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.
Avoid brittle fracture in pressure vessels
Key points identify effects from auto-refrigeration on steel vessels
During an emergency, equipment failure or a planned maintenance event, hydrocarbon-processing industry (HPI) pressure vessels are normally depressurized. This action may cause auto-refrigeration and low-metal temperature situations in which the likelihood of brittle fracture may occur in steel vessels and reactors. This case history describes the results from a simulation regarding auto-refrigeration effects on HPI reactors. The study also included investigation on brittle-fracture phenomenon and recommendations for a proactive engineering approach to mitigate such failures. Key points highlighted from the study are:
- Although the process-fluid temperature from auto-refrigeration drops to –86°C, considering the vessel’s metal-mass heat capacity and ambient temperature, the short-term vessel minimum metal temperature does not become colder than –28°C. Therefore, selecting expensive material of construction can be avoided.
- Complying with the ASME rules or other internationally recognized codes for minimum requirements is crucial to the structural integrity of a pressure vessel. However, proactive engineering practices and precautions pertaining to the design, materials, fabrication, nondestructive examinations and operation are also required to ensure that the vessels are resistant to brittle fracture.
This case study focuses on a gas field production facility, which uses several separation vessels and a stabilization unit to obtain dew-point control for the natural gas products and Rvp-controlled condensate products. The process vessels operate as three-phase separators containing vapor, light-liquid hydrocarbons and heavy-liquid phase. The study focused on the simulation and design of three interconnected separation vessels—V-100, V-101 and V-102 (Fig. 1).
Since in accordance with API 521, all process equipment with operating pressure higher than 18 barg must be depressurized in case of an incidence, the fluid pressure should be reduced to 6.9
barg and the blowdown lines including the restricted orifice were designed based on depressurizing to 6.9 barg within 15 minutes. In practice during depressurization and blowdown events, the actual vessel-fluid pressure drops from operating pressure (initial pressure) down to almost atmospheric pressure (final pressure). The general assumptions and process design basis parameters used in the simulation include:
- Minimum ambient temperature of –13°C is the minimum outer metal-wall temperature of the vessels
- “PV work term contribution” is defined as isentropic expansion efficiency and assumed as 100% (a conservative assumption)
- Construction material is carbon steel
- Other design basis parameters are listed in Table 1.
The first simulation was done without including the vessels’ metal mass. In other words, the control volume of depressurization study was limited to the fluid inside the vessel, and the metal wall temperature was assumed to be the same as the inventory fluid temperature. Also, the temperature difference between vapor and liquid was assumed to be negligible. Table 2 lists the final fluid temperatures obtained in the first simulation.
In the second depressurization simulation, the metal mass of each vessel was included in the control volume of the model. Table 3 shows the results of the second simulation based on the metal mass values. As shown in Table 3, the calculated inner-wall temperatures are considerably higher than the calculated fluid final temperatures listed in Table 2 from the first simulation.
The simulation work indicates that including the metal-mass heat capacity into evaluation increases the accuracy of estimated minimum metal temperature of the vessels. Consequently, more accurate vessel wall temperatures aid cost-effective selection of construction materials for the separation vessels.
All three separation vessels were designed using ASME Code Section VIII, Division 2. Table 4 lists the design data for the separation vessles.
MDMT of vessels. The minimum design metal temperature (MDMT) of a vessel is the minimum metal temperature in which the vessel can sustain its full design pressure without having to be impact tested. When the vessel operates at pressures less than its full design pressure, concessions on MDMT are allowed based on ASME Section VIII. Table 5 lists the result of MDMT calculations for this study’s vessels based on ASME, Section VIII, Div. 2.
Minimum allowable temperature (MAT), as defined in API 579, is “the lowest (coldest) permissible metal temperature for a given material and thickness based on its resistance to brittle fracture. It may be a single temperature or an envelope of allowable operating temperatures as a function of pressure. The MAT is derived from mechanical design information and material specification. MAT at design pressure is MDMT.
