Pressure Vessel Design against Wind and Seismic Load

Abstract

Pressure Vessels are used in different applications such as in distillation towers, refineries, petrochemical industries & reactors. Failure of vessel can cause the damage to the human life within the plan or in the adjacent area to the plant. A failure can occur due to the excessive loading on the vessel. One of the critical loading conditions is Wind-Earthquake loading. ASME Section-VIII, Division -1 does give the information regarding the wind-seismic loading on the vessel, but it does not provide any method to calculate wind-seismic load. To consider the wind-seismic load, different countries has provided different building codes. At the time of the enquiry, it may possible that in the customer specification for wind and seismic conditions, proper data are not given and because of that, designer has to read the code to obtain that data. This consumes lot of time to design a vessel at enquiry. So, to avoid such situations a proper guideline is prepared for the codes mostly used at the industry. When the column or vessel is slender i.e. height to diameter ratio more than 15, then the column or vessel is subjected to wind induced vibration due to the cross wind and along wind response of vessel. When the frequency of vortex shedding coincides with the natural frequency of the vessel, it results in resonance and large amount of deflection and vibration. Few methods are defined in various codes to consider cross wind effect and along wind effect. PV-Elite software uses the method of National Building Code of Canada for calculating force due to vortex shedding and Zorilla & Mahajan method to check for wind induced vibration. A modal analysis was done to cut down the time to obtain the frequency of uniform as well as non uniform vessel by plotting the L/D vs. f plot, which is presented here. The work presented includes calculation of Wind-Seismic loads as per Indian Standards used for wind-seismic load i.e. IS: 875 & IS: 1893. The same is compared with PV-Elite Software. Skirt supported vessels are mounted on to the basering. The maximum bending moment can be seen at the basering only. The one side of the basering will be in tension and other side will be in compression. Four types of baserings are used to support the vessel. Though, basering with the top continuous ring is used in most of the vertical vessels. The Brownell & Young method and Jawad & Farr methods are used to design the basering. Both methods are discussed in the work presented. As the vessel is subjected to various loadings i.e. load due to internal or external pressure, load due to the dead weight and bending moment due to wind-seismic load; the vessels are checked against combined stress. ASME Section-VIII, Division- 1 & 2, gives the method for combined stress analysis. Here, combined stress analysis has been shown as per Section-VIII, Division-1 and successfully validated with the PV-Elite software output. PV-Elite software, is used widely at the industry to design the pressure vessel, is generating notes and errors in the output file of the basering design calculations. The justification of which, is prepared for a customer of industry and accepted by them. Apart from it, while calculating the combined stress as per ASME Section-VIII, Div-2, for cone to shell junction at both the end, few mismatch of data, equation and answer were found by carrying out the comparison of manual calculation to the software output. The bugs were sent to the COADE Engineering Software Inc. They are accepted and are rectified in the newer version of the software.