NBSIR 76-1077 (

Analysis of Failure of a

Polyethylene (PE) Gas Service

Pipe (Lenoir County, NC)

S. D. Toner

Product Safety Engineering Section

Product Engineering Division

Center for Consumer Product Technology

June 1976

Final Report

Report for North Carolina Utilities Commission

Raleigh, North Carolina

NBSIR 76-1077

ANALYSIS OF FAILURE OF A

POLYETHYLENE (PE) GAS SERVICE

PIPE (LENOIR COUNTY, NC)

S. D. Toner

Product Safety Engineering Section

Product Engineering Division

Center for Consumer Product Technology

June 1976

Final Report

Report for North Carolina Utilities Commission

Raleigh, North Carolina

U.S. DEPARTMENT OF COMMERCE. Elliot L. Richardson, Secretary

James A. Baker, III, Urtder Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Analysis of Failure of a Polyethylene (PE) Gas Service Pipe

(Lenoir County, North Carolina)

A section of polyethylene (PE) gas service pipe was submitted to this

laboratory for an evaluation of the possible cause of in-service failure.

The pipe was personally delivered to this laboratory on November 21,

1975 by Mr, Thomas L. Dixon, Pipeline Safety Engineer for the North

Carolina Utilities Commission (NCUC) , who had conducted a field investigation of the incident on September 30, 1975.

Prior to receipt of the PE pipe the NCUC had requested the NBS to

conduct an analysis on the pipe. This request was contained in a document entitled: "Notice of Analysis of Pipe, Docket No. G-21, Sub 1^3,"

dated October 29, 1975, and subsequently amended by an Errata Order

dated November 10, 1975. This document stated that an explosion had

occurred on September 29, 1975, on the Tull Hill Farm on North Carolina

State Road 1004 in Lenoir County, North Carolina. It also stated that

the explosion, which killed three people, appeared to have occurred in

conjunction with an escape of gas from a section of pipeline of the North

Carolina Natural Gas Corporation.

Mr. Dixon provided the following verbal information:

1. The pipe had been purchased and Installed by the North

Carolina Natural Gas Corporation.

2. The only known purchase specification required the use of a specific, modified high density PE pipe compound to

produce a nominal 3/4 inch diameter Standard Thermoplastic

Pipe Dimension Ratio (SDR) pipe capable of withstanding

pressures to 150 pounds-force per square inch (psi).

3. The specified pipe compound and Its manufacturer were

i dent i f i ed

4. The purported pipe fabricator was identified.

5. The SDR ratio was not known.

6. The gas service pipe was installed In 19^4, at a depth of

approximately 32 Inches and was connected to a gas main

equipped with 52 psi relief valves.

7. The gas service line provided fuel to a furnace located in

a building component attached to a tobacco barn.

8. In 1965, a 16-i nch-diameter concrete sewer pipe leading to

a floor grating in the same attached structure was installed

above the PE gas service line. The sewer pipe was described

as being constructed of sections of corrugated bell and

spigot pipe, the joints of which were sealed by means of a "tar paper" wrap.

- 2

-

9. After the explosion a trench was dug to expose the gas service

pipe. The sewer pipe was reportedly found to be in direct

contact with a portion of the gas pipe. Photographs taken at

this point, and provided to the author, i nd i cated that the

visible part of the sewer pipe crossed over the gas pipe at an estimated angle of 10°, but did not show whether the two pipes were in actual contact.

10. A bell and spigot joint was located approximately one foot

from the area of the apparent leaks in the gas service pipe.

The pipe wrap seal on the bottom of this joint was said to

exhibit considerable damage.

11. When the partially exhumed gas pipe was cut upstream from the

apparent leak (i.e., in a section away from the building) and was pressure tested with air, air was noted to be escaping

through the sewer grating in the building. Therefore, it was surmised that gas had leaked from the service pipe, entered

the sewer line through the nearby partially unprotected pipe

joint, and collected in the building structure housing the

gas furnace.

12. When a section of the gas service pipe was exhumed and further

tested by air pressure at about 40 psi , two leaks were noted

and subsequently marked. This section of pipe was the one received

at this laboratory, and still remains in the author's custody.

