Many industrial processes involve application of very high pressure and temperatures which is achieved by pressure vessels. Since they are to withstand very high values of pressure and temperature, it goes without saying that safety in designing of pressure vessel is of the utmost importance and if compromised; it can lead to catastrophic damages. This makes a strong call that the pressure vessels are operated within the safety levels, by qualified operators and under the guidance of regulatory authorities to ensure that the vessel meets the safety standards.
Furthermore, installation needs to be done as per the details in design drawings, to avoid the severe accidents caused by the vibrations during operation. Owing to this, most companies insist and have a strong urge to design and manufacture them only as per the National or International standards like ASME or BS, and provide sufficient design allowances.
Pressure Vessel Design Approach
Engineers and designers make sure that the pressure vessel design conforms to the strict safety standards and guidelines as per ASME, BPVC – Boiler and pressure vessel code, or BS – British Standards. Almost, everywhere across the globe, pressure vessels designed are based on ASME standards. This is either by ‘Design by Rule’ or ‘Design by Analysis’ approach of ASME Section VIII, Division 1 or ASME Section VIII, Division 2.
However, the Design by Rule has now become redundant with the introduction of Design by Analysis approach, with the evolution of nuclear technology. This is mainly because the design by rule approach is based on several empirical formulas and doesn’t facilitate designers with every little detail.
On the other hand, the Design by Analysis approach involves the analysis of every single element of the vessel and is more consistent with the higher pressure, and safety allowances. At later stages, to analyze the fluid flow from nozzles and heat transfer in the vessels, the study through Computational Fluid Dynamics – CFD is useful for internal fluid dynamics.
There are companies who offer pressure vessel design support solutons to manufacturers and fabricators, for various forms of vessels like rectangular, circular, spherical, cylindrical, and others. Pressure vessels have their applications in mostly all sorts of industries like nuclear power plant, heating systems, chemicals, food and beverages, oil and gas, cement plants, and are manufactured in various shapes and sizes, based on their prescribed application. Some commonly known pressure vessels in the industry are diving cylinders, recompression chambers, distillation towers, pressure reactors.
Pressure Vessel Design Factors
According to the classification of pressure vessels into thick and thin walled category, their design procedure also differs. Generally, since the storage tanks and thin walled vessels are made of ductile material, the design is based on maximum shear stress theory and the major reasons for vessel failure are due to stress rupture, elastic deformation or by excessive plastic deformation caused at the inner walls of the vessel.
In case of thick walled pressure vessel, there are several types of loads and stresses developed, such as radial stress, hoop or cylindrical stress (acting perpendicular to the axis and radius of the vessel), stresses due to external pressure and internal fluid pressure. The design of pressure vessels thus involves multiple calculations and considerations.
Apart from this, there are factors like thermal stresses and vibrations to be considered as well while planning the Pressure Vessel design. In addition, the design for heads (covers) of the vessel of various types is also crucial as the surface area of the head plays a major role in heat transfer. Corrosion, manufacturing, and safety allowances are provided according to ASME or BS standard systems.
Importance of Design Analysis
One of the case studies of fatigue life analysis of cyclic pressure vessel according to ASME Boiler and Pressur Vessel Code (BPVC) shows the necessity of analysis. The client, an American Pressure Vessel manufacturer, wanted to have their design tested for fatigue life estimation for different pressure limits. The project involved the fatigue life estimation using finite element analysis as per the ASME BPVC section VIII, Div. 2 Part 5. This section of ASME has alternative set of rules for the specifications of material, design, fabrication, inspection and testing of pressure vessels having internal or external pressure exceeding 15psi and provisions for finite element analysis by calculations or by analysis.
Upon analysis of the client’s design of pressure vessel, it was found that the design was not safe, since it showed pre mature failure in the material, as it could not stand the desired pressure loading cycles. As a solution, engineers at Hi-Tech provided information pertaining to the critical cross sections for necessary design alterations.
Sourced by ekomeri.com