Flat heads stress

Unstayed flat heads and covers can be designed by very specific rules and formulas given in this subsection. The stresses caused by pressure on these members are bending stresses, and the formulas include an allowance for additional edge moments induced when the head, cover, or blind flange is attached By bolts. Rules are provided for quick-opening closures because of the risk of incomplete attachment or opening while the vessel is pressurized. Rules for braced and stayed surfaces are also provided. 

UG-23 Maximum Allowable Stress Values UG-27 Thickness of Shells Under Internal Pressure UG-32 Formed Heads, Pressure on Concave Side UG-34 Unstayed Flat Heads and Covers... 

The ASME design formula from UG-34 for circular flat heads subjected to internal pressure is t = d CP/SEf, where t is the minimum thickness of the head (in.), d is the internal diameter of the vessel (in.), P is the MAWP (psi), S is the allowable stress in the material listed in ASME Section II, and E is the weld joint efficiency. The value of C ranges from 0.10 to 0.75 depending on the method of attachment of the head and the shell dimensions. For preliminary designs, a value of 0.33 for C will lead to a good approximation of the required head thickness. 

Watts, C. W., and Lang, H. A., The Stresses in a Pressure Vessel with a Flat Head Closure, ASME Paper No. 51-A-146, 1951. 

Particular attention should be paid to the head of the fastener. Conical heads, called flat heads, produce undesirable tensile stresses and should not be used. Bolt or screw heads with a flat imderside, such as pan heads, roimd heads, and so forth (Pig. 9.9), are preferred because the stress produced is more compressive. Flat washers are also suggested and should be used mder both the nut and the fastener head. Sufficient diametrical clearance for the body of the fastener should always be provided in the parts to be joined. This clearance can nominally be 0.25 mm (0.010 in). 

The thickness of unstayed flat heads and covers subjected to a pressure P, and an allowable stress S with a joint efficiency E, for a variety of cases characterized by the constant C, is given by... 

Whatever mechanical fastener is nsed, particular attention should be paid to the head of the fastener. Conical heads, called flat heads, produce undesirable tensile stresses and... 

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. 

Example 10.1. Determine the maximum stress in a flat head under internal pressure of 1000 psi, diameter d = 48 in., and thickness / = 7 in. for both simple-supported and fixed-edge conditions. Assume carbon steel with p = 0.3. 

Appendix 4 gives definitions and rules for stress analysis for shells, flat and formed heads, and tube sheets, layered vessels, and nozzles including discontinuity stresses. Of particular importance are Table 4-120.1, Classification of Stresses for Some Typical Cases, and Fig. 4-130.1, Stress Categories and Limits of Stress Intensity. These are veiy useful in that they clarify a number of paragraphs and simphfy stress analysis. 

Flat plates are used as covers for manways and as the channel covers of heat exchangers. Formed flat ends, known as flange-only ends, are manufactured by turning over a flange with a small radius on a flat plate, as shown in Figure 13.9a. The corner radius reduces the abrupt change of shape, at the junction with the cylindrical section, which reduces the local stresses to some extent flange-only heads are the cheapest type of formed head to manufacture, but their use is limited to low-pressure and small-diameter vessels. 

There is no optimum design, since the shape of the surface struck is unpredictable. A foam that is ideal for an impact with a flat surface has a too low yield stress to be ideal for an impact on a kerbstone. Designers must also consider ventilation, and compensate for the lower radius of curvature R at the front and rear of the helmet. Ventilation holes reduce the contact area of the foam on the head, so the foam density must be increased to compensate. Ventilation holes are particularly important at the front and rear, to achieve the required air flow. However, since accident surveys show the front and sides of helmets are the most frequently impacted sites, it is important that there is a good thickness of foam in these regions. 

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