13- An on-site investigation had also been conducted by

Mr. H.M. Shepherd of the Pipeline Safety Division, National

Transportation Safety Board.

In addition to the above information, it was reported that the

pipe may have been fabricated to meet the requirements of the Department

of Commerce, National Bureau of Standards, Commercial Standard CS255~63

for Polyethylene (PE) Plastic Pipe (SDR-PR) , issued July 1, I963. The

requirements of this Standard were essentially the same as a current

American Society for Testing and Materials (ASTM) D2239-73a, Standard

Specification for Polyethylene (PE) Plastic Pipe (SDR-PR).

Description of Pipe •

The submitted sample of black, plastic pipe was approximately 1.9

meters (m) in length, and had a nominal inside diameter of 0.8 inch (in).

The two points at which leaks had been noted during field tests of the

exhumed pipe had been marked. These points were located approximately

15 centimeters (cm) apart and about 65 and 80 cm from one end of the

sample. The section of pipe was permanently bowed. The areas marking

the two leaks were located on the side of the pipe with respect to the

bow (see figure 1), and were in essentially a straight line parallel to

the longitudinal direction of the pipe. At least a portion of one crack was visible on the outer wall of the pipe, but the other crack

was not discernible.

- 3

-

There was evidence that the pipe had been marked with a legend in

at least three places. However, the surface of the pipe in the marl<ed area had been abraded to the extent that the legend could not be

deciphered, either visually, by low power stereo microscopy, or by long

wavelength ultraviolet radiation. When plastic pipe is claimed by the

manufacturer to meet the requirements of a specific standard, it must

be marked to identify the nominal pipe size, type of plastic pipe

material, the SDR or pressure rating for water at 23°C (73. ^°F), the

designation of the appropriate standard, and the manufacturer's name

(or trade mark) and code. However, there is no known requirement that

this information remain legible after final installation.

In the absence of any identifying markings on the pipe, it was

not possible to conduct this analysis with respect to compliance or non-compliance to a specification or standard. However, on the basis of

information received at the time the pipe was submitted for analysis,

appropriate testing procedures set forth in CS-255 and ASTM D2239 were

used as guidelines in characterizing the pipe.

Eval uat i on of P i pe

Visual

The two marked areas of leakage were designated as A and B. These

two areas were, respectively, 80 and 65 cm from the nearest end of the

overall sample, figure 1. A portion of crack A was visible on the outer

wall of the pipe. A crack somewhat transverse to the longitudinal

direction of the pipe was visible. This crack was approximately 2 cm in

length but curved into a longitudinal groove, apparently the result of

abrasion, which extended for about 11.5 cm along the outer pipe wall,

consequently, the actual length of the crack could not be determined

initially. Microscopic examination of area B did not reveal the presence

of a crack, although numerous pits, scratches and embedded particles

were observed. The pipe was then pressurized to h psi gauge (psig) air

pressure. The leak at A was obvious, but no leak was detected at B. Subsequently, a liquid leak detector was applied over area B and the

approximate location of two small leaks was determined. The pipe was

then placed under the microscope, again pressurized and the leak

detector applied to this area. The two leaks were located and found to

be about 3 millimeters (mm) apart in the longitudinal direction and 5 mm

apart in the transverse direction of the pipe. No holes were actually

observed under magnification, but one leak appeared to be at or adjacent

to a small piece of embedded debris. The latter leak was located in a shallow furrow in the pipe wall that appeared to have resulted from

external abrasive damage.

A section of pipe, containing the two cracks, was cut from the

sample. This section was about cm in length with the two cuts being

made approximately 15 cm to the left of crack A and to the right of

crack B (see figure 1). Examination of the inside of this section showed

the presence of two cracks on the inner wall at the approximate location
r.:, L.

^1
- k

-

of the leaks. The inner wall was found to contain corrugations in the

transverse direction, extending over approximately one-fourth of the

circumference. There were at least six of these corrugations which

appeared to be located about 6 cm apart. Both cracks were oriented

perpendicularly at the approximate center of these corrugations.