Lowest metal temperature (LMT). LMT as defined and used in this article is the lowest metal temperature due to the operating condition and minimum ambient temperature. The LMT may be a single temperature at an operating pressure or an envelope of temperatures and coincident pressures. Actually, the LMT, in this case, is derived from the calculated inner wall temperature due to the contained process fluid temperature and also the minimum ambient temperature. The LMTs of the vessels coincident with final pressures (after depressurization and blowdown) are shown in Table 6.
As shown in Figs. 2–4 and Table 6, the LMTs for all of the vessels at the final pressure as well as other coincident pressures are on the safe side based on the rules and design philosophy of ASME Section VIII Div. 2. Although the code requirements have been satisfied, further considerations and precautions are required to ensure the design and construction of the vessels are resistant against brittle fracture. Several key factors in combination can contribute to brittle fracture of steel vessels; a proactive engineering approach is recommended.
BRITTLE FRACTURE PHENOMENA
The major concern for low-temperature vessels is brittle-fracture phenomenon, which can be a cause for vessel failure. Many metals lose their ductility and toughness; they become susceptible to brittle fracture as the metal temperature decreases. At normal or higher temperatures, a warning is
normally given by plastic deformation (bulging, stretching or leaking) as signs of potential vessel failure. However, under low-temperature conditions, no such warnings of plastic deformation are given. Unfortunately, an abrupt fracture can cause a catastrophic event.
Only materials that have been impact tested to ensure metal toughness at or above a specified metal temperature should be used. However, certain paragraphs in the ASME Pressure Vessel Code applying to low-temperature vessels indicate when impact testing may not be required for a pressure-vessel component material (impact test exemptions). In general, four main factors, in combination,
can cause brittle fracture of steel vessels. These factors are represented in the form of “brittle fracture square” as shown in Fig. 5. The factors that contribute to the brittle fracture of carbon or low-alloy steel pressure vessels are reviewed briefly here:
Low temperature. A metal depending on its toughness property has a transition temperature range within which it is in a semi-brittle condition (ductile to brittle transition). Within this range, a notch or crack may cause brittle fracture (notch brittleness). Above the transition range (warmer), brittle fracture will not happen even if a notch exists. Below the transition range (colder), brittle fracture can happen even though no notches or cracks may exist. Although the transition from ductile to brittle fracture actually occurs over a temperature range, a point within this range is selected as the “transition temperature” to delineate the boundaries of ductile and brittle zones. One of the ways to determine this temperature is by performing many impact tests on the construction material.
Loading. The type and level of mechanical/thermal loading will affect the vessel’s susceptibility to brittle fracture. Dynamic loading associated with cyclic mechanical/thermal or impact loading, as opposed to quasi-static loading, is a brittle-fracture contributing factor. Furthermore, shock-chilling effects, defined as rapid decreases in equipment temperatures, can be a cause for brittle fracture. Based on the stress levels applied (in a quasi-static loading), component material, effective thickness and minimum metal temperature, ASME Section VIII, Divisions 1 and 2 present criteria for vessel-component material-impact test requirements and/or exemptions.
Susceptible steel. Susceptibility of steels depends on several parameters such as poor toughness, material flaws (cracks and notches), corrosion vulnerability, large thickness, etc.:
• Steel composition. Steels with lower carbon content (C) are proven to have higher toughness at lower temperatures. Also, phosphorous (P) present in steels decreases the transition temperature of steel and improves weldability. In general, steel-transition temperature is a function of carbon content percent plus 20 times the percentage of phosphorous. Furthermore, adding nickel to steel can increase steel toughness and decrease its transition temperature. Stainless steel 304 with 8% nickel can resist impact loads at –320°F. Furthermore, sufficiently low carbon equivalents contribute to the weldability of the material (reducing hardness and cold-cracking susceptibility) and, thus, making metal crack-free girth welds. Selecting the appropriate welding material also is a determining factor to ensure a crack-free weld.
