In the context of the ASME Boiler and Pressure Vessel Code (BPVC), when referring to the “ASME factor” for pressure vessels, it’s often about the design safety factor or the allowable stress factor. This factor represents the relationship between the material’s ultimate tensile strength (or yield strength) and the allowable stress value used in design calculations.
For many materials and applications under the ASME BPVC Section VIII (which governs the design of pressure vessels), this factor is set at:
Allowable Stress (S)=Yield Strength (or Tensile Strength)3.5Allowable Stress (S)=3.5Yield Strength (or Tensile Strength)
So, the safety factor is typically 3.5. However, it’s crucial to note that this is a simplified representation, and the actual factor can vary based on several parameters:
The factor can change depending on the material used and the operating temperature. ASME BPVC Section II, Part D, provides allowable stress values for various materials at different temperatures.
The allowable stress can also be affected by the efficiency of the weld joints. Full penetration welds that are radiographically examined, for example, may have a higher joint efficiency compared to those that aren’t.
The ASME BPVC Section VIII has multiple divisions. Division 1 is based more on design-by-rule principles and uses the 3.5 factor commonly. Division 2 (Alternative Rules) can have a more detailed design-by-analysis approach and might use different safety factors based on a more nuanced understanding of material behavior, stress concentrations, and other parameters.
The ASME BPVC differentiates between primary stresses (like those caused directly by pressure) and secondary stresses (like those from thermal expansion). The treatment and allowable values might differ based on the nature of the stress.
When designing a pressure vessel or any related component, it’s imperative to refer directly to the relevant sections and tables of the ASME BPVC to determine the appropriate allowable stress and associated safety factors for the specific material and conditions in question.
For pressure vessels that are expected to undergo cyclic loading or unloading (e.g., pressure changes, thermal cycles), fatigue can be a significant concern. The ASME BPVC Section VIII, Division 2 provides rules and methodologies to account for fatigue. It involves determining the number of cycles the vessel will experience over its lifetime and comparing it to fatigue curves generated for the material.
For vessels operating at high temperatures, creep (time-dependent deformation) can become a significant design constraint. ASME provides guidelines for evaluating the long-term effects of elevated temperatures on materials and ensuring that vessels can withstand these effects.
When pressure vessels have multiple openings or closely spaced penetrations, the material between these openings (ligaments) becomes critical. The ASME BPVC provides rules to calculate the efficiency of these ligaments and adjust the allowable stress accordingly.
Vessels often incorporate formed heads (like elliptical, hemispherical, or torispherical heads) and transitional sections. The ASME BPVC provides formulas and rules to determine the thickness and design of these formed components based on the type of head and the applied pressure.
While internal pressure is the primary load for many vessels, external loads like wind, seismic, weight of attached equipment, and piping-induced loads can also be significant. The ASME BPVC provides guidance on evaluating and accounting for these external loads in the design and support of pressure vessels.
Pressure vessels often have flanged connections secured with bolts. The design, selection, and application of these bolts are crucial for the vessel’s integrity. The ASME BPVC offers guidelines for the proper design and selection of bolts and gaskets for flanged connections.
Over time, vessels can undergo corrosion or wear, which can reduce wall thickness. The ASME BPVC allows designers to add a corrosion or wear allowance to the required wall thickness to ensure that the vessel remains safe throughout its operational life.
Fabrication quality is paramount for the safety of pressure vessels. The ASME BPVC mandates certain quality control and assurance measures, including material traceability, welding procedure, and welder qualifications, non-destructive examination, and hydrostatic testing.
For a vessel to be stamped with the ASME “U” symbol, comprehensive documentation needs to be maintained and provided. This includes material test reports, welder qualifications, non-destructive testing reports, and more. This documentation serves as a record of the vessel’s compliance with ASME standards and provides crucial information for future inspections and maintenance.
In summary, the design and fabrication of pressure vessels according to the ASME BPVC is a multifaceted process. Every aspect, from material selection to fabrication techniques, is governed by rules aimed at ensuring the highest levels of safety and reliability. When adhered to diligently, these standards and criteria help prevent failures and the associated catastrophic consequences.
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