This section of pipe was subsequently separated into five pieces by

transverse cuts. Cuts were made at about 3 cm from the midpoints of each

of the two areas containing the leaks. Examination of the interior of each

section resulted in the discovery of a third crack located about 3.5 cm from the end of crack A on the side away from crack B, i.e., to the left

of A as viewed in figure 1. This crack appeared to be about 2.5 cm in

length. Subsequently, a section of pipe about 7-5 cm in length containing

this crack, which was designated as C, was removed.

Each of the three sections of pipe containing the cracks were then

split longitudinally at points approximately 90°, c i rcumf e ren t i a 1 1 y , on each side of the crack. The cracks on the inner wall were then subjected

to microscopic examination in an effort to locate the probable point of

crack initiation and the reasons for failure. A unique feature of the

cracks was that all three exhibited the same general configuration. The

major portion of each crack, although exhibiting varying degrees of

curvature, was generally in the longitudinal direction of the pipe. One

end of each crack exhibited a rather abrupt change in direction,

partially transverse to the major crack. When the three cracks were

observed in the positions in which they were located in the pipe, this

change was in the same, somewhat transverse, direction and at the same

end of the crack, e.g., the left end. In the case of crack B, this

portion of the crack was on the end nearest crack A.

The overall length of the cracks In the longitudinal direction,

disregarding curvatures and directional changes, were: 3-6 cm for

crack A, 2.6 cm for crack B, and 2.8 cm for crack C. The major,

essentially longitudinal portion of each crack was: 2.7 cm for crack A,

1.9 cm for crack B, and 2.0 cm for crack C. In the cases of cracks A

and B, these were the regions In which leakage occurred.

Examination of crack A revealed the presence of two particles, which

looked like sand, embedded in the crack near the mid wall (the cylindrical surface parallel with and halfway between the Inner and outer walls

of the pipe) of the pipe. These particles were located about k mm from

the end of the crack on the inner wall and were near the right end of

the visible crack on the outer wall as illustrated in figure 1. The

crack In the 2.7 cm section appeared to be a stress crack.

In examining crack B, special attention was given to those points

on the inner wall in the area of the two leaks. These occurred near one

end of the 1.9 cm segment of the crack or the end most distant from

crack A. There was no evidence of any occluded particles, voids, or other Irregularities at these points on the Inner wall edges of the

crack. The edges of the crack in the 1.9 cm section did exhibit a
I
- 5

-

reddish discoloration. The edges of the remainder of the crack, however,

did not exhibit this discoloration but were rather translucent and

reminiscent of a tensile failure.

Examination of crack C did not reveal anything of significance with

regard to its cause. However, the edges of the crack at the inner wall

appeared to have been subjected to a tensile failure. There was no visual evidence that any portion of this crack completely penetrated the

pi pe wal 1

After the preliminary microscopic examinations were completed the

sections containing the three cracks were cut to expose the crack

surfaces in the pipe wall.

That portion of crack A which completely penetrated the outer wall was found to be about 2.3 cm in length. When the pipe was cut at each

end of this crack, the two sections readily separated as illustrated in

figure 2. The red mark on the inner pipe wall near the left end of the

crack marks the location of the apparent origin of the stress crack.

There was a small, raised irregularity in the crack surface of the smaller

piece located about 0.25 min from the inner wall. It was somewhat less

than 0.25 mm in length in the longitudinal direction of the pipe, and was estimated to be on the order of O.OA to 0.05 mm in width. A corresponding

matching cavity was observed on the opposite crack surface on the larger

piece of pipe. The yellow mark on the inner pipe wall near the right end

of the crack in figure 2 indicates the locations of the two particles

previously noted. These two particles fell out when the crack was separated. Examination of the crack surfaces in this area showed the

presence of three very small voids and a small pit near the mid wall. It could not be determined whether the pit had been caused by the presence

of one of the particles. Two small reddish spots were also observed in

this area, but it could not be determined whether they were due to

occlusions or to debris.

