Code Case N-755 Revision 1 –
Allowable Hydrostatic Stresses for PE4710 Material
William I Adams
Senior Engineer
WL Plastics Corp
June 14, 2011
Revised by:
Douglas D Keller
Technical Lead
LyondellBasell Industries
January 23, 2015
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Table of Contents
1.0 Background – Rev. 0 PE3408 v. Rev. 1 PE4710
3
1.1 Material Designation Codes
3
1.2 HDB, HDS and DF – ASTM D2837 and PPI TR-3 Requirements
3
1.2.1 HDB
3
1.2.2 HDS
4
1.2.3 DF
4
1.2.4 ASTM D2837 and PPI TR-3
5
1.3 Long-Term Performance of PE Pressure Pipe
6
1.3.1 Long-Term Hydrostatic Strength
6
1.3.2 Slow Crack Growth
6
1.3.3 Predicting the Onset of SCG
6
1.3.4 PE Pressure Pipe Rating
6
1.3.5 Short-Term Pressure Capability
7
1.4 Comparing PE4710 and PE3408
7
1.4.1 Illustrating Minimum PE3408 Performance
8
1.4.2 Illustration Minimum PE4710 Performance
9
2.0 CC N-755 Revision 1 PE4710 Requirements
10
2.1 Mandatory Appendix IV
10
2.2 Allowable Stress Tables
10
2.2.1 Tables -3131-1(a) and -3131-1(b)
10
2.2.2 Tables -3131-1(c) and -3131-1(d)
11
Appendix A – Technical Basis for HDB Used in Code Case N-755, Rev. 1.
12
Appendix B – Barlow’s Formulation
16
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
1. Background – Rev 0 PE3408 vs. Rev 1 PE4710
1.1. Material Designation Codes
Material Designation Codes are used in North America by ASTM International (ASTM), American Water
Works Association (AWWA), the Plastics Pipe Institute (PPI) and industry to identify thermoplastic
pressure piping compounds. The Material Designation Code is the abbreviation for the thermoplastic
st
nd
rd
th
followed by four numbers. The 1 and 2 numbers are key physical properties. The 3 and 4 numbers
1
are the hydrostatic design stress (HDS) rating for water at 73°F in accordance with ASTM D2837 and
2
3
PPI TR-3 in hundreds of psi with tens and units omitted. PPI TR-4 is a document that lists hydrostatic
design basis (HDB) and HDS ratings according to the PPI Hydrostatic Stress Board for polyethylene and
other thermoplastic pipe materials in groups according to the material designations. Depending on the
st
nd
thermoplastic, the 1 and 2 digit physical properties will be different. (Reference – see ASTM F412 for
4
definition and F714 for an example of use.)
For polyethylene pressure piping compounds, the Material Designation Code is:
•
•
•
•
The abbreviation for polyethylene
st
5
1 digit – The ASTM D3350 cell classification property value for density
nd
2 digit – The ASTM D3350 cell classification property value for slow crack growth (SCG)
resistance
rd
th
3 and 4 digits – The HDS for water at 73°F in hundreds of psi with tens and units omitted
PE3408
•
•
•
•
PE
3
6
3 = ASTM D3350 density cell 3 (>0.940 – 0.947 g/cm density per ASTM D1505 )
7
4 = ASTM D3350 SCG resistance cell 4 (>10 hours SCG resistance per ASTM F1473 )
08 = 800 psi HDS for water at 73°F per PPI TR-3 (HDS = HDB x DF; 800 = 1600 x 0.50)
PE4710
•
•
•
•
PE
3
4 = ASTM D3350 density cell 4 (>0.947 – 0.955 g/cm density per ASTM D1505)
7 = ASTM D3350 SCG resistance cell 7 (>500 hours SCG resistance per ASTM F1473)
10 = 1000 psi HDS for water at 73°F per PPI TR-3 (HDS = HDB x DF; 1000 = 1600 x 0.63)
1.2. HDB, HDS and DF – ASTM D2837 and PPI TR-3 Requirements
1.2.1.
HDB
Thermoplastic pressure piping compounds are rated for internal pressure service in accordance with
ASTM D2837 and PPI TR-3. ASTM D2837 is an extrapolation method based on analysis of hydrostatic
8
stress-rupture data generated in accordance with ASTM D1598 . It involves conducting a series of
sustained internal pressure tests for up to 10,000 hours, and then analyzing the data using a two
parameter linear regression model and determine a long-term hydrostatic strength (LTHS) at 100,000
hours that when categorized becomes the HDB rating for the thermoplastic compound. HDB is
1
ASTM D2837 Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design
Basis for Thermoplastic Pipe Products
2
PPI TR-3 Policies and Procedures for Developing Hydrostatic Design Basis (HDB), Pressure Design Basis (PDB), Strength Design
Basis (SDB), and Minimum Required Strength (MRS) Ratings for Thermoplastic Piping Materials or Pipe
3
PPI TR-4 Listing of Hydrostatic Design Basis (HDB), Strength Design Basis (SDB), Pressure Design Basis (PDB) and Minimum
Required Strength (MRS) Ratings for Thermoplastic Piping Materials or Pipe
4
ASTM F714 Standard Specification for Polyethylene (PE) Plastic Pipe (DR-PR) Based on Outside Diameter.
