Stress Analysis of GRP / GRE / FRP piping system using Caesar II: Part 2 of 2

Continued from part 1…. Click here to go to the part 1…

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.. 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 . 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
  1. WW+HP …………………….HYDRO
  2. W+T1+P1 …………………..OPERATING-DESIGN TEMPERATURE
  3. W+T2+P1 …………………..OPERATING-OPERATING TEMPERATURE
  4. 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. 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.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.
  Output 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%).

Stress Analysis of GRP / GRE / FRP piping system using Caesar II: Part 1 of 2

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 design stage to determine possible limitations of component availability. The mechanical properties and design parameters varies from vendor to vendor. So it is utmost important 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 vendor.

Stress analysis of 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 GRP piping system in a very simple format.

Inputs Required for 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.

Modelling 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.

• 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 few parameters from Caesar 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 line list.
• Update pipe density from vendor information sheet, if vendor does not provide density of pipe then you can keep this value unchanged.
• On the right side below the code, enter the failure envelop data received from vendor.
• Enter thermal factor=0.85 if pipe is carrying liquid, enter 0.8 if the pipe carries gas.
• After you have mentioned all the highlighted fields proceed modelling by providing dimensions from the isometric/piping GA drawing. Add supports at proper location from isometric drawing.
• Now click on environment button and then on special execution parameter.
• Enter the GRP/FRP co-efficient of thermal expansion received from 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 ok button.
• While modelling remember to change the OD and thickness of elbows/bends.

Click here to read Part 2 of this article

Must have Load cases for stress analysis of a typical piping system using Caesar II

The main objectives of stress analysis is to ensure

A. Structural Integrity (Design adequacy for the pressure of the carrying fluid,Failure against various loading in the life cycle and Limiting stresses below code allowable.)

B. Operational Integrity (Limiting nozzle loads of the connected equipment within allowable values, Avoiding leakage at joints, Limiting sagging & displacement within allowable values.)

C. Optimal Design (Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.)

To meet these objectives several load cases are required during stress analysis. This article will guide all the beginners with the methodology to build several load cases which will be required for stress analysis.

In this article we will use following notations for building load cases:

WW=water filled weight of piping system,

HP=Hydrotest Pressure,

insulation,

P1=Internal Design pressure,

T1=Operating temperature,
T2=Maximum temperature,

T3= Minimum temperature,

WIN1, WIN2, WIN3 AND WIN4: wind loads acting in some specific direction,

U1, U2, U3 AND U4: uniform (seismic) loads acting in some specific direction.

While analysis at a minimum the stress check is required for the below mentioned cases:

a. Hydrotesting case: Pipelines are normally hydrotested before actual operation to ensure absence of leakage. Water is used as the testing medium. So during this situation pipe will be subjected to water weight and hydrotest pressure.
Accordingly our first load case in Caesar II will be as mentioned below

1.                    WW+HP                          HYD

b. Operating case: When operation starts working fluid will flow through the piping at a temperature and pressure. So accordingly our operating load cases will be as mentioned below:

2.            W+T1+P1                OPE                   for operating temperature case
3.            W+T2+P1                OPE                   for maximum system temperature case
4.            W+T3+P1                OPE                   for minimum system temperature case

c.  Sustained Case: Sustained loads will exist throughout the plant operation. Weight and pressure are known as sustained loads.  So our sustained load case will be as follows:

5.             W+P1                           SUS

d. Occasional Cases:  Piping may be subjected to occassional wind and seismic forces. So to check stresses in those situations we have to build the
following load cases:

6.                W+T1+P1+WIN1                       OPE                      Considering wind from +X direction
7.                W+T1+P1+WIN2                       OPE                      Considering wind from -X direction
8.                W+T1+P1+WIN3                      OPE                      Considering wind from +Z direction
9.                W+T1+P1+WIN4                       OPE                     Considering wind from -Z direction
10.              W+T1+P1+U1                            OPE                      Considering seismic from +X direction
11.              W+T1+P1-U1                              OPE                      Considering seismic from -X direction
12               W+T1+P1+U2                            OPE                      Considering seismic from +Z direction
13               W+T1+P1-U2                              OPE                      Considering seismic from -Z direction

While stress analysis the above load cases form load case 6 to load case 13 is generated only to check loads at node points.

To find occasional stresses we need to add pure occassional cases with sustained load and then compare with code allowable values. Following sets of  load cases are built for that purpose.
14.                 L6-L2                      OCC                         Pure wind from +X direction
15.                 L7-L2                      OCC                         Pure wind from -X direction
16.                 L8-L2                      OCC                         Pure wind from +Z direction
17.                 L9-L2                      OCC                         Pure wind from -Z direction
18.                 L10-L2                    OCC                        Pure seismic from +X direction
19.                 L11-L2                    OCC                         Pure seismic from -X direction
20.                L12-L2                    OCC                         Pure seismic from +Z direction
21.                 L13-L2                    OCC                         Pure seismic from -Z direction
22.                 L14+L5                  OCC                         Pure wind+Sustained
23.                 L15+L5                  OCC                         Pure wind+Sustained
24.                 L16+L5                  OCC                         Pure wind+Sustained
25.                 L17+L5                  OCC                         Pure wind+Sustained
26.                 L18+L5                  OCC                         Pure seismic+Sustained
27.                 L19+L5                  OCC                         Pure seismic+Sustained
28.                 L20+L5                  OCC                        Pure seismic+Sustained
29.                 L21+L5                   OCC                       Pure seismic+Sustained