Crack B is illustrated in figure 3. The two portions of this

crack did not separate when the pipe was cut. Examination of the surfaces of the crack in the area of the two leaks observed on the outer

wall did not indicate any obvious defects in the pipe. The two sections were then bent and broken apart. There was some visual evidence that

the crack had penetrated the outer wall for a distance of about 6 mm at

the right end of the crack in the area of the two leaks. The red mark

on the inner wall is the location of the probable point of initiation

of the stress crack. The larger piece of pipe had a very small

irregularly shaped pit, located about 0.25 mm from the inner wall, and

approximately 0.25 mm in diameter, at the apparent initiation point. A

similar, but much shallower pit was observed on the opposite crack

surface. It could not be determined whether this pit was a void or had

been caused by the presence of an occluded particle. Two small voids were found near the mid wall of the pipe, in the crack surface, but

neither appeared to have been a crack initiation point.

- 6

-

Attempts to separate crack C indicated that this crack did not

extend much beyond the mid wall of the pipe. The crack was forced open,

resulting in some tensile failure, in order to examine the crack surfaces.

Although the point of origin of the crack seemed quite obvious, there were no indications of any artifacts, voids, or occlusions to enhance

crack formation. The apparent origin of the crack was near the point of

directional change in the crack, as illustrated In figure A.

In the section of pipe between the two sections containing cracks

A and B, a severely abraded area was noted on the same side of the pipe

as crack A. This area is located on the outer pipe wall just to the

right of the two arrows showing the location of crack A (figure 1).

This area was approximately 2.5 by 2.5 cm and its center was located

about k cm from the nearest end of the visible part of crack A. The

outer wall of the pipe at this point was concave. It was also noted that

three similar abraded areas exhibiting less severe damage occurred a distance of 15, 30 and 45 cm to the left of the above area as viev;ed in

figure 1. The outer wall of the pipe was visibly deformed and concave

in these areas. This concavity is typical of compressive deformation.

It was noted that the abraded area located 15 cm from the one between

cracks A and B, was in the area of crack C at the point of the directional

change in the crack configuration. This was also the location of the

suspected point of initiation of this stress crack.

Examination of the pipe revealed a series of abrasion marks

associated with shallower deformations of the outer pipe wall at intervals of approximately 7.5 cm, including those previously observed.

That is, starting at the abrasion mark indicated in figure 1 there was evidence of a similar mark 7-5 cm to the right, located at the suspected

origin of stress crack B, while to the left there was a mark 7«5 cm away located at the suspected origin of crack A. The next mark to the

left was the one observed at crack C, while further to the left there

were five additional marks located at 7-5 cm intervals, including the

two previously noted marks at the 30 cm and 45 cm intervals from the one

in figure 1. There was also some evidence, though not conclusive, of

three or four additional abrasion marks to the right of crack B in

figure 1. These marks appeared to be associated with the rib-like projections previously noted on the inner wall of the pipe. As a point of

verification, wall thickness measurements made adjacent to the cracks

and parallel to the longitudinal direction of the pipe across these

areas indicated that thickness did not vary by more than about 0.0004 - 0.0005 in (0.010 - 0.013 mm), over a distance of 1 to 1.5 cm. This

compares to wall thickness variations of .007 ~ -010 in (0.18 - 0.25 mm)

when a cross section of pipe was measured ci rcumferenti al ly

In the normal process of examining the inner wall of the split

sections of pipe, the three areas opposite the crack positions were

found to have a pair or irregularly shaped white lines parallel to the

longitudinal direction of the pipe. Similar small white spots and other

- 7

-

particulate matter were observed between these lines. Microscopic

examination of these lines revealed that they were composed predominately

of opaque white particles. These particles appeared to have collected

along the top of ridges extending away from the inner wall for a distance

estimated to be of the order of 0.5 mm or less. Other darker particles,

which may have been sand, were observed in the area between the two lines

and particularly in the white spots. Microscopic examination indicated

that none of these particles were embedded in the plastic. They seemed

to be loosely adhered to the pipe wall, and could best be described as debris. It was felt that the white particles, in particular, might

