ag hdpe stress analysis

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2010 TECHNICAL REPORT

Electric Power Research Institute 3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA


Evaluation of Design Methods for Above Ground High Density Polyethylene Pipe

EPRI Project Manager J. Hamel

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA


This document does NOT meet the requirements of 10CFR50 Appendix B, 10CFR Part 21,

ANSI N45.2-1977 and/or the intent of ISO-9001 (1994)

Evaluation of Design Methods for Above Ground High Density Polyethylene Pipe 1021094

Final Report, December 2010

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

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THE FOLLOWING ORGANIZATION(S), UNDER CONTRACT TO EPRI, PREPARED THIS REPORT: Becht Nuclear Services

THE TECHNICAL CONTENTS OF THIS DOCUMENT WERE NOT PREPARED IN ACCORDANCE WITH THE EPRI NUCLEAR QUALITY ASSURANCE PROGRAM MANUAL THAT FULFILLS THE REQUIREMENTS OF 10 CFR 50, APPENDIX B AND 10 CFR PART 21, ANSI N45.2-1977 AND/OR THE INTENT OF ISO-9001 (1994). USE OF THE CONTENTS OF THIS DOCUMENT IN NUCLEAR SAFETY OR NUCLEAR QUALITY APPLICATIONS REQUIRES ADDITIONAL ACTIONS BY USER PURSUANT TO THEIR INTERNAL PROCEDURES.

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail [email protected].

Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc.

Copyright 2010 Electric Power Research Institute, Inc. All rights reserved.

ACKNOWLEDGMENTS

The following organization, under contract to the Electric Power Research Institute (EPRI), prepared this report:

Becht Nuclear Services 5225 Woodside Executive Court Aiken, SC 29803

Principal Investigators G.A. Antaki C. Becht V

This report describes research sponsored by EPRI.

This publication is a corporate document that should be cited in the literature in the following manner:

Evaluation of Design Methods for Above Ground High Density Polyethylene Pipe. EPRI, Palo Alto, CA: 2010. 1021094.

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REPORT SUMMARY

The results in this report are intended to support the development of an American Society of Mechanical Engineers (ASME) Section III Code Case for use of high-density polyethylene (HDPE) in above ground safety related piping applications. It examines the some of the differences in material and system behavior from metal pipe, at least at low temperatures. These differences include low temperature creep, potentially large displacements associated with the high coefficient of thermal expansion and low modulus of elasticity, strain rate sensitivity, and existence of slow crack growth (SCG) as a credible failure mechanism. It includes development of proposed ASME Code design rules.

The report is intended to be complementary to other ongoing EPRI activities that have the overall objectives to determine the material and engineering properties needed for the design of safety related buried and above ground piping systems. That work includes determining full-range stress-strain data, fatigue data, stress intensification factors and flexibility factors for selected piping components, determination of SCG behavior, damping values for the seismic event, determination of modulus of elasticity at seismic strain rates, develop and demonstrate methods to protect HDPE piping from postulated fire events, and perform seismic qualification of candidate vent and drain valve configurations.

Background Degradation of raw water piping systems is a major issue facing nuclear power plant owners, and many plants will require repair or replacement of existing carbon steel piping components. New plants wish to build on the lessons learned from operating plants and use piping materials that are expected to last the design lifetime. HDPE has been used in non-safety service water systems for over nine years, in both below ground and above ground applications, and found to perform well. Since the cost of installing HDPE piping is much lower than the cost for steel pipe, the use of HDPE pipe in safety related applications is desirable. ASME Code Case 755 was initiated to establish requirements for the use of HDPE piping in buried safety related systems, and it is desired to extend this work to above ground applications..

Objective To evaluate the issues associated with the use of HDPE for above ground safety related piping systems and support the development of appropriate ASME Code rules to achieve a safe and reliable design.

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Approach Material properties and failure modes applicable to the design of HDPE piping systems for above ground use were identified and evaluated. Existing Section III Code rules for safety class 2 and 3 metal piping systems, and rules for the design of below ground safety related HDPE piping systems as contained in Code Case N-755, and current code committee activities were evaluated. A set of proposed above ground rules were developed and applied to an example piping system with pressure, thermal, weight, and seismic loads. The piping system was evaluated using a standard piping analysis code (Caesar II) as well as the Abaqus code with both a beam and a 3-D formulations including large displacement effects. Results in terms of pipe stresses and deflections, as well as support loads were compared.

Results and Findings Evaluation of a sample problem using proposed code rules found that they could be successfully implemented using standard piping software.

The example problem evaluated in this report found that for well restrained piping systems, designed to withstand seismic events, minimize sag between supports, and prevent displacement interferences with other plant equipment, results from a standard piping stress code based on small displacement theory compared well with a finite element code based on large displacement theory. This conclusion may or may not be applicable to systems which are not well restrained.

EPRI Perspective Successful application of HDPE for below ground ASME Class 3 piping systems has resulted in increased industry interest in the use of HDPE for above ground piping systems, particularly for new plant builds. New ASME Code rules will be needed to support such use, either as a revision to Code Case N-755, or as a new parallel code case. This report is intended to provide a starting point for the development of such rules. Additional evaluations of candidate piping systems will likely be required after current EPRI and industry efforts to further quantify engineering and material properties of HDPE, and further industry development of ASME code rules. Additional information about the overall HDPE project is available in reports 1011836, 1013549, 1013572, 1013479, 1014902, 1018351, 1019180 and 1020439.

Keywords High-density polyethylene HDPE ASME piping design Above ground piping

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ABSTRACT

The purpose of this report is to present and illustrate the method for the design analysis and qualification of safety class 2 and 3 above ground high density polyethylene (HDPE) piping systems.

There are potential economic and safety benefits for pursuing the use of HDPE pipe above-ground due to its resistance to microbial attack and corrosion. Buried HDPE pipe has been used successfully used in many industries, including the nuclear power industry. HDPE has also been used extensively above-ground, typically near the ground on closely spaced, ground-mounted supports. In this report we examine its use in suspended systems, a viable option, as evidenced by applications in non-nuclear industries, and in a non-safety application in the turbine building at the Catawba Nuclear Station.

ASME Code Case N-755 has established a method for the design analysis and qualification of buried class 2 and 3 HDPE piping systems. Enclosed in this report are proposed design rules for above ground piping which relies primarily on the technical basis of Code Case N-755, but also addresses issues specific to above-ground piping. Design issues addressed by the proposed above-ground code case include sustained loads, seismic loads, thermal expansion loads, joint flexibility, piping supports and the concept of long-term and short-term HDPE properties.

Included is an example problem for which all of the criteria in the proposed code rules are analyzed. This example problem incorporates both hand solutions to some of the code-case equations and numerical solution utilizing Caesar II v5.20 software. Attached in Appendix B is an independent check of the CAESAR II software with the Finite Element Analysis (FEA) software package, Abaqus 6.10-1.

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CONTENTS

1 INTRODUCTION ....................................................................................................................1-1 1.1 Scope ..............................................................................................................................1-1 1.2 Background .....................................................................................................................1-1

2 DESIGN AND ENGINEERING OF HDPE PIPING SYSTEMS...............................................2-1 2.1 Material Properties ..........................................................................................................2-1 2.2 Failure Mechanisms ........................................................................................................2-1

2.2.1 Creep Rupture .........................................................................................................2-1 2.2.2 Slow Crack Growth..................................................................................................2-4 2.2.3 Fatigue.....................................................................................................................2-5 2.2.4 Creep-Fatigue Interaction........................................................................................2-6 2.2.5 Burst or Balloon Failure ...........................................................................................2-8

2.3 Design .............................................................................................................................2-8 2.3.1 Pressure Design ......................................................................................................2-8 2.3.2 Pipe Support............................................................................................................2-9 2.3.3 Thermal Flexibility..................................................................................................2-11 2.3.4 Seismic Design......................................................................................................2-12 2.3.5 Consideration of Large Displacement Theory .......................................................2-12