5
ASTM D3350 Standard Specification for Polyethylene Plastics Pipe and Fittings Materials
6
ASTM D1505 Standard Test Method for Density of Plastics by the Density-Gradient Technique
7
ASTM F1473 Standard Test Method for Notch Tensile Test to Measure the Resistance to Slow Crack Growth of Polyethylene
Pipes and Resins
8
ASTM D1598 Standard Test Method for Time-to-Failure of Plastic Pipe Under Constant Internal Pressure.
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
determined at a specific temperature such as 73°F (23°C) or 140°F (60°C) or 180°F (82°C), etc. The
ASTM D2837 data regression analysis method requires a specific distribution of data points and that all
failures in the data set are ductile. See Appendix A for more information on how HDB is demonstrated.
1.2.2.
HDS
ASTM D2837 includes a procedure for determining HDS, which is the HDB multiplied by a service
(design) factor (DF) for an application such as water. ASTM D2837 does not specify DF values for
applications; however, the HDS determination procedure prescribes how a DF for an application is to be
developed.
“5.5 Hydrostatic Design Stress—Obtain the hydrostatic design stress by multiplying the hydrostatic
design basis by a service (design) factor selected for the application on the basis of two general
groups of conditions. The first group considers the manufacturing and testing variables, specifically
normal variations in the material, manufacture, dimensions, good handling techniques, and in the
evaluation procedures in this test method and in Test Method D1598 (Note 8). The second group
considers the application or use, specifically installation, environment, temperature, hazard
involved, life expectancy desired, and the degree of reliability selected (Note 9). Select the service
factor so that the hydrostatic design stress obtained provides a service life for an indefinite period
beyond the actual test period.” (Note 8 and Note 9 are informational notes in ASTM D2837.)
1.2.3.
DF
From the ASTM D2837 excerpt above, the design factor is the combination of two distinctly different
groups of variables. The first group is directly related to the thermoplastic compound, pipe, and testing. It
addresses variability in manufacturing, compound and testing precision that affects the quality of the
LTHS data used to determine the HDB of the thermoplastic compound. The second group is based on
the product application or use; that is, installation quality, operating pressure and temperature stability,
and internal and external environment effects.
Regarding the first group of DF variables, ASTM D2837 LTHS data is developed using sustained internal
pressure testing of pipe samples. Resin manufacturing will result in lot to lot variations in PE resins.
Differences in pipe extrusion equipment and processing procedures introduce variability in the pipe.
Different laboratories conducting sustained pressure tests introduce variability in test results through
different laboratory equipment. That is, the resin, the pipe and its testing are subject to variability. Higher
quality, reduced variability data strengthens the compound’s HDB rating
The second group addresses uncertainty in installation and operation for an application. Despite the best
efforts of installers, installation quality will vary. Likewise operating conditions are seldom as benign as a
sample resting quietly in a laboratory test tank. The installed internal and external environment generally
introduces variability in temperature, internal pressure and dynamic loads and stresses such as varying
water table or traffic loads.
The point of this design factor discussion is that the design factor applied to the long-term strength of a
thermoplastic pressure piping compound does not correlate with a safety factor applied to a material
property such as tensile strength. The design factor incorporates variability in manufacture, testing and
the application to reduce design strength developed from a long-term analysis to a lower acceptable risk
level. A safety factor is a reduction of a short-term physical property that may have no correlation with
long-term performance. When properly selected, a design factor in accordance with Section 5.5 of ASTM
D2837 provides for an indefinite hydrostatic service life.
PPI’s Hydrostatic Stress Board (PPI HSB) developed ASTM and PPI procedures for thermoplastic
pressure pipe material stress ratings in the 1960’s. The PPI HSB is an internationally recognized board
9
for thermoplastic material stress ratings and service design factors (see PPI TR-9 .) The PPI HSB
developed the first service design factor for thermoplastic water piping, which is the 0.50 DF that has
been used for over five decades for North American water pipe made from conventional thermoplastic
compounds such as PVC, CPVC, HDPE, ABS, PP, etc.
1.2.4.
9
ASTM D2837 and PPI TR-3
PPI TR-9 Recommended Design Factors and Design Coefficients for Thermoplastic Pipe
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
PPI TR-3 is the PPI HSB policy for evaluating and listing long-term internal pressure strengths for
thermoplastic (and other) pressure pipe compounds. Participation in the PPI HSB listing program
continues to be voluntary. The listings are based on meeting requirements as defined in PPI TR-3. PPI
TR-3 listing policy is based on ASTM D2837, but extends D2837 by requiring separate data sets from
three commercial resin lots, and pipe from one of the lots must be manufactured on commercial
production equipment. These requirements for listing apply to all thermoplastic compounds.
