ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
MATERIAL SELECTION FOR SEVERE SERVICES
Materials are selected on the basis of service requirements, notably strength, so corrosion resistance (stability) may not be the primary design consideration. Assemblies need to be strong and resilient to the unique loads and stresses imparted on them, which can include significant temperature changes and thermal gradients for many high-temperature applications.
In making a choice, it is necessary to know what materials are available and to what extent they are suited to the specific application. The decision is quite involved and the choice is significantly affected by the environment and the intended use, be it a reactor vessel, tubes, supports, shields, springs, or others.
Some problems may occur because of distortion and cracking caused by thermal expansion/contraction; typically, a high-temperature alloy might change 1/4 in./ft from ambient to 1,000°C (1,832°F).
The user or designer needs to properly understand that the environment dictates the materials selection process at all stages of the process or application. For example, an alloy that performs well at the service temperature may corrode because of aqueous (dew point) corrosion at lower temperatures during off-load periods, or through some lack of design detail or poor maintenance procedures that introduce local air draughts that cool the system (e.g., at access doors, inspection ports, etc.).
To provide as optimum performance as possible, it is necessary for a supplier to be aware of the application, and for the user to be aware of the general range of available materials. Otherwise, severe problems can result. For example, a catastrophic failure occurred within weeks for an ignitor, made with Type 304 stainless steel (UNS S30400, iron, 19% Cr, 9% Ni, 0.08% C). Type 304 stainless steel would have been suitable for clean oxidizing conditions to about 1,650°F in continuous service, or 1,550°F in intermittent (temperature cycling) service (1). The failure occurred because of overheating with contributions from sulfidation (hot corrosion). The true cause of failure was a material mix-up, because Type 304 was not specified, but was inadvertently used.
Mechanical limits of materials
In considering traditional alloys, it is important for the designer and user to be fully aware of the mechanical limits of a material. For example, the ASME Pressure Vessel Codes advise that the maximum allowable stress shall not exceed whichever is the lowest of: (i) 100% of the average stress to produce a creep rate of 0.01% in 1,000 h; (ii) 67% of the average stress to cause rupture after 100,000 h; and (iii) 80% of the minimum stress to cause rupture after 100,000 h.
These recommendations may be better appreciated by extracting typical data for Type 304 intended for use in a pressure vessel up to 1,500°F.
Based upon ASME tables, for a load of 2.5 ksi at 1,400°F, the expected design life would be 24 yr; at 1,450°F, the life falls to 7 yr; and at 1,500°F, it is only 2.2 yr. Thus, a short-term temperature excursion can have a significant effect on equipment life. Also to be noted is that a small increase in loading, for example, from 2.5 to 3 ksi at 1,400°F, can markedly reduce the life expectancy, here, from 24 to 9 yr.
Overheating is the most common cause of high-temperature corrosion failure, but the temperature influence on mechanical properties is of equal or even more significance in that many failures occur because of creep deformation (creep voids) and thermal fatigue. Overheating can arise for various reasons, including an unexpected accumulation of tenacious deposits that can foul tubes in a heat exchanger.
High-temperature alloys and uses
High-temperature alloys are typically iron-, nickel- or cobalt-based alloys containing >20% chromium (or 30% for cobalt), which is sufficient to form a protective oxide against further oxidation. The basic alloys include various additional elements that aid in corrosion resistance, notably aluminum (typically >4% to develop an alumina scale), silicon (up to 5% to develop an amorphous (glass-like) scale that is complementary to chromia), and rare earth elements (typically <1%, e.g., yttrium, cerium, and lanthanum, that improve scale adhesion). Other additions, such as the reactive metals, the refractory metals, and carbon, primarily improve mechanical properties. The beneficial and detrimental roles of common alloying elements on the anticipated performance of alloys at high-temperatures is covered by Agarwal and Brill (2).
Types of high-temperature corrosion
There are certain distinguishing features about the morphology of high-temperature corrosion that aid in deciding upon the cause of damage. Some typical indications include thick scales, grossly thinned metal, burnt (blackened) or charred surfaces, molten phases, deposits of various colors, distortion and cracking, and magnetism in what was first a nonmagnetic (e.g., austenitic) matrix
Damage varies significantly based upon the environment, and will be most severe when a material’s oxidation limits are exceeded, notably when an alloy sustains breakaway attack by oxygen/sulfur, halogen/oxygen, low-melting fluxing salts, molten glasses, or molten metals, especially after fires.
