous substances, Corrosion Materials commented that Hastelloy C-276 is one of the most corrosion-resistant materials on the market today.
Heating up Hastelloy
As metals warm up, their physical properties begin to change. It's important to know what limitations your alloy has under certain temperature conditions. The nature of the operation, as well as environmental factors, must be taken into consideration before choosing an alloy. For example, temperatures around oil drilling operations can rise quickly, regardless of climate. But an oil rig near the equator will likely have different requirements than a rig in Alaska or Russia.
Hastelloy C-276 has a melting point of between 2,415 and 2,500 degrees Fahrenheit, according to Corrosion Materials. As such, it can withstand incredibly high temperatures:
- At 2,000 degrees Fahrenheit, it maintains oxidation resistance.
- At 1,900 degrees Fahrenheit, it continues to fight pitting, corrosion and cracking.
- At 1,600 degrees Fahrenheit, it can still carry loads.
- At 1,000 degrees Fahrenheit, it has a thermal conductivity of 11 Btu/ft•h•°F.
Choosing a powerful alloy for your shell and tube heat exchanger is a critical decision to make. Depending on where you are operating, you may have very specific needs. In many cases, Hastelloy C-276 has the ability to withstand the harshest environments while continuing to work efficiently. However, it's best to speak with a knowledgeable professional before making any concrete decisions.
If you're looking to invest in equipment that is highly corrosion resistant, durable and long-lasting, give consideration to a nickel-based alloy. For more information about how your operation can benefit from incorporating Hastelloy C-276, reach out to the engineers at Enerquip. These professionals are among a select few in the shell and tube heat exchanger space that have experience incorporating Hastelloy C-276 in process equipment.