3 PROPOSED CODE CASE DESIGN REQUIREMENTS .....................................................3-1 -1000 GENERAL REQUIREMENTS.....................................................................................3-1 -1100 SCOPE .......................................................................................................................3-1 -1200 QUALIFICATION OF SUPPLIERS .............................................................................3-1 -1300 OPEN ITEMS..............................................................................................................3-1 -2000 MATERIALS................................................................................................................3-2 -3000 DESIGN ......................................................................................................................3-3 -3100 SCOPE .......................................................................................................................3-3 -3110 NOMENCLATURE......................................................................................................3-3

-3120 DESIGN LIFE..............................................................................................................3-3 -3130 DESIGN AND SERVICE LOADING............................................................................3-3 -3131 Pressure Design of Pipe .............................................................................................3-4 -3131.1 Minimum Required Wall Thickness..........................................................................3-4 -3131.2 Allowable Service Level Spikes Due to Transients Pressure...................................3-4 -3132 Pressure Design of Joints and Fittings........................................................................3-4 -3223 Longitudinal Stress Design .........................................................................................3-5 -3223.1 Longitudinal Applied Mechanical Loads...................................................................3-5 -3223.2 Short Duration Longitudinal Applied Mechanical Loads...........................................3-5 -3300 TEMPERATURE DESIGN ..........................................................................................3-5 -3310 MINIMUM TEMPERATURE........................................................................................3-5 -3311 Design for Expansion and Contraction........................................................................3-5 -3400 SEISMIC DESIGN.......................................................................................................3-6 -3410 SEISMIC INDUCED STRESSES................................................................................3-6 -3420 NONREPEATED ANCHOR MOTIONS ......................................................................3-6

4 EXAMPLE HDPE PIPING SYSTEM ANALYSIS ...................................................................4-1 4.1 System Description .........................................................................................................4-1 4.2 Design Analysis Work Flow.............................................................................................4-2 4.3 Loading Conditions..........................................................................................................4-3

4.3.1 Design Life...............................................................................................................4-3 4.3.2 Design Pressure and Temperature .........................................................................4-3 4.3.3 Service Level Pressure and Temperature ...............................................................4-3 4.3.4 Weights and Mechanical Loads...............................................................................4-4 4.3.5 Seismic Input ...........................................................................................................4-4

4.4 Piping System Model.......................................................................................................4-4 4.4.1 Caesar Model ..........................................................................................................4-5 4.4.2 Abaqus Models........................................................................................................4-5 4.4.3 Load Cases .............................................................................................................4-6

4.5 Mechanical Properties.....................................................................................................4-6 4.6 Physical Properties..........................................................................................................4-6

4.6.1 Modulus of Elasticity................................................................................................4-6 4.6.2 Poissons Ratio........................................................................................................4-6 4.6.3 Coefficient of Thermal Expansion............................................................................4-6

4.7 Preliminary Support Arrangement ...................................................................................4-7

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4.8 Results ............................................................................................................................4-7 4.9 Abaqus and CAESAR II Comparison ..............................................................................4-7

5 CONCLUSIONS .....................................................................................................................5-1

6 REFERENCES .......................................................................................................................6-1

A PIPE STRESS ANALYSIS (CAESAR II) INPUT .................................................................. A-1

B FINITE ELEMENT ANALYSIS USING ABAQUS................................................................. B-1

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LIST OF FIGURES

Figure 2-1 Load-Controlled Loading under Creep Conditions ...................................................2-3

Figure 2-2 Strain Controlled Loading under Creep Conditions ..................................................2-3

Figure 2-3 Stress Relaxation from a 2.7% strain induced in PE4710 Material using the Apparent Modulus from Code Case N-755 ........................................................................2-4

Figure 2-4 Stress-Rupture Characteristics of a Material Which Shows Two Unique Type of Failure Mechanisms [7] ..................................................................................................2-5

Figure 2-5 Damage Interaction Diagram for Type 304SS and 9Cr-1Mo-V ................................2-7

Figure 2-6 Recommended Pipe Support Arrangement-1 [1] ...................................................2-10 Figure 2-7 Recommended Pipe Support Arrangement-2 [3] ...................................................2-11 Figure 2-8 Recommended Pipe Support Arrangement-3 [3] ...................................................2-11 Figure 4-1 Caesar II Model Layout ............................................................................................4-1

Figure 4-2 Nodal Layout of Caesar II Model ..............................................................................4-2

Figure 4-3 Workflow for Analysis and Qualification ...................................................................4-3

Figure 4-4 Seismic Response Spectrum ...................................................................................4-4

Figure 4-5 Convergence of Seismic Stresses due to Increased Nodal Density ........................4-5

Figure B-1 Layout of Simplified Pipe Section............................................................................ B-2

Figure B-2 Restraint Layout ...................................................................................................... B-4

Figure B-3 Abaqus Model ......................................................................................................... B-5

Figure B-4 Long Term Deflection (in) Calculated by Abaqus.................................................. B-10

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LIST OF TABLES

Table 2-1 ASME Code References for Calculating Expansion Stresses...................................2-6

Table 3-1 Table -3131-1(a) Long Term Allowable Stress S for Polyethylene (psi) ....................3-7 Table 3-2 Table 3223-1 Stress Indices B1 and B2 ....................................................................3-7

Table 3-3 Table 3223-2 Design and Service Level Longitudinal Stress Factors, k....................3-8

Table 3-4 Table 3223-3 Short Duration (5 minutes) Allowable Longitudinal Tensile Stress Factors, k ................................................................................................................3-8

Table 3-5 Table 3311.2-1 Stress Intensification Factor i ...........................................................3-8

Table 4-1 Valve Weights and Dimensions ................................................................................4-2

Table 4-2 CAESAR II Results for Design Conditions.................................................................4-7

Table 4-3 CAESAR II Results for Service Level D.....................................................................4-7

Table 4-4 Comparison of Displacements due to Thermal Expansion........................................4-8

Table B-1 Force versus Displacement...................................................................................... B-2

Table B-2 Reaction Forces due to 30F Temperature Increase ............................................. B-3

Table B-3 Results for Gravity.................................................................................................... B-6

Table B-4 Results for Thermal Expansion ................................................................................ B-7

Table B-5 Results for Seismic Analysis .................................................................................... B-8

Table B-6 Natural Frequencies ................................................................................................. B-9

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1 INTRODUCTION

1.1 Scope

The purpose of this project is to present a set of proposed American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) rules for design of Class 2 and 3 above ground piping constructed of high density polyethylene (HDPE). The rules are similar in many ways to those contained in Code Case N-755 for buried HDPE, but addressing the unique features of above-ground, suspended HDPE pipe. Additionally this report discusses the method for the design analysis and qualification of above ground HDPE piping systems. There is an example problem which demonstrates how to apply the proposed code rules. The example problem uses the closed form equations from the proposed code rules, a numerical solution using the piping code Caesar II v5.20, and the Abaqus 6.10-1 finite element analysis code to solve the load cases.

1.2 Background

The interest in the use of HDPE piping at nuclear facilities is to provide a cost-effective solution to prevent corrosion and microbial attack often found in raw water systems. HDPE piping is impervious to these deterioration mechanisms and is a reliable and economical alternative to metallic pipe. Many non-nuclear industries apply HDPE above-ground. The Catawba Nuclear Station has had successful operating experience with both above-ground and below ground HDPE piping in both non-safety related and safety-related piping systems.

Code Case N-755 for buried HDPE pipe, and HDPE pipe manufacturer guidelines serve as the starting point for the rules for above-ground HDPE.

.

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2 DESIGN AND ENGINEERING OF HDPE PIPING SYSTEMS

2.1 Material Properties

Understanding the similarities and differences between metal and HDPE is essential for establishing a safe and appropriate set of design analysis and qualification rules for above-ground HDPE. HDPE material response is driven by a semi-crystalline polymeric structure which behaves differently than the crystalline and grain structure of steel [1]. However, like steel, HDPE experiences an elastic response to a load up to yield at which point both materials develop plastic strains with increasing load. However, the HDPE yield point is not as clearly defined and its elastic behavior is different than metal. Both materials have a visco-elastic response to a load after a critical temperature. For metal this temperature is above 700F; for HDPE this visco-elastic response occurs at ambient temperature [1].

