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RELIABILITY EVALUATION OF GAS DISTRIBUTION NETWORK OF

POLYETHYLENE PIPES

Hiroyuki Nishimura1 , Takafumi Kawaguchi

1, and Seiji Miaki

2

1. Energy Technology Laboratories, Osaka Gas Co., Konohana-ku, Osaka 554-0051, JAPAN

2. Pipeline Business Unit, Osaka Gas Co., Konohana-ku, Osaka 554-0051, JAPAN

Keywords: 1. PE; 2. creep test; 3. reliability; 4. long-term strength; 5. earthquake resistance

Introduction

Polyethylene (PE) pipes have been widely used in Japan since they were put into practical use in

1979. The use of PE pipes was greatly extended since the Great Hanshin-Awaji Earthquake in

1995. The quality of PE pipes and joints have been maintained at a high level as products and

fusion jointing techniques for 30 years. There are few leakages and failures in the field except

those by third-party construction work.

PE pipes are suitable for gas distribution and in widespread use globally. PE pipes once buried

under the ground are extremely difficult to repair or replace. It is required that installed PE pipes

have a long life for at least several decades to a century. Installed PE pipes must endure a

continuous stress by internal pressure and bending due to soil subsidence. Although there are

many predictions of the life time of PE pipes by accelerated tests, there are few comparisons of the

life time between actual field failures and accelerated experimental failures. In addition, there are

few results of deformation behavior of PE pipes although they are mentioned to be also extremely

safe in differential settlements and earthquakes.

Objectives of the paper

The first objective is to study the slow crack growth resistance and to predict the service life of PE

pipes referring an analysis of actual field failures and results of accelerated long-term tests. The

second is to study the earthquake resistance conducted by a survey of damage to pipes due to

earthquake and by experiments of deformation behavior of PE pipes. Totally, the reliability

evaluation of gas distribution network of PE pipes is discussed in this paper.

Methods

The full-notch creep test (FNCT) originally developed in Japan is specified as ISO167701), 2)

as well

as specified in JIS K67743)

for evaluating the slow crack growth resistance and is now widely used

in the world pipe industries. A strip of specimen cut out of the PE pipe along its axial direction is

used for the FNCT. The specimen is notched at the middle circumferentially using a razor blade,

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so that the stress is concentrated on the axial center. It is possible to simulate an extremely severe

stress concentration condition of PE pipes that have a sharp flaw around its full circumference on

both exterior and interior surfaces.

PE pipeline systems survived the Great Hanshin-Awaji Earthquake without failures. High-speed

tensile tests were conducted on actual pipes to study pipe deformation behavior between joints

when experimentally subjected to displacement.

Results

There are a few field failures reported on PE pipes installed in the 1960’s and early 1970’s in US

as summarized by the GRI report4)

and on trial construction of PE pipes in the late 1970’s in Japan

although resin grades of PE pipes have been improved many times. Field failures mainly occurred

at a joint part by slow crack growth. The life time of PE pipes installed in the early 1970’s in US was

reported to approximately from 10 years to 15 years in the case of incomplete fusion joint and

excessive bending. A field failure on trial construction of PE pipes in the late 1970’s in Japan is

shown in Figure1. A crack was firstly generated from the bottom of the bead at the corner of the

fusion saddle joint subjected to longitudinal bending, and a few of cracks were combined and a

combined creep crack grew through the pipe wall. The crack reached the inside of the wall,

resulting in gas leakage.

Figure 1. Photographs of a field failure on trial construction of PE pipes installed in the late 1970’s

in Japan

There are a few field failures reported on PE pipes installed in the 1960’s and early 1970’s

although resin grades of PE pipes have been improved many times. The life of PE pipes early

installed such as US C-1 was reported to approximately from 10 years to 15 years in the case of

incomplete fusion joint and excessive bending. It was found that the predicted life time of US C-1

in the range of stress from 3 to 4 MPa at 20 degrees C by the FNCT as shown in Figure 2 was

approximately equal to the actual field failure time.

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Figure 2. Results of the FNCT on PE pipes installed in the early 1970’s (US C-1)

Figure 3 shows the relation between stress and time to failure on several PE pipes obtained by the

FNCT at 80 degrees C. Compared with the time to failure of US C-1 whose life time is

approximately from 10 years to 15 years, the time to failure of actual installed PE pipes in Japan

such as JPN AI-1 through AIII and EU A-IV has from 10 times to 100 times or more.

