Finite Element Analysis on Condensate Tanks & Nozzles
We performed a finite element stress analysis on condensate receiving tanks – including the shells for external loads on the nozzles and to recommend design changes if necessary. The tanks are constructed of ASTM SA516 Grade 70 carbon steel and the nozzles were SA53 Grade B carbon steel pipe.
The receiving tanks’ shells were evaluated for specified external nozzle loads. The purpose of the evaluations was to ensure the structural integrity of each receiving tanks’ shell when subjected to external loads on the attached nozzles. Evaluation of the nozzles, flanges, and the tank support structure were not part of the work scope and were therefore not evaluated. A 0.0625 inch corrosion allowance on both the tanks’ shells and nozzles, as specified on the client’s drawings, was taken into account in the evaluations.
Tank shells were evaluated using either hand calculations or finite element analysis. The “hand calculations” consisted of determining the stresses in the shell using the method described in the Welding Research Council Bulletin 107 “Local Stresses in Spherical and Cylindrical Shells due to External Loadings.” These calculations were performed using the computer program CodeCalc.
Finite element analyses of several of the receiving tanks were also performed when the WRC 107 evaluation showed shell stresses requiring significant tank design modifications. Finite element models were created for three of the receiving tanks. Each of the models included the tank shell and the nozzles on top of the tank. The tank head, attached piping, and support structure were not included. Welds were not explicitly included in the model. Each of the models was created using approximately 45,000 to 75,000 linear order shell elements (SHELL63).
The receiving tanks are constructed of ASTM SA516 Grade 70 carbon steel, and the nozzles are SA53 Grade B carbon steel pipe. Material properties used in the finite element analyses were for 300 F and are as follows: Young’s Modulus: 27 x 106 psi Poisson’s Ratio: 0.3 Density: 0.283 lb/in3
The bottom of each model was completely constrained at four locations where the tank is supported by structural angles acting as support columns.
Two load cases were evaluated for each finite element model: an internal pressure load case of 125 psig, and an external pressure (vacuum) load case of 14.7 psig. The results of the finite element analyses of tanks showed that the original shell thickness was not sufficient for the nozzle loads, and that the nozzle reinforcement plates would have to extend beyond typical dimensions. The reinforcement plates necessary to reduce shell stresses to a magnitude below the allowable were determined to be approximately 75% of the tank diameter. This size requirement was the result of the significant moment loads on the nozzles. Due to the size of the required reinforcement plates, it was considered more cost effective to increase the thickness of the entire top half of the tanks, rather than use individual reinforcement plates.
We perform (thermal, stress, vibration and fatigue) finite element analysis to ASME Code on equipment including tanks and heat exchangers. Give us a call to talk about your engineering needs.