NBSIR 74-492

Failure Analysis of a Polyvinyl Chloride)

Natural Gas Urn Pipe From Las Vegas,

New [.lexico (NBS Failure Analysis. Ho. 105)

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 Office of Pipeline Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20590

NBSIR 74-492

FAILURE ANALYSIS OF A POLY (VINYL CHLORIDE)

NATURAL GAS MAIN PIPE FROM LAS VEGAS,

NEW MEXICO

(NBS Failure Analysis No. 105)

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 Office of Pipeline Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20590

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

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

A section of poly(vinyl chloride) (PVC) pipe from a natural gas

main pipe was submitted to the Metallurgy Division, National Bureau of

Standards by the State Corporation Commission of New Mexico at the

request of the Office of Pipeline Safety, National Transportation Safety

Board. The latter agency requested an evaluation of the cause of an in-service failure. Gas leakage from a failure crack in the pipe was said to have been the attributable cause of a fatal explosion, on October 21, 1972, in a private residence located at 120 Alamo Street,

Las Vegas, New Mexico.

The pipe was received by this laboratory in two pieces, which

based on accompanying reports, resulted from accidental breakage in the

transverse direction through the site of the original failure crack,

during exhumation and removal of the section of the gas pipe from the

trench. In addition, one section of pipe contained three longitudinal

cracks extending away from this break, and a small portion of the pipe

was missing at a point where one of these cracks emanated away from

the transverse break. The following excerpts are taken from the

accompanying letter of transmittal from a Safety Engineer in the Pipeline Division, State Corporation Commission, State of New Mexico; and

purportedly describe a series of events preceeding the explosion, as well as those occurring during and after exhumation of the pipe:

"At station 7+25 to 8+50, being in the lower portion of

the grade, a concentration of water collected during some

very heavy rainfall and I am informed that the contractor

was attempting to remove this very wet soil and in so doing

a piece of their machinery got too deep and struck the pipe."

"On inspection you will note that I have marked the top

of the pipe in red. Also, I marked a part of the pipe break

in red." (Author's note: The marked portion of the break

indicated that it had extended from the top of the pipe in a circumferential direction to about the mid-points of the

two sides of the pipe, in its installed configuration.)

"This, I am told by the workmen who removed the section

of pipe, was about the limits of the break; the remainder

of the break occurred when the pipe was removed from the

ditch."

"You will notice at the top of the edge of the coupling

a "nick" which was made by an outside force. Also, along the

top of the pipe on each section you will note a "scratch" or

abrasion showing on each of the sections we are sending you."

This laboratory was requested by the Office of Pipeline Safety to

evaluate the pipe for compliance to specifications, because the information quoted above from the letter of transmittal indicated that being

struck by construction equipment was the obvious cause of failure.

Consequently, the pipe was evaluated for compliance, and some additional

observations were made of the fracture surfaces.

- 2 -

Several weeks after the analysis had been completed, and a report

of test drafted for the Office of Pipeline Safety, the author was informed

by counsel for plaintiff in a lawsuit involved with the explosion that

testimony in earlier depositions indicated that the original crack, that

is, the one from which leakage occurred, was on the bottom of the pipe in

its original orientation. In addition, counsel for plaintiff showed this

author several photographs which had been included as exhibits in the

earlier depositions. Although all of the photographs were taken after

the pipe had been broken in two during removal from the trench, a careful

examination of them indicated conclusively to us that .the original

fracture was a transverse, or circumferential one, and that both sections

of pipe received by this laboratory had been further severely damaged at some time after the pipe was broken in two during removal from the trench.

This additional damage caused considerable difficulty in attempting to

determine the cause of failure, since it was in the critical area of the

original failure, and the information supplied in the original letter of

transmittal, i.e., that the original crack was on the top of the pipe 'in

the installed configuration, was erroneous. Independently, in this

laboratory, and prior to receipt of information that the crack had been

at the bottom of the pipe circumference, microscopic examination of the

surfaces of the transverse crack had indicated that the point of in-service,

fatigue-type failure of the pipe occurred at the bottom, if failure were

due to other than external mechanical damage, i.e., being struck by

construction equipment.

In reassessing the problem, the original package was carefully

examined. The pipe had been wrapped in several layers of brown kraft

paper, and mailed as "printed matter". At the time the pipe was received

in this laboratory, one end of the package had been opened. However, the

unopened end showed evidence of damage, including a hole in the package.

