Expansion Vessel. Pressurized Equipment Design. PED.

Abstract

With the publication 2020-21.06 we dealt with the design of an expansion vessel. After identifying the normative references that regulate the pressure equipment, then the PED directive (2014/68 / EU), with reference to the project data that classify the pressure equipment to category IV (Annex II of the PED directive) we chose as a design method, the DBF analytical method envisaged by the EN 13445 standard that the PED directive has adopted. Having identified the mechanical characteristics of the material used for the construction of the vessel (yield point, traction limit and other mechanical properties), we calculated the safety factor based on the analysis of the loads that the pressure equipment will have to support under operating conditions (both normal and exceptional). Since 6 nozzles were connected to the expansion tank, it was necessary to check the reinforcement of the nozzle which confirms the request for reinforcement for the nozzles connected to the casing, with a larger diameter. Assuming three types of (periodic) oscillating loads, we carry out the fatigue test in all the load hypotheses considered. The checks and analyzes carried out on the results and calculations have demonstrated the required design safety even in exceptional cases such as temperature and pressure peaks, confirming the elastic regime (even if at the limit) with a temperature more than double the operating temperature (from 220 ° C at 450 ° C) in combination with a peak pressure equal to twice the operating pressure (from 25 bar to 50 bar). With the intention of achieving certainty of the results obtained, it was appropriate to size the container using an alternative calculation method to compare the results; for this purpose we used the calculation model provided by ASME VIII, which provides for the calculation of the pressure necessary for the complete plasticization of the section according to the Tresca criterion and which having provided, the same results as the model provided for by the EN 13445 DBF standard (model calculation based on the hypothesis of not exceeding the yield point of the material, therefore in the hypothesis of linearity determined by Hooke's law) it allows us to believe that the certainty of the results has been reached, aimed at operating safety of the pressure equipment. The project calculation ends with the Elasto Plastic Stress Analysis: deformation analysis according to the DBA Method (ASME VIII Div. 2) which allows us to calculate the plastic deformation, the collapse limit of the material and to carry out a study of the deformations at the elastic field limit and in the plastic field by Navier's theory (Claude-Louis-Marie Navier) which applies the constitutional laws of materials known as Hooke's law for the elastic field and the constitutional laws of Ramberg-Osgood material for the field plastic.