PPI TR-4 is a published listing of thermoplastic pressure pipe compounds that meet standard or
10
listing requirements and higher performing PE compounds that meet Part F.7
experimental
requirements. Listings in PPI TR-4 include HDB’s at various temperatures, and HDS ratings for water at
73°F (23°C).
In 2004, the PPI HSB published additional PE material performance requirements in PPI TR-3, Part F.7
11
Pipe
so that higher performing PE pipe compounds such as PE4710 could qualify for a 0.63 DF.
compounds such as PE3408 do not meet these requirements and must use the 0.5 DF. The additional
performance requirements allow higher performing PE compounds to be operated at higher internal
pressure stress with a comparable or lower risk level.
PPI TR-3 Part F.7 sets three additional requirements for higher performing PE compounds:
•
Reduced variability in pipe test data
ASTM D2837 requires the Lower Confidence Limit (LCL) to the LTHS ratio (LCL/LTHS) meet a
minimum of 85%. TR-3 Part F.7 requires a minimum LCL/LTHS ratio of 90%.
•
Increased SCG resistance
PPI TR-3 Part F.7 requires performance equal to or greater than 500 hours SCG resistance per
ASTM F1473 (PENT). Conventional HDPE compounds typically meet 10 to 100 hours PENT
SCG resistance.
•
Substantiation at 50 years
ASTM D2837 and PPI TR-3 use elevated temperature, accelerated testing to assure that the
onset of the SCG failure mechanism is beyond a required limit. ASTM D2837 and PPI TR-3
requires validation of the PE pipe compound to assure linearity of the regression to 100,000
hours, which is the stress-time intercept for the LTHS at the test temperature. PPI TR-3 applies a
o
similar substantiation methodology to demonstrate linearity of the 73 F regression to 50 years
(438,300 hours).
Conventional thermoplastics and PE3408 compounds meet standard ASTM D2837 and PPI TR-3
requirements, but do not meet the additional performance requirements of PPI TR-3 Part F.7. PE4710
compounds meet ASTM D2837 and TR-3 requirements, and the additional Part F.7 requirements.
Therefore, PE4710 compounds qualify for the 0.63 DF and in turn the higher calculated HDS. PPI TR-3
Part F.7 requirements allow higher operating stress for PE4710 pipe compounds relative to PE3408 pipe
compounds.
10
While PPI TR-4 defines requirements for experimental listings, the ASME code for safety related cooling water applications only
allows standard grade listings.
11
PPI TR-3 refers to testing of the pipe compound. ASME terminology defines the term “polyethylene compound” as equivalent to
the “pipe compound” as defined by PPI.
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
1.3. Long-Term Performance of PE Pressure Piping
1.3.1.
Long-Term Hydrostatic Strength
When subjected to sustained stress and temperature, thermoplastic materials such as PE exhibit a time
dependent response. This knowledge led to the development of ASTM D2837 in the mid 1960’s. As
discussed above, ASTM D2837 is a method for determining a categorized HDB for thermoplastic
compounds that are subjected to constant stress at constant temperature.
LTHS is developed by
evaluating stress vs. time-to-failure experimental data at constant temperature. The LTHS is then
compared to LTHS ranges as defined per HDB category. This HDB is then assigned to the pressure pipe
compound. (See Appendix A, Table A.1 for the categorization of HDB values based on LTHS
extrapolation.)
The purpose of the HDB is to provide a standardized design basis for determining an internal pressure
rating at a given temperature. The 100,000 hour intercept was chosen for its statistical significance.
ASTM D2837 requires testing to 10,000 hours, and 100,000 hours is the next log-decade. However, the
HDB does not represent a service life.
1.3.2.
Slow Crack Growth
PE pressure pipe compounds (and many other conventional thermoplastics) were developed in the
1950’s. Early pipe pressure ratings were calculated by applying a safety factor to the ductile tensile yield
strength of the material. After a few years, however, some PE pipes began failing prematurely by a
previously unknown failure mechanism. The assumption that instantaneous ductile tensile yield strength
could be used for pipe pressure rating was wrong. Research, testing and evaluation determined that the
previously unknown failure mechanism was slow crack growth (SCG). Unlike ductile tensile failure where
the material elongates and then fails in the elongated region, SCG is a stress-cracking mechanism where
cracks advance slowly through the pipe wall without evident ductile elongation. SCG is not chemical or
stress embrittlement. It is a fracture mechanics mechanism that arises from sustained tensile stress that
is less than the ductile failure tensile stress.
1.3.3.
Predicting the Onset of SCG
After the discovery of polyethylene’s SCG susceptibility, ASTM D2837 and PPI TR-3 were revised to
include a validation procedure to assure that SCG would not occur before the 100,000 hour intercept of
the ductile failure LTHS curve. HDB determination requires data having a common ductile tensile mode
of failure. Because SCG is not ductile tensile failure, SCG failures must be excluded from the ductile
HDB data. Validation assures that SCG failures are excluded from the HDB data set.
