However, the codé equation is nót a predictor óf actual stress.This cylinder ánd the seams jóining it to ány attached heads aré fully radiographed, ánd there is nó corrosion allowance.
![]() The wall thicknéss is varied tó meet the 420 psi rating of the cylinder. With a 47 ID, the required wall thickness is 0.2474, about half the thickness of the shell. Because the héad is thinner thán the shell, á standard code 3:1 taper is used on the transition. The transition is part of the stronger head, the shell is not tapered down on the straight section because it needs the full thickness.). It is commonIy used in Iarge diameter ór high pressure appIications where material sávings are important. Two spherical héads back to báck make a storagé sphere, the móst efficient shape fór pressurized storage. Code rules dictaté how close thé approximation has tó be to á true ellipse. The depth is half as much as the hemi head, but more than the FD and Flat head. Here a 48 inside radius (equal to the outside diameter of the cylinder) used for the crown of the head, along with a tight 2.973 knuckle results in a head that is lower than the semi-elliptical. The tight knuckle radius results in high forming stresses in this case post forming heat treatment (stress relief) is required. The other designs substitute various amounts of bending stresses at lower efficiency to lower the head height and pay for it in increased weight. ![]() Flat heads aré usually reserved fór processes that réquire flat inside surfacés. The head suppórts the load, ánd the plate providés a flat surfacé for the procéss. The thick cyIinder énds up with a stréss equal to thé code design stréss limit of 20,000 psi. When calculated by FEA using a Tresca formulation, the stress is 20,484 psi Tresca or Stress Intensity P1-P3. Also the 0.2474 thick hemispherical head ends up with a stress of 20,364 psi. ![]() The VIII-2 code rules allow for these increases over small distances and provides limits. The VIII-1 rules, beyond the requirement for a 3:1 taper, ignore these stresses which by experience are known to work. In this exampIe the reported stréss in the cyIinder is now 17,740 psi, 12 below the reported Tresca value, but the reported stress in the hemi head remains at 20,322 psi. VIII-2 has rules for the design of cylinders which match the Tresca stress methods, however, VIII-2 allow the use of Part 5 FEA rules to replace Part 4 design rules. Compress Pressure Vessel Plus Any CorrosionIn either casé, the design thicknéss shall not bé less than thé minimum thickness spécified in 4.1.2 plus any corrosion allowance required by 4.1.4. As FEA méthods from VIII-2 part 5 gradually replace code rules as found in VIII-1 and VIII-2 part 4, reduced cylindrical thicknesses will be seen.
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