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Identifying and Reducing Stresses in Pressure Vessels

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Pressure vessels are common equipments utilized in industries to store liquids and gases under high pressure. It is certain that pressurized fluids will develop stresses in the vessel, which when exceeds the limiting value, will lead to hazardous incidents and fatalities.

Reports from the National Board of Boiler and Pressure Vessel showed that during the period from 1999-2000, an increase of 24% in accidents were recorded due to pressure vessel failure. Fatalities due to accidents directly involved with pressure vessels are also showing no signs of reduction. It is therefore mandatory for engineers to design pressure vessels that withstand stresses effectively without failure. The design of these vessels is usually governed by standard codes such as BPVC defined by ASME.

It is important however to identify the stresses acting on pressure vessels to develop design that can withstand the loads effectively. Stresses are generally categorized as primary and secondary; where primary stresses are the ones induced due to internal and external pressure, mechanical loads and wind loads that can result into total collapse of the vessel.

Secondary stresses on the contrary are strain-induced stresses, which develop due to radial loads acting at junctions of various components of the vessel and thermal expansion.

To determine the effects of primary and secondary stresses, the vessel can be studied using finite element analysis. Through proper application of boundary conditions such as pressure, temperature and material properties, finite element solvers can provide stress distribution across the vessel geometry. The division 3 of ASME BPVC section 8 provides the procedure to design pressure vessels using finite element analysis, which must be followed to determine limiting pressure values, stress intensity factor and thickness of the shell.

Based on the values obtained, the CAD model can be prepared to perform structural analysis. By converting the model into mesh of elements and applying required boundary conditions, a finite element model will be created which can be solved using stress and thermal equations to determine stress distribution and deformation in the geometry.

As mentioned, stresses are most commonly found near the junctions such as nozzles meant for pressure relief, which weakens the pressure vessel design. The results of finite element analysis can be utilized to identify such regions and possible action can be taken to improve the design. Modification can include implementing reinforcement pads around the nozzle area or increasing the thickness of the shell. Joints of enclosure heads are also one of the areas with high stress concentration, which can be eliminated by increasing the skirt length at the end of enclosure heads.

About Author: Mehul Patel specializes in handling CFD projects for Automobile, Aerospace, Oil and Gas and building HVAC sectors. He works as a CFD consultant with Hi-Tech CFD for the past 5 years and has successfully executed numerous CFD projects of high complexities. He is an expert in turbo-machinery, gas dynamics, Combustion, Fluid Dynamics, multiphase flow analysis, computational fluid dynamics etc.

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