SCG research showed that log-stress vs. log-time SCG failure data would also be a straight line, but that
the slope of the SCG failure curve would be steeper than the slope of the ductile failure curve. The
transition of a material from ductile to a SCG failure mode is commonly known as the “knee.” Research
showed that SCG failure could be accelerated by testing at elevated temperature, and that data at several
elevated temperatures could be shifted to predict the onset of SCG failure at a lower temperature. The
validation procedure that was added to ASTM D2837 and PPI TR-3 requires elevated temperature
sustained pressure testing and analysis to demonstrate that SCG does not occur before the 100,000 hour
intercept with the ductile tensile LTHS curve used for HDB determination. (See Appendix A for more
information.)
1.3.4.
PE Pipe Pressure Rating
The relationship between PE compound stress rating (HDB), pressure rating, and pipe is described by the
following equation. (Reference – ASTM D2837 Sections 3.1.8 and 3.1.8.1)
PR =
where
PR =
HDS =
DR =
2 × HDS 2 × HDB × DF
=
(DR − 1)
(DR − 1)
Equation (1)
pressure rating (or Pressure Class, PC) for water at 73°F, (23°C) psi (MPa)
PE compound hydrostatic design stress for water at 73°F (23°C), psi (MPa)
dimension ratio
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
=
Do
tmin
HDB
DF
=
=
=
=
Draft D – January 23, 2015
Do
t min
pipe average outside diameter, in. (mm)
pipe minimum wall thickness, in. (mm)
PE compound hydrostatic design basis at 73°F (23°C), psi (MPa)
service (design) factor for application such as water at 73°F (23°C)
Although similar, Equation (1) is not Barlow’s Formula. Appendix B to this paper is an excerpt from The
Engineering Toolbox (www.engineeringtoolbox.com) that describes Barlow’s Formula and its use for
estimating the bursting pressure of (metallic) pipes and tubes at ideal room temperature conditions. The
significant difference between Equation (1) and Barlow’s Formula is that the allowable stress for the
compound in Equation (1) is the categorized long-term sustained hoop stress, HDB, which is reduced by
the overall design factor, DF, where Barlow’s Formula uses short-term material tensile strength reduced
by a safety factor. Equation (1) provides for sustained long-term strength, where Barlow’s Formula is for
instantaneous bursting strength.
Tensile strength is an instantaneous value that for PE compounds is directly related to density, but the
long-term strength of a PE compound is dependent on molecular structure, which is described by
molecular weight (molecule length), molecular weight distribution and modality, comonomer, side chain
branching, tie molecules, etc. These molecular structure properties are for the most part independent of
density.
As discussed above, using tensile strength for pressure rating was an especially poor indicator of field
performance because the long-term failure mode for PE is SCG, not ductile bursting. That is, using
Barlow’s Formula to pressure rate PE pipes resulted in premature service failures. From this field
experience, Barlow’s Formula is an improper and inadequate model for determining pressure rating for
PE pressure pipes. Using Barlow’s Formula for thermoplastic pipe pressure rating is a misapplication of
Barlow’s formula.
1.3.5.
Short-Term Pressure Capability
In ASTM and AWWA PE piping standards (current and proposed), Barlow’s Formula is incorporated in
short-term requirements where the minimum bursting stress or hoop tensile for PE pipe must exceed
12
2900 psi for high density PE compounds. Quick burst testing is conducted per ASTM D1599 . Hoop
13
tensile testing for larger pipes is conducted per ASTM D2290 .
1.4. Comparing PE4710 to PE3408
PE4710 is not the same as PE3408 for a number of reasons.
•
PE4710 complies with PPI TR-3 Part F.7 requirements for higher performing PE compounds;
PE3408 does not.
•
PPI TR-3 Part F.7 requires substantiation at 50 years in addition to validation at 100,000 hours.
PE4710 complies; PE3408 does not require substantiation.
•
A higher performing PE polymer structure is required to meet PPI TR-3 Part F.7 requirements.
PE4710 has a molecular structure that provides for compliance with PPI TR-3, Part F.7
requirements; PE3408 does not.
•
PE4710 has higher density and meets higher SCG requirements compared to PE3408.
3
3
o
>0.947 – 0.955 g/cm for PE4710 vs. >0.940 – 0.947 g/cm for PE3408
o
Allowed minimum of 500 hours SCG resistance for PE4710 vs. allowed minimum of 10
hours SCG resistance for PE3408
12
ASTM D1599 Standard Test Method for Resistance to Short-Time Hydraulic Pressure of Plastic Pipe, Tubing, and Fittings
ASTM D2290 Standard Test Method for Apparent Hoop Tensile Strength of Plastic or Reinforced Plastic Pipe by Split Disk
Method
13
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Compliance with PPI TR-3 Part F.7 requires improved resistance to long-term SCG fracture. PPI TR-3
Part F.7 compliant PE4710 compounds must be more consistent because of the tighter data quality
o
requirements. PPI TR-3 Part F.7 substantiation requires linearity of the 73 F regression to 438,300 hours
(50 years). ASTM D2837 validation is at 100,000 hours (11.4 years).
