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

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 7^^193

2. Gov't Acct ssion No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Failiir<:> An3lvc;i<; of a PnlvTVinvl Chloride) (PVC) Plastic

Natural Gas Transmission Pipe from Las Vegas, Nevada

(NBS Failure Analysis No. 106)

5. Publication Date May 197^

6. Performing Organization Code

7. AUTHOR(S)

Samuel D. Toner

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/^S'ork Unit No. 3120^18 {k]Ok)

11. Contract/Grant No. ^

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

Office of Pipeline Safety

National Transportation Safety Board

Department of Transportation

Washinqton. D.C. 29590

13. Type of Report & Period Covered

Fi na 1 14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

A natural gas leak from a poly (vinyl chloride) (PVC) gas transmission line was the reported cause of a fatal explosion in a motel complex in Las Vegas,

Nevada. An analysis of the pipe was conducted to determine the cause of

failure. The results indicated that the pipe compound had not been properly

fused during the extrusion process. In connecting the plastic pipe to a metal

pipe, a solvent-based primer had been used to activate the adhesive on a pipe

wrap tape, applied over the plastic pipe and a compression fitting. The solvent

apparently caused gross swelling of the pipe, beneath the tape, negating the

effect of a metal insert stiffener. Subsequent creep deformation of an adjacent section of the plastic pipe apparently exerted sufficient tensile stress

on the swollen portion of the plastic pipe to induce the formation of a stress

crack.

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; Natural gas pipe; Plastic gas pipe;

Poly(vinyl chloride) gas pipe

18. AVAILABILITY Unlimited

7^ For Official D istribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Uashington, D.C. 20402, SD Cat. No. CH

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

11

22. Pr

SCOMM-DC 29042-P74

NBSIR 74-494

Failure Analysis of a Polyethylene |PEj Natural

Gas Service Line from Bowie, Maryland

(NBS Failure Analysis No. 107)

Samuel D. Toner

Measurement Engineering Division

Consumer Product Systems Section

National Bureau of Standards

Washington, D. C. 20234

May 1974

This is a final report.

Prepared for Pipeline Safety Division

National Transportation Safety Board

Department of Transportation

Washington, D, C. 20591

NBSIR 74-494

FAILURE ANALYSIS OF A POLYETHYLENE (PE) NATURAL

GAS SERVICE LINE FROM BOWIE, MARYLAND

(NBS Failure Analysis No. 107)

Samuel D. Toner

Measurement Engineering Division

Consumer Product Systems Section

National Bureau of Standards

Washington, D. C. 20234

May 1974

This is a final report.

Prepared for Pipeline Safety Division

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

A section of polyethylene plastic pipe from a natural gas service

line was received for evaluation of the cause of an in-service failure.

This failure analysis was conducted at the request of, and under the

sponsorship of, the Pipeline Safety Division, National Transportation

Safety Board. Immediately prior to receipt of the pipe, it had

been in the possession of the Office of the Fire Marshall, Prince

Georges County, Maryland. The pipe contained a single crack in the

wall. It was reported to us that gas leakage from this crack was the

substantive cause of a fatal explosion and fire on June 23, 1973, in a private residence served by the line, located at 12321 Welling Lane,

Bowie, Maryland.

The damaged section of pipe was obtained at a meeting held in this

laboratory, attended by several persons, the purpose of which was to

discuss the possible approaches to the conducting of a failure analysis.

Among those present were a representative of the manufacturer of the

basic plastic resin, and a representative of the resin manufacturer's

subsidiary which fabricated the pipe. These individuals provided the

following information:

1. The basic pipe compound consisted of a carbon-black filled

polyethylene- 1-butene copolymer,

2. The pipe compound contained a tracer compound, consisting

of a metal oxide, used exclusively by the manufacturer as a means of identifying its materials, in which the metal

content was nominally 5 ppm.

3. Copies of the specifications for both the pipe compound

and the fabricated pipe, which were in effect at the time

of manufacture of this particular pipe, which was estimated

to have been in the latter part of 1965, were provided.

These specifications included data on the properties of

the pipe compound, as well as dimensions, tolerances, pressure

ratings and similar data on the fabricated pipe, including

recommended installation procedures. Although the pipe was

not so marked, the technical brochures indicated that it was

in compliance with Commercial Standards CS197-60 and CS2S5-63,

and was in compliance with type PE3306 plastic pipe as described in the latter standard. This code indicates that

the pipe compound was produced from Type 3, Grade 3, high

density polyethylene resin and had a hydrostatic design stress

rating of 600 psi for water at 23°C (73.4°F).