Figure 3. Results of the FNCT on several PE pipes

Figure 4 also shows the relation between the specified tensile stress and the time to failures at 80

degrees C and 65 degrees C. The time to failure of JPN AI-1 at 20 degrees C can be predicted by

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the following equation. It has been verified that PE pipes currently used are highly resistant to

cracks and have a long life.5), 6)

log t = 1/T(8610-946logσ)-20.14

where t is time to failure (hr), T is temperature (K), and σ is stress (MPa).

Even if PE pipes with full circumferential sharp flaws on both exterior and interior surfaces are

exposed to a bending stress which is caused by differential settlement etc. and exerts a stress load

of approximately 5 MPa on the pipe surface, the pipe is likely to be serviceable with no failure for

several hundred years insofar as it is used at a normal temperature of 20 degrees C. In other

words, it has been verified that PE pipes currently used by the gas industry are highly resistant to

cracks.

Figure 4. Results of the FNCT on JPN AI-1

Figure 5 shows comparison of results of the FNCT between newly installed pipes in 1986 and

dug-up pipes in 2004 on JPN AII. With respect to chemical degradation of PE pipes in the ground,

it was also found that the time to failure of JPN AII pipes at 80 degrees C dug-up in 2004 was no

difference with that at newly installation in 1986. As the environment is quite stable in the ground,

there was no degradation of pipes for 18 years although the degree of crystallinity of newly

installed pipes in 1986 has gradually increased and the impact strength of them has slightly

reduced.

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Figure 5. Comparison of results of the FNCT between installed new pipes and dug-up pipes on

JPN AII

The gas distribution network of PE pipes survived the Niigata Prefecture Chuetsu earthquake in

2004 and the Niigataken Chuetsu-oki earthquake in 2007 following the great Hanshin-Awaji

earthquake in 1995.

Concerning evaluation of earthquake resistance, in practice, when the ground subsides, the piping

system is held by the binding force of the ground, so that pipes slip as they are pulled. Since PE

pipes have smooth surfaces, the binding force of the ground is low, so that they slip easily.

(Before test) (After test)

Figure 6. Photographs of a test specimen before and after the high-speed tensile tests

The photograph on the left shows a test specimen and mechanical joints for fixing the specimen on

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the high-speed tensile machine in Figure 6. The tensile speeds were 1 m/s, which was equivalent

to the maximum velocity of the Great Hanshin-Awaji Earthquake. The pipes used were 50 mm in

nominal diameter. The pipe lengths were selected from 21 cm to 30 cm. The photograph on the

right shows the fracture of the pipe which occurred at the middle in Figure 6.

The high-speed tensile test results on actual pipes revealed that deformation of pipes subjected to

displacement occurred only in the pipe, not the joint, and that the deformation did not depend on

the type of the joint, whether the joint was mechanical or EF. The elongation at breakage was

approximately 9 cm with the minimum single pipe length of 24 cm when pulled at a tensile speed

of 1 m/s at -5 degrees C as shown in Figure 7.

The high-speed tensile tests on actual pipes were conducted to investigate their deformation

behavior. A 5 cm deformation absorption for a service pipe and a house pipe should be required in

the guidelines for aseismic designs.). Based on the results of the high-speed tensile tests of PE

pipes, the pipe elongation length at breakage met the 5 cm deformation absorption requirement

specified in the guidelines for aseismic designs even if the pipe length is the minimum pipe length

between joints.

Figure 7. Results of high-speed tensile tests for a pipe with a socket joint

It can be said that even if the pipe length is minimum pipe length between joints and pipes do not

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slip, PE pipes conform to the guidelines in terms of elongation at breakage.

Conclusions

It was found that PE pipes have a long product life and no maintenance cost for operation. Based

on the results of high-speed tensile tests of PE pipes, the pipe elongation length at breakage met

the 5 cm deformation absorption requirement specified in the guidelines for aseismic designs even

if the pipe length is the minimum pipe length between joints.

However, the percentage of total installed PE pipes compared with total installed all pipes for low

pressure networks is still now 30 % or less in Japan. Both manufacturers and users should

continue to make an effort to complete highly reliable integrated gas distribution networks of PE

pipes.

References

1. ISO16770 : Determination of environmental stress cracking (ESC) of polyethylene -- Full-notch

creep test (FNCT) (2004)

2. N. Nishio and S. Iimura: Proc. Eighth Plastic Fuel Gas Pipe Symp., 29(1983)

3. JIS K6774 : Polyethylene pipes for the supply of gaseous fuels(2005)

4. A GRI report titled “Field Failure Reference Catalog for PE Gas Piping”, (1989)

5. H. Nishimura : Chemistry and Education, Vol.51, 11, 654(2003)

6. H. Nishimura: Seikei-Kakou, Vol.20, 11, 790(2008)

7. Japan Gas Association: “Guidelines for aseismic designs of gas pipelines”, 347(1982)