One method for calculating long-term creep due to primary loads and the stress relaxation of secondary loads for a material experiencing creep is an isochronous stress-strain curve. Isochronous means to occur at the same time, and with respect to creep this means the stress-strain curve at a given point in time. The HDPE industry uses a term called apparent modulus, which is defined as the initial applied stress divided by the creep-strain at a given time and pressure, this ratio clearly decreases as the duration of loading increases [1]. This definition appears to exclude elastic strain; however the elastic strain would be small compared to the creep strain. If a point is chosen on the isochronous stress-strain curve for a given time at load and a line is drawn back to the origin from this point, then the slope of the line would be the apparent modulus, excluding the elastic-strain component. It does not represent the actual modulus of elasticity of the material, which the HDPE industry defines as the flexural or tensile modulus [1].

2.2 Failure Mechanisms

Establishing criteria for design of above-ground HDPE pipe requires understanding the failure mechanisms. This report will discuss five failure mechanisms, Creep Rupture, Slow Crack Growth (SCG), Fatigue, Burst (balloon), and Creep-Fatigue Interaction.

2.2.1 Creep Rupture

Time to creep-rupture is the method the HDPE industry uses to qualify pipe for long-term service. ASTM D 2837-08 [12] contains the methodology for performing the creep rupture tests

2-1

Design and Engineering of HDPE Piping Systems

and how the data is extrapolated to long-term service. This specification states that in order to acquire a hydrostatic design basis (HDB) rating of 1,600 psi, the pipe must undergo a 10,000 hr creep rupture test which qualifies it for a minimum of 100,000 hours at a stress of 1,530 psi [7]. A design factor of 0.50 is then applied which qualifies the pipe for 50 year service at 800 psi stress.

The ASME Code Section II, Part D has a different margin for stress for creep rupture in metals and has one additional requirement. For metals operating at less than 1,500F the allowable stress per ASME Section VIII Part D is the minimum of (100% the avg. stress to produce a creep rate of 0.01%/1,000 hour, 67% of the avg. stress to cause rupture at 100,000 hours, 80% of the min. stress to cause rupture at the 100,000 hours) [9].

Creep damage can be tracked for different operating conditions. For example, excursions to higher temperatures and pressures would accumulate creep damage at a rate faster than the design temperature and pressure condition and therefore would reduce the overall life of the pipe.

1Tt

kd

Where

(t)k = actual time at stress level, k (Td)k = allowable time at stress level, k

A paper by Ifwarson discusses testing done to validate long term cyclic service for medium density polyethylene (MDPE), PE, and cross linked polyethylene (PEX) pipe. Unfortunately, it does not address HDPE pipe. The testing only focused on different creep conditions via temperature cycles and did not study cyclic stress. Table 1 from the paper shows that the above equation was non conservative for MDPE, PEX, and in some cases PB material [10]. This implies that HDPE might need to be tested for the effect of different creep conditions. However, this should not be a priority as it has been demonstrated that creep rupture is not a realistic long term failure mode, and the margin the plastic industry uses (50%) is much more conservative than the margin that the metal industry uses (80%).

It is important to distinguish between load-controlled, or primary, and displacement controlled, or secondary, loads. Some typical primary loads are internal pressure, mechanical loading, weight, inertial seismic forces, and elastic follow up. Typical secondary loads include thermal expansion, anchor motion, and through-wall thermal gradients. Figure 2-1 and Figure 2-2 show how primary and secondary loads, respectively, behave differently in the creep regime. In a load-controlled loading the stress remains constant and creep strain continues to accumulate until a creep-rupture failure occurs. In a strain-controlled loading the strain remains constant and the stress decreases as the elastic strain is replaced by creep strain.

2-2


Figure 2-1 Load-Controlled Loading under Creep Conditions

Figure 2-2 Strain Controlled Loading under Creep Conditions

The HDB is a design allowable based on a primary load failure mechanism, creep rupture. A strain controlled loading would behave differently because the stress would relax over time and there would be no additional elongation. Figure 2-3 plots a 2.7% strain induced in PE4710 using the apparent modulus at ambient temperature from Code Case N-755. The stress was calculated by multiplying the strain by the apparently modulus at each point in time.

2-3


Figure 2-3 Stress Relaxation from a 2.7% strain induced in PE4710 Material using the Apparent Modulus from Code Case N-755

On the reverse cycle the total strain would go to zero, and the elastic strain would be the negative of the creep strain thereby introducing a negative stress in the piping system. Therefore, even with creep relaxation, the total range of stress due to cyclic strain ranges would be the same. Eventually, the system would reach a quasi equilibrium when the creep strain is equal to half of the total strain range, and the elastic range would go from negative the half of the total strain range to the positive of half the total strain range.

2.2.2 Slow Crack Growth

Another long term failure mechanism for HDPE pipe is the initiation and growth of a crack under a constant load condition. A similar type of crack growth can occur in metals when a crack reaches a critical crack size. However, in metal the failure is much more rapid once a critical crack size is achieved.

The initiation and growth of a crack can result in a shift in the slope of the stress-rupture lifetime plot of a material. Once the slow crack mechanism starts to dominate over the creep-rupture mechanism the slope decreases significantly and it is impossible to achieve a long term life. Appendix X1 in ASTM D 2837-08 discusses this behavior which is illustrated in Figure 2-4.

2-4


Figure 2-4 Stress-Rupture Characteristics of a Material Which Shows Two Unique Type of Failure Mechanisms [7]

Quantifying an allowable crack size will depend on understanding the visco-elastic fracture mechanics of HDPE material. Work is currently progressing on developing the fracture mechanics methodology for HDPE Pipe. At the Vancouver ASME BPVC meeting in November 2010, work done by the Task Group on Flaw Evaluation for HDPE Pipe was presented on developing KI, the Stress Intensity Factor. The next step in developing the fracture mechanics methodology will be determining the crack growth rates for a given stress intensity or range of stress intensity.

2.2.3 Fatigue

Fatigue failure is a strain controlled mechanism that traditionally results from alternating displacement controlled loads. For above ground pipe the most common source of the alternating displacement controlled loads is thermal expansion and contraction with startup and shutdown. For above-ground metallic pipe, Markls work in the 1940s and 1950s is at the origin of the current design analysis rules in ASME Section III and in ASME B31. The allowable stress range is artificial in that the Markl tests were strain controlled and were then multiplied by a reference modulus of elasticity. This is why B31.3 states the range of bending and torsional stresses shall be computed using the reference modulus of elasticity at 21C (70F), except as provided in Para. 319.2.2(b)(4) [4]. Therefore, when metallic pipe is designed at creep temperatures, such as 1,200F, the modulus at 70F is used to calculate the stress range, even though an apparent modulus could be calculated. This is because the most accurate comparison to an S-N curve

2-5


generated by using a reference modulus is to perform a piping evaluation at with the same reference modulus. Table 2-1 references the ASME precedence for calculating the expansion stress range.

Table 2-1 ASME Code References for Calculating Expansion Stresses

ASME Code Para. Method Section III - Div 1 - NB NB - 3672.5 calculate expansion stress with Eh and then multiply by Ec/EhSection III - Div 1 - NC NC - 3672.6 Use Ec to calculate expansion stress Section III - Div 1 - ND ND - 3672.6 Use Ec to calculate expansion stress Section VIII - Div 2 3.F.1.2 Calculate expansion stress with Et and then multiply by Efc/Eh B31.3 319.4.4(a) Use Ea to calculate expansion stress

Where: EFC = modulus of elasticity used to establish the design fatigue curve Et = modulus of elasticity of the material under evaluation at the average temperature of the cycle EC = modulus at room temperature Eh = hot modulus Ea = reference modulus of elasticity at 21C (70F)

EPRI is sponsoring a study that is investigating the fatigue life of 4710 material which includes a butt fusion joint. This study follows the methods used by Markl and has completed its work on PE4710, cell classification 445474C material. These findings show that the 1,100 psi limit found in CC N-755 has approximately a factor of 2 on stress to failure for temperatures ranging from 70F to 140F [5, 6, 11], See Figure 3-1 of [11]. This factor is the same as for metallic pipe and therefore provides support to the 1,100 psi limit. When a piping stress analysis is performed for an above ground piping system the reference tensile modulus at 70F should be used for comparison to the fatigue limit. Upcoming EPRI fatigue studies will be performed on a cell classification 445574C that is compliant to the material requirements of the November 2010 draft revision 1 to Code Case N-755.

