"Quick on the Uptake," Feature Article, December 2002

quick on the uptake

Examining a boiler's pipeline becomes the job of an afternoon.

This article was prepared by staff writers in collaboration with outside contributors.

When the Arizona Health Science Center decided to install a power plant, it hired a San Diego contractor, Solar Turbines Inc., to design and build a cogeneration system, a gas turbine to generate electricity, and a heat-recovery boiler to produce steam.

Rentech Boiler Systems Inc. of Abilene, Texas, was called in to supply the boiler. The water will be carried to the boiler by a complex system of pipes that weave around various features of the power plant. For added efficiency, the water will pass through a serpentine coil called the economizer, which will make use of the boiler's own exhaust for preheating. It was the design of the piping system that landed on the desk of Rentech's chief engineer, Don Morran, for structural analysis.

"Before water goes into the boiler, preprocessing operations heat the water and pressurize the piping," Morran said. "Analysis of the piping system was required to verify its structural integrity under pressure, temperature, and site loadings, and to ensure compliance with the applicable ASME piping code."

The piping system started at a deaeration tank in which oxygen was removed from the water to prolong the life of the boiler. The piping was routed around existing features of the installation site, including platforming, forklift trails, building barriers, and property lines.

A PipePak model shows most of the pipeline for a cogeneration plant. Terminals at the left enter and exit the economizer.

A control station equipped with a sensor monitored the water level inside the boiler. The control station's valve regulated the flow of water to keep the level consistent. Over time, due to repetitive motion, this type of valve tends to wear out. Hence, the piping system included a bypass around the valve so it could be replaced while the boiler was still in operation.The economizer came farther along the line, just before it emptied into the boiler.

According to Morran, the on-site project manager provided him with a drawing in AutoCAD, software from Autodesk of San Rafael, Calif. Morran entered the geometry into PipePak, which is analysis software from Algor of Pittsburgh.

Morran specified ASME B31.1-2001 as the applicable piping code by choosing from a menu that is contained in the analysis software. Specifying the piping code automatically set several other model parameters, including the default load combinations.

Morran defined the piping system geometry, including pipes, bends, reducers, valves, and other elements. He said that the model had 60 nodes, one for each significant change along the 80.8 feet of pipeline.

Morran specified pipe data, including size, corrosion allowance, insulation, and contents; material data, including allowable stresses from the standard library; and load data, including temperature and pressure loads for various operating and test conditions.

He then used the software to view the analysis results. There was a particular span of pipe that he identified as needing more support to protect its ends.

"In general, the end points should be as well supported as possible because experience has shown that reducing or eliminating stresses at the ends can prolong the life of a piping system and, more importantly, the terminal connections," Morran said. He added supports at appropriate locations along the line and then analyzed the revised model. Morran concluded that stresses at the end points had been sufficiently reduced.

To verify that the piping structure complied with the ASME code, Morran displayed the code stress ratios for each load combination, which indicated that all stresses were safely below the allowable value. He said, "Displaying the ratios between code stress and allowable stress is a quick and easy way to indicate whether or not the piping system is in compliance with code requirements. It shows immediately if there is a problem and where it's located."

Morran used the software to automatically generate a report that summarized input, analysis results, equipment data, and more. He said, "Probably two hours after the job hit my desk, the report was back in the draftsman's hands and he was drawing the additional supports that were specified as a result of the analysis."

The power plant under construction at the center is part of the University of Arizona in Tucson.