be deposits of salts remaining from evaporated ground water which had

seeped into the piping system. The presence of such salt residues,

particularly if they contained cations such as calcium and magnesium

would provide unequivocal verification that the cracks located opposite

to them were located on the top of the pipe in its installed configuration. Subsequently, the two pipe sections opposite cracks A and B were submitted to the NBS Analytical Chemistry Division for possible identification of these residual materials. A qualitative electron microprobe

analysis indicated a preponderance of silicon cations and considerable

amounts of aluminum. Traces of calcium, iron, potassium, sulfur and

titanium were also found. Pure clay consists of a compound composed of

hydrogen, aluminum, silicon and oxygen with the following empirical

formula: HAlSiO/^. Ordinary clay is a complex mixture of aluminum

silicates frequently containing combined iron and other metals, and

often admixed with iron compounds, calcium carbonate (limestone), and

silicon dioxide (sand). Clay particles, although essentially insoluble

in water, are capable of forming colloidal suspensions in water. Ground

water containing such suspensions could have seeped into the pipe, for

example through crack A, flowed to the bottom and on subsequent evaporation resulted in deposition of the particles on the pipe wall.

Dimensional Measurements

The inside diameter (l.D.) was measured at four cross sections of

the pipe where cuts had been made to remove the cracked sections. The

average l.D. was of the order of 0.805 in (2.0^5 cm) or slightly below

the allowable minimum of 0.809 in (2.055 cm) for Ilk inch l.D., SDR PE

pipe (CS255) . (In Voluntary U.S. Standards for plastic pipe, U.S. customary units are regarded as standard.) The inside diameter of SDR PE pipe

is controlled to allow use of standard size insert fittings. For any

given nominal pipe size the l.D. is the same for all PE standard dimension

ratio pipe. The SDR is determined by the wall thickness.

Wall thickness measurements were made on cross sections of the

pipe at each end of each of the three cracks. Using standard procedures,

the wall thickness was determined by averaging the maximum and minimum

values obtained on each cross section. The wall thickness at each of

the six measured areas exceeded the minimum requirements, 0.118 in (0.30

cm), for SDR7 PE pipe (CS255) . In every case the minimum wall thickness

was located at least 90°, in the circumferential direction, from the areas containing the cracks.

- 8

-

The pipe was also found to meet the requirements of CS 255 for the

wall thickness range, or eccentricity.

Since the wall thici<ness and wall thickness range met the requirements

for SDR 7 PE pipe, there was no reason to suspect that the substandard

I.D. would have any detrimental effect on either the pressure rating or the useful service life of the pipe.

Density of Pipe Compound

Density measurements were conducted in accordance with the procedures

set forth in ASTM D1505-68, Standard Method of Test for Density of

Plastics by the Density-Gradient Technique.

Two sets of three specimens were measured. One set consisted of

specimens cut from the end of a pipe section cut about 3 cm to the left

of crack B, in figure 1, at points approximately 120° from each other in

the circumferential direction of the pipe. The other set consisted of

specimens cut from the pipe at points about 1.5 to 2.0 mm from each of

the three cracks at locations adjacent to the suspected points of crack

initiation. Both sets of specimens had an average density of 0.957 grams

per cubic centimeter (g/cm3) , indicating that i t was a Type III, high

density PE resin as defined in CS-255. The specified pipe compound

reported used to manufacture this pipe has a nominal density of 0.960

g/cm^.

Infrared Spectrographi c Analysis

A hot pressed film specimen prepared from a sample of the pipe was analyzed by infrared spectrographi c techniques. The resulting spectrum

indicated that the polymeric component of the pipe was a high density

polyethylene resin copolymer i zed with 1-butene. This was the type of

compound reportedly specified for this pipe.

Resu Its

The surfaces of all three cracks found in the pipe exhibited

characteristic conchoidal fractures normally associated with brittle

failure. Among the causes of brittle failure,, in the type of plastic

used in the submitted sample, are the intermittent or continuous application of stress which over a period of time may result in a stress crack

due to fatigue.