Therefore, it seemed apparent that the pipe was most likely damaged in

the mails, either as a result of being dropped, or crushed. When received

a portion of the pipe was missing, as noted above, and the hole in the

package was of sufficient size that the missing material could have fallen

out of the package.

On the basis of the above information, the original draft report of

test was rescinded, since much of it was concerned with the determination

of the possible cause of damage which occurred after the pipe was removed

from the trench, which therefore would be later information now indicated

to have been totally unrelated to the condition of the pipe at the time

of the accident. Figure 1 shows the pipe in the as received condition,

with the broken ends placed together. Point "A" indicates the location

of a nick in the outer edge of the coupling. Point "B" locates what

initially was shown as being the approximate midpoint of the original

crack, and was reported to be in the immediate area of the top of the

pipe in its installed configuration. Consequently, terms such as upper,

lower, and side, used in this report are in reference to locations on the pipe, with respect to point "B", as they would have been while the

- 3 -

pipe was in service. A longitudinal crack, between points "B" and "C", was one of three such cracks present in this section of the pipe when it was received in this laboratory, which were later found to have been

produced after the pipe was exhumed.

Description of Sample

Hie sample consisted of a section of bell-end pipe into which a section of straight pipe was cemented. In addition to the name of the

manufacturer, the pipe was marked with legends containing the following

information: 2" SDR21 200PSI WP @ 73°F PVC1120 CS 256-63. This indicated

the following: that the pipe had a nominal outside diameter of 2 inches;

that the standard thermoplastic pipe dimension ratio, i.e., the ratio of

the pipe diameter to the wall thickness, was 21; that it was pressure

rated for continuous use for up to 200 pounds per square inch of water

pressure at 73°F; that the pipe was produced from a compound based on

rigid poly (vinyl chloride) (PVC), Type 1, Grade 1, a normal impact

strength and high chemical resistance material, to provide a hydrostatic

design stress of 2000 pounds per square inch for water at 73°F; and that

the pipe conformed to the specifications and requirements set forth in

Commercial Standard No. 256' issued in 1963 by the National Bureau of

Standards, Department of Commerce, for 'Poly(Vinyl Chloride) Plastic Pipe

(SDR-PR and Class T) . The complete specifications for the basic PVC

compounds required to be used in pipe covered by CS 256-63 are included in

the Standard by means of a reference to the American Society for Testing

and Materials (ASTM) , Method D1784-69, Standard Specification for Rigid

Poly (Vinyl Chloride) Compounds and Chlorinated Poly (Vinyl Chloride)

Compounds, which fully describes them. Although this Standard was issued

three or four years after the pipe in question had been installed, pre- cursor Tentative Standards were in effect at the time the Commercial

Standard was issued and these contained the same fundamental physical

properties requirements for Type 1, Grade 1, PVC as the current issue,

although the latter now classifies this type of PVC as "Class 12454-B rigid

PVC." In addition to the above legends, each piece was marked with what

appeared to be an identification number, such as might be used to identify

a lot or batch of material. The bell-end piece was marked, A22E863,

and the straight piece, B17A85348.

The overall length of both sections of the pipe was about 160 cm

(63 in), with the bell-end section measuring about 77 cm (30 in). The

length of the straight pipe was 84 cm (33 in) exclusive of a section

about 5.7 cm long (2.5 in) inserted into the bell socket.

'Hie transverse crack in question was in the bell-end section, at

the lower half of the pipe, and was located along the bottom of the socket.

At nbout the midpoints of the sides of the pipe the crack curved away

from the socket base to point "B", in Figure 2, which was located about

H.25 cm (3.25 in) from the outer end of the bell socket, or approximately

2.3 cm (0.9 in) from the point where the outside diameter of the pipe

began to increase to form the bell socket.