2.2.4 Creep-Fatigue Interaction

In metallic piping it is sometimes possible to focus only on fatigue as a failure mechanism. This is not possible for HDPE above ground piping because creep occurs in conjunction with fatigue.

The mechanisms for creep failure and fatigue failure are different and therefore when both mechanisms occur their interaction must be taken into consideration. For metals this interaction is modeled by using Miners rule which is shown in the below equation [8]. This is explained in Para 12.3.7.1 in the Companion Guide to ASME for Subsection NH.

2-6


+

DTt

Nn

kdjD

Where:

D = allowable creep-fatigue damage factor (Nd)j = number of design allowable cycles of type, j. (n)j = actual number of cycles of type, j

The factor D depends on how much fatigue damage and creep damage has been accumulated. For instance for Type 304SS the damage interaction curve, taken from the Companion Guide is plotted in Figure 2-5 [8]. This material, which demonstrates strong resistance to creep-fatigue failure, has its focal point of the curve at Df = Dc = 0.3. This means that if 30% of the allowable fatigue damage is accumulated with 30% of the allowable creep damage then the material has reached its design life. If 0% of the allowable fatigue damage is accumulated then 100% of the allowable creep damage may be accumulated. A metal like 9Cr-1Mo-V exhibits extremely poor creep-fatigue interaction and has a focal point at 10% fatigue, 1% creep damage [8].

Figure 2-5 Damage Interaction Diagram for Type 304SS and 9Cr-1Mo-V

While HDPE creep and fatigue mechanisms are different than metal, it is likely that a creep-fatigue interaction diagram, with an appropriate focal point, could be constructed from fatigue

2-7


tests performed over long time periods. Understanding how well or poor HDPE responds to creep-fatigue interaction is required for establishing an allowable, D.

2.2.5 Burst or Balloon Failure

This failure mode is demonstrated in quick pressure burst tests. If a critical pressure is achieved in the pipe, then the pipe continues to expand until it experiences a ductile burst. The margin on this failure mode could be used as a basis for establishing a short term transient allowable. However if the transient occurs over a long enough duration then its effect on the creep life should be considered as described in the Creep-Rupture section.

2.3 Design

2.3.1 Pressure Design

HDPE Pipe can be specified as Iron Pipe Size (IPS) or Ductile Iron Pipe Size (DIPS) and as OD or ID controlled. The industry classifies different thicknesses of a nominal pipe size by the dimensional ratio. For OD-controlled pipe this term is DR, for ID-controlled pipe the term is IDR. Pressure rating of pipe is typically quantified in the HDPE industry with the hydrostatic design basis (HDB). The following equation can be used to calculate a required thickness of pipe using the typical industry practice [1]. However, this equation should not be used in lieu of Code requirements; it is merely representative of how vendors specify their pipe.

+=

1P

FT*DF*HBD*2D

t

D

omin

Where:

Do = OD-controlled pipe outside diameter, in HDB = Hydrostatic Design Basis, psi DF = design factor FT = service temperature design factor, if elevated temperature HDB is not used PDesign = design pressure tmin = minimum thickness for pressure design, in

Once a minimum required thickness is calculated then an appropriate DR can be chosen. For fittings, CC N-755 states that mitered elbows shall be one DR less than that of the connecting pipe [2]. Therefore, the elbows will be thicker than the pipe.

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2.3.2 Pipe Support

A preliminary support arrangement, before performing the stress analysis, may be established by limiting the long-term deflection between supports to 1 inch, as recommend by Performance Pipe Technical Note PP 815-TN. The corresponding support spacing is shown below [3].

( )4 fp yS ww5384E/S

L +=

Where:

E = apparent modulus for the design life, psi Ls = spacing between supports, in Sy = yield stress, psi Wp = Weight of Pipe per unit Length, lb/in WF = Weight of Fluid per unit Length, lb/in

Pipe supports must be carefully designed in HDPE applications because of multiple issues unique to HDPE piping. The first issue is that HDPE material is very soft and therefore it is easy to damage the piping with rough surfaces or lack of a large enough support area to distribute the load. Additionally, HDPE piping requires significantly more pipe supports than steel piping and therefore the piping system could become over constrained for thermal expansion if not carefully designed. Chapter 8 in the PE handbook has figures for typical support designs for HDPE piping systems, as shown below.

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Figure 2-6 Recommended Pipe Support Arrangement-1 [1]

Pipe Performance Technical Note PP 815-TN also provides some guidance for pipe support as shown in the following figures.

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The qualification of pipe supports should be performed in the same manner as if the pipe was metallic, following the project-specific methods and criteria. The loads on the supports shall consider a full load based on tensile modulus and also a reverse load that results from the residual stress due to creep relaxation.

2.3.3 Thermal Flexibility

A reference for flexibility analysis of above-ground piping systems may be gleaned from ASME B31.3 which contains a chapter on non-metallic piping. However, ASME B31.3-2008 broadly states that if a system replicates a previous design or can be readily judged, then it is acceptable [4]. If the HDPE piping system does not meet these criteria then B31.3 states the designer shall demonstrate adequate flexibility[Para A319.4.2. 7]. Unfortunately, B31.3 provides no guidance on what the appropriate allowable stress basis for the stress amplitude is, nor does it gives insight into the appropriate stress intensification factors (SIFs).

Code Case N-755, which was developed for buried HDPE systems, provides more thorough guidance for alternative thermal expansion or contraction evaluation [2]. This assumes a limited number of cycles.

2-11


EPRI has sponsored significant work in the area of fatigue testing of HDPE and this section will reference some of the key points for justifying the application of the Code Case N-755 stress limit to above ground piping. Fatigue testing detailed in the Section 2.6 in the 2007 Technical Update of project 1014902 suggested the 1,100 psi limit has a margin of 2 on fatigue capacity [5]. It demonstrates this capacity for both the tests at 70F and 140F. The final report supports this claim as shown in Figure 3-1 of [11].

A lot of work went into the EPRI sponsored tests and this report will attempt to summarize the key points. The fatigue tests were performed following the practice from Markls work on metal. A pipe under slight pressure was fixed at one end near a butt fusion joint and then the opposite end of the pipe was displaced a fixed distance in both directions, with a mean displacement of zero. The testing differs from Markls work in the conversion of the displacement range to stress. Markl only used one modulus to convert all the displacement tests to a unified S-N curve. The EPRI report used the stiffness from the first half cycle of each specimen to develop that specimens stress [see 2.4.2 of 11]. This naturally results in the 160F and 140F tests having a lower allowable stress range because the stiffness is lower, with the 50F and 70F tests having a higher allowable stress range because the stiffness is higher. This report recommends analysis of the strain-N data to see if it collapses when the modulus is simply ignored. Given the scatter in fatigue tests, it is likely that it wouldnt collapse completely. The goal of having a collapsed strain-N curve would be to produce a more uniform process of pipe analysis that would allow for a pipe designer to only consider one modulus, which is the current practice of the ASME code. In the case that the data doesnt collapse, then temperature effects would have to be included as described in Section 3.7 of EPRIs fatigue report [11].

2.3.4 Seismic Design

For above ground piping, stress due to seismic events is primarily calculated using a response spectrum analysis. This involves first calculating the natural frequencies of the piping system. The natural frequencies depend on the elastic modulus and, because of the short duration of the seismic event, the tensile modulus should be used. Once the accelerations are calculated using the natural frequencies and response spectrum, then the problem becomes load-controlled and therefore the modulus is no longer relevant for calculating stresses.

Testing is being done by EPRI to determine if the tensile modulus is different for the strain rates experienced during seismic events. Work is also being done by EPRI to calculate the damping values of HDPE. EPRI plans on using the log decrement method to determine damping from cantilevered pipe vibration tests.

2.3.5 Consideration of Large Displacement Theory

In Appendix B, this report compares an analysis by the pipe stress software package Caesar II to an analysis by the FEA software package Abaqus. This comparison was done to address the possibility that codes which do not consider large displacement theory may not accurately capture HDPE behavior. The comparison is made by studying the restraint forces because comparing stresses would result in an erroneous comparison because Caesar II modifies the

2-12


stress output to meet Code rules. The Appendix shows almost uniform agreement for the thermal expansion case; however the results differ for the seismic case, ~50% in some cases. It is possible that not enough nodes were placed between the supports in the Caesar II model, as Figure 4-5 shows that convergence for seismic stresses depends on a sufficient number of nodes between supports in Caesar II.