4. This particular pipe was pressure rated for continuous use

at pressures up to 190 psi for water and 95 psi with natural

gas at 23°C (73.4°F)

.
- 2 -

A representative of the gas company, also present at the meeting,

provided the following:

1. The pipe had been purchased by the gas company and then

given to a contractor, who installed the service line in

December 1965. The final connection of the service line to

the main had been conducted by gas company personnel. The

service line had been installed at a depth of 102 cm (40 in)

below ground level, and in the process of coupling it to the

steel main, had been bent upward at an angle of approximately

30 degrees and attached to a tee on the main by means of a standard compression fitting.

2. The nominal working pressure in the service line was about

20 psig.

3. In the months subsequent to the installation, a paved street

and sidewalks had been installed in such a location that the

steel main was beneath the sidewalk on the opposite side of

the street from that on which the house in question was

located.

4. Hie crack in the service line was located approximately 46 cm

(18 in) from the line-to-main coupling.

5. An additional 1.86 m (6 ft) loi^g section from the same continuous

length of pipe as that containing the crack was submitted for

use as needed in the test program. This section had been

exhumed from the service line trench at a site across the

street from the cracked section, and had been pressure tested

for leaks following the explosion.

In addition to the above information, a request was also made to

attempt to determine the approximate volume leakage rate, using air,

rather than natural gas, while the cracked section was subjected to a pressure of about 20 psig.

Preliminary Examination of the Pipe

The damaged section of pipe was about 46 cm (1,5 ft) in length,

with the crack located at approximately the mid-point of the length of

the exhumed section. Identifying marks on the pipe included the producer's tradename and the pipe size, i.e. Schedule 40, 1/2-inch. The

section of pipe was permanently bowed as illustrated in Figure 1. The

crack was approximately 3.8 cm (1.5 in) long and in a modified "J"

shape, with the stem of the "J" extending in the longitudinal direction

of the pipe, as illustrated in Figure 2. The longitudinal portion was

on the order of 2 cm (0.8 in) in length, with the remaining curved

portion extending about an additional 0.5 cm (0.2 in) in the longitudinal

direction. The curved portion of the crack extended to a point where it
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was about 45 degrees to the transverse and longitudinal axes. The

longitudinal section of the crack was located essentially on the inside

of the bow.

The pipe was visibly flattened at a point just beyond the end of

the longitudinal portion of the crack. At the point where the longitudinal portion began to curve, microscopic examination revealed the

presence of a slight gap in the outer surface of the pipe. Preliminary

tests, using 10 psig of nitrogen pressure, indicated that this part of

the crack seemed to have the highest leakage rate. This portion of the

crack, measuring about 0.5 cm (0.2 in) in length, was the only part of

the crack in which there was an obvious separation of the two crack

surfaces in the outer surface of the pipe. Unfortunately, there was no way of determining whether this gap was present prior to exhumation of

the section, or whether it occurred as a result of intentional attempts

to straighten the section, e.g., to visually attempt to examine the

interior of the pipe, by any of the several persons known to have had

access to the pipe prior to its acceptance in this laboratory. This

particular section of the crack, because of its location on the inside

of the permanent bow in the pipe, would most probably have been subjected

to more tensile stress than any other part of the crack if the pipe were manually straightened. Consequently, the flow rates, subsequently

measured and reported in another section of this report, are probably

higher than those which would have occurred while the pipe was still in

service

.

Following this preliminary work, the cracked section of pipe, still

in its "as received" condition was submitted to the NBS Fluid Meters

Section, Mechanics Division, for the air leakage tests. The pipe was pressurized to approximately 20 psig (34-35 psia) and maintained at

that pressure for 30 minutes. The results are given in Table 1. The

slight increase in flow rate observed after 30 minutes of test was attributed to a possible slight straightening of the pipe.

Materials Tests

All of the following tests were conducted from specimens taken

from the 1.86 meter (6 foot) section of pipe.

The pipe compound was subjected to an infrared spectrographic

analysis, and subsequently qualitatively identified as a polyethylene-1-

butene copolymer.

