GRP products being proprietary the choice of component sizes, fittings, and material types are limited depending on the supplier. Potential GRP vendors need to be identified early in the design stage to determine possible limitations of component availability. The mechanical properties and design parameters vary from vendor to vendor. So it is of utmost importance that before you proceed for stress analysis of such systems you must finalize the GRP/FRP/GRE vendor.
Several parameters (Fig. 1) for stress analysis have to be taken from the vendor.
Stress analysis of the GRP piping system is governed by ISO 14692 part 3. The GRP material being orthotropic the stress values in axial as well as hoop direction need to be considered during analysis. The following article will provide a guideline for stress analysis of the GRP piping system in a very simple format.
GRP/FRP Information Required from Vendor
Before you open the input spreadsheet of Caesar II communicate with the vendor through the mail and collect the following parameters as listed in Fig.1.
The values shown in the above figure is for example only. Actual values will differ from vendor to vendor. The above parameters are shown for a 6” pipe.
Inputs Required for GRP Stress Analysis
For performing the stress analysis of a GRP piping system following inputs are required:
- GRP pipe parameters as shown in Fig. 1.
- Pipe routing plan in form of isometrics or piping GA.
- Analysis parameters like design temperature, operating temperature, design pressure, fluid density, hydro test pressure, pipe diameter, and thickness, etc.
Modeling GRP/FRP/GRE in Caesar II
Once all inputs as mentioned above are ready with you open the Caesar II spreadsheet. By default, Caesar will show B 31.3 as governing code. Now refer to Fig. 2 and change the parameters as mentioned below:
- Change the default code to ISO 14692.
- Change the material to FRP (Caesar Database Material Number 20) as shown in Fig. 2. It will fill a few parameters from Caesar’s database. Update those parameters from vendor information.
- Enter pipe OD and thickness from vendor information.
- Keep corrosion allowance as 0.
- Input T1, T2, P1, HP, and fluid density from the line list.
- Update pipe density from vendor information sheet, if the vendor does not provide the density of pipe then you can keep this value unchanged.
- On the right side below the code, enter the failure envelop data received from the vendor.
- Enter thermal factor=0.85 if the pipe is carrying liquid, enter 0.8 if the pipe carries gas.
- After you have mentioned all the highlighted fields proceed to model by providing dimensions from the isometric/piping GA drawing. Add supports at the proper location from the isometric drawing.
- Now click on the environment button and then on special execution parameter. It will open the window as mentioned in figure 3.
Now Refer Fig. 3 and change the highlighted parts from available data.
- Enter the GRP/FRP co-efficient of thermal expansion received from the vendor
- Calculate the ratio of Shear Modulus and Axial modulus and input in the location.
- In FRP laminate keep the default value if data is not available.
- After the above changes click on the ok button.
- While modeling, remember to change the OD and thickness of elbows/bends.
Modeling of GRE Bend and Tee Connections
- The modeling 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 the 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 its 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 out plane SIF to adopt a 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.
However, if you are analyzing a piping system consisting of GRE pipe plus metallic piping then expansion load cases need to be prepared.
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 the occasional case, 1.24 for operating case and 1.0 for the sustained case. These 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 underground then two different caesar II files are required. One for sustained and operating stress check. And the other for hydro testing stress check as the buried depth during hydro testing is different from the original operation. Also, buried depth may vary in many places. So caesar II modelling should be done meticulously to take care of exact effects.
For buried modelling, one needs 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 a slope, Those slopes are required to model properly to get accurate results.
Output Results from GRP Stress Analysis
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%).