1.4.1.
Illustrating Minimum PE3408 Performance
When ASTM D2837 data is plotted, the regression analysis yields a straight line using log-stress vs. logtime coordinates. This graphical presentation (see Figure 1) is typically called a stress-rupture curve.
The ductile curve is typically referred to as Stage I failure curve and the brittle curve is referred to as
Stage II (for polyethylene, this is caused by SCG). The following details the key parts of the stressrupture curve (Figure 1).
a. The ductile stress-rupture curve for a PE3408 compound (thick blue line).
b. The long-term hydrostatic strength (LTHS), which is the intercept at 100,000 hours (dashed
vertical line). The LTHS is then categorized to yield the HDB value.
c. The onset of the brittle curve, Stage II (dashed red line). The SCG curve has a steeper slope
compared to the ductile failure curve.
d. The maximum HDS for the PE compound is calculated by multiplying the HDB by the service
design factor for water, in this case 0.5 DF – the 800 psi HDS line (horizontal black line).
Note: Figure 1 is not drawn to scale. It is only meant as an illustration.
PE 3408 Ductile Curve (Stage I)
HDB (categorized LTHS @ 100,000 hour
intercept)
D2837 Validation requires SCG
Transition > 100,000 hours
Log (stress)
LTHS
Brittle Curve (Stage II, SCG)
1600 x 0.05 = 800 psi HDS
o
for water at 73 F
Log (time)
At continuous 800 psi HDS, minimum
expectation for
SCG leakage > 100 years
100,000 hours
(11.4 years)
Figure 1. An Illustration of ASTM D2837 Stress-Rupture Requirements for PE3408 Compound for
Water at 73°F.
o
o
For qualified PE compounds, PPI TR-4 lists the recommended HDS for water at 73 F (23 C). For
PE3408 compounds that comply with ASTM D2837 and PPI TR-3 requirements, the HDS is 800 psi for
o
o
water at 73 F (23 C). For a validated PE3408 compound that is operated continuously at maximum HDS,
the earliest potential for the onset of SCG pipe wall leakage is when the HDS curve intersects the SCG
curve. Water systems seldom operate continuously at the maximum pressure rating so the actual
operating (pressure) stress is below the HDS curve (horizontal black line) in Figure 1.
Page 8 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
1.4.2.
Draft D – January 23, 2015
Illustrating Minimum PE4710 Performance
Figure 2 illustrates PE4710 performance for water service. The following details the key parts of the
stress-rupture curve.
a. The ductile stress-rupture curve for a PE4710 compound (thick blue line).
b. The long-term hydrostatic strength (LTHS), which is the intercept at 100,000 hours (dashed
vertical line). The LTHS value is then categorized to yield the HDB value.
c. The onset of the brittle curve, Stage II (dashed red line). The SCG curve has a steeper slope
compared to the ductile failure curve.
The items below are the differences from the PE 3408 chart.
d. The maximum HDS for the PE compound is calculated by multiplying the HDB by the service
design factor for water, in this case 0.63 DF - the 1000 psi HDS line (horizontal black line).
e. The 50 year substantiation requirements for PE4710 compounds. This is the intercept at 438,000
hours (50 years).
PE4710 Ductile Curve (Stage I)
HDB (categorized LTHS @ 100,000
hour intercept)
PPI TR-3 requires substantiation
SCG Transition > 438,000 hours (50 years)
Log (stress)
LTHS
Brittle Curve
(Stage II, SCG)
1600 x 0.63 = 1000 psi HDS
for water at 73F
At continuous 1000 psi HDS, minimum
expectation for SCG leakage greatly
exceeds 100 years
Log (time)
100,000 hours
(11.4 years)
438,000 hours
(50 years)
Figure 2. An Ilustration of PPI TR-3 Stress-Rupture Requirements for PE4710 Compound for Water
at 73°F
The PE4710 depicted in Figure 2 HDB and substantiation requirements for a 73°F (23°C) water system
operating continuously at the maximum HDS of 1000 psi. For a substantiated PE4710 compound that is
operated continuously at its maximum HDS, which is 25% greater than the maximum HDS for a PE3408,
the earliest potential for the onset of SCG pipe wall leakage is when the HDS curve intersects the SCG
curve. Water systems seldom operate continuously at the maximum pressure rating so the actual
operating (pressure) stress is lower than the HDS curve (horizontal black line) in Figure 2.
This discussion illustrates the improved requirements for PE4710 compared to PE3408, and how the
improved requirements either significantly reduce the statistical risk of PE4710 SCG failure if operated at
lower than maximum HDS stress, or that the statistical risk of PE4710 SCG failure if operated at
maximum HDS is comparable to that of aPE3408 material that does not meet the improved requirements.
Appendix A offers an analysis based on data and mathematical analysis for PE4710.