Additionally, it should be noted that under design conditions HDPE pipe may deflect less than metallic pipe under design conditions. Metallic pipe systems can experience more thermal expansion that HDPE pipe. This is because the allowable temperature range for metal far exceeds that of HDPE, which makes up for the difference in the coefficient of thermal expansion. Additionally, metal pipe can creep further than HDPE because the HDPE design basis creep strain at 50 years is 3.1%. However metal pipe has no limitation under Section VIII Division 1 & 2 and can creep up to 1% per 100,000 hours which can exceed the 3% creep strain. Piping analysis has always been a more crude analysis compared to vessel design. However the safety margins are sufficient enough that decades of experience with pipe that sometimes has greater growth than HDPE under design conditions show advanced analysis is not required.

2-13

3 PROPOSED CODE CASE DESIGN REQUIREMENTS

-1000 GENERAL REQUIREMENTS

-1100 SCOPE

(a) This Case contains rules for the construction of Class 2 and Class 3 polyethylene pressure piping components at Design Temperatures not exceeding 140F (60C), and for maximum Service Levels B, C or D temperatures not exceeding those for which allowable stresses are provided in this Case. Use of these materials is permitted only for above ground plant service and cooling water systems that are classified as Class 2 and 3.

(b) Terms relating to polyethylene as used in this Case are defined in -9000

-1200 QUALIFICATION OF SUPPLIERS

Not covered in this proposed Code Case.

-1300 OPEN ITEMS

This proposed code case only addresses the design requirements and will rely on the extensive work done on Code Case N-755 for addressing the many requirements of manufacturing, fabricating, qualifying and installing HDPE pipe systems.

To address the proposed failure mechanics in Chapter 2, this code case relies on a combination of rules from Code Case N-755, new criteria, and also leaves some failure mechanisms as open items. This code case refers to Code Case N-755 to protect against slow crack growth, creep rupture, and burst (short-term) failure. The slow crack growth and creep rupture are addressed by the long term allowable stress in Table 3131-1(a) and the limitation on allowable scratch depth of article 2310 of [2]. This long term allowable stress is based on the Hydrostatic Design Basis (HDB) which protects against both creep rupture and slow crack growth. The quick burst failure is protected against by the short term allowable in 3131.2 and 3223.2, which come from Code Case N-755.

This code case proposes a slightly different method than Code Case N-755 for addressing fatigue. It is proposed that a reference modulus of elasticity be used for calculating stress ranges resulting from secondary load cycles. For now, the allowable range is set to 1,100 psi with the

3-1

Proposed Code Case Design Requirements

stipulation that the number of design cycles shall not exceed 10,000. Ultimately it is proposed that the qualification of fatigue cycles in the code case rely on an S-N curve generated from a displacement controlled test -N curve multiplied by the reference modulus proposed in Para. 3311.

This proposed code case does not address creep-fatigue interaction; however it is possible to demonstrate why it may not be a large concern for HDPE. Table 1 in PP 820-TN [13] shows that the projected stress life intercept for 1,000 psi is 1010 years. This means that almost none of the creep life for a pipe operating at 800 psi for 60 years will be consumed. Discussions with industry experts seems to confirm that creep rupture is not a realistic failure mode and rather slow crack growth is a more realistic long-term failure mechanism for pipe operating at the allowable stress. So in Figure 2-5 if a HDPE condition was plotted after 60 years of service Dc would be practically zero. However, this does not guarantee that there is no creep-fatigue interaction. The only way to confirm this is to perform a step-wise fatigue tests (rather than saw tooth) with long holds (10 minutes to an hour).

A final consideration for HDPE is whether or not the piping systems will deflect so greatly as compared to steel design in the past that new analysis techniques are required. However, under design conditions HDPE deflection is not very different from high temperature steel piping. For one, the amount of creep strain accumulated from load controlled stresses within the allowable after 50 years is only 3.1% (670/21,200 @ 100F). Additionally, while the thermal expansion may be upwards of 10 times higher than steel, the maximum allowable temperature is far below that of steel. The maximum realistic range allowed in this code case would be around 50F to 176F (for loads to 1 year) which is a range of 126F. Meanwhile metal piping in other industries can be designed for thermal ranges of 50F to 1200F and greater, which is a range of 1,150F or ~10 times larger than a realistic range for HDPE pipe. So the thermal growths are quite comparable. These high temperature metal piping systems have been designed quite successfully for decades using the same rules proposed in this code case.

Additionally, enclosed in Appendix B is a check of Caesar II using Abaqus with large displacement theory. Although the sample problem is fairly well restrained, the calculated restraint forces due to thermal expansion were almost identical, while the seismic stresses were under predicted in the Caesar model. This implies that more nodes should have been introduced in the Caesar model to capture all the modes. However, it shows that fundamentally Caesar can capture all the important affects that a large-scale FEA model would capture

-2000 MATERIALS

Materials will be referenced to the ASME Section II Code.

3-2


-3000 DESIGN

-3100 SCOPE

The design rules of this Section are limited to above ground polyethylene piping systems constructed of straight pipe, three and five segment mitered elbows, fusion joints, and flanged connections.

-3110 NOMENCLATURE A = cross-sectional area of pipe, in2BB1 = stress index, Table-3223-1 [2] BB2 = stress index, Table-3223-1 [2] c = corrosion allowance, in D = outside diameter of pipe, in Fa = axial force due to the specified Design applied mechanical loads, lb FaC = axial force range due to thermal expansion or contraction FaD = axial force due to the non repeated anchor motion, lb FaE = axial force range due to the effects of seismic inertia, lb i = stress intensification factor k = factor from Table-3223-2 [2] M = resultant bending moment due to the specified Design applied mechanical loads, in-lb MC = resultant moment range due to thermal expansion or contraction, in-lb MD = resultant moment due to the non repeated anchor motion, in-lb ME = resultant moment due to the effects of seismic inertia, in-lb PD = design pressure, psi tdesign = design thickness, in tmin = minimum thickness for pressure design, in t = thickness of pipe, in S = allowable stress, psi Z = section modulus of pipe cross section

-3120 DESIGN LIFE

(a) The Design Specification shall specify the design life of the system, not to exceed 60 years.

(b) The duration of load shall be specified for each load case, and the PE pipe physical and mechanical properties shall be based on the duration of load.

-3130 DESIGN AND SERVICE LOADING

Design loads shall be as defined in NC/ND-3112.1 through NC/ND-3112.3. Loads applied to PE pipe shall be defined in the Design Specification, and shall include, as a minimum the following:

3-3


(a) Maximum and minimum internal design gage pressure PD, for pressure design in accordance with -3131 and -3132.

(b) Maximum and minimum temperature T, for the selection of allowable stress and design for temperature effects in accordance with -3300. The maximum Service Level A temperature shall be the Design Temperature, TD.

(c) The stress limits for the loads resulting from the maximum flow velocity, v, shall be as provided in -3131.2.

(d) Non-repeated anchor movements in accordance with 3420

(e) Seismic inertia for Seismic design in accordance with 3410

-3131 Pressure Design of Pipe

-3131.1 Minimum Required Wall Thickness

The minimum required wall thickness of straight sections of pipe for pressure design shall be determined by the following:

ctt mindesign +=

( )DD

min P2SDPt +=

-3131.2 Allowable Service Level Spikes Due to Transients Pressure

The sum of the maximum anticipated operating pressure plus the maximum anticipated Level B pressure spikes due to transients shall be no greater than 1.5 times the piping system Design Pressure, PD, (see -3131.1). The sum of the maximum anticipated operating pressure plus the maximum anticipated Level C and D pressure spikes due to transients shall be no greater than 2 times the piping system Design Pressure (see -3131.1).

-3132 Pressure Design of Joints and Fittings

(a) Polyethylene pipe shall be joined using the butt fusion process. All connections to metallic piping shall be flanged joints.

(b) Sustained pressure and pressure rating of polyethylene pipe fittings shall comply with the specifications listed in Code Case N-755 Mandatory Appendix III [2]. The Design Pressure, PD, for the fittings shall be greater than or equal to the Design Pressure of the attached pipe.