Load cases from 22 to 29 will be used for checking occasional stresses with respect to code B 31.3 allowable (=1.33 times Sh value from code). Use scalar combination for load cases 22 to 29 above and algebraic combination for others as shown in figure attached below:

e. Expansion Case: Following load cases are required for checking expansion stress range as per code

30.                 L2-L5                     EXP
31.                  L3-L5                    EXP
32.                 L4-L5                     EXP
33.                 L3-L4                    EXP                            for complete stress range

The above load cases (from 30 to 33) are used to check expansion stress

The above mentioned load cases are minimum required load cases to analysis any stress system. Out of the above load cases the load cases mentioned in point number 1, 5, and 22-33 are used for stress check. And load cases mentioned in point number 1 to 13 are used for checking restraint forces, displacements and nozzle load checking.

Few additional load cases may be required for PSV connected systems, Rotary equipment connected systems.

Seismic and Wind analysis may not be required every time. So those load cases can be deleted if the piping system does not fall under the purview of seismic and wind analysis by project specification.  However to perform wind and seismic analysis proper related data must have to be entered in Caesar II spreadsheet (Will be discussed in my future posts).

If the stress system involves use of imposed displacements (D) and forces (F) then those have to be added with the above load cases in the form of D1, D2 or F1, F2 as applicable.

It is a better practice to keep

1. Hydro and sustained stresses below 60% of code allowable

2. Expansion and occasional stresses below 80% of code allowable

3. Sustained sagging below 10 mm for process lines and below 3 mm for steam, two phase and flare lines

4. Design/Maximum displacement below 75 mm for unit piping and below 200 mm in rack piping.

BASICS OF PIPE STRESS ANALYSIS: A PRESENTATION-Part 2 of 2

Continue from Part 1 of this topic…..

Basic Allowable Stress:

Minimum of (As per ASME B 31.3)

1. 1/3rd of Ultimate Tensile Strength (UTS) of Material at operating temperature.
2. 1/3rd of UTS of material at room temperature.
3. 2/3rd of Yield Tensile Strength (YTS) of material at operating temperature.
4. 2/3rd of YTS of material at room temp.
5. 100% of average stress for a creep rate of 0.01% per 1000 hr.
6. For structural grade materials basic allowable stress=0.92 times the lowest value obtained from 1 through 5 above.

Loads on a Piping System:

There are two types of loads which acts on a piping system: Static loads and Dynamic Loads,Static loads are those loads which acts very slowly and the system gets enough time to react against it. Examples of static loads are shown in Fig.1

Fig.1: Examples of Static Loads

On the other hand dynamic loads acts so quickly that the system does not get enough time to react against it. Examples of dynamic loads are shown in Fig.2

Fig.2: Examples of Dynamic Loads

other disciplines in any organization are shown in Fig. 3:

Fig.3: Inter Departmental Interaction with Stress Team

Stress Criticality and Analysis Methods:

•  Highly Critical Lines (Steam turbine, Compressor connected pipelines): By Computer Analysis
•  Moderately Critical Lines (AFC connected lines): By Computer Analysis
•  Low critical Lines : Visual/Simple Manual Calculation/Computer analysis and
•  Non Critical Lines: Visual Inspection

Stress Analysis using Caesar II :

Inputs:

• Stress Isometric from Layout Group
• LDT And P&ID from Process
• Equipment GA and Other detailed drawings from Mechanical
• Process flow diagram/datasheet if required from process
• Piping Material Specification
• PSV/ Control Valve GA and Datasheet from Instrumentation
• Soil Characteristics from civil for underground analysis
• Nozzle load limiting Standards
• Plot Plan for finding HPP elevation and equipment orientation.
• Governing Code

Analysis:

• Checking the completeness of the piping system received as a stress package.
• Node numbering on stress Iso.
• Filling the design parameters (Design temp, pressure, Ope. Temp, Min. Temp, Fluid density, Material, Line Size and
  thickness, Insulation thk and density, Corrosion allowance etc) on stress Iso.
• Modeling the piping system in Caesar using parameters from stress Iso.
• Analyzing the system and obtaining results.

Conclusion & Recommendation : Whether to accept the system or to suggest necessary changes in layout and supporting to make the system acceptable as per standard requirements.

Output:

•  Final marked up Iso’s to Layout
•  Support Loads to Civil
•  Spring Hanger Datasheets.
•  Datasheets for Special Supports like Sway brace, Struts, Snubbers etc.
•  SPS drawings
•  Stress Package final documentation for records

 Type of Supports:

• Rest
• Guide
• Line Stop
• Anchor
• Variable Spring Hanger
• Constant Spring Hanger
• Rigid Hanger
• Struts
• Snubbers
• Sway Braces etc

Questionnaire:

• What are the various types of loads which cause stresses in the piping system?
• Which code do we refer for Refinery Piping?
• Which standard governs the design of Pumps?
• The coefficient of thermal expansion of a substance is 1.8 mm/m/Deg.F. What is its value in mm/mm/Deg.C.?
• Calculate the minimum pipe thickness of a seamless 10” NB A106- Gr B material with design pressure of 20 bars. (Design Temp= 350 degree C and Corrosion allowance= 1.6 mm)