- 4 -

Visual Examination

tl The visual examination was concerned with three areas of potential v i damage, including longitudinal abrasion marks along the upper surface w of the pipe, a nick on the outer edge of the bell socket near the top of c the pipe, point "A" in Figure 1, and the transverse crack surfaces. ;

In the vicinity of the coupling of the bell-end section to the

straight section of the pipe, there were a series of shallow abrasion

marks. These marks covered an area approximately one inch in width

along the top portion of the pipe. Because of the ductility of the plastic,

it could not be determined with certainty in which direction the abrasions

occurred, with respect to the nick in the end of the bell socket. The

nick on the outer edge of the bell socket was near some of the abrasion

marks on the straight section of the pipe. Whether there was any connection between these marks and the nick, could not be determined with

certainty. The nick which was approximately 0.5 cm (0.19 in) long in the

circumferential direction was not very deep. About 10% of the total wall

thickness had been removed at the socket entrance. Since the socket

appeared to be firmly cemented to the straight section of pipe, the

latter of which had been inserted about 5.7 cm (2.25 in) into the socket,

it seemed unlikely that the presence of the nick would have any detrimental

effect on the strength of the pipe at that point. Microscopic examination

of the surface of the nicked area indicated that it was unlikely that it was the result of a brittle fracture, such as one caused by a sharp blow,

but rather appeared to have been the result of a scraping action. Except

for a few particles of embedded soil, the remainder of the surface was relatively clean. However, there was nothing to indicate a specific

time at which this particular damage occurred, other than that it happened

sometime after the two pipe sections had been joined together. This

was indicated by the fact that the surrounding area was coated with a light grey pipe cement.

A microscopic examination of the transverse crack surfaces disclosed

an occluded particle embedded in the pipe wall near the bottom of the

pipe. This particle appeared to be a piece of sand and was slightly

less than 0.02 cm (0.01 in) in width and appeared to be essentially

spherical. Some discoloration in the pipe wall, as well as the presence

of other debris was noted in this area.

In order to observe these features more carefully, the straight

section of pipe was cut off, in the circumferential direction at a point

approximately 6.35 cm (2.5 in) beyond the outer edge of the bell socket.

In cutting the pipe, the particle was vibrated loose and was lost, leaving

a discrete cavity in the wall of the pipe. The bell-end section of the

pipe was also cut off in such a manner as to reveal the internal crack

surfaces of the longitudinal cracks, originally believed to have been part

of the overall failure of the pipe. The first transverse cut was made at

point "C" in Figure 1. Two additional cuts, nearer the socket end of the

section, were made in order to open up two shorter longitudinal cracks

along the sides of the pipe.

- 5

-

'Hie cavity left by the missing particle was located in the wall of

the pipe near the inner surface, nearly diametrically opposite point "B"

in F

: igure 1. The cavity was slightly less than 0.02 cm (0.01 in) in both

width and length. Its depth indicated that most of the particle had been

embedded in the wall on the socket side of the crack, see Figure 3. Only

a shallow matching cavity was observed on the opposite crack surface.

The surface of the pipe wall surrounding the cavity, on the socket side,

exhibited a brownish discoloration that appeared to be rather typical of

discoloration due to thermal degradation of the material in the pipe

compound. This discoloration was in contrast to the dark grey color of

the pipe. An alternative cause of the discoloration could have been the

presence of a foreign material capable of compatible fusion with the PVC

pipe compound, but this is an unlikely situation.

Approximately 1 cm (0.4 in) further along the bottom of the pipe,

there was another area near the inner wall that was discolored and pitted.

However, it could not be determined by microscopic examination if there

were any discrete particulate matter within the pits, or whether the discoloration was due to the presence of other foreign matter or to the presence

of degraded pipe compound. Immediately adjacent to the shallow cavity

formed by the occluded particle on the opposite surface of the crack, there

was a small area, extending from the inner surface of the pipe wall to

approximately the mid-wall of the pipe, which appeared to be coated with

rust. On the surface of the crack containing the particle, a thin reddishbrown line containing some small particles of debris extended for about

1.9 cm (0.75 in) in the circumferential direction along the mid-wall of

the pipe, see Figure 3. This line began just opposite the rust-like stain

and progressed upward toward the side of the pipe. These small particles

although adhering to the surface of the crack did not appear to have been

molded into the pipe compound. Further up the side of the pipe another

thin line of a lighter brown stain was observed. This stain was about

0.75 cm (0.3 in) in length and was located in an area about one-third of

the way through the pipe wall, as measured from the inner pipe surface.

