Vessel thickness

This subsec tion includes rules governing weided-joint designs and the degree of radiography, with efficiencies for welded joints specified as functions of the quahty of joint. These efficiencies are used in the formulas in Subsec. A for determining vessel thicknesses. 

Equation 1 gives the required vessel thickness based P = Pressure, psig... 

Vessel thickness is quickly checked using standard equations ... 

P = Pressure, psia for fractionator F factor correlation or psig for vessel thickness calculation... 

The principal stresses (see Section 13.3.1) acting at a point in the wall of a vessel, due to a pressure load, are shown in Figure 13.1. If the wall is thin, the radial stress comparison with the other stresses, and the longitudinal and circumferential stresses o and <72 can be taken as constant over the wall thickness. In a thick wall, the magnitude of the radial stress will be significant, and the circumferential stress will vary across the wall. The majority of the vessels used in the chemical and allied industries are classified as thin-walled vessels. Thick-walled vessels are used for high pressures, and are discussed in Section 13.15. 

A vacuum distillation column is to operate under a top pressure of 50 mmHg. The plates are supported on rings 75 mm wide, 10 mm deep. The column diameter is 1 m and the plate spacing 0.5 m. Check if the support rings will act as effective stiffening rings. The material of construction is carbon steel and the maximum operating temperature 50° C. If the vessel thickness is 10 mm, check if this is sufficient. 

Pressure vessels are subjected to other loads in addition to pressure (see Section 13.4.7) and must be designed to withstand the worst combination of loading without failure It is not practical to give an explicit relationship for the vessel thickness to resist combined loads. A trial thickness must be assumed (based on that calculated for pressure alone) and the resultant stress from all loads determined to ensure that the maximum allowable stress intensity is not exceeded at any point. 

Table 3, taken from data on pp 203 04 of Ref 11, gives relationship betw deton velocity, material of confining vessel, thickness of wall and chge density for some pure HE s... 

Even in a thin-walled vessel the radial stress is not exactly imiform over the vessel thickness. To correct for this, the internal pressiue in the denominator of Equation 6.4 is multiplied by 1.2 to obtain a more accurate formula. Thus,... 

To account for corrosion, the vessel thickness is increased by adding a corrosion allowance, tc, to assure fliat the vessel operates safely during the hfetime of a process. Therefore, Equation 6.5 becomes... 

Excessive corrosion allowances and minimum vessel thickness. 

The calculations described in this chapter apply only to cylindrical vessels whose lining is thin relative to the radius of the vessel. Thick-walled vessels, and vessels of other shapes such as rectangular or spherical, will require considerably more complex mathematical analysis which is beyond the scope of this handbook. In such cases, exact formulae are difficult or impossible to obtain, and the designer must resort to computer programs for performing the complex calculations. 

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