NBSIR 74-493

Failure Analysis of a Poly (Vinyl Chloride) (PVCj

Plastic Natural Gas Transmission Pipe From

Las Vegas, Nevada (NBS Failure Analysis No. 106J

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„ 29590

NBSIR 74-493

FAILURE ANALYSIS OF A POLY (VINYL CHLORIDE) (PVC)

PLASTIC NATURAL GAS TRANSMISSION PIPE FROM

LAS VEGAS, NEVADA (NBS Failure Analysis No. 106)

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. 29590

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

A section of cracked poly(vinyl chloride) (PVC) plastic

natural gas transmission line was submitted to this laboratory

for the purpose of conducting a failure analysis. The analysis

was requested by the Office of Pipeline Safety, National Transportation Board, and the sample was received through the NBS

Metallurgy Division. The pipe, according to information supplied

in an accompanying letter from the Public Service Commission of

Nevada, had been located beneath the intersection of Commerce

Street and Oakey Blvd., Las Vegas, Nevada. Gas leaking from the

crack was determined to be the cause of an explosion and fire in a nearby section of the Monte Carlo Motel on June 2h , 1973.

Description of the Pipe

In addition to the manufacturer's tradename, the pipe was marked with the following legend: 2" IPS SCH A PVC2110 88 PS I WP (3 73. -nSf- CS 237-61. This translates as follows:

nominal 2- i nch O.D.; Iron Pipe Size; lightweight pipe; produced

from rigid poly(vinyl chloride), Type 2, Grade 1, (high impact,

normal chemical resistance); with a hydrostatic design stress

of 1000 ps i ; pressure rated for continuous use to 88 ps i water

pressure at 73. ^°F; conforming to the requirements of the National

Sanitary Foundation for use with potable water; and conforming to

Commercial Standard No. 237, issued in 1961 by the National Bureau

of Standards, for Dimensions and Tolerances of Schedule A, Type 1 and Type 1 1 PVC p i pe

.

The sample was a composite consisting of two sections of pipe,

one about 32 cm (12.5 in) long in which the crack was located,

and one about 132 cm (52 in) long, both of which had been joined

by means of a 7.6-cm (3-in) long, in-line socket fitting. The

short section also had attached to it a metal nut, a metal seal

retainer, and a rubber seal located about 18 cm (7 in) from the

socket. These components were part of a typical compression

coupling, reportedly used to make an in-line attachment of the PVC

pipe to a 5-cm (2-in) metal pipe. Figure 1 shows the pipe in the

"as received" condition, although it does not include all of the

attached long section of the pipe. A 20-cm (8-in) long section

of steel pipe had been inserted into the coupled end of the

plastic pipe as a reinforcement. Several turns of a black pipe

wrap had been applied over the nut and tapered along the adjacent

four inches of the plastic pipe. In the process of disconnecting

the coupling, the pipe wrap covering the nut had been completely

severed from that on the plastic pipe. The visible section of the

crack, which was reported to have been on the top of the pipe in

its installed position, extended for about 3-8 cm (1.5 in) from

the wrap towards the socket fitting.
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Figure 2 is a close-up illustration of the crack, in the "as

received" condition. There was also a crack in the pipe wrap,

which is shown in this figure. However, it was later found, on removal of the tape wrap, that the crack in the pipe wrap was superficial and had no apparent bearing on the failure of the pipe.

The long section of the pipe had been permanently deformed

in the downward direction with respect to the location of the crack.

The total deflection at the outer end, away from the socket, was

on the order of 8 cm (3 . 1 5 i n) . This downward bending of the

pipe appeared to be a typical example of creep deformation and

would be expected to result in an increase in tensile stress on

the upper surface of the pipe, near the coupling, where the plastic

pipe presumably was rigidly constrained against bending by the

metal insert. At several places on this section of the pipe there were chunks of very hard, clay-like soil firmly adhered to the

outer pipe wall. Consequently, it v;as surmised that deformation

may have been caused by forces exerted on the pipe, over a long

period of time, probably due to the expansion of the soil whenever

it became wet. Although the installed depth of the pipe was not

definitely stated, from the information supplied, it apparently

was approximately 1 m (3 ft) below the pavement. Consequently,

any expansion of the soil in the upward direction would have been

limited by the presence of the pavement, thereby increasing the

amount of downward (compressive) forces on the pipe, and resulting

in the deformation. Lateral expansion forces could reasonably

be expected to have little, or no, significant effect on the pipe.

