Stiffening rings external pressure

Any stiffening rings used must be spaced closer than Lc. Equation 13.52 can be used to determine the critical buckling pressure and hence the thickness required to resist a given external pressure see Example 13.2. A factor of safety of at least 3 should be applied to the values predicted using equation 13.52. 

Figure 13.15. Effective length, vessel under external pressure (a) Plain vessel (b) With stiffeners (use smaller of 1J and Ls) (c) I — section stiffening rings (d) Jacketed vessel...

A support ring that forms a closed ring will help stiffen the column to withstand a high external pressure. However, this construction reduces the effective downcomer area at the top and can cause premature flooding if not allowed for in downcomer design. Unless the downcomer has excess area, it has been recommended (48, 86) that the support ring should not extend into the downcomer area. 

Step 8 If the computed allowable external pressure is less than the design external pressure, then a decision must be made on how to proceed. Either (a) select a new thickness and start the procedure from the beginning or (b) elect to use stiffening rings to reduce the L dimension. If stiffening rings are to be utilized, then proceed with the following steps. 

Fl = longitudinal force due to wind, seismic, expansion, contraction, etc., lb Ft = transverse force, wind or seismic, lb CTx = longitudinal stress, internal pressure, psi = circumferential stress, internal pressure, psi CT,. = longitudinal stress, external pressure, psi CTs = circumferential stress in stiffening ring, psi <7h = latitudinal stress in head due to internal pressure, psi... 

Vacuum Stiffener Rings Rings made of flat bar or plate, or structural shapes welded around the circumference of the vessel. These rings are installed on vessels operating under external pressure to prevent collapse of the vessel. Also used as insulation support rings. 

The spiral baffles may not be considered as contributing stiffness to the shell for the case of external pressure since they are not closed circumferential rings. If stiffening rings are required, they must be placed inside the vessel. This may be a problem from a process standpoint. The alternatives are to increase the shell thickness, or... 

The moments of inertia of the stiffening ring and the shell act together to resist collapse of the ves l under external pressure. Timoshenko (42) has shown that the combing moment of inertia of the shell and stiffener may be considered as equivalent to that of a thicker shell, or... 

External pressure— All walls -cylinders without stiffening rings n 2.0 2.0 1.35... 

In deriving Eq. 5.17 for the maximum strength of a cylindrical shell under external pressure, it was assumed that the ends of the shell were simply supported. For this to be true, stiffening rings, flanges, and so on (Fig. 8.13) are needed as lines of supports. These supports are assumed to carry all the load that (he shell carries due to external pressure. By referring to Figs. 8.13 and 8.14, total force in stiffener is... 

Example 8.6. A long cylindrical shell is constructed of carbon steel with a yield stress of 38,000 psi and a radius of 36 in. If the stiffeners are spaced at a 4-ft interval, calculate the required shell thickness and the size of stiffening rings for an external pressure of 10 psi at 100 F. 

Figure 8.18 Maximum arc of shell left unsupported becouse of gap in stiffening ring of cyilndrico) shell under external pressure (Ref. 2).

The stiffening ring must also be checked against buckling. The classical expression for the buckling of a ring due to external pressure is... 

A = strain obtained from external pressure charts As = area of stiffening ring... 

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