Pressure vessels design loads

The maximum shear-stress theory has been found to be suitable for predicting the failure of ductile materials under complex loading and is the criterion normally used in the pressure-vessel design. 

Two examples -one for static and one for cyclic load- will be reviewed from analysing customer needs to realising a modem pressure vessel design. 

An elementary understanding of pressure vessel design is needed in the preliminary stages of design, as most correlations for pressure vessel costs are based on the weight of metal required, and hence require an estimate of the vessel wall thickness as well as its volume. In many cases the required wall thickness will be determined by the combination of loads acting on the vessel rather than internal pressure alone. 

The cylindrical shell is frequently used in pressure vessel design. For initial designs, it is useful to calculate the stresses in a thin-walled cylindrical shell that is uniformly loaded with internal pressure. For thin-walled pressure vessel calculations to be valid, the radial stresses in the shell need to be negligible. This is usually taken to be a valid assumption when the ratio of the vessel inner radius to the wall thickness (R/t) is greater than 10. "... 

Standard calculation forms can save considerable time in pressure vessel design. These forms also systematize the mechanical design procedure to insure that nothing is omitted. Most engineering contractors have developed their own vessel calculation forms. Basically, all are alike in that they correlate, in easy-to-use fashion, the design rules set forth in Section VIII of the ASME Boiler and Pressure Vessel Code for Unfired Pressure Vessels. They also include design considerations not covered by the code, such as wind loading for tall vessels. (Text continues on p. 139.)... 

Pressure Vessel Design Manual Load Condition 1... 

In this context, it is important for a pressure vessel designer to understand the nature of loading and the structural response to the loading. This generally decides what type of analysis needs to be performed, as well as what would be the magnitude of the allowable stresses or strains. Generally the loads acting on a structure can be classified as sustained, deformation controlled, or thermal. These three load types may be applied in a steady or a cyclic manner. The structure under the action of these loads may respond in a number of ways ... 

A cylindrical polypropylene pressure vessel of 150 mm outside diameter is to be pressurised to 0.5 MN/m for 6 hours each day for a projected service life of 1 year. If the material can be described by an equation of the form s(t) = At" where A and n are constants and the maximum strain in the material is not to exceed 1.5% estimate a suitable wall thickness for the vessel on the assumption that it is loaded for 6 hours and unloaded for 18 hours each day. Estimate the material saved compared with a design in which it is assumed that the pressure is constant at 0.5 MN/m throughout the service life. The creep curves in Fig. 2.5 may be used. 

Pressure vessels are subjected to other loads in addition to pressure (see Section 13.4.7) and must be designed to withstand the worst combination of loading without failure It is not practical to give an explicit relationship for the vessel thickness to resist combined loads. A trial thickness must be assumed (based on that calculated for pressure alone) and the resultant stress from all loads determined to ensure that the maximum allowable stress intensity is not exceeded at any point. 

Any horizontal force imposed on the vessel by ancillary equipment, the line of thrust of which does not pass through the centre line of the vessel, will produce a torque on the vessel. Such loads can arise through wind pressure on piping and other attachments. However, the torque will normally be small and usually can be disregarded. The pipe work and the connections for any ancillary equipment will be designed so as not to impose a significant load on the vessel. 

The ARC is described in Chapter 2 (Section 2.3.2.3). Measurements can be made to determine dT/dt, dp/dt, and pmax. The important AHd or AHt can then be calculated. Using pmax, the gas production per unit mass can also be calculated. This value is used for estimating the pressure load for the plant unit in the vessel design and the plant layout design. 

Many aspects of quench pool design (vessel, spargers, mechanical design loads, etc.) and operation are covered in Guidelines for Pressure Relief and Effluent Handling Systems (AIChE-CCPS, 1998), and it should be consulted when a quench pool has to be designed. 

The selection of a specific fuel cell pressure will affect numerous design parameters and considerations such as the current collector width, gas flow pattern, pressure vessel size, pipe and insulation size, blower size and design, compressor auxiliary load, and the selection of a bottoming cycle and its operating conditions. 

It should be noted that vapor depressuring may not be practical when the vessel design pressure is less than 100 psig (690 kPa) because valves and piping can become unreasonably large and costly or when the vapor depressuring load governs the size of pressure relief and flare headers. Refer to API RP 521,... 

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