There was a possibility that the amount of deformation in the sample

as received in this laboratory was less than that in the actual

installation. The end of the long section of the pipe was badly

damaged. Visual examination indicated that it had been manually,

or mechanically broken off by an upward force applied to the pipe,

when it was removed from the ground. This rather poor technique

of removing the sample could have resulted in some straightening

of the deformed section of the pipe.

Following the initial visual examination, the cracked section

of pipe was cut off at the socket. Starting at the point of emergence of the crack, a half-inch wide longitudinal strip of the

pipe wrap was removed, revealing that the crack extended to within

a fraction of an inch of the compression fitting and was approximately 14.5 cm (5-7 in) in total length. The crack was essentially

straight in the longitudinal direction, with two exceptions noted

below. Following the examination of the crack and surrounding

pipe surface, the remainder of the pipe wrap was removed, essentially in one p i ece

.

Figure 3 is an illustration of the crack between points A

and B, after the initial cutting away of part of the wrap. The

following are combined observations made after initial exposure of

the crack and after complete removal of the wrap:
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1. The pipe beneath the wrap was found to be discolored

and swollen. In contrast to the light grey color of the

pipe, all portions of the outer surface of the pipe,

in direct contact with the wrap, were dark brown. Those

areas not in direct contact with the wrap, i.e., where

overlaps between turns occurred, were light green in

color. An example of this can be seem in Figure 3- The

light colored, roughly longitudinal linear area passing

below the letters C and D and extending to the area of

point E resulted from lack of contact of the wrap with

the pipe surface at the point where the second turn of

the wrap passed over the starting end. The surfaces of

the brown areas were slightly raised with respect to

that of the green areas, indicating that the former were

subjected to a greater degree of swelling.

2. There was a slight deviation in the linearity of the

crack, in the form of a small arc, about 0.6 cm (0.25 in)

from the end nearest the coupling, at point E in Figure 3-

3. The point of maximum swelling of the pipe occurred just

beyond the arc on the side away from the coupling, i.e.

just to- the left of point E in Figure 3.

k. At about the midpoint of the length of the crack there

was a dislocation in the form of a short, curved trans- verse crack approximately 0.08 to 0.10 cm (0.03 to 0.04 in)

in length at point D in Figure 3-

5. Towards the outer end of the wrapped area, at a point

just beyond the inner end of the metal pipe insert, at

point C in Figure 3, there appeared to be two or three

grains of sand, embedded deep in the crack. These

particles protruded through the inner wall of the pipe,

and felt hard and rather sharp to the touch.

6. Pressure exerted by the coupling had permanently com- pressed the pipe, to the extend that the O.D. was approximately 0.1 cm (O.Ok in) less than the average

O.D. of the pipe, in the area indicated by F in Figure 3-

7. There was a ballooned area on the bottom of the pipe,

directly opposite of, and in the general area delineated

by points C and D in Figure 3 of the crack. It was centered about 9 cm (3-5 in) from the. coupling, and

ranged from about 3-2 to 3-8 cm (1.25 to 1.50 in ) in

diameter.

Examination of the Pipe Wrap

The pipe wrap was a composite consisting of a thin, essentially

transparent, backing film which had been coated with a relatively

thick, black, bitumen-like "adhesive" layer of material. Infrared
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analysis indicated that the backing film was a poly(vinyl

chloride/vinyl acetate) ( PVCA) copolymer. The "adhesive" layer was composed predominantly of the same copolymer with some unidentifiable additives. There was visual evidence, in the form

of brush marks on the surface of the "adhesive" and wrinkling of

the backing film, that the pipe wrap had been treated with a primer prior to its application to the pipe and coupling, and that

the materials in the wrap had been subjected to an active solvent.