3-4


(c) Flanged connections shall include a metallic backup ring and shall provide a leak tight joint up to and including the piping hydrostatic test pressure. In addition, the maximum surge pressure per -3131.2 shall not cause permanent deformation of the pipe.

(d) Mitered elbows shall comply with the requirements of ND-3644, excluding ND-3644(b). In place of ND-3644(e) butt fusion joints shall be used in accordance with this standard. In addition, the mitered elbows shall be one dimension ratio (DR) lower than the DR of the attached straight pipe.

-3223 Longitudinal Stress Design

-3223.1 Longitudinal Applied Mechanical Loads

Longitudinal stresses due to axial forces and bending moments resulting from applied mechanical loads shall not exceed k x S.

Where

kxSZMxB

AF

x2xB2xt

xDPxB 2a

1D

1 ++

-3223.2 Short Duration Longitudinal Applied Mechanical Loads

For the assessment of short duration loads (less than five minutes) the allowable stress, S, may be replaced by one of the two following two alternatives

(a) 40% of the material tensile strength at yield

(b) The values in Table 3223-3

-3300 TEMPERATURE DESIGN

-3310 MINIMUM TEMPERATURE

The polyethylene material shall not be used at a temperature below the manufacturer limit, but in no case shall the temperature be less than minus 50F.

-3311 Design for Expansion and Contraction

The stresses shall satisfy the following equation:

3-5


1100psiA

FZ

iM aCc +

For displacement controlled conditions, the effective loads shall be calculated using the reference tensile modulus of elasticity from the fatigue tests that ultimately establish the allowable stress range. However, for the time being it is recommended that the tensile modulus of elasticity at 70F be used, 110,000 psi.

-3400 SEISMIC DESIGN

-3410 SEISMIC INDUCED STRESSES

The stresses in the above ground PE piping system due to inertial seismic loading, or repeated anchor motion, shall satisfy the following equation:

1100psiAF

ZiM aEE +

Seismic inertial loadings shall be combined by square root sum of the squares. The calculation of natural frequency depends on the modulus of elasticity; however the stresses resulting from the inertial loading, which is based on the natural frequency, are independent of modulus. The designer shall consider the strain rate effect on the modulus of elasticity. (Note to reviewers: this is the subject of a current EPRI test program.)

-3420 NONREPEATED ANCHOR MOTIONS

The effects of any single non-repeated anchor movement shall meet the requirements of the following equation:

2SA

FZ

iM aDD


Table 3-1 Table -3131-1(a) Long Term Allowable Stress S for Polyethylene (psi)

Temperature (F)

50 years

Temperature (F)

50 years

Temperature (F)

50 years

73 800 96 689 119 587 74 795 97 684 120 582 75 790 98 680 121 578 76 785 99 675 122 574 77 780 100 670 123 570 78 775 101 666 124 565 79 770 102 661 125 561 80 765 103 657 126 557 81 760 104 652 127 553 82 755 105 648 128 549 83 751 106 643 129 545 84 746 107 639 130 540 85 741 108 634 131 536 86 736 109 630 132 532 87 731 110 626 133 528 88 726 111 621 134 524 89 722 112 617 135 520 90 717 113 612 136 516 91 712 114 608 137 512 92 708 115 604 138 508 93 703 116 599 139 504 94 698 117 595 140 500 95 694 118 591

Table 3-2 Table 3223-1 Stress Indices B1 and B2

DR 7 DR 9 DR 11 DR 13.5

BB1 Straight and Butt Fused Joint 0.5 0.5 0.5 0.5

BB2 Straight and Butt Fused Joint 1.0 1.0 1.0 1.0

BB1 Miter [Note (1)] 0.69 0.69 0.69 0.69

BB2 Miter [Note (1)] 1.38 1.64 1.91 2.21

NOTE: (1) Mitered elbows shall not exceed 22.5 (3) angle of change in direction at mitered joint

3-7


Table 3-3 Table 3223-2 Design and Service Level Longitudinal Stress Factors, k

Service Level Design A B C D

k 1.0 1.0 1.1 1.33 1.33

Table 3-4 Table 3223-3 Short Duration (5 minutes) Allowable Longitudinal Tensile Stress Factors, k

Temp, F 70 100 120 140 176

S, psi 1200 940 770 630 400

Table 3-5 Table 3311.2-1 Stress Intensification Factor i

Fitting or Joint i

Straight Pipe 1.0

Butt Fusion 1.0

Mitered Elbows 2.2

3-8

4 EXAMPLE HDPE PIPING SYSTEM ANALYSIS

4.1 System Description

The piping system is illustrated in Figure 4-1 and 4-2. It is HDPE 4710 pipe, and is 4 inch nominal size. The system has a bypass line and a pressure relief valve. Butterfly valves are used to isolate the bypass line and the main control valve which is a globe valve. The valves are carbon steel, class 150, with weights and lengths shown in Table 4-1.

Figure 4-1 Caesar II Model Layout

4-1

Example HDPE Piping System Analysis

Figure 4-2 Nodal Layout of Caesar II Model

Table 4-1 Valve Weights and Dimensions

Valve Nominal Size Weight, lb Length, in

Butterfly 4 20 2.12

Globe 4 110 9

4.2 Design Analysis Work Flow

Figure 4-3 details the analysis and qualification workflow that was followed for this example problem.

4-2


Define Loading Conditions

Develop Piping System Stress Model

Select a Preliminary Support Arrangement

Select Pipe Size by Pressure Design

System LayoutMechanical Properties

Physical Properties

Run Stress Analysis

Qualify Stresses and Loads

Design Support Attachments and Qualify Supports

Figure 4-3 Workflow for Analysis and Qualification

4.3 Loading Conditions

4.3.1 Design Life

The design life of the system is 60 years.

4.3.2 Design Pressure and Temperature

PD = 150 psi TD = 100 F

4.3.3 Service Level Pressure and Temperature

PA_service = PB_service = PC_service = 150 psi PD_service = 200 psi for 30 days maximum TA_service = TB_service = TC_service = 100 F TD_service = 140 F for 30 days maximum

4-3


4.3.4 Weights and Mechanical Loads

Pipe size = 4 nominal (4.5 OD) Pipe Weight = 2.51 lb/ft Fluid Weight = 4.25 lb/ft Valve Weights are listed in Table 4-1.

There are no additional mechanical loads in this example problem.

4.3.5 Seismic Input

The seismic input response spectra are plotted in Figure 4-4. The seismic damping is 3% (Note to reviewers: This is the subject of a current EPRI sponsored test program). The seismic input is 3 dimensional (North-South, East-West, vertical) with component vectors combined by SRSS. In this example there was no seismic anchor motion, the end vessels are assumed to be rigid and rigidly anchored.

Figure 4-4 Seismic Response Spectrum

4.4 Piping System Model

The system layout and the model geometry are based on the isometric, and are illustrated in Figure 4-1.

4-4


4.4.1 Caesar Model

Caesar II v5.20 was used in this example to calculate the stresses due to sustained (deadweight) loads, seismic loads and thermal expansion loads. The model is shown in Figure 4-1, with guide restraints approximately every 4 feet or less. Appendix A contains the input echo of the Caesar II v5.20 model.

Consideration was given to the mesh density of the model (number of nodes between each restraint) for lumped mass programs. Figure 4-5 shows the convergence of stress due to inertial loading for a simply supported beam. Based on this convergence study, three nodes were placed in between every support.

Figure 4-5 Convergence of Seismic Stresses due to Increased Nodal Density

This example primarily follows Code Case N-755 which uses Section III code stress calculations, therefore Class III Subsection ND was chosen as the code in Caesar II [14]. This code allows the use of user-defined stress indexes, B1 & B2, and SIFs.

4.4.2 Abaqus Models

The FEA software package Abaqus version 6.10-1 was used to check the CAESAR II results. Initially, two models were made for the elbow shown in Figure B-1. The first model was a full 3D model and the second model used Abaqus beam elements which use beam theory based on the pipes section properties. This elbow model tested the performance of the Abaqus beam model which would be required for modeling the full system because a 3D analysis would have been too computationally demanding. A full model of the piping system was made with Abaqus beam elements after the elbow test demonstrated that the beam model would be conservative.

