Power Company Uses Analysis to Prevent Piping Failure

POWER COMPANY USES ANALYSIS TO HELP PREVENT PIPING FAILURE

PipePlus was used to determine critical areas in the piping system. This stress contour of a pipe elbow was then performed with Algor FEA.

The merging of PipePlus with Algor FEA has played a crucial role in preventing failure in powerplant piping. At Mississippi Power and Light Co. (MP&L), a piping system was analyzed with PipePlus, the Algor pipe stress analysis system, then further studied with the Algor Design and Finite Element Analysis (FEA) system. Together, the two analysis packages revealed questionable strength-to-stress ratios in pipe fittings and high stresses in pipe elbows. The results, in turn, have been used to guide inspection efforts aimed at catching piping defects or other problems that could produce time consuming and expensive power outages.

The piping system, constructed at the MP&L plant, is used to generate steam energy. The system is a high energy type that carries superheated (1000^oF) steam at 800 PSI from a boiler to a 771,000 KW turbine-generator. The system is referred to as a "Hot Reheat" system because it carries steam that has already passed through the high-pressure section of a turbine and through a boiler (where it is reheated to 1000^oF) back to the turbine and into the intermediate-pressure section of the turbine.

Two analyses (PipePlus and Algor FEA) were performed by Keith J. Orgeron, an engineer with MP&L. The models were created and analyzed using an IBM PC AT with EGA graphics, and an IBM PS/2-80 with VGA graphics.

This FEA model of a pipe elbow was created with ViziCad Plus.

Finding High-stress Areas with PipePlus

To begin the project, Orgeron constructed a model of the entire piping system using PipePlus. The purpose of the PipePlus analysis was to identify the relative strength-to-stress levels of all possible seam-welded fittings in the system. Seam-welded fittings have been known to contain significant fabrication weld defects. The fittings Orgeron was most concerned with connected the high energy piping section with the overall line.

After analyzing the model, Orgeron found the highest strength-to-stress ratio in 42" OD elbow fittings. The ratio was .60, indicating strength well over the ANSI B31.1 Code of 1.00. However since ANSI B31.1 Code assumes the fittings are flawless, and since weld defects are not uncommon in seam-welded fittings, four of these fittings were selected for further analysis.

This precision contour of the elbow helped Orgeron determine the accuracy of the analysis.

Analyzing Pipe Elbows with Algor FEA

Orgeron used Algor's linear stress analysis package to model and analyze the elbow fitting enabling him to locate and evaluate the highest stresses. The stress profiles indicated precisely where peak stresses in the elbow occurred. The region was the outside surface of the inboard bend (interdos) which is shown in red in the picture beginning this case history.

By determining that the interdos are the most highly stressed areas in piping elbows, and by determining the critical elbows in the system (with PipePlus), MP&L was able to concentrate inspection efforts at the interdos of the seam-welded elbows. Once any defects were uncovered, MP&L could predict the importance by reviewing the stress plots of the elbow. By considering where the defect was located, especially whether it was in a low or high stress region, MP&L was able to decide the course of action to take, if any.