Many industrial processes involve oxidation, i.e., a metal reacts in air to form and sustain a protective oxide. There can be several oxide products, some of which are less desirable, for example, wustite, a defective oxide of iron that forms rapidly at about 1,000°F on steel.
Most high-temperature alloys are oxidation resistant, so price, availability, experience, and the type of application usually dictate choice. There are no significant problems up to 750°F, few up to 1,380°F, but the choice of successful alloys becomes somewhat limited above about 1,470°F.
Sulfurous gases are common to many applications, including fuel combustion atmospheres, petrochemical processing, gas turbines, and coal gasification. Sulfides (e.g., sulfur vapor, hydrogen sulfide) can be very damaging, because metal sulfides form at faster rates than do metal oxides. Sulfides have low melting points and produce voluminous scales (scale spallation).
Halogen attack is commonly manifested as a combination of scale spallation with internal alloy damage including voids that form as a result of highly volatile species (5). Material performance is dictated by the unique properties of the halides, including high vapor pressures, high volatility (vaporization), low melting points, mismatched expansion coefficients with metal substrates, and the effects of displacement reactions whereby oxide or sulfide are thermodynamically favored over the halides
Several environments are synonymous with carburization, including pyrolysis and gas-cracking processes, reforming plants, and heat-treating facilities that involve carbon monoxide, methane, and hydrocarbon gases. Damage is usually manifested as internal carbides, notably in grain boundaries and is generally worst above 1,922°F. When carburizing conditions alternate with oxidizing ones, carbides can become oxidized to oxides, which yields carbon monoxide that can weaken the grain boundaries in an alloy. Such an alloy fails by “green rot,” a name that describes the green fractured surface that results (chromium oxide).
Nitriding is a surface hardening treatment, where nitrogen is aded to the surface of steel parts either using a gaseous process where dissociated ammonia as the source or an ion or plasma process where nitrogen ions diffuse into the surface of components. Gas nitriding develops a very hard case in a part at relatively low temperature, without the need for quenching.
To sum up
Ideally, the material choice is based on known data and experience, which implies communication between a user and a supplier. A better knowledge of anticipated component requirements in addition to corrosion behavior provides for a better choice and the expectation of more reliable service. Proper identification and recording of damage from prior systems is a positive benefit in deciding upon an alternative alloy or coated system. Wherever possible, and certainly for new and complex environments, testing is to be recommended.
Choose Materials for High-Temperature Environments: 1. Rothman, M.F., ed, " High Temperature Property Data: Ferrous Alloys, "ASM International, Metals Park, OH, p9.26 (1989)
2. Agarwal, D.C., and U. Brill, "MaterialDegradation Problems in High Temperature Environments, "Industrial Heating, p.56 (Oct 1994)
3. Elliott, P., "Practical Guide to High-Temperature Alloys, "Materials Performance, 29(4), p. 57 (Apr. 1989)
4. Lai, G.A., "HIgh Temperature Corrosion of Engineering Alloys, " ASM International, Metals Park, OH (1990)
5. Hussain, M.S., "Aspects of HOt Corrosion Attack on High Temperature Materials, "PhD thesis, University of Manchester (1984); C.J. Tyreman, "The High Temperature Corrosion of Metals and Alloys in HF-containing Environments, " PhD thesis, University of Manchester (1986).
6. Grabke, H.J., "Carburization - A High Temperature Corrosion Phenomenon," Publication No 52, Materials Technology Institute of the Chemical Process INdustires, Inc, St. Louis (1998)
7. Elliott, P., "Materials Performance in High Temperature Waste Combustion Systems, "Materials Performance, 33(2), p.82 (1993).
8. Goebel, J.A., et al., "Mechanism for Hot Corrosion of Nickel Base Alloys, "Met. Trans., 4(1), p.261 (1973)
9. Eliott P., and T.J. Taylor, "Some Aspects of Silicon Coatings Under Vanadic Attack," in "Materials and Coatings to Resist High Temperature Oxidation and Corrosion, " A. Rahmel and D.R. HOlmes, eds., Applied Science Publishers, London, p. 353 (1978).
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