The presence of the metallic oxide tracer was established by means

of neutron activation analysis, and quantitatively verified as being

present in approximately the specified amount of 5 ppm, reported as the

metal. These tests were performed by a member of the staff of the NBS

Activation Analysis Section, Analytical Chemistry Division. When the

sample was analyzed in triplicate the metal content was found to be 4.1,

4.6, and 18 ppm. Since the abnormally high value of the one sample was
- 4 -

believed to be due to inhomogeneity of the oxide in the pipe compound,

or due to accidental contamination of the sample during preparation, a fourth analysis was conducted and the metal content found to be 4.1 ppm.

The density measurements on specimens of the pipe were obtained

using the procedures described in the American Society for Testing and

Materials (ASTM) Method D1505-68, Standard Method of Test for Density

of Plastics by the Density Gradient Technique. The average density was found to be 0.9545 gm/cm-^, within the range expected for the specified

nominal density of 0.960 gm/cm^ for the basic pipe compound.

ITie melt index (M.I.) of the pipe was determined using the procedures

described in the ASTM Method D1238-70, Standard Method of Measuring Flow

Rates of Thermoplastics by Extrusion Plastometer. The measured M.I.

Value of 0.28 was within the range expected for the specified nominal

value of 0.30.

Flattening tests were conducted on specimens of pipe. The

flattening test is a standard test procedure for most plastic pipe,

although it is not an applicable test for new polyethylene pipe. In

this analysis it was used simply to determine possible embrittlement of

the pipe compound. These tests normally consist of placing a specimen

between parallel plates, and applying a compressive force over a period

of 2 to 5 minutes, until the specimen is flattened to 40% of its original

actual measured outside diameter. In this case, however, the pipe was completely crushed until the inner walls were in intimate contact. This

required an applied force of approximately 45 kg/cm (250 lb/in) of

length of the pipe specimen. Neither of two specimens tested showed

any evidence of splitting, crazing, or cracking that might indicate

deterioration of the pipe. One additional specimen was first prepared

by putting a longitudinal cut, using a surgical scapel, about 0.6 cm

(0.25 in) in length along the inner wall. The nicked section was aligned with the upper plate and the flattening test repeated, but the

crushing force was maintained for 5 seconds after the inner walls had

made contact. This test was strictly an arbitrary one intended to

induce stress cracking. It was felt that any significant extention of

the crack might be indicative of poor molding techniques or deterioration

due to aging. No evidence of either were noted when the specimen was cross-sectioned for the purpose of microscopically examining the cut.

The density and M.I. values, as well as the absence of cracking

or splitting of the pipe or lengthening of the nick in the flattening

tests, were considered to be relatively good indications that there had

been no apparent deterioration of the pipe compound due to poor extruding

or fabricating techniques or subsequent aging.

Dimensional Measurements

Three specimens, cut from the 1.86 meter section of pipe, were measured to determine compliance with the manufacturer's specifications

' with respect to the outside diameter and the wall thickness. The
- 5 -

average O.D., based on the minimum and maximum measured values, met

the requirements. All three specimens met the maximum, but two were

slightly below the minimum wall thickness requirements. However, in

the latter case, the standard micrometer normally recommended for

measuring pipe wall thickness was unavailable, and a modified micrometer

of slightly less accuracy was used. In view of this, and considering

the low working pressure in the pipe, this minor variation was not

considered to have a significant bearing on the strength of the pipe.

Later when the crack was opened, wall thickness measurements along the

edges of the crack, particularly in the immediate area of the probable

point of initiation of the crack, indicated that the wall thickness was within compliance, with respect to the manufacturer's specification.

The cracked section of the pipe contained a visibly flattened area

just beyond the end of the longitudinal portion of the crack. Consequently, an O.D. profile was made of the pipe over a 30.5 cm (12 in)

length. Starting at a point on the longitudinal section of the crack,

located approximately 1.5 cm (0.6 in) from the end of the longitudinal

portion and about 0.25 to 0.50 cm (0.1 to 0.2 in) from where the crack

began to curve, the pipe was marked off in 1.27 cm (0.5 in) increments

for a distance of 15 cm (6 in) in each direction from this central point.

A diameter of the pipe (intersecting the crack) in a transverse plane

passing through the midpoint of the crack was arbitrarily considered to

be at 0°. In this plane, and in parallel planes at 1.27 cm (0.5 in)

increments, the diameter of the pipe was measured at approximately 45,

90, and 135°, considering the 0° point to be in the same circumferential

position in each plane. Of all of these measurements only three 0°

measurements, located at 1.27-, 2.54-, and 3.8-cm (0.5-, 1.0-, and

1.5- in) from the central point, in the direction away from the curved

portion of the crack failed to meet minimum O.D. requirement, with the

flattest point at the 2.54-cm (1.0-in) location. At each of these three

locations the 90° measurement exceeded the maximum O.D. requirement.