A similar though much lighter brown stain, about 0.63 cm (0.25 in)

in length and containing no apparent particulate matter, was noted near

the mid-wall on the opposite side of the area containing the occluded

part ic le

There were several small cavities in the crack surface, in the area

of the occluded particle, distributed in a random pattern. A matching

pattern of cavities was located on the opposite crack surface. Since

these cavities were essentially clean, apparently contained no particulate

matter and were not associated with any apparent discoloration of the pipe

compound, they were considered to be indicative of the presence of small

voids, or bubbles, in the interior of the pipe wall. If it were assumed

that the discoloration previously noted in the pipe wall was due to thermal

degradation, then the presence of these voids could be explained by related

overheating of the pipe compound at a temperature not quite sufficient to

result in discoloration of the pipe compound in these areas.

- 6 -

Vertical striae were observed on the surface of the crack extending

from the inner surface to approximately the mid-wall of the pipe. This we' area extended for about 3 cm (1.2 in) in one direction from the occluded c> particle including the area containing the small cavities, and for about ot 0.5 cm (0.2 in) in the other direction to the beginning of the thin line Kx containing the particles of apparent debris. These striae are typical oi

indications of a stress crack. d

a There was an excess of pipe cement on both sides of the crack,

extending away from the bell end and also into the straight section of

pipe inserted in the socket. However, it did not appear that the cement

was present in such quantity that residual solvent would have had a detrimental effect on the strength of the pipe. In fact, the visual

evidence available indicated good workmanship in the assembled joint. In

addition, assuming that the cementing of this coupling was performed with

the pipe sections in the trench, a fairly common installation procedure,

then the presence of the excess cement at this location would definitelyestablish the fact that it was at the bottom of the pipe in its installed

configuration

Compliance Tests

The pipe was tested for compliance with several sections of CS 256-63,

for SDR21 pipe, with respect to dimensions, resistance to flattening, and

for extrusion quality, as required for new pipe. Since each of the two

sections of pipe were marked with different code numbers, as described

above, these series of tests were conducted on specimens from each section.

Two specimens approximately 5 - 6.4 cm (2 - 2.5 in) in length were

cut from the outer ends of each of the two pipe sections. The ends of the

specimens were squared off and smoothly finished. Measurements were then

made of the outer diameter and wall thickness. The specimens were not

checked for compliance to the requirements for out-of-roundness , since

these are applicable only to pipe prior to shipment from the point of

manufacture. The specifications were met with the exception that each

of two sets of measurements on the straight section of pipe indicated that

the minimum wall thickness was 0.003 inch less than that specified

although the average wall thickness was within specifications. However,

this could be attributed to the uncertainty of the measuring procedures.

The type of micrometer specified in the standard for making these measurements was not used. A substitute instrument of less accuracy was used.

Consequently, considering these factors, plus the actual application of

the pipe, this apparent failure to meet specifications was considered

inconsequential with regard to the strength of the pipe.

Following the dimensional measurements, the same specimens were subjected to the resistance to flattening test. This test requires the

specimen to be slowly crushed between two parallel plates until the distance between the plates is equal to 40 percent of the measured outside

diameter. None of the specimens exhibited any visual evidence of cracking,

crazing, or splitting that would have indicated poor quality or the possible

onset of degradation due to aging.

- 7 -

Two additional specimens approximately 0.6 cm (0.25 in) in length

were removed from the outer ends of each pipe section for use in the

extrusion quality test. The test method referenced in CS256-63 is that

of the ASTM Method D2152-67, Standard Method of Test for Quality of

lixtruded Poly (Vinyl Chloride) Pipe by Acetone Immersion. On completion

of' the test the specimens exhibited no evidence of swelling, flaking or disintegration, indicating that the compound had been properly fused

during the molding operation.

It was not possible to check the pipe for notched impact strength,

since this test requires the use of flat specimens. The current ASTM

Standard for Type 1, Grade 1 PVC requires a minimum notched Izod impact

strength of 0.65 foot-pounds per inch of notch. Although there is a current ASTM Specification for bell-end PVC pipe containing requirements

for dimensions of the bell socket, with respect to wall thickness,

length, and entrance and bottom diameters, CS256-63 requirements were

limited to straight pipe. As a result, no attempt was made to determine

these dimensions, except on a cursory basis.

On the basis of an infrared spectrographic analysis, the pipe

compound was found to be based on a PVC homopolymer.

