Monobloc vessels

If the maximum shear stress theory is taken as the criterion of failure (Section 13.3.2), then the maximum pressure that a monobloc vessel can be designed to withstand without failure is given by ... 

The term solid-wall or monobloc vessel is applied to all components where the cylindrical wall consists of a single layer. Solid-wall vessels are suitable for all types of pressure vessels, in particular for those operated under high temperatures. Thermal stresses arising during heating or cooling are smaller than in multilayer vessels because of the good thermal conduction across the wall. Therefore solid-wall vessels are especially suitable for batchwise operation. 

ASiVlE Code, Section VIII, Division I, has developed alternate equations for tliick-walled monobloc vessels. The equations for thickness of cylindrical shells and spherical shells are as follows ... 

Figure 2.8 shows the fragments of a 5000 psi monobloc Vessel purposely tested to destruction. The fragmentation lb typical of brittle rupture. Brittle rupture may result from the use of brittle materials, notch brittleness, ... 

Ea< h of the indiyidual shells can be treated as a monobloc vessel as follows. 

Figure 15.2 is a graphical representation of the super-iraposecl LamS pressure-stress curve for a monobloc vessel... 

HPR monobloc (Fig. 3.8) For applications at high pressure, the HPR monobloc is available. This tool features six 100 mL PTFE-TFM vessels fixed in a strong casing, serving operation limits of 300 °C and 100 bar. 

The vessels are used as monobloc resp. solid-wall or multilayer features with appropriate connections, mainly consisting of endpieces and covers (Fig. 4.3-1). 

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].

Fig. 4.3-7. Comparison of different design strategies of high pressure cylindrical vessels A, Monobloc (eq. 4.3-9) B, Two piece shrink fit Rm = M-, R0 (eq. 4.3-12) C, Partially autof-rettaged cylinder (eq. 4.3-13) D, collapse pressure (pCOmPi. pi) of a Monobloc (eq. 4.3-10) a0,2 yield strength p imposed internal pressure, Rm radius of the shrink fit.

Flat end pieces (plates) are frequently chosen for production reasons. For monobloc forgings of high pressure vessels or welded on end pieces it is important to avoid stress risers at the transition locations. For conical transition the stresses yield only slightly larger than those in the cylindrical wall (Fig. 4.3-8). 

The bodies of thick-walled pressure-vessels with an outer- to inner-diameter ratio d /dj > 1.2 can either be manufacured as monobloc (solid-wall vessels) or be constructed of several layers (multi-wall vessels). During the process of design and fabrication of such vessels some significant and common rules must be observed ... 

As decribed in Chapter 4.3.2, it is increasingly common to design and manufacture such apparatus as monobloc reactors. Furthermore with modem testing facilities with, for example, ultrasonic methods it is possible to check the finished apparatus up to wall-thicknesses of 350 mm. These developments are an excellent contribution for the safe operation of high-pressure vessels. 

Monobloc Solid vessel wall by forging or rolled plate... 

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. 

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. 

Motoihaga H, Tahara T. Manufacturing monobloc ultra high pressure vessels. High Pressure Technology, PVP-Vol. 344, ASME 1997. 

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