4-5


4.4.3 Load Cases

Four cases were run, for Service Levels A-C. One case was run for Service level D to check the deflection due to sustained weight and another to calculate the stress results for the sustained, thermal and seismic load cases. The apparent modulus from Table -3210-1(a) [2] was used for the deflection check and the tensile modulus was used for the code stress calculations.

4.5 Mechanical Properties

The long term allowable stresses for PE4710 are 800 psi and 670 psi, for ambient and design temperatures, respectively. These stresses are from Code Case N-755 Table -3210-3(a) and are used for evaluating the long term applied loads to the piping system. The HDB, which comes from the Plastic Pipe Institute listed material, is 1277 psi [2]. The HDB is used for initially sizing the pipe, although the thickness it calculates was checked against -3131 in the proposed code case.

The short term allowable stresses for PE4710 are 1200 psi and 940 psi, for ambient and design temperatures, respectively. The stresses are from Code Case N-755, Table 3223-3 and are used for evaluating any short duration ( 5 minutes) loads. These allowable stresses would be used for evaluating transients; however this example does not contain any.

4.6 Physical Properties

4.6.1 Modulus of Elasticity

The modulus of elasticity for HDPE Pipe for PE4710 comes from Code Case N-755 Table 3210-1(a) [2]. The long-term apparent modulus is 29 ksi and 21.2 ksi for ambient and design temperatures, respectively. The tensile modulus, are 110 ksi and 100 ksi for ambient and design temperatures, respectively. The short term modulus is used for calculating the code stresses for the different load cases and the long-term apparent modulus is used for calculating the deflection of the piping system.

4.6.2 Poissons Ratio

Poissons ratio varies depending on whether the load is a short duration of long duration load. Code Case N-755 states the value for Poissons ratio is either 0.35 (5 minutes duration) or 0.45 [3]. The same issues regarding the apparent modulus might apply here, however this was not been investigated in this report.

4.6.3 Coefficient of Thermal Expansion

The coefficient of thermal expansion used in this study was = 1.0E-4 in/in-F [6].

4-6


4.7 Preliminary Support Arrangement

In this example the Performance Pipe recommended spacing equation results in a support spacing of 5.5 ft. However, the supports were placed on 4 horizontal spacing due to the layout of the system. A hanger support was also added to the long vertical section. Guides were placed at either end of the metal valves.

4.8 Results

The results for the sustained, thermal, seismic and deflection analyses for the design conditions are shown in Table 4-2. This table shows that all the requirements are met for the design conditions.

Table 4-2 CAESAR II Results for Design Conditions

Node Check Value Allow Sustained 1018 Stress (psi) 366.3 670

Thermal 230 Stress (psi) 900.9 1100 Seismic 510 Stress (psi) 567.6 1100

Long-term Deflection 1000 Y-Displacement (in) 0.0175 1

The results for the Service Level D analysis are shown in Table 4-3. In this case the design fails to meet the thermal expansion criteria, but it meets all the other criteria.

Table 4-3 CAESAR II Results for Service Level D

Node Check Value Allow Sustained 1018 Stress (psi) 477 500

Thermal 230 Stress (psi) 2109.4 1100 Seismic 510 Stress (psi) 575.3 1100

Long-term Deflection 1000 Y-Displacement (in) 0.0175 1

4.9 Abaqus and CAESAR II Comparison

Appendix B details the Abaqus analysis that was used to check the CAESAR II results. As a first step, a simple elbow was used to compare a full 3D analysis in Abaqus, a beam model in Abaqus, and a CAESAR II analysis. This study showed that the CAESAR II model calculates the highest elbow stiffness. This is because CAESAR II incorporates the Code flexibility factors which are more conservative than the true elbow flexibility.

The full model analysis showed that for the thermal expansion case, the Abaqus beam model calculates the same restraint reactions as the CAESAR II model, as shown in Table B-5. However, Table 4-4 shows that the calculated displacements dont exactly line up. This could be

4-7


related to the elbow model in CAESAR II being stiffer so the pipe is more restrained. However, the restraint reactions end up being the same because more force is exerted for a given displacement.

Table 4-4 Comparison of Displacements due to Thermal Expansion

Displacement (in) Node Program x y z

Abaqus 0.00 0.09 0.16 620 CAESAR 0.00 0.10 0.16 Abaqus 0.10 0.06 0.01 330

CAESAR 0.08 0.07 0.05 Abaqus 0.27 0.00 -0.30 230

CAESAR 0.14 0.00 -0.25 Abaqus -0.41 0.00 0.00 120

CAESAR -0.34 0.00 0.00

4-8

5 CONCLUSIONS

This report has presented a number of potential failure modes that may require attention for above ground piping applications. Existing work addresses most of these failure modes, but more work might need to be done to ensure that HDPE can be designed to protect against all failure modes for above ground applications. This report also notes that in some cases there is not a significant difference between HDPE and steel behavior, and that the method of analyses for above ground applications can be similar. Therefore, the design analysis that includes all the failure modes doesnt necessarily require starting from scratch, but rather it can borrow from previous experience with metallic pipe in the creep regime.

The example problem contained in this report demonstrates that a lump mass analysis tool can match the results from a more detailed FEA analysis. While the seismic load case did not match exactly, a solution is presented that would work and the lack of a match is a demonstration of why proper nodal density must be included in the lumped mass analysis package. The example problem is a relatively small scale piping system so it might fully capture behavior that would happen in something like a 40 length of pipe.

5-1

6 REFERENCES

1. PPI Handbook of Polyethylene Pipe. Plastics Pipe Institute, www.plasticpipe.org. 2. ASME Code Case N-755 Use of Polyethylene (PE) Plastic Pipe Section III, Division 1, and

Section XI. proposed revision 1 dated 9-02-2010.

3. Performance Pipe Technical Note PP-815-TN, Above Grade Pipe Supports, Chevron Phillips Chemical Company LP, 2007.

4. ASME Code for Pressure Piping, B31, B31.3 Process Piping, 2008. 5. Fatigue and Capacity Testing of High Density Polyethylene Pipe and Pipe Components

Fabricated from PE4710. EPRI, Palo Alto, CA: 2007. 1015062. 6. Fatigue Testing of High-Density Polyethylene Pipe and Pipe Components Fabricated from

PE 4710 2008 Update. EPRI, Palo Alto, CA: 2008. 1016719. 7. Annual Book of ASTM Standards-2004, Section 8 Plastics Volume 08.04 Plastic Pipe and

Building Products. 2004 ASTM International, West Conshohocken, PA. 8. Companion Guide to the ASME Boiler & Pressure Vessel Code - Criteria and commentary

on Select Aspects of the Boiler & Pressure Vessel and Piping Codes Second Edition. Volume 1. 2006 by ASME, New York, NY.

9. 2010 ASME Boiler and Pressure Vessel Code Section II Part D Properties (Customary). 2010 by ASME, New York, NY.

10. Ifwarson M, Leijstrm H. Results and Experiences Obtained by Studsvik from Long-Term Pressure Tests on Plastic Pipes for Validation of Miners Rule. Studsvik Polymer AB, www.studsvik.se/polymer.

11. Stress Intensification and Flexibility Factors of High Density Polyethylene Pipe Fittings: Volume 1: Testing Results. EPRI, Palo Alto, CA: 2010. 1020439, V1.

12. ASTM D2837 - 08 Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for Thermoplastic Pipe Products, West Conshohocken, PA.

13. Performance Pipe Technical Note PP-820-TN, Design Factor for HDPE Pipe, Chevron Phillips Chemical Company LP, 2007.

14. Boiler & Pressure Vessel Code, Section III, Division 1, Nuclear Components, American Society of Mechanical Engineers, New York, NY.

6-1

A PIPE STRESS ANALYSIS (CAESAR II) INPUT

LISTING OF STATIC LOAD CASES FOR THIS ANALYSIS

1 (OPE) W+T1+P1

2 (SUS) W+P1

3 (EXP) L3=L1-L2

Job Description:

PROJECT:

CLIENT :

ANALYST:

NOTES :

PIPE DATA

-----------------------------------------------------------------------------

-----------------------------------------------------------------------------

A-1

Pipe Stress Analysis (Caesar II) Input

From 10 To 20 DY= 1.000 ft.

