Modelling of Bend and Tee Connections:
- Modelling of bends is a bit different as compared to CS piping. Normally bend thicknesses are higher than the corresponding piping thickness. Additionally you have to specify the parameter, (EpTp)/(EbTb), which is located at the Bend auxiliary dialogue box as shown in Fig. 4. This value affects the calculation of the flexibility factor for bends.
- When you click on SIF and Tee box in Caesar II spreadsheet, you will find that only three options (Tee, Joint and Qualified Tee) are available for you as shown in Fig. 4. Each type has their own code equation for SIF calculation. Use the proper connection judiciously. It is always better to use SIF as 2.3 for both inplane and outplane SIF to adopt maximum conservative approach.
Load Cases for Analysis:
ISO 14692 informs to prepare 3 load cases: Sustained, Sustained with thermal and Hydro test. So accordingly the following load cases are sufficient to analyse GRP piping system
- WW+HP …………………….HYDRO
- W+T1+P1 …………………..OPERATING-DESIGN TEMPERATURE
- W+T2+P1 …………………..OPERATING-OPERATING TEMPERATURE
- W+P1 ………………………..SUSTAINED
The expansion load cases are not required to create as no allowable stress is available for them as per the code.
While preparing the above load cases you have to specify the occasional load factors for each load case in load case options menu as shown in Fig. 5. ISO 14692 considers hydro test case as an occasional case. In higher versions of Caesar II software (Caesar II-2016 and Caesar II-2017) these load factors are taken care by default. So you need not enter the values. The option of these value entry will be available only if you define the stress type as occasional for those software versions.
The default values of occasional load factors are 1.33 for occasional case, 1.24 for operating case and 1.0 for sustained case. This occasional load factors are multiplied with system design factor (normally 0.67) to calculate the part factor for loading f2.
For aboveground GRP piping the above load cases are sufficient. But if the Line is laid undeground then two different caesar II files are required. One for sustained and operating stress check. And the other for hydrotesting stress check as the buried depth during hydrotesting is different from the original operation. Also buried depth may vary in many places. So caesar II modelling should be done meticulosly to take care exact effects.
For buried modelling one need to split the long lengths into shorter elements to get proper results. Element length of 3 m or less is advisable. Sometimes buried model contains slope, Those sloved are required to model properly to get accurate results.
Both stress and load data need to be checked for GRP piping. Normally the stresses are more than 90% (Even sometimes it may be as high as 99.9%).