Layered Vessels

The rest of the rules in Part AD for flat heads, bolted and studded connections, quick-actuating closures, and layered vessels essentially duphcate Division 1. The rules for support skirts are more definitive in Division 2. 

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. 

There are several methods of layering in common use ( ) thick layers shrunk together (2) thin layers, each wrapped over the other and the longitudinal seam welded by using the prior layer as backup and (3) thin layers spirally wrapped. The code rules are written for either thick or thin layers. Rules and details are provided for all the usual welded joints and nozzle reinforcement. Supports for layered vessels require special consideration, in that only the outer layer could con-triDute to the support. For lethal service only the inner shell and inner heads need comply with the requirements in Subsec. B. Inasmuch as radiography would not be practical for inspection of many of the welds, extensive use is made of magnetic-particle and ultrasonic inspection. When radiography is required, the code warns the inspector... 

Fig. 4.3-4 (ABC) gives the superimposed stress distribution in the walls of a two-layered vessel under internal pressure. It can be clearly recognized that the compressive tangential prestresses by shrink-fitting (Fig. 4.3- 4B) are decreased at the inner layer and increased at the outer layer towards a more even stress distribution (Fig. 4.3- 4 C) compared to that for a monobloc cylinder (Fig. 4.3- 4A). The theoretical fundamentals for the dimensioning of shrink-fit multilayer cylinders can be taken from [2][8][9].

Three further processes to manufacture multi-layer vessels are applied ... 

The urea reactor is usually a multi-layered vessel that is made of carbon steel with a corrosion-resistant interior liner. Reactor liner materials are usually 316L stainless steel (urea grade) with a lower ferrite content (<0.6%) for pipes and (<1%) for forgings), (see Table 11.4. for the composition of urea grade 316L stainless steel88. 

PS is limited in its use and is mostly applied as a film-forming material because of its brittleness, insufEciently high heat resistance and instability in organic solvents. To improve its impact resistance and elasticity PS is often modified by rubbers [20,22]. Foamed impact-resistant PS is rather seldom used as a layer-vessel incorporating Cl in multilayered anticorrosion films. 

Multi-Layer Pressure Vessels. Multi-layer vessels have been known by various names over the years. The most common names are ... 

A multi-layer vessel is a vessel in which the cyhndrical portion is made up of two or more contacting bands or layers. The inner shell is the innermost band of a multilayer vessel and is made in the same manner as any solid wall vessel. The inner shell can be of any suitable material to resist corrosion by the contents. This is one of the unique advantages of a multi-layer vessel. Subsequent layers are added on top of this inner layer by a variety of techniques to achieve the ultimate wall thickness required. 

A Chinese manufacturing firm developed a ribbon wound technique for multi-layered vessel construction in the 1960s and has subsequently produced over 7,000 ribbon wound vessels. Kobe Steel in Japan, was originally a multi-layer vessel manufacturer and produced approximately 1,000 units of the concentrically wrapped types. They currently do not produce multi-layer vessels any longer but still engage in solid wall, monobloc construction. 

In general, the thicker the vessel, and the longer the vessel, the more attractive the multi-layer option becomes. It is usually more economical to design a multi-layer vessel with a large L/D ratio for a given volume. The selection of the multi-layer option is usually determined by economics. However, once the practical manufacturing limits of solid wall construction are exceeded, multi-layer may be the only option. 

It was not until January 1979 that layered vessels were included in die ASME Code, almost 50 years after they were first introduced. Also in 1979 the ASME Policy Board, Codes and Standards, approved the establishment of a special working group for high pressure vessels under Section VIII. This effort would ultimately culminate in the issuance of Division 3 of Section VIII in 1998. 

Multi-layer vessels can be built to any of the three sections of ASME Section Vm, Divisions 1, 2 or 3. However, only Division 3 allows the designer to take credit for the residual compressive stresses induced by the autofrettage technique. This is a big advantage in the design of the shell. The appropriate ASME Code Sections that apply to multi layer vessels are as follows ... 

Layered vessels are constnicted by successively wrapping thin layers around a center core until the desired wall thickness is achieved. One advantage of this process over monobloc construction, is that the layers each have uniform chemical and mechanical properties. Optimum properties cannot always be achieved across thick sections or maintained during the whole fabrication sequence. With the multi-layer concept, no matter how thick the shell, it does not suffer from lack of material uniformity. 

Multi-layer vessels can be used in hot hydrogen service for hydrotreating and hydrocracking applications. 

Multi-layer vessels can be field fabricated. Although this would be unusual, it is possible and has been done before. 

Nozzles can be welded through the shells of a multi-layer vessel but this is frequently avoided due to the extreme thickness. It is more common for all nozzles to be placed in the heads. In any case nozzles through multi-layered vessels should be minimized. 

Autofrettage technique Although multi-layer vessels can be built to either Divisions 1, 2 or 3, only Division 3 allows credit for the residual compressive stresses induced by the autofrettage technique. 

Example Co IculoHon 15.5. The interferences for the multilay vessel d ribed in Example Calculation 15.1 will be determined. The nominal diniensions of the three-layer vessel from Example Calculaticm li.l are ... 

EX Extractors may be solid or multi-layer vessels. Quick closure system required. Special inner baskets may be required. 

A fhick- ll layered vessel (Courtesy of the Noofer Corporolion, St. Louis.)... 

World War II the technology of building layered vessels has improved substantially. Today layered vessels are used in a wide range of high-pressure applications in the petrochemical industry such as heat exchangers, urea reactors, ammonia converters, autoclaves, and coal gasification reactors. 

Layered vessels are constructed by various methods. The difference between these methods is in the thickness of individual layers, wrapping procedure, and welding technique. In general, layered-vessel construction can be divided into three categories. The first is the concentric- or spiral-wrapped method where the layers consist of segments welded together in a spiral or concentric fashion to form the required thickness, as shown in Fig. 15.6u and b. The second method is the shrink fit method whereby layers are individually formed into cylinders and shrunk on each other to form the required total thickness (Fig. 15. ). The third is the coil-wrapped method whereby a continuous sheet or strip is wound in a spiral or helical fashion to form a cylinder as in Fig. 15.6d. 

Example 15.3. A layered vessel with a 42-in. inner diameter is constructed of carbon steel with E — 29,000,000 psi. Determine the maximum allowable gap if P = 4000 psi, S = 20,000 psi, and the maximum cycle life of the vessel is 10,000 cycles. 

Equation 15.19 determines the maximum value of any one gap in a layered vessel. Where there is more than one gap in a given cross section, a criterion is needed to take accumulated strains of all gaps into account. This can be accomplished by determining first the strain developed in closing each gap. The total strain is ften summed up and compared with an allowable strain. 

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