| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | ALL | ||
| Area: | modeling | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | May. 2015 09.06.02.06 |
Problem:
On a riser pipe (i.e. vertical pipe) with
a. elbows attached to the top and bottom of the vertical pipe
b. intermediate supports on the vertical run of pipe to support the weight of pipe
c. Support near the bottom elbow.
The total pipe weight is spread among all the supports however, the liquid weight in the vertical column is concentrated on the bottom support. However, when reviewing the results, the total Pipe and Liquid weight is distributed among all the supports. This is the error. How to best model this scenario?
Solution:
Most stress analysis programs, including AutoPIPE, distribute the weight of the pipe's contents over the support for both horizontal and vertical pipe runs i.e. per unit length the same as the pipe steel weight.
Consider this, a model is made of up of multiple node points,
Upon closer inspection, the model is essentially made up of multiple piping elements. Consider that each consecutive pairs of node points ( ie #1 - A00 to A01N, #2 - A03 to A04N) defines a piping element, where properties are applied.
Again, all the properties (i.e. piping material, contents, loads, etc) are applied to each of these individual elements and are not shared, this is an important concept, Properties are Not Shared.
Example: segment C below is made up of 6 node points or 5 individual elements connected together.
The properties of these elements do not flow from one element to the next regardless of the angle of incline (i.e. 0 deg to 90 deg); horizontal to vertical and every angle in between.
In addition, with this methodology there is no internal Hydrostatic pressures calculation. Different pressures will need to be manually calculated and applied to each pipe element as needed.
Note:
As an alternative to manually calculating and applying the different pressure values to sloped / vertical piping, AutoPIPE V8i 09.06.02.xx has a feature, "Local Incidental Pressure Calculation", available for when Piping code = DNV. This feature is located under Tools> Model Options> DNV 2012 Result Options> General. An enhancement has been logged (TFS-E229493) to make this calculation available for all piping codes.
Update, AutoPIPE v.10.0 and higher has a new feature, Incidental Pressure (Loads > Pressure > Incidental Pressure) that is available in all codes.
Basically for any non-horizontal pipe (pipe angel = 1 - 90 deg) to accurately account for the weight of the pipe contents column in vertical piping, calculate and adding extra loads at the base support / anchor and intermediate supports/anchors to manually correct the software. Note, Rigid Anchors at intermediate levels on a vertical riser are only suitable for cold pipework
Model Riser Piping Approach #1:
model Riser piping with contents turned ON, with vertical supports at each floor, and base support at the bottom.
Comments: this model will produce incorrect static results as the total weight of pipe and contents are spread among all the supports. However, for earthquake loads and Dynamic loads involving Modal Analysis, the correct mass is captured along the vertical piping.
Model Riser Piping Approach #2:
model Riser piping with contents turned OFF, with vertical supports at each floor, base support a bottom, and insert additional weights representing weight of contents at all supports except those v-stops on the vertical piping.
Comments: this model should produce better static analysis results, but should not be used for any Dynamic analysis as the correct mass is not distributed on the vertical piping.
Conclusion, suggest creating 2 models, one for static load cases and one for dynamic load cases.