Design of Pressure Vessels to Code Specifications

Lnited States and Canada (227). Regaihil of the method of design, pressure vessels within the liu ts of tihe ASME code specifications are usually checked against th specifications. Before discussing the code it kt of interest to briefly review the antecedents of the code and its develop-mmt. A. M. Greene, Jr. has published an extensive history of tte ASME boBer code (177-184). 

Although steam devices were used to some extent prior to 1800, the pressures involved were low. Significant steam pres.sures were not used until the development of the Watt steam engine in the early part of the nineteenth century. 

The iniroduction to the code stated that public hearings on the code should be held ev y two y os. The first of hearings was held in Decanter of 1916. A large representation of manufacturers, bufldera, d %ners, and users was present as a r ilt of the teuance ISOO invitations. Most of the dter hms were concemM with the code requiremrats dealing with boiler construction. 

In 1916 it was reco ntized that there was a need for a safety code for unfired pressure v eis (181), but becaure the revisions for the 1918 code were well under way, no 

In 1918 a revised edition of the ASME code was is.sued. This edition contained a number of new sections and therefore constituted an expansion of the code. In 1924 the code was revised with the addition of a new section, VIII, which represented a new code for unfired pres.sure vessels. 

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The ASME code consists of establishing the rules of safety governing the (i) design (ii) fabrication (iii) inspection during construction (materials, design, fabrication, examination, inspection, testing, certification and pressure relief) of boilers and pressure vessels. The rules were formulated to meet the needs and objectives of users, manufacturers and inspectors of pressure vessels. The formulated rules afford reasonable protection of life and property and provide a margin for deterioration in service in the useful lifetime. Any pressure boiler that is fabricated must meet the specifications and, after inspection, it is certified and provided with a stamp of approval. 

AH pressure vessels designed to code specifications except those exempted because of small size must Ih lesled either hydrostalicaUy, pneumatically, or by means of the proof-test (11). 

Maximum Allowable Working Pressure (MAWP) the maximum pressure pounds per square inch gauge permissible at the top of a completed vessel in its operating position for a specific designated temperature corresponding to the MAWP pressure. This pressure is calculated in accordance with the ASME code (Par. UG-98) [1] for all parts or elements of the vessel using closest next larger to calculated value nominal thickness (closest standard for steel... 

The mechanical design of the column incorporates three main stages. The first specification is for the materials of construction. Second, the shell and head thickness must be chosen in order to withstand the operating conditions and also extraneous forces. Finally, consideration of the construction and assembly. The design was performed according to the relevent Australian Standard, AS1 210 SAA Unfired Pressure Vessels Code (Ref. Al 0). 

The tank is specified to have a capacity of 1950 m3. This figure is the sum of one week production of nitric acid (1500 m3) plus an extra 450 m3. This extra 450 m3 capacity will be the normal tank operating level and is available for product sales to external markets. This represents approximately 20 standard road-tanker loads. The dimensions of the tank represent a standard specification available through the Denver Company (United States). The tank internal diameter is 15.2 m and the tank height required is 10.7 m. The internal and external pressure loads require a wall, base and roof plate thickness of 16 mm (a standard plate thickness available through BHP, Australia) to meet the Australian design code for pressure vessels (AS1 210). This thickness gives a 100% safety factor over the maximum anticipated stresses. 

The PED, as does the ASME VIII, accepts that the SRV is set at the MAP or PS that is the exact equivalent to MAWP in the ASME code. Also in PED, MAP or PS are the design pressure of the weakest component of the equipment used in a pressure vessel, which needs protecting from potential overpressures. This pressure vessel may be an assembly of different components - pipes, flanges, nozzles, shells, and so on - and each may have a different design pressure. It establishes the limit of the pressure vessel for very short and exceptional increases of pressure above MAP, specifically to enable the SRV to operate properly and reach its rated capacity. This exceptional increase is called accumulation. So accumulation is specific to the individual pressure vessel and does not relate to the SRV. The pressure increase of the SRV to nominal flow is called overpressure. 

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