Response to Negative Regarding Design Factor on AWWA C906 ballot, item #9
Stephen Boros, Chairman, Hydrostatic Stress Board
March 4, 2009 (Revised for by Committee Chair for Subcommittee use , June 2009)
General Comment
The comment submitted suggesting a design factor of 0.5 is confusing the design factor
(DF) for thermoplastics in a piping application with an engineering safety factor (SF).
They are very different concepts in this application and must be treated as such. There
are many considerations that go into a design factor for thermoplastic pipe – not the least
of which is recognition of the potential failure mode for each type of plastic.
It is not the hoop stress from internal pressure that will determine the service life of a
plastic pipe, but rather how the material responds to other induced stresses during service
and the corresponding stress intensifications (point loading, fatigue, surge, etc...). Ductile
materials will shed these stress concentrations by distributing the stress into the
surrounding matrix, while more brittle materials will tend to be further affected and result
in failure. Add to this the effects of fatigue and it is not a simple system. Due to these
complex issues, the Hydrostatic Stress Board has been the source of the hydrostatic
design stress (HDS) and the corresponding DF for thermoplastic materials for more than
40 years. The Hydrostatic Stress Board was initiated in 1958 to develop methodology for
the determination of the long-term strength of thermoplastic materials (such as PVC and
PE) intended for piping applications. It is made up of a cross section of knowledgeable
experts from across the plastics industry. The application of the strength reduction factor
(i.e. design factor in this case) to the HDB results is a maximum hydrostatic design stress
(HDS) which takes into account that there will be additional stresses imposed on the pipe
as well as how the material will respond to these conditions. (A list of HSB current
members is shown at the end of this document).
The Hydrostatic Stress Board, after studying the subject for several years set three
additional performance criteria in order for a PE material to qualify for the higher 0.63
design factor. PE materials not meeting these requirements continue to utilize the 0.50
DF for determining the maximum design stress for water at 73°F.
1. A demonstration, in accordance with Plastics Pipe Institutes requirements, that in its
stress-rupture evaluation a PE material shall continue to fail by the ductile mode
through at least the 50-year intercept. This requirement ensures that the PE material
continues to operate in the ductile state even after very prolonged periods of sustained
stress.
2. The lower confidence limit (LCL) of the projected average value of the material’s
long term hydrostatic strength cannot be less than 90% of the average value that is
forecast by means of method ASTM D2837 (i.e. the LCL/LTHS ratio must be greater
than 0.90) – A high ratio enhances the statistical reliability of the forecast of the
long-term strength. It is also another indication of high resistance to failure by a
brittle-like mechanism, a mechanism that leads to greater scatter in stress-rupture
data.
3. The minimum failure time under ASTM test method F1473, a fracture mechanics
based method that uses a combination of a significantly elevated test temperature and
a very sharp notch that induces a high localized stress intensification in the test
specimen, cannot be less than 500hrs – This time is about 5 times greater than the test
time which based on a calibration of test results versus actual quality of field
experience has been found to result in essential immunity to the effect of localized
stress intensifications that occur under field conditions (by comparison, the previous
edition of C906 required only 10 hours on this test).
Additional background information on the development and evolution of the design
factor is attached in a memo from Stan Mruk, past chair of the Hydrostatic Stress Board.
A- Safety factor (SF) against failure due to over-pressurization exceeds two to one
(2:1).
All PE pipe conforming to the standard has a safety factor against burst from over
pressurization that greatly exceeds 2 to 1. The importance of such a safety factor
cannot be minimized as it gives protection to the pipeline operator against accidental
temporary over-pressurization and surge pressures. The burst strength of PE pipe
remains constant through out the life of the pipe, and should not be confused with the
long-term strength as determined by the HDB. The attached report shows that PE
pipes that have been under constant stress (i.e. pressurized) in excess of 100,000 hours
at 60°C (more than 10 years) retain the same burst pressure as new pipe. (see attached
reference paper by Sandstrum, et al ..). Taking into account the accelerating effects of
the elevated temperature, this would be equivalent to more than 50 years at normal
water service temperatures of 73°F. Therefore the safety factor against burst remains
constant throughout the pipe’s life at greater than 2:1 even with the application of a
0.63 DF.
B- Design factor increase based on improvements in PE materials.
While long-term hydrostatic strength and burst strength are important in pipe design,
it is not the limiting factor to PE pipe life. Most often the lifetime limiting mechanism
in PE pipe is slow crack growth. In certain polyethylene pipe-grade materials, over
long periods of time, microscopic cracks may develop, which then slowly grow into
minute cracks (SCG). While not the end of the useful service life, this is typically
considered failure of the pipe. Long term design involves estimating the time to
failure under conditions that lead to slow crack growth (SCG) and applying a design
factor, so as to lower the instigating stress well below the failure curve. The design
factor for PE materials meeting the performance requirements in C906-07 continues
to be limited to 0.5. The proposed changes to C906-07 provides for the introduction
of newer and improved materials such as PE4710 and PE3710. These materials are
considered 3rd and 4th generation improvements and have vastly improved slow crack
growth resistance. One measure of slow crack growth resistance is test method
ASTM F1473 referred to as the PENT test. PENT values for materials in C906-07 are
required to equal or exceed 10 hours by cell class definition. The PE4710 and PE
3710 materials are required to equal or exceed 500 hours. Research indicates that this
level of PENT performance moves the potential for brittle type failures to hundreds of
years in service, essentially eliminating this as a potential failure mechanism. In
addition to PENT the newer materials must also demonstrate a linear stress rupture
curve to at least 50 years—that is, no downturn which might indicate a tendency
toward slow crack growth. These additional requirements assure longevity, such that
the occurrence of slow crack growth has been shifted out to hundreds of years and
any additional stress from the application of the 0.63 DF will not result in a reduced
operating safety of the system over the design life.