Conclusions

The presence of the sand-like particle embedded in the pipe wall,

and located at the bottom of the pipe, together with the presence of

apparently degraded pipe compound, was considered to be the most probable

initiation source of a stress crack. Such a stress crack could be initiated

by various combinations of external forces, including normal expansion

and contraction of the pipe, compaction of surrounding soil, or compressive

forces at ground level that would result in the development of tensile

stresses on the inner surface of the bottom of the pipe wall.

The presence of the stain, apparently rust, adjacent to the location

of the occluded particle, and the presence of debris in the reddish-brown

line adjacent to the particle, strongly suggest that there was a stress

crack in the inner wall of the pipe extending from the inner surface to

approximately the mid-wall of the pipe, and that it was undoubtedly present

for some extended period of time. The crack was located in such a position

that it probably would not have been observed in a cursory visual examination of the interior of the bell socket of the pipe. The absence of any

pipe cement inside the crack indicates that it developed sometime after

the two sections of pipe had been assembled. Voids in the area of the

stress crack could reasonably be expected to result in additional weakening

of the pipe wall. Other stained areas observed on the surfaces of the

cracks seemed to indicate the possible presence of degraded pipe compound.

All of these factors could result in a gross weakening of the pipe wall

near the base of the bell socket, particularly in the area along the bottom

of the pipe in its installed configuration. Absence of similar debris

and rust stains, on the crack surfaces, between the mid-wall and the

outer wall indicated that failure in this area occurred at a later period

in time, probably as a result of some applied external force which caused

the crack to propagate to the outer wall. The relatively smooth fracture

- 8 -

surfaces in this area, as opposed to striated surfaces in the stress

crack area, suggested the possibility of a brittle fracture which may have

occurred rather suddenly.

This analysis indicates that the strength of the pipe was reduced

by the presence of a stress crack along the inner surface of the bottom

of the pipe. The stress crack appeared to have been caused by flaws

introduced into the pipe during the molding process. Subsequent external

forces of undetermined origin caused failure of the pipe wall and the

resultant gas leak.

Figure 1. Illustration of the fracture area on the pipe,

as received. "A" shows location of a nick on the

outer edge of the bell socket; "B", the apparent

terminal point of an accidental fracture; and "CM

, the terminus of a longitudinal crack on the outer

surface of the pipe due to subsequent damage in

shipment
V
„^-„^,„..^,J,„..„^. — • «—

-'V

B

\

\

Figure 2. Illustration of the upper part of the pipe, in

the vicinity of point "B".

Figure 3. Illustration of crack surface on socket side near

bottom of pipe. Arrow shows location of cavity in

which the occluded particle had been embedded.

The dark line near the mid wall of the socket is

that containing rust-like particles and stains.

MA ("IV. 7-73) DEPT. O F COMM.

ilBLlOGRAPHIC DATA

SHEET

1. PUBLICATION OR HI. PORT NO.

NBS 1 R 7^-^92

2. Gov't Accession

No.

3. Recipient's Accession No.

Tl'l-E AND SUBTITLE

failure Analysis of a Poly(Vinyl Chloride) Natural

Gas Main Pipe from Las Vegas, New Mexico

(NBS Failure Analysis No. 105)

5. Publication Date May 197^

6. Performing Organization Code

rUTHOR(S)

amuel D. Toner

8. Performing Organ. Report No.

PERFORMING ORGANIZATION NAMF AND AfinRFSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projecr/Task/U'ork Unit No. 3120*418 (4103)

11. Contract/Grant No.

[ ^pon s or 1 0£ Or£ on i z sit i on Name an d Completer Address free f Cj 5tsto ZIP)

Office of Pipeline Safety

National Transportation Safety Board

Department of Transportation

Washington. D.C. 20590

13. Type of Report & Period

Covered

Fi na 1 14. Sponsoring Agency Code

5. SUPPLEMENTARY NOTKS

i. ABSTRACT (A 200-word or Jess (actual summary ol most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A natural gas leak from a polyvinyl chloride) (PVC) gas main was

considered to be the attributable cause of a fatal explosion in a private

residence in Las Vegas, New Mexico. An analysis of the pipe was conducted

in an effort to determine the possible cause of failure. The results

indicated that failure occurred as a result of weakening of the pipedue to

the presence of a stress crack initiated by an occluded particle, which had

apparently been molded into the pipe wall during its fabrication.

7. KFY WORDS (six to twelve entries; alphabetical order; capitalize only tfie 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

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