• Steel structure. A correlation was developed between steel structure (microstructure and grain size) and fracture-toughness by numerous fracture toughness tests at different low temperatures. Based on this correlation, steels with coarse-grained microstructures have lower toughness at low temperatures as compared to steels with the fine-grained microstructure. During an 1999 incidence with a high-density polyethylene (HDPE) reactor, a brittle fracture occurred at a temperature of –12°C in a 24-in. flange of ASTM A105 material that had a coarse-grain microstructure (ASTM grain size number 5 to 6 ferrite-pearlite microstructure).
• Hydrogen cracks (hydrogen-induced cracks or so-called flakes). When hydrogen atoms diffuse into the metal during material manufacturing operations such as forming, forging and welding or when hydrogen is introduced to the metal through a galvanic or hydrogen sulfide (H2S) corrosion process, the metal is prone to hydrogen cracks.
There are various techniques to prevent hydrogen cracks, including appropriate heat treatments or slow cooling after forging, in which the hydrogen within the metal diffuses out. In the case of welding, usually pre-heating and post-heating are applied to diffuse out the hydrogen and to prevent any cracks and brittleness.
• Environmental stress fracture. Steels exposed to corrosive fluids such as wet H2S, moist air or sea water are prone to premature fracture under tensile stresses, considerably below their “fracture toughness” threshold. Suitable steel materials should be used when exposure to corrosive fluids is possible.
Crack/stress risers. Steel vessels with thicker walls have a greater probability potential for brittle fracture due to the larger thermal gradient across the wall thickness. Thicker metal walls can result in differential expansion of material across the wall thickness and could possibly lead to a crack occurrence and eventually brittle fracture.
Stress raisers such as sharp or abrupt transitions or changes of sections, corners or notches (as may be found in weld defects) as a result of design or fabrication processes are all stress risers, which can cause stress intensification. The weak points are prone to brittle fracture when other susceptible conditions exist.
Based on the brief technical information given here, several proactive measures can ensure resistance of carbon or low-alloy steel vessels against brittle fracture under quasi-static loading:
- Design pressure vessels, if justifiable, by analysis in accordance with the ASME Section VIII, Div 2 part 5, or other internationally recognized codes that result in lower wall thicknesses.
- Order vessel materials from reliable and capable manufacturers. Key vessel components still require attention to proper heat treatment, avoiding hydrogen cracks, quality control, etc.
- Specify fine-grain steel materials with appropriate specifications and require production tests for plate/piece (from the same heat) if an impact test is not requested. Ensure that the steel with fine-grain microstructure/toughness is supplied; do not rely just on the material certificates. Also, conduct impact tests on test pieces to verify required toughness.
- Take benefit of the recommendations contained in the document indicated in reference 1 for ordering pipe flanges made in forged steel complying with ASTM A105.
- Ask the material manufacturer for effective construction/fabrication methods such as vacuum degassing to prevent hydrogen-crack formation in the metal and require stringent nondestructive examinations and quality control.
- Do nondestructive testing (NDT) to identify cracks or reject materials with detectable cracks. • Eliminate “stress risers,” at the design and fabrication stages
- Verify full-penetration welds with adequate toughness using appropriate welding material/processes and require weld procedure qualification and production-weld test specimens for both the weld and heat-affected zone for each weld process.
- Conduct proper vessel post-weld heat treatment (PWHT), preferably in a furnace in one piece whenever practical, and examine heat-affected zone hardness to ensure the beneficial effects of the performed PWHT.
- Perform the vessel hydrostatic test in accordance with the rules of the ASME Section VIII Code or other internationally recognized codes.
- Apply “control of operation” proactively, whenever practical, (e.g., after a depressurizing to ensure that the vessel metal temperature is sufficiently warm prior to re-pressurization).
A proactive engineering program, as envisioned in Fig. 6, can incorporate the listed measures during vessels design, procurement and construction stages.
Sources: Khazrai, F. Haghighi, H.B., Kordabadi, H., Chagalesh Consulting Engineers – Hydrocarbon Processing March 2011
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