Due to the linearity of the polyethylene molecules and the

orientation effects induced in the extrusion molding process, stress

cracks in the type of pipe that was evaluated are generally parallel

to the length of the pipe. The partially transverse portions found at

the end of each crack were atypical of stress crack failure in high

density polyethylene pipe. However, this directional change occurred

at points where both the inner and outer pipe wall exhibited permanent

deformation. These deformed areas were typical of creep deformation due

to compressive stresses on the outer wall and tensile stresses on the

i nner wal 1

- 9

-

Creep deformation occurring over a long period of time may result

in a change in the morphology of a plastic material, for example by

inducing molecular disorientation and perhaps resulting in a change in

the crystal 1 i n i ty of the material.

The regularly spaced abrasion marks found over at least a 60-cm

length of the gas pipe can reasonably be assumed to have been caused

by contact with the concrete sewer pipe installed above it. Abrasion,

at these contact points, could have occurred as a result of movement of

either or both pipes against one another due to such factors as differences in the thermal expansion characteristics of the two pipe

materials, or to expansion, contraction or compacting of the surrounding

soil. The varying degree of severity of damage noted at the different,

apparent, contact points could be attributed In part to the fact that

the concrete pipe was rigid and unyielding, whereas the more resilient

plastic pipe could yield to some extent from compressive forces applied

to one side. However, the degree to which the pipe was capable of

yielding could well be expected to be either limited or enhanced by

the localized condition of the surrounding soil.

Since small artifacts were observed at the apparent origin of both

cracks A and B, the possibility of stress cracks developing at these

points without pressure being exerted by the sewer pipe cannot be

eliminated. However, the permanent deformation, or corrugations,

previously noted on the inner pipe wall, would indicate that the plastic

material at the inner wall in the vicinity of these artifacts would have

been subjected to abnormal tensile stresses. Since the same conditions were found to exist at crack C, but no apparent artifacts were observed,

it would appear that external forces were the primary cause of failure,

while the artifacts in cracks A and B may have been contributory

causes to the extent of the failure.

NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUHI i( A'l lON OK KI-I'OK r NO. NBSIR76-1077

2. Cidv't AcilSSKlll No.3. K( I ipii Mi '.^ Ai t cssion No.

4. TlTLl' .^N^ .sUl^in i.!-;

Analysis of Failure of a Polyethylene (PE) Gas

Service Pipe (Lenoir County, North Carolina)

5. Piihl u ;\l ion n.iic June 1976

kh6.03

7. AL'THOR(S)

Samuel D. Toner

8. Pi'rforming Organ. Kcport No.

9. PKRFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 202j4

10. Proic-ct/Ta.sk/Work Unit No. 4460127

11. ( onlrac t /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

A255 - 225

Washington, D.C. 2023A

13. Type of Report & Period

(--overed Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. .ABSTRACT (A 200-word or less (actual summary ol most si^idcant information. If document includes a significant

bibliography or literature survey, mention it here.)

A fatal explosion in Lenoir County, North Carolina was apparently caused by the

accumulation of natural gas from leaks, due to in-service failure, in a 3/4-inch,

inside diameter polyethylene (PE) gas service pipe. Three stress cracks were

found on the inner wall of the pipe, two of which had penetrated the outer pipe

wall. The apparent primary cause of the stress cracks was due to creep deformation of the pipe wall at the site of each crack, resulting from pressure exerted

on the plastic pipe by a 16-inch diameter concrete sewer pipe which had been

installed across the plastic pipe. These deformations appeared to have resulted

from compressive stresses on the outer wall of the pipe. Small artifacts near

the apparent origins of the two cracks, which penetrated the pipe wall, may have

been contributory causes to the extent of the failure.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Failure analysis; gas pipe failure; gas pipe, plastic; natural gas explosion;

plastic gas pipe failure; polyethylene gas pipe

18. AVAILABILITY ^ Unlimited

jCX. For Official Distribution. Do Not Release to NTIS

\~ Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C13

1

' Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

16

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74

Figure 2

Figure 3

Figure h



•vVVKT

:r-rMETOWN PA

.ir.H 79