This article will enable you to quickly verify for yourself whether or not your current piping stress analysis program may have limitations which can prevent proper calculation of basic ASME B31.1 orB31.3 piping code stresses when more than one thermal operating condition is analyzed. In a typical plant, valves are opening and closing, pumps and other equipment are running hot and cold, shutdowns may occur in the middle of a cold winter, or cryogenic applications may be required, to name a few of the possible scenarios which may occur. Given these possibilities, the piping engineer often does not know which scenario will result in the worst-case stress condition. As a result, it is often necessary to analyze more than one operating condition in order to accurately determine worst-case stresses and loads.
The ASME B31.1 and B31.3 piping codes require users to calculate stress ranges not only from ambient temperature to the various thermal operating conditions, but also the stress ranges between these operating conditions. The piping codes further state that users must consider externally imposed displacements together with thermal displacements in order to properly calculate the total displacement stress range.
When thermal conditions change, externally imposed displacements will change as well. Vessels, pumps, turbines and other equipment connected to the piping system will grow or shrink, depending on the thermal conditions. Please consider the following example of an auxiliary steam system analyzed with the AutoPIPE piping stress analysis software, (see Figure 1 below).
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| Figure 1 |
The piping system has two operating conditions to be analyzed. The first one is all hot (T1), and the second condition (T2) is when the valve at node. B2 is closed, thereby resulting in a cool down of the branch line from 600°F to 120°F starting at the valve, and a change in the externally imposed displacement at anchor node B6 from 1" growth in the Y direction in the first thermal condition case (T1) to only 1/10" growth in the Y direction in the cool down condition case (T2).
Please remember that the ASME B31.1 and B31.3 piping codes require a stress range to be calculated not only from ambient to thermal condition 1 (cold to T1) and ambient to thermal condition 2 (cold to T2), the codes also require that a stress range be calculated between the two operating conditions (T1 to T2).
As you can see from the analysis results (see Figure 2), the worst case B31.3 piping code stress calculated by AutoPIPE was in stress range case T1 to T2 at branch point A7 with a 43,837 psi stress that failed piping code compliance. You can quickly input and analyze this system for yourself with another piping stress program. Results from piping stress analysis programs other than AutoPIPE will likely show that the system passed code compliance with only about 26,700 psi stress (compared to 43,837 psi!) at branch point A7, when in fact, the system was considerably over-stressed at that branch. Run the same analysis using ASME B31.1 and you will see an even larger disparity between AutoPIPE and other piping stress programs.
Why would other piping stress programs differ from AutoPIPE by such a wide margin in the calculation of B31.1 and B31.3 piping code stresses? Answer: Some programs are limited to only one (1) set of anchor displacements and one (1) set of support displacements, no matter how many thermal operating conditions must be analyzed. Since equipment grows or shrinks depending on thermal conditions, such a limitation on anchor/support displacements can have a profound impact on the calculation of piping code stresses when more than one thermal operating condition is analyzed. Other piping stress programs are incapable of calculating a stress range between multiple thermal operating conditions (some can't calculate the Tl to T2 code compliance case).
The Wrong Approach
"No problem", we often hear from users, "I'll just run two jobs, one job with the anchor displacement from thermal condition 1 and another separate job using the anchor displacement and thermal conditions of case 2". This idea sounds acceptable, although time consuming, until you consider the fact that you cannot calculate a stress range between 2 different thermal operating conditions (you can't run the Tl to T2 case) when these two conditions are run as separate jobs. Try the example for yourself. The results from most piping stress programs other than AutoPIPE will show that the system has passed code compliance when, in fact, the system failed.
Since the worst case stress in piping code compliance is quite often the stress range between different thermal operating conditions (T1 to T2, T1 to T3, T2 to T3, etc.), the ability to calculate a stress range between multiple operating conditions, while considering different external displacements in each thermal condition, can make the difference between passing or failing ' code compliance as the steam system example demonstrates.
Cryogenic applications, pumps in series with different pumps operational and idle, winter shutdowns, and safety relief systems, arc situations, which typically require the analysis of multiple thermal operating, conditions.
As a footnote, most piping stress software programs other than AutoPIPE are limited to only one earthquake load direction and/or only one wind load direction for analysis (typically, wind and earthquake loads need to be analyzed in at least 2 directions). This limitation makes it very time consuming and error prone to manually sort through output from separate job runs to determine worst case support loads, worst case equipment loads, and worst case stresses from multiple wind directions and/or multiple earthquake loads.
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| Figure 2 |
AutoPIPE Capabilities
In one analysis, AutoPIPE will automatically perform piping code compliance, calculating stress ranges with up to three (3) sets of temperatures and three (3) sets of pressures using different anchor and support displacements for each different thermal condition. In the same analysis, AutoPIPE will analyze support and equipment loads with consideration of multiple wind directions, multiple earthquake loads, and up to 13 dynamic occasional loads. AutoPIPE output displays the greater of earthquake stress and wind stress at each point, while summarizing all loads on each restraint and equipment nozzle with rotating equipment compliance in one analysis. If your piping stress software does not perform these functions automatically, then you can determine for yourself how many problematic piping code compliance stress calculations may have been made, and how many man-hours have been spent running separate jobs. Bentley strongly emphasizes the CAD interfaces offered by AutoPIPE for AutoPLANT, PRO-PIPE. Intergraph PDS and other plant design systems. Although AutoPIPE's commitment to CAD interfaces is a strong benefit for users, it is often overlooked that AutoPIPE provides important stand-alone capabilities not available with other piping stress programs, enabling users to more quickly and reliably analyze their piping systems.