C- Design factor change approved by HSB and by AWWA.
As explained in the General Comments section above, the increase in design factor
was determined and approved by the Hydrostatic Stress Board. The Hydrostatic
Stress Board is a body within the Plastics Pipe Institute and has historically
established the design factors for all thermoplastics materials including HDPE and
PVC. Membership in the Hydrostatic Stress Board includes representatives from all
thermoplastic materials industries including PVC, PE, PEX, CPVC, PA, and
thermoplastic composites. The Hydrostatic Stress Board members develop polices on
how to establish the long-term strength of thermoplastic compounds for piping
applications and issue recommendations for the Hydrostatic Design Stress (HDS).
The use of a higher Design factor for the improved PE materials has been accepted in
Europe, Australia, and other parts of the world for years, the U.S. has been in the
process of developing the appropriate protocols and standards since 2002. It might be
noted that the most recent edition of AWWA C901 now contains the Design factor of
0.63 for these higher performing materials. This was approved and passed through
AWWA last year. So, there is already precedent in AWWA to use the higher Design
factor for the improved materials. The AWWA PE standards C901 and C906 need to
be consistent as was done last year for PVC standards C900 and C905.
D- Design factor is lower for natural gas applications.
The commenter misinterpreted how PE pipe is utilized in natural gas distribution
systems. The design factors for regulated natural gas distribution service are
established by the Department of Transportation and published in the Title 49 Code
of Federal Regulations, Part 192. The transportation of a fuel gas demands a higher
degree of conservatism in design. The design factor for this application takes many
factors into account, such as the potential for the formation of gas condensates and
elevated temperatures, along with an additional factor of safety due to the serious
consequences of a failure in a piping system conveying a compressed combustible gas
– even a pinhole leak. While PE pipes have a more conservative design factor in
these applications, the Federal Code no longer permits the use of PVC pipe for new
gas pipeline construction. However, if it did the design factor for PVC would be no
higher than 0.32 and possibly as low as 0.25, and gasket joints would not be allowed
at all. As an additional point of information, the same PE materials that are being
proposed for use with a Design factor of 0.63 for water, are also being proposed for
an increased design factor by the Federal DOT from 0.32 to 0.40 (the same 25%
increase in working stress). Because of the improved performance of these PE
materials, this is not being considered a decrease in the ―safety‖ of these critical
systems nor should it be for water.
E- Design factor placed in body of standard.
In response to received comments, the definition for the design factor has been
placed in the body of the standard, as well as additional verbiage to show how the
Hydrostatic Design Stress is calculated. It is further clarified that the DF of 0.63 does
not apply to all PE materials, but only those that qualify for the enhanced
performance properties as determined by the Hydrostatic Stress Board and published
in TR-3. Previously approved PE materials will continue to have the HDS determined
using the 0.50 DF. These materials are also listed in PPIs TR-4 publication.
F- Summary
The increase in Design factor from 0.5 to 0.63 is based on marked improvements in
PE material’s long-term performance. The increase is consistent with changes
approved by the Hydrostatic Stress Board and by AWWA in C901, as well as
numerous other standards organization including ASTM, CSA, and ASME. The
safety factor against burst continues to exceed two to one (2:1)
.
List of 2009 HSB members
Pipe Suppliers
1
2
3
4
5
6
Richard Kolasa – PE
Gary Runyan - PEX
Peter Cook - PVC/Composite/PEX/CPVC
Ron Bishop - PVC
Keith Steinbruck – PVC
Bob Walker – PVC
Resin Suppliers
7
8
9
10
11
Andy Olah - PEX/CPVC
Richard Guise - PVC
Brian Cole – PE
Nancy Conley – PE
Bill Michie – PE
Distributor/End User
12
13
Steve Sandstrum – ISCO
Will Bezner - Complete Production
HSB Consultants
14
15
16
17
Bob Ayres
Michael Byrne
Stan Mruk
Frank Volgstadt
Laboratories
18
19
Ken Oliphant
Jarno Hassenin
Certifying Bodies
20
21
Nasrin Kashefi – NSF
Nils Holm– CSA
Trade Associations
22
23
24
PPI Technical Director - Stephen Boros
Uni-Bell PVC Pipe Assoc - TBD
PPI Tech. Com. Chair- Adel Haddad