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Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
2. CC N-755 Revision 1 PE4710 Requirements
2.1. Mandatory Appendix IV
Revision 1 of Code Case N-755 allows only the use of certain PE4710 compounds and materials in
accordance with Mandatory Appendix IV. Mandatory Appendix IV is designed to assure the procurement
of PE4710 compounds and materials that meet or exceed the allowable stresses in Tables -3131-1(a), 3131-1(b), -3131-1(c), and -3131-1(d).
The background section of this paper describes the basis for determining long-term stress capabilities of
PE3408 and PE4710 compounds. Mandatory Appendix IV further restricts PE4710 compounds to a
higher performing subset of PE4710 compounds. In many cases, meeting one requirement invokes
additional requirements thereby allowing Mandatory Appendix IV to be relatively concise, but thorough
and specific.
Mandatory Appendix IV requirements are summarized as follows:
•
PE4710 compound in accordance with PPI TR-4
Requiring PE4710 specifies compound density, SCG resistance, and 1000 psi (6.90 MPa) HDS
for water at 73°F (23°C). To comply with the HDS requirement, the compound must meet PPI
TR-3 Part F.7 requirements DF (>500 h SCG resistance per ASTM F1473, 90% LCL/LTHS ratio,
and 50-year substantiation) to qualify for the PPI TR-3 0.63.
•
PPI TR-4 listed Standard Grade HDB at 73°F (23°C) of 1600 psi (11.03 MPa) for water
•
PPI TR-4 listed Standard Grade HDB at 140°F (60°C) of 1000 psi (6.90 MPa) for water
PPI TR-3 allows experimental grade listings for compounds that are under development. A PPI
TR-3 Standard Grade listing excludes developmental compounds. HDB ratings incorporate SCG
onset validation.
•
Minimum of 2000 hours SCG resistance in accordance with ASTM F1473 (PENT)
14
Preliminary research suggests that larger diameter, thicker wall pipe made from PE4710
compounds having >2000 hour PENT SCG resistance can tolerate up to 10% OD damage
without compromising service life. Additional research is ongoing to confirm SCG requirements
and other requirements for damage tolerance.
•
Independent and Dependent PPI TR-4 HDB and HDS listings
PPI TR-4 independent listings apply to PE4710 pipe compound from the Polyethylene Compound
(PC) manufacturer. Dependent listings apply to PE4710 pipe compound in pipe form from the
Polyethylene Material (PM) Manufacturer. Identical listings are required of both, which assures
the processing capability of the pipe manufacturer.
2.2. Allowable Stress Tables
2.2.1.Tables -3131-1(a) and -3131-1(b)
Tables -3131-1(a) and -3131-1(b) are derived directly from Mandatory Appendix IV HDB requirements for
PE4710 compounds at 73°F (23°C) and 140°F (60°C). Table -3131-1(a) is in inch-pound units and Table
-3131-1(b) in metric units. For both tables, a 0.50 DF is applied to the HDB. The lower DF was chosen
by the ASME SWG HDPE Pipe to accommodate negatives from the US Nuclear Regulatory Commission
and others. The limitation to a 0.50 DF for PE4710 materials did not restrict the design of safety systems
and provides an additional 25% margin of safety from the pressure rating of PE4710 polyethylene
material.
14
“Stress Intensity Effect on Slow Crack in Scratched PE Pipe”, Z. J. Zhou, Plastics Pipe XV, Vancouver, BC, September, 2010
Page 10 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Table -3131-1(a) and Table -3131-1(b) allowable stress values for temperatures between 73°F (23°C) and
140°F (60°C) were calculated in accordance with the PPI Handbook of Polyethylene Pipe, Second
Edition, Chapter 3, Appendix A.2. This reference provides an interpolation equation (see Equation 2) for
determining temperature multipliers between lower and higher HDB.
 1
1 
  −  
 HDBB − HDBH    TB TI  

FI = 1 − 
HDBB

   1 − 1  


  TB TH  
where
FI =
HDBB =
HDBH =
T =
Equation (2)
multiplier for intermediate temperature TI
HDB at base temperature, 73°F (23°C), psi (MPa)
HDB at high temperature, 140°F (60°C), psi (MPa)
temperature in °R (°K)
Intermediate temperature allowable stress values in Table -3131-1(a) and Table -3131-1(b) were
calculated using Equation 3.
S I at t I = FI × HDBB × DF
where
SI =
tI =
DF =
2.2.2.
Equation (3)
allowable stress at intermediate temperature, tI, psi (MPa)
intermediate temperature, °F (°C)
design factor = 0.50 for Revision 1
Tables -3131-1(c) and -3131-1(d)
In accordance with ASTM D2837 and PPI TR-3, HDB’s must validate or prove that the onset of SCG
failure is beyond the 100,000 hour intercept on the ductile stress-rupture curve. The specified validation
procedure is to conduct testing at elevated temperature. To validate the Mandatory Appendix IV required
1000 psi HDB at 140°F (60°C), several alternative methods are prescribed in PPI TR-3, Part F.4. One of
these methods, F.4.1 “Standard Method – Validation of HBB”, is described below for illustrative purposes.
