the only technique available for their fabrication. When single-wall thicknesses 

 of more than 2 inches were required for vessels fabricated from welded plate, 

 a point of diminishing returns was reached, as the tensile strength properties of 

 rolled alloy steel plate began to decrease with further increase in plate thickness. 



The introduction of the multilayer pressure vessel construction 

 technique overcame this wall thickness limitation. This technique permitted 

 thick pressure vessel walls to be built up from thin sheets or plates, thus 

 obtaining thick walls with material properties equal to those found in thin 

 sheets or plates. Because of this, the multilayer construction technique has 

 been widely accepted and remains today the most reliable and proven technique 

 for fabricating large-diameter, high-pressure vessels. 



There are, however, two shortcomings inherent in the multilayer 

 construction technique that become more and more pronounced as the sizes 

 and operational pressures of the pressure vessels increase. The first shortcoming 

 is the reliance on longitudinal and circumferential welds for joining the many 

 layers in the wall. Since reliable welding methods as a rule lag behind the 

 development of new steel alloys, reliance on welding forces the multilayer 

 fabricators to use generally only lower strength alloys for which reliable welding 

 techniques have been already developed. At a first glance this does not seem 

 to be much of a disadvantage, as instead of the thin vessel walls of new, higher 

 strength alloys, the old, lower strength alloys could be utilized in thicker vessel 

 walls. Such a substitution would be quite acceptable if the distribution of 

 stresses in the vessel wall remained the same regardless of wall thickness. 

 Unfortunately, this is not the case; the distribution of stresses becomes less 

 and less uniform as the wall-thickness-to-vessel-diameter ratio increases 

 (Figure 1 ), making thick-wall vessels uneconomical in terms of their internal 

 pressure capability (Figure 2). 



Jhe second shortcoming of the layered pressure vessel construction 

 is its monolithic mass that makes it impossible to transport such a vessel by 

 land if its dimensions are large and its pressure capability is high. There is a 

 limited solution to this problem whereby the individual vessel layers are welded 

 on site, and the finished assembly is never moved again from its foundations. 

 This solution is acceptable, but the welding and stress relieving is done under 

 conditions less than ideal and future removal of the vessel for repair or main- 

 tenance is extremely difficult and expensive. 



Although the two above-mentioned shortcomings of layered vessel 

 construction are not serious enough to preclude its use for pressure vessels of 

 any size or pressure capability, they are grave enough to warrant investigation 

 of other types of vessel construction. I n the previous survey of pressure vessel 

 construction conducted at NCEL (Appendix A), all the available and proposed 

 methods of vessel construction were reviewed. Only two were found to merit 