PIPE

Dia= 4.500 in. Wall= .484 in. Insul= .000 in. Mill%(-)= .00

GENERAL

T1= 100 F P1= 150.0000 lb./sq.in. Mat= (454)PE 4710 ST

E= 110,000 lb./sq.in. EH1= 100,000 lb./sq.in. EH2= 110,000 lb./sq.in.

EH3= 110,000 lb./sq.in. EH4= 110,000 lb./sq.in. EH5= 110,000 lb./sq.in.


EH9= 110,000 lb./sq.in. v = .350 Density= .0343 lb./cu.in.

Fluid= .0361100 lb./cu.in.

RESTRAINTS

Node 10 ANC

ALLOWABLE STRESSES

ASME ND (2007) Sc= 800 lb./sq.in. Sh1= 627 lb./sq.in.

Sh2= 800 lb./sq.in. Sh3= 800 lb./sq.in. Sh4= 800 lb./sq.in.


Sh8= 800 lb./sq.in. Sh9= 800 lb./sq.in.

-----------------------------------------------------------------------------

From 20 To 30 DY= 1.000 ft.

ALLOWABLE STRESSES




A-2



-----------------------------------------------------------------------------

From 30 To 40 DY= 1.000 ft.

ALLOWABLE STRESSES





-----------------------------------------------------------------------------

From 40 To 50 DY= 1.000 ft.

PIPE

Dia= 4.500 in. Wall= .484 in. Insul= .000 in.

GENERAL

Mat= (454)PE 4710 ST E= 110,000 lb./sq.in. EH1= 100,000 lb./sq.in.



EH8= 110,000 lb./sq.in. EH9= 110,000 lb./sq.in. v = .350

Density= .0343 lb./cu.in.

BEND at "TO" end

Radius= 6.000 in. (LONG) Bend Angle= 90.000 Angle/Node @1= 45.00 49

Angle/Node @2= .00 48 Miters= 4 Ftg Thk= .616 in.

SIF's & TEE's

Node 50 Sif(in)= 2.200 B1= .690 B2= 1.640

A-3


Meets 3673.2b-2, Notes 10,11 = ON Ferric Mat, Note 3673.2b-1.3 = OFF

ALLOWABLE STRESSES





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From 50 To 60 DX= -.562 ft.

PIPE


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From 60 To 70 DX= -1.146 ft.

RESTRAINTS

Node 60 Y

Node 60 Z

-----------------------------------------------------------------------------

From 70 To 80 DX= -1.146 ft.

-----------------------------------------------------------------------------

From 80 To 90 DX= -1.146 ft.

RESTRAINTS

Node 90 Z

Node 90 Y

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A-4


From 90 To 100 DX= -1.146 ft.

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From 100 To 110 DX= -1.146 ft.

-----------------------------------------------------------------------------

From 110 To 120 DX= -1.146 ft.

RESTRAINTS

Node 120 Y

Node 120 Z

-----------------------------------------------------------------------------

From 120 To 130 DX= -.562 ft.

PIPE


BEND at "TO" end



SIF's & TEE's

Node 130 Sif(in)= 2.200 B1= .690 B2= 1.640


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From 130 To 140 DY= -1.000 ft.

PIPE


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A-5


From 140 To 150 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 150 To 160 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 160 To 170 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 170 To 180 DY= -1.000 ft.

RESTRAINTS

Node 180 X

Node 180 Y

Node 180 Z

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From 180 To 190 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 190 To 200 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 200 To 210 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 210 To 220 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 220 To 230 DY= -1.000 ft.

PIPE


A-6


BEND at "TO" end



RESTRAINTS

Node 230 Y

SIF's & TEE's

Node 230 Sif(in)= 2.200 B1= .690 B2= 1.640


-----------------------------------------------------------------------------

From 230 To 240 DZ= 1.000 ft.

PIPE


-----------------------------------------------------------------------------

From 240 To 250 DZ= .500 ft.

-----------------------------------------------------------------------------

From 250 To 260 DZ= .500 ft.

-----------------------------------------------------------------------------

From 260 To 270 DZ= .500 ft.

RESTRAINTS

Node 270 X

Node 270 Y

-----------------------------------------------------------------------------

From 270 To 280 DZ= 1.000 ft.

A-7


SIF's & TEE's

Node 280 Reinforced Tee Sif(in)= 1.000 B1= .500 B2= 1.000


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From 280 To 290 DZ= 1.000 ft.

RESTRAINTS

Node 290 X

Node 290 Y

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From 290 To 300 DZ= .500 ft.

-----------------------------------------------------------------------------

From 300 To 310 DZ= .500 ft.

-----------------------------------------------------------------------------

From 310 To 320 DZ= .500 ft.

-----------------------------------------------------------------------------

From 320 To 330 DZ= 1.000 ft.

PIPE


BEND at "TO" end



RESTRAINTS

Node 328 Y

A-8


SIF's & TEE's

Node 330 Sif(in)= 2.200 B1= .690 B2= 1.640


-----------------------------------------------------------------------------

From 330 To 340 DY= -1.000 ft.

PIPE


-----------------------------------------------------------------------------

From 340 To 350 DY= -1.000 ft.

-----------------------------------------------------------------------------

From 350 To 360 DY= -1.000 ft.

RESTRAINTS

Node 360 X

Node 360 Y

Node 360 Z

-----------------------------------------------------------------------------

From 360 To 370 DY= -1.000 ft.

SIF's & TEE's



-----------------------------------------------------------------------------

From 370 To 380 DY= -1.000 ft.

-----------------------------------------------------------------------------

A-9


From 380 To 390 DY= -.823 ft.

-----------------------------------------------------------------------------

From 390 To 400 DY= -.177 ft.

RIGID Weight= 22.00 lb.

-----------------------------------------------------------------------------

From 400 To 410 DY= -2.000 ft.

PIPE


BEND at "TO" end



RESTRAINTS

Node 420 Z

Node 420 Y

SIF's & TEE's

Node 410 Sif(in)= 2.200 B1= .690 B2= 1.640


-----------------------------------------------------------------------------

From 410 To 420 DX= 1.000 ft.

PIPE


-----------------------------------------------------------------------------

From 420 To 430 DX= 1.000 ft.

A-10


-----------------------------------------------------------------------------

From 430 To 440 DX= 1.000 ft.

-----------------------------------------------------------------------------

From 440 To 450 DX= 1.000 ft.

RESTRAINTS

Node 450 Z

Node 450 Y

-----------------------------------------------------------------------------

From 450 To 460 DX= .625 ft.

-----------------------------------------------------------------------------

From 460 To 470 DX= .750 ft.


-----------------------------------------------------------------------------

From 470 To 480 DX= .625 ft.

RESTRAINTS

Node 480 Z

Node 480 Y

-----------------------------------------------------------------------------

From 480 To 490 DX= 1.000 ft.

-----------------------------------------------------------------------------

From 490 To 500 DX= 2.000 ft.

-----------------------------------------------------------------------------

From 500 To 510 DX= 1.000 ft.

A-11


PIPE


BEND at "TO" end



RESTRAINTS

Node 500 Z

Node 500 Y

SIF's & TEE's

Node 510 Sif(in)= 2.200 B1= .690 B2= 1.640


-----------------------------------------------------------------------------

From 510 To 520 DY= 2.000 ft.

PIPE


-----------------------------------------------------------------------------

From 520 To 530 DY= .177 ft.


-----------------------------------------------------------------------------

From 530 To 540 DY= .823 ft.

-----------------------------------------------------------------------------

From 540 To 550 DY= 1.000 ft.

SIF's & TEE's

A-12




-----------------------------------------------------------------------------

From 550 To 560 DY= 1.000 ft.

RESTRAINTS

Node 560 Y

Node 560 X

Node 560 Z

-----------------------------------------------------------------------------

From 560 To 570 DY= 1.000 ft.

-----------------------------------------------------------------------------

From 570 To 580 DY= 1.000 ft.

-----------------------------------------------------------------------------

From 580 To 590 DY= 1.000 ft.

-----------------------------------------------------------------------------

From 590 To 600 DY= 1.000 ft.

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ag hdpe stress analysis

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