Stiffeners external ring

An external ring stiffener is not very desirable from the appearance standpoint and is even less desirable from the strength standpoint because the maximum stress may be either compression in the outer flange or tension in the vessel shell due to load Q. 

Note If the cargo tank is constructed with external ring stiffeners, with an air space, check to see that a drainage hole has been provided and is open. 

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... 

Whenever possible, shipping saddles should be located adjacent to some major stiffening element. Some common stiffening elements include stiffening rings, heads (both internal and external), or cones. If necessary, temporaiy internal spiders can be used and removed after shipment is complete. 

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... 

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