•
Demonstration of 690 psi (4.76 MPa) hoop stress for 3,800 hours at 194°F (90°C), or
•
Demonstration of 775 psi (5.34 MPa) hoop stress for 11,300 hours at 176°F (80°C).
For AmerenUE’s Callaway facility relief request, the elevated temperature excursion design required 341
psi (2.35 MPa) allowable stress at 176°F (80°C) for approximately one month.
Applying the 0.50 DF to the validation required testing at 176°F (80°C), an allowable stress of 387 psi
(2.67 MPa) is obtained, which exceeds the 341 psi (2.35 MPa) temperature excursion design stress
required for the Callaway project. Further, 176°F (80°C) validation testing is for a minimum of 11,300
hours (1.29 years), which significantly exceeds the Callaway temperature excursion time requirement.
Accordingly, the Mandatory Appendix IV requirement for a 1000 psi (6.90 MPa) HDB at 140°F (60°C)
provides for Table -3131-1(c) (inch-pound units) and Table -3131-1(d) (metric units) allowable stresses at
176°F and 80°C respectively because the HDB must be validated in accordance with ASTM D2837 and
PPI TR-3. It is to be noted that higher allowable stress and temperature excursion duration are possible;
however, SWG HDPE Pipe decided to use the more conservative allowable stress that was suitable for
the Callaway project design requirements. If higher allowable stress or greater duration would be needed
for another project, it would be a consideration for a subsequent revision to the Code Case.
Page 11 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Appendix A – Technical Basis for HDB Used in Code Case N-755, Rev. 1.
Note: Information contained in this appendix is based on a presentation by Mr. Stephen Boros, Technical
Director of PPI and Chairman of the PPI Hydrostatic Stress Board, at the ASME BPV Committee PE Pipe
Symposium in February, 2010.
The relationship between hoop stress (HDB and/or HDS) and internal pressure from water in a
polyethylene pipe is determined by application of the thin walled pressure vessel equation (A.1):
where
𝑆=
𝑃(𝐷 − 𝑡)
2𝑡
Equation A.1
S= Hoop Stress (either HDS or HDB x DF), psi
P= internal pressure, psig
D= average outside diameter, inches
t = minimum wall thickness, inches
The hoop stress generated on the pipe wall by the internal pressure in the pipe is calculated by the thin
walled pressure vessel equation. Even for thicker walled pipe, this is appropriate because the modulus of
the thermoplastic is low compared to steel pipes.
For polyethylene, the response to an applied stress is time dependent. Figure A.1 below shows a stress
regression curve on Cartesian coordinates for a polyethylene material. An initial steep drop in hoop
stress is seen as the material relaxes. After a short period of time, the drop in stress becomes more
gradual.
Figure A.1. This graph shows the effect of time upon stress for a typical polyethylene pipe material using Cartesian
coordinates.
By testing at several temperatures and several hoop stresses at each temperature, a graph can be
generated to demonstrate the long-term stress performance of the polyethylene material (see Figure A.2).
Page 12 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Figure A.2. This graph shows the typical hydrostatic stress data for a Revision 1 Code Case N-755-compliant
o
PE4710 polyethylene. Data is shown for the three typical testing temperatures of 23, 60 and 90 C.
The blue points in Figure A.2 indicate a typical data set generated per ASTM D1598 as required by ASTM
D2837 and PPI TR-3 for determination of the 23°C (73°F) HDB. The second set of data points (green) is
a typical data set required to determine 60°C (140°F) HDB. The orange points are those required at 90°C
(193°F) for the different stress levels and corresponding times to validate operation at 140°F. The orange
o
data can also be used to substantiate the 23 C data linearity to 50 years.
o
The darker line for both the 23 & 60 C data sets represent the linear least squares fit (LTHS) to the data
and is typically extrapolated out to 100,000 hours The lighter line for each data set is the 97.5% lower
confidence limit.
o
A regression analysis of the 23 and 60 C data is carried out following the ASTM D2837 and PPI TR-3
procedures. The reader is referred to these documents for more information. In order for the PE4710
regressions to be valid, all specimens in the data set must fail in the ductile mode.
The short-term tensile stress (burst stress) of a polyethylene pipe is determined by either ASTM D1599
(internal pressure burst) or ASTM D2290 (split-ring tensile test). The result is shown in Figure A.3 as the
top red line. The short-term stress is a useful quality control test for PE pipe manufacturer and also
represents an absolute maximum allowable hoop stress during pressure testing of an installed pipe line.
o
The blue line represents the LTHS regression of the sustained pressure test data, in this case for 73 F
testing. The intercept of the extrapolation (blue dashed line) of the sustained pressure data to the
100,000 hour line (solid black vertical line) is used to categorize the HDB. The lower confidence limit
Page 13 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
(LCL) line (red) represents a 97.5% confidence limit based on the sustained pressure test data. In order
to meet the PE4710 requirements, the ratio of intercepts at 100,000 hours for the LTHS and LCL must be
> 90% indicating limited scatter in the sustained pressure data. Additional testing of sustained pressure
at elevated temperatures per PPI TR-3, section F.4 validates the linearity of the LTHS extrapolation to
100,000 hours.
