Pressure vessels minimum wall thickness

The wall thickness of the pipe or plate used for the shell is normally determined from the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. TEMA standards also specify some minimum wall thicknesses for the shell. 

This concept can be especially significant for a low-pressure vessel wl minimum wall thickness is desired. For example, assume the caiv .,.i i.ons for a 50-psig MAWP vessel indicate a wall thickness of 0.20 in., and it is decided to use K-in. plate. This same plate might be used if a MAWP of 83.3 psig were specified. Thus, by specifying the hi MAWP (83.3 psig), additional operating flexibility is available at e dally no increase in cost. Many operators specify the MAWP based on... 

The analysis of the membrane stresses induced in shells of revolution by internal pressure gives a basis for determining the minimum wall thickness required for vessel shells. The actual thickness required will also depend on the stresses arising from the other loads to which the vessel is subjected. 

A horizontal, cylindrical, tank, with hemispherical ends, is used to store liquid chlorine at 10 bar. The vessel is 4 m internal diameter and 20 m long. Estimate the minimum wall thickness required to resist this pressure, for the cylindrical section and the heads. Take the design pressure as 12 bar and the allowable design stress for the material as 110 MN/m2. 

For the design of internal-pressure cylindrical vessels, the API-ASME Code for Unified Pressure Vessels recommends the following equations for determining the minimum wall thickness when extreme operating pressures are not involved ... 

The fluid in a rotating centrifuge exerts pressure on the walls of the bowl or basket. The minimum wall thickness required to contain this pressure load can be determined in a similar manner to that used for determining the wall thickness of a pressure vessel under internal pressure. If the bowl contains a single homogeneous liquid. Figure 13.48a, the fluid pressure is given by ... 

The structure of this formula can quickly be related to the thin-walled pressure vessel cylinder equation. Using the equation that calculates the stress at the center of the vessel wall, ux = P R + 0.5t)/t, and rearranging to solve for the thickness, results m. t = PR/ ux — 0.5P. The addition of the weld joint efficiency, E, and changing the coefficient before P to 0.6 results in the ASME code formula, t = PR/ SE — 0.6P), which they feel best represents the minimum wall thickness required to contain an internal pressure, P, in a cylindrical vessel having a radius, R, and made of a material with an allowable stress, S. 

A chemical reactor is a vessel manufactured from metal, often stainless steel or high resistant alloys. A minimum wall thickness is required to resist to different toads, and mainly to internal pressure. Accurate mechanical calculation of process equipment is a matter of mechanical engineering, and out of the scope of this book. Here we present only simple relations that permit the evaluation of weight and cost of vessels in a preliminary economic analysis. More information about mechanical design can be found in Coulson Richardson (1993). 

Beside the resistance to internal pressure, a minimum wall thickness is required to ensure that the vessel resists to deformation due to its own weight, to incidental loads. 

At low pressures, wall thicknesses calculated from Eq. (16.60) may be too small to give sufficient rigidity to vessels. Accordingly, the following minimum wall thicknesses should be used ... 

Tst = equilibrium temperature at which Cst exists over liquid in dry air at one atmosphere, °C or °F Tu = equilibrium temperature at which the upper flammable limit composition exists over liquid in dry air at one atmosphere, °C or °F Tw = vessel wall temperature, °R Ti = gas temperature, °R, at the upstream pressure, determined fromTt = (Pi/P,) (T,) t = minimum required thickness of shell of vessel, no corrosion, inches... 

The formulas for thin-walled pressure vessels are first-order equations and are easier to rearrange and solve for minimum thickness and maximum stress values. The thick-walled vessel formulas provide the most accurate value for the stresses in the pressure vessel wall, but solving the thin-walled equations provides comparatively accurate results and is, therefore, quite useful for preliminary design estimates. 

Pressure vessels are subject to thinning by corrosion, erosion, or mechanical abrasion. To increase the desired useful life of the vessel, the design should include a suitable increase in wall thickness over the minimum design thickness required for safe pressure containment. In most cases, there is no specific code requirement for how much corrosion allowance a vessel requires. Vessels subject to corrosion should have provisions for complete draining as well as openings to allow for the inspection of internal surfaces. 

Many pressure vessels used for isostatic pressing must be classified as thick walled . The literature on this subject is considerable but, unfortunately, in many countries rules for minimum requirements of design, fabrication, operation, inspection, and certification of such vessels is lacking. Design and fabrication are very much in the domain of specialists who rely to a great extent on past experience. 

Various theories have been formulated in which the above stresses are used to compute worst stresses Ov within given structures (such as cylindrical shells). These theories have been taken into account in the formulation of the national codes for pressure vessel design, as indicated in Table 8.4. This table lists several relationships for calculating minimum permissible wall thickness for cylindrical shells as a function of internal radius and pressure. Geode in these... 

What is the circumferential stress based on the ASME Code, Vin-1, at the bottom of a tall vessel that contains fluid at 50 Ib/ft and an internal design pressure of 400 psi The vessel is 36-in. inside diameter by 0.5-in, minimum wall by 45 ft 0 in. overall length with 3-in. thick flat heads on each end. =1.0 and the vessel is supported at the bottom. 

The minimum required wall thickness for a component can be taken as the thickness in the new condition minus the original specified corrosion allowance. The minimum required wall thickness for pressure vessel components can be computed if the component geometry, design pressure (including liquid head) and temperature, specifications for the material of construction, allowable stress, and thicknesses required for supplemental loads are known. The values for thickness calculations must include future corrosion allowance—the amount of corrosion expected after several field inspections are performed. Refer to the API 579, Fimess-for-Service [2], for additional discussion. 

Chapman [10] has shown in an analysis of thermal stresses in spherical reactor vessels that minimum thermal stresses are obtained when the inner and outer vessel wall temperatures are approximately equal. Pressure stresses decrease and thermal stresses increase as shell thickness is increased a minimum combined stress occurs at an optimum wall thickness. Often this stress is greater than the permissible design stress thermal shielding must then be provided between the reactor and pressure vessel to reduce heat production and obtain a reasonable stress. 

The thickness of the vessel walls will depend on the design pressure. If the reagents have the physical properties of benzene their vapor pressure will be appreciable and the design pressure P psia) should exceed this by 25 psia for safety. If the minimum design pressure is 50 psia, we have... 

The usefulness of water sprinklers in protecting vessels exposed to the direct action of fire has been proven over many years. It is important to use the water from the first moments, with a layer of a certain thickness totally covering the wall to be cooled, especially those areas directly in contact with the flame. The required flowrate of water should be kept constant—in some cases, the action of firefighters and the consequent increase in water consumption have considerably decreased the pressure in the network and, thus, the water flowrate to the vessels—with a minimum value that will depend on the circumstances. 

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