Visual and Microscopic Examination of the Crack

A 13-cm (5-in) long section of the pipe containing the crack was removed by making transverse cuts about 5 cm (2 in) beyond each end of

the crack. This section was then cut open in the longitudinal direction

to expose the inner wall. Two items were immediately observed: 1) the

crack extended for an additional 3.2 cm (1.25 in) in the longitudinal

direction along the inner wall, although not in as straight a line

as that on the outer pipe surface; and 2) most importantly, an occluded

particle was observed embedded in the crack and protruding through the

inner wall surface. The particle was located approximately 0.5 cm

(0.2 in) from the end of the curved portion of the crack. The protruding

part of the particle was sharp to the touch and appeared to be firmly

embedded in the crack. Under microscopic examination the particle

appeared to be a grain of sand. In addition, it was noted that some of

the plastic had been extruded downward, that is, into the interior of

the pipe, around the particle on both sides of the crack. The distortion
- 6 -

of the inner wall surface caused by the displaced plastic material

appeared to be somewhat greater along the inner side of the curvature

of the crack. The occluded particle is shown in Figure? 5.

The crack was then opened by making transverse cuts to points as closely as possible to the ends of the crack as it appeared on the outer

surface of the pipe. When the second cut was completed the two pieces

separated readily, and although the occluded particle fell out at this

time, it was recovered. It should be noted that this situation had been

considered prior to the opening of the crack and, consequently, the exact

location of the particle had been marked on the inner surface of the

pipe.

Subsequently, the two surfaces of the crack were carefully examined

by means of a stereo microscope over an approximate range of 12X to 60X

magnification. The most significant points of interest were those areas including and surrounding the location of the occluded particle. At

the point where the particle was located, each crack surface contained a discrete cavity. The one on the crack surface along the inner part of the

curvature appeared to be slightly deeper than that on the opposite surface.

However, each cavity seems to have rather discrete characteristics which

would indicate, at the first approximation, that the particle had become

occluded while the pipe compound was still soft, i.e., during the initial

extrusion of the pipe. One of the recommended procedures in the extrusion

of this type of polyethylene pipe compound was that a 20/80/20 - mesh

breaker screen pack be used in the extruder. Assuming this technique

was used, the particle, if it had been a contaminant in the original

pipe compound, would not have passed through an undamaged 80 - mesh

screen, although it was small enough to pass through a 20 mesh screen. However, this does not preclude the possibility of accidental contamination

of the screen pack or extruder on the downstream side of the 80 - mesh

screen. The inner wall of the pipe directly opposite the particle was carefully examined on the assumption that if the particle had gotten into

the pipe during the installation, and was subsequently crushed with

sufficient force to embed the particle into the pipe wall, that there

should have been some visible damage on the opposite wall. No such damage

was evident under microscopic examination. In addition, when a section

of pipe was crushed, in a laboratory test, until the inner surfaces were

in intimate contact, the specimen was permanently distorted out-of-round

with respect to the specification requirements, a condition not observed

in the immediate area of the occluded particle.

The sides of the crack above the particle extending to the outer

pipe surface were examined for abrasion marks, such as could be expected

to have been caused by the particle being forced into the crack from the

outside. No such damage was observed. However, if such a situation had

occurred, this would mean that the crack was already present when the

particle was forced for fell into the crack. Since the site of the

occluded particle was believed to be the obvious point of initiation of

the stress crack, it would appear to have been too fortuitous for the

stress crack to have been present and that the particle just happened to
- 7 -

fall into and become lodged at this exact point. In addition, when the

sample was received there was no visible gap in the outer surface of the

pipe wall through which the particle could have fallen. This portion

of the crack was towards the inside of the bow in the pipe, and presumably

could have been held closed by slight compressive forces at or near the

outer surface of the pipe.

Above and around the particle cavity, the surfaces exhibited

evidence of a brittle fracture of the type typically associated with

stress cracking. One surface, in particular, was evenly coated with a rust-like stain, approximately 0.75 cm (0.3 in) in width, surrounding the

particle cavity and extending to the outer surface of the pipe wall.

