Piping Design and Analysis Software Provides Convenience and Confidence

By Don Morran

Piping design and analysis software is an invaluable tool for performing structural analysis of piping systems. When I began doing piping analysis back in the early 1980s, I solved problems either longhand or with a calculator, which quickly grew unwieldy. An engineering consultant recommended ALGOR's PipePak software as a good value for what I needed to do, which was flexibility analysis of relatively short piping runs to connect pressure vessel equipment. I've used PipePak ever since. It can solve complex pipe routings, create useful reports of the model and results and help to ensure compliance with piping codes for allowable stress under applicable pressure, temperature and site loadings.

PipePak provides comprehensive design, analysis, visualization and reporting capabilities. The software is easy to install and starts up quickly. The initial PipePak screen provides access to all of the major functions including the following:

• Superdraw, a CAD-like environment for building models including piping features such as valves, tees or supports.
• A built-in spreadsheet environment for defining piping models.
• A built-in visualization environment that enables you to display the piping system and various properties and results.
• A report generator that enables you to create formatted, fully customizable reports in either text or HTML format including graphics.
• An optional capability for importing piping model files from CADPIPE, Intergraph PDS and CAESAR II.

If you need help when using any of the functions, a pull-down menu provides links to information about the software including: the PipePak User's Guide for complete documentation of software features plus a verification examples manual with Nuclear Regulatory Commission benchmarks (NUREG/CR-1677); on-line software demonstration Webcasts available at www.eTechLearning.com; a keystroke-specific tutorial, which guides you through the steps of defining and analyzing a model; and the ALGOR websites (www.ALGOR.com and www.PipePak.com).

Defining the Model

PipePak offers several different ways to define a model. You can: import a universal geometry file (such as a .dxf file) into Superdraw and then transfer it to PipePak; import piping model files from other piping software packages with the optional Piping Import Extender; draw the model in Superdraw and transfer it to PipePak; or define the model data directly in the built-in spreadsheet.

Typically, I receive a draftsman's AutoCAD drawing of a preliminary piping layout. Because I am experienced with the software, I prefer to enter the model geometry manually in the spreadsheet. Entering the data by hand gives me a feel for the piping system and enables me to make changes, if need be, as I go along.

Upon opening a new model, the "Global Parameters" pop-up form appears, which enables me to define global parameters for the model including the applicable piping code. PipePak supports the following piping codes including revisions through:

• ASME B31.1-2001 Power Piping
• ASME B31.3b-2001 Process Piping
• ASME B31.4a-2001 Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids
• ASME B31.8a-2000 Gas Transmission and Distribution Piping Systems
• ASME Section III Division 1 - Subsection NC Class 2 Components (2001)
• ASME Section III Division 1 - Subsection ND Class 3 Components (2001)
• British Standard BS 806 (1993)

Mostly, I deal with ASME B31.1. Defining the piping code in the "Global Parameters" form automatically sets several other model parameters including the default load combinations. Custom global settings can also be specified as default values in a configuration file.

The unit system can be specified for the model through a "Unit Selection Wizard", which enables you to choose from English, metric, SI and custom options. The unit of measure that applies to the current field is always clearly shown. The software accepts dimensions in various notations, even within the same model. For example, foot and inch notations can be used in one field and decimal notation in another.

For each point of the piping system, I enter data in one row of the spreadsheet. Each row is divided into 12 data fields including: X, Y and Z geometric offsets; bend radius; pipe, material and loading data; and extra data for defining a support, anchor, stress intensification factor, weight, mass, cut, force, valve, flange, reducer, bellows, tee, history, frequency or spectrum. Every time you press the "Enter" key to advance the cursor, the software reads the current field and performs error checking on the value. In addition, a context-sensitive help message can be accessed for each field.

For the pipe, material, loading and extra data fields, when you enter an identifier, a pop-up data entry form will appear. For example, the "Pipe Definition" form is used to specify pipe size, corrosion allowance, insulation and pipe contents; the "Material and Allowable Data" form is used to specify material properties and allowable stresses from standard or custom libraries; and the "Load Data" form is used to specify temperature and pressure loads, which enables you to analyze various operating and test conditions in a single analysis run. Values specified in the pipe, material and loading fields become the default for subsequent rows until a different value is entered in that column, which saves time by eliminating the need to enter duplicate information.

In the "From/To" field, you can designate a row as a comment line and insert text comments into the spreadsheet. Another way of using comments is to temporarily disable certain rows. For example, if you want to perform an analysis without a piping segment, instead of removing the segment, you can comment it out. Later, if you wish to include this segment, you can recover it by simply removing the comment line designation.