In section F.5 of PPI TR-3, additional testing for PE4710 resins is required to substantiate the linearity of
o
the LTHS extrapolation at 73 F to 50 years. The ratio of the LTHS at 50 years to the LTHS at 100,000
hours must be greater than 80%.
o
Figure A.3. A simplified regression analysis for 73 F LTHS for a polyethylene material showing the relationship of
LTHS and LCL to the hydrostatic burst stress (short-term ductile performance per ASTM D1599) and the HDS after
application of the 0.5 design factor. Note: The bracket for HDB is placed at the 50 year timeline for clarity purposes
only. HDB is categorized based on the intercept of the regression line and the 100,000 time line.
The key points shown in the regression analysis in Figure A.3 are:
1) The regression line is below the burst stress line.
2) The Lower Confidence Limit (LCL) is below the regression line.
3) The HDS is below the LCL.
Long-term strength is categorized, or standardized, within established ranges and referred to as an HDB.
The categories can be found in ASTM D2837 and are reproduced below in Table A.1.
Page 14 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Table A.1. Relationship of Calculated LTHS to Categorized HDB at 73oF
Range of Calculated LTHS Values
Categorized Hydrostatic Design Basis
psi
(MPa)
psi
(MPA)
760 to < 960
(5.24 to < 6.62)
800
(5.52)
960 to < 1200
(6.62 to 8.274)
1000
(6.89)
1200 to < 1530
(8.27 to < 10.55)
1250
(8.62)
1530 to < 1920
(10.55 to < 13.24)
1600
(11.03)
1920 to < 2400
(13.24 to <16.55)
2000
(13.79)
2400 to < 3020
(16.55 to <20.82)
2500
(17.24)
3020 to < 3830
(20.82 to < 26.41)
3150
(21.72)
3830 to < 4800
(26.41 to < 33.09)
4000
(27.58)
o
According to Table A.1 PE4710 materials will be categorized with a HDB at 73 F of 1600 psi if the
regression analysis of experimental data demonstrates an LTHS value at 100,000 hours between 1530 to
1920 psi. This is highlighted in yellow.
Typical PE4710 Regression Analysis follows PPI TR-3 policies for establishing a HDB based on these
rules:
• Three distinct and separate lots of the compound to demonstrate consistency.
• One lot with 73°F and 140°F full datasets to 10,000 hours meeting the full requirements of ASTM
D2837.
o Minimum of 18 data points with proper time distribution.
o LTHS > minimum for HDB.
o LCL/LTHS ratio > 0.90.
o LTHS @ 50 years > 80% LTHS.
• Two additional lots with 73°F and 140°F datasets to 2000 hours containing a minimum of 10 data
points with proper distribution
• The regression analysis must also meet the ASTM D2837 requirements (HDB, LCL, etc.).
o
o
o
o
o Validation of 73 F and 140 F HDB with two datasets at 80 C or 90 C.
o Substantiation of the 73 °F HDB at either:
o
 80 C with log average minimum of 6 specimens is 6000 hours, or
o
 90 C with log average minimum of 6 specimens is 2400 hours.
Summary
This Appendix contains composite data for regression analysis of actual PE4710 materials and discusses
how the Long-Term Hydrostatic Strength is determined. Reference is also made to the regression
analysis method in ASTM D2837 and the additional requirements of PPI TR-3, including substantiation of
the linearity of the regression analysis to 50 years.
Page 15 of 16
Code Case N-755 Revision 1 – Allowable Hydrostatic Stresses for PE4710 Material
Draft D – January 23, 2015
Appendix B – Barlow’s Formula
Resources, Tools and Basic Information for Engineering and Design of Technical Applications!
Bursting Pressure
Barlow’s Formula and bursting pressure of pipes and tubes
Barlow's formula can be used to estimate burst pressure of pipes or tubes.
P = 2 s t / (do SF)
(1)
where
P = max. working pressure (psig)
s = material strength (psi)
t = wall thickness (in)
do = outside diameter (in)
SF = safety factor (in general 1.5 to 10)
The Barlow's estimate is based on ideal conditions at room temperature.
Material Strength
The strength of a material is determined by the tension test, which measure the tension force and the
deformation of the test specimen.
•
•
the stress which gives a permanent deformation of 0.2% is called the yield strength
the stress which gives rupture is called the ultimate strength
Strength of some common materials:
Material
Yield Strength
(psi)
Ultimate Strength
(psi)
Stainless Steel,
304
30,000
75,000
6 Moly, S31254
45,000
98,000
Duplex, S31803
65,000
90,000
Nickel, N02200
15,000
55,000
Summary
This section highlights the application of Barlow’s equation to short-term burst pressure
calculations and not to long-term creep rupture failure.
Page 16 of 16