The opposing surface also exhibited staining, although not as evenly

distributed over the entire surface, but also extending to the outer

surface. As a result of these observations it was concluded that the

occluded particle created a stress point which later resulted in the

formation of a stress crack extending through the pipe wall at that

location. This crack presumably, because of the observed stains,

occurred and resulted in the initial gas leak, at some extended period

of time prior to the development of the crack to the size observed in

the exhumed pipe. There was one other place in the crack, approximately

0.25 cm (0.1 in) in width, located at approximately the outer end of

the longitudinal portion of the crack, as observed on the outer surface,

where similar rust-like stains covered both surfaces of the crack from

the inner to the outer surface. However, the density of the color of

these stains subjectively appeared less than those surrounding the

occluded particle. This site appeared to have been the possible origin

of a second gas leak. One possible assumption of its occurrence is that

the crack propagated along the inner wall of the pipe, from the site of

the occluded particle, and that a rupture at this point could have

resulted from stresses in the pipe wall, since this site was located

just within the area where the pipe began to be flattened out-of-round

.

Although the pipe wall was completely cracked through between the particle

and the other site of stains, there were no stains that extended to the

outer surface, although some staining of the sides of the crack were

observed near the inner surface. This seemed to indicate that this portion of the crack, at least near the outer surface, was relatively

recent. Tliere were other random rust-like stains on the inner wall of

the pipe, which indicated that their source, as well as those in the

crack, could have been the nearby steel main.

Two other potential sources of stress were noted as a result of

the visual examinations. The flattened portion of the pipe, noted above,

was in the area where the inner wall was cracked. The outer wall con- tained a group of pits with embedded dirt and sand as though someone had

stepped on the pipe. However, after noting comparable abrasions at

various points around the circumference of the long section, where the

pipe was not out-of-round, it was surmised that they may have been due

to soil compaction, expansion and contraction or the like and that the

flattening may have been due to straightening of the pipe. This assumes
- 8 -

that the pipe was firmly held by the nearby compression fitting, and

that flattening was not a fabrication defect. Such straightening could

have produced tensile stresses near the outer surface in the cracked

area. There was also the possibility that the flattened area was due

to kinking of the pipe at some point in time prior to, or during,

installation, a situation which could result in permanent out-of-roundness

.

The other observation was that both sections of pipe contained a pair of

shallow, parallel grooves along the inner surface that were probably

caused by the extruder die. Although both sections were bowed, these

grooves were out of phase by about 90° with respect to their location

and the direction of bowing. One of the grooves was immediately adjacent

to the occluded particle on the side towards the curved portion of the

crack, and the other somewhat closer to the curve, and both were located

essentially along the inside of the bow in this section of pipe. The

two grooves in the long section were located along the side of the inner

wall with respect to the bow. This indicated that the pipe may have been

twisted during installation, since the pipe is normally snaked back and

forth along the bottom of the trench. If twisting occurred as a result

of installation, then residual torsional stresses could also have been

present in the crack area. Stresses due to either or both of these

possibilities would be expected to decrease the resistance of the plastic

to stress cracking.

Conclusions

From this investigation, it was concluded that the cause of failure

appeared to be due to a stress crack initiated by the presence of an occluded particle, which acted as a stress point, in the inner wall of

the pipe. All observations seemed to indicate that the particle became

lodged in the wall during some stage of its fabrication, and that in

acting as a stress point, weakened the pipe. The stress crack probably

propagated to its final size as a result of exposure to a variety of

possible forces on, and possibly within, the pipe wall.

Evidence obtained by microscopic examination indicated the occurrence

of a small leak through the pipe wall at a point directly above the site

of the occluded particle, and a second probable small leak that appeared

to have been caused in part by the presence of the stress crack which had

propagated along the inner wall from the site of the occluded particle.

The full crack between, and extending slightly beyond these two sites,

may have been very recent; and as far as final propagation through the

outer surface in the pipe wall was concerned, may have been abrupt in

nature

.
Table 1. Results of Air Leakage Tests

Time P T Air Leak Rate

psia °F CFM at P, T a/ CFM at 14.7 psia, 70°F b/

Start 34,,0 71,.2 2.,31 5.,32

35.,1 71..2 2.,55 6,,08

1/2 hr. later 35.,1 70.,5 2,.86 6.,80

34.,0 70.,5 2,,82 6.,50

£/ CFM (ft min" ) at conditions inside the tube of pressure, P, and

temperature, T.

b/ Same leak at the same test conditions expressed as a volume rate

of air at 14.7 psia and 70°F.

Figure 1. Cracked section of pipe "as received", arrows indicate location of the crack.

Figure 2. Close-up of crack with arrows showing

extent of crack on the outer surface

of the pipe.