Periodically, as I build a model, I use the "Review Graphically" command to graphically display the piping system. After entering 3 or 4 lines, I'll render the model just to ensure the geometry that I've entered matches what the draftsman has given to me for analysis. The 3-D graphical rendering capability has been available for as long as I've been using the software, but the latest revision is a great improvement. The rendering of the piping model includes all of the details down to the valve hand wheels. It's almost like a photograph of the physical object.

A feed water piping line, like the one shown in the inset photograph, was modeled with ALGOR's PipePak piping design and analysis software. The built-in graphics environment displays a 3-D, shaded view of the control valve section. (Courtesy of Rentech Boiler Systems, Inc.)

After all points have been defined, I save the model. Before analysis, I visually check the model for errors to verify the geometry, supports and other piping attributes including diameter, schedule, wall thickness, corrosion allowance, thermal insulation, content specific gravity, material, pressure and temperature. Then, I use the "Action:Validate" command to check the model data for errors. If there are any errors, the software will display messages that let me know where and what kind they are.

Analyzing the Model

The software can perform static stress analysis, modal analysis, response spectrum analysis, time history (or transient) analysis and harmonic response analysis as well as weight, center of gravity and mass moment of inertia calculations. In addition to piping systems, frame structures modeled with beam/truss elements in Superdraw can also be analyzed.

When I use the "Action:Analyze" command, the "Analysis Progress" screen appears and displays messages indicating the progress of the analysis. For most models, the analysis run time is only a few seconds. For a large piping system with many frictional restraints that interact, calculations can take longer. For example, a giant watertube boiler model took 32 hours to analyze. In these cases, the "File:Global" command can be used to modify the allowed number of iterations or the force tolerances for the iteration convergence in order to speed up the analysis run time.

Viewing Results and Making Reports

After the analysis is completed, I view the results both graphically and in tabular report format.

Analysis results are displayed both graphically and in tabular report format. On the left, a display of code stress ratios illustrates that all stresses are below allowable values in compliance with the ASME B31.1-2001 power piping code. On the right, an HTML report lists the system stress values.

The built-in graphics environment provides a 3-D, full-color, OpenGL display of the piping system and a tree view listing all system components. It can display system properties and results for all load cases including stress, forces and moments, deflection and mode shapes. Display options provide control over colors, shading, text labels, symbols and dimensions. Viewing options include dynamic rotation, zooming and panning commands as well as the definition of dynamic clipping planes. Right-click functionality can be used for parts selected either through the tree view or by clicking on the system, and point-and-click inquiry features offer information on results or component properties. You can save a display as an image file in any standard format and create an .avi file of animated deflections or mode shapes.

Typically, the way I examine results is, first, I look at stresses throughout the entire piping line. Then, I examine the anchors at each end. Generally, 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. Therefore, often I add supports at appropriate locations along the line in order to reduce stresses at the ends.

To verify that the piping structure complies with the applicable piping code, I display the code stress ratios for each load combination. Displaying the ratios between code stress and allowable stress is a quick and easy way to indicate whether the piping system is in compliance with code requirements. It will immediately show if there is a problem and where it exists both numerically and by color code. If the maximum code stress ratio is less than one, then you know your routing is okay. You don't have to examine each node to ensure that the piping structure meets code.

Once the results have been reviewed, I use the "Report" pull-down menu to generate a report that summarizes input, analysis results, equipment data and more. The "Report Wizard" guides you through the steps of creating a report in text or HTML format.

For HTML reports, you can scroll through the report and click on links in a menu frame to jump to sections. Image files and animation files can be included in the report. The software automatically highlights any row showing a stress result above the acceptable value (producing a stress ratio greater than 1.0). This highlighting feature quickly pinpoints problem areas to simplify troubleshooting.

I return a printout of the report along with graphical displays of results to the draftsman so that any necessary changes indicated by the analysis can be made. Often, there are additional changes to the piping system during installation. For example, a particular valve might not be available and so a different valve with a different weight will be used. The on-site project manager can look at the printout of stress results and see if the proposed change is in an area of high or low stress and make an informed decision about whether the flexibility analysis should be re-examined. The ability to share model and analysis information with anybody who needs to see it, through reports and graphics in both electronic and hard copy format, is a great time saver.

In conclusion, ALGOR's PipePak software provides engineers with a powerful, easy-to-use tool for designing complete piping systems and performing structural analysis in compliance with well-known piping codes. ALGOR incorporates the latest technology in PipePak to provide flexible design methods, highly accurate analysis capabilities, powerful visualization and versatile report generation, which enables engineers to create better, more reliable piping systems.

Don Morran is a chief engineer with Rentech Boiler Systems, Inc., a supplier of custom steam-generating pressure vessels located in Abilene, Texas. He holds a Bachelor of Science degree in Aerospace Engineering from the University of Texas at Austin and has more than 20 years of experience in the design and analysis of pressure vessels, piping systems and structures.

Click here for more on how Rentech Boiler Systems, Inc. used ALGOR software.