At the request of this laboratory, information was subsequently

received from the Public Service Commission indicating that a primer, of unknown composition, was normally used to activate

the surface of the adhesive layer. Acetone, or one of its higher

boiling homologs, notably methyl ethyl ketone, is frequently used

in this type of application since these ketones readily soften or dissolve PVCA copolymers, but have essentially no solvating action,

at ambient temperatures, on rigid PVC homopolymers the types

used in pipe extrusion compounds. The use of any solvent-based

primer in an application such as this could be expected to result

in some permeation of solvent, or solvent vapor, into the pipe

wall. Since specimens of this pipe failed the acetone immersion

test which is discussed below, it is reasonable to assume that

if ketone solvents, or any material with similar solvating action

toward unfused PVC were present in the primer, normal permeation

of such materials into the pipe wall would be expected to cause some swelling and also enhance migration, into the pipe wall, of

soluble pipe wrap components. Such actions, if detrimental to

improperly fused pipe could reasonably be expected to result in

the swelling and staining noted in this sample. Thus, the brown

stains were probably due to direct solvent action where the tape

was in contact with the outer pipe surface, and the green stains

to solvent vapor action. The maximum degree of swelling, believed

to have been caused by the pipe wrap and excluding the ballooned

area on the bottom of the pipe, occurred in the area just to the

left of point E in Figure 3, which could be expected since this area was covered with the maximum thickness of the tapered wrap,

and consequently was undoubtedly subjected to the highest concentration of solvent and solvent vapor, for the longest period of

time. The maximum degree of out-of- roundness was on the top of

the pipe, just beyond the small arc in the crack. Swelling caused

the inner wall of the pipe, particularly the upper half, to pull

away from the metal pipe insert, reducing its effectiveness as a reinforcing agent. This could be expected to result in the swollen

portions, i.e., along the top, of the pipe being subjected to

higher than normal tensile stresses due to the previously noted

deformation of the adjacent longer section of the pipe. It also

seems obvious that the previously noted reduction in the pipe

diameter due to the adjacent compression fitting would induce

inherent compression stresses that could feasibly produce a synergistic effect, in conjunction with adjacent tensile stresses,

in reduction of the strength of the pipe. Since the above types
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of solvents are relatively volatile, it would appear most likely

tliat the swelling began to occur almost immediately after the

wrap had been applied; and as a result, swelling may well have

occurred prior to the initial pressur i zat i on of the pipe.

Microscopic Examination of the Crack

In order to study the surfaces of the crack, as well as the

inner wall of the pipe, the cracked section was opened by making

transverse cuts through the pipe at points A and B (Figure 3),

and a longitudinal cut opposite the crack, on the bottom of the

pipe. Microscopic examination of the crack surfaces indicated

that a stress crack probably was initiated at the small arc,

point E (Figure 3)) which was located between the point of maxi- mum swelling and the area that had been compressed by the coupling

seal. However, microscopic examination of the surface and

surrounding areas of the small transverse crack, located near the

midpoint, point D (Figure 3) » indicated that this was also a possible point of a stress crack initiation. There was also the

possibility that the transverse crack was a junction of two adjacent^ parallel, slightly overlapping, longitudinal cracks. A small

piece of the outer pipe wall was missing at this point. In addition, there was a small indentation at the inner wall that

could have been due to the presence of a void or a foreign

occlusion. This indentation was approximately equal in width to

the length of the transverse crack.

The small sand- like particles, observed embedded in the crack,

fell out and were not recovered during attempts to remove the

metal pipe insert. The insert was quite difficult to remove due

to the extremely tight seal, affected by the compression seal,

between it and the plastic pipe. However, the location of the

particles had previously been marked on the outer pipe surface..

Subsequent microscopic examination of the crack surfaces revealed

the presence of two discrete cavities on each side of the pipe

wall at and near the inner surface. The sharpness of detail of

the cavities indicated that they were apparently present in the

molding compound at the time the pipe was extruded, as opposed to

having fallen into the crack after it had formed. Examination of

the crack surfaces surrounding the pits did not reveal any visual

evidence that their presence resulted in the initiation of a stress crack, but their presence could reasonably be expected to

be conducive to propagation of a crack, initiated at some other

point in the vicinity of the particles.

The ballooning observed in the bottom wall of the pipe was apparently caused by use of an excessive amount of pipe cement

in joining the cracked section to the socket. Cement had flowed

along the bottom of the pipe and collected in a pool between the

metal insert and the inner wall of the plastic pipe. In addition

excess cement had flowed into the metal insert for approximately

11 cm (^.5 in). Analysis of this material indicated that it was a PVC
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homopo lymer . It is not unusual for this type of cement to contain

(tyc 1 ohexanone and/or dimethyl formamide as active solvents for the

PVC homopolymer pipe compound. Such solvents attacl< or partially

solvate the pipe surface, ensuring a strong bond between pipe and

fitting. However, these solvents have low volatilities and if

entrapped, such as between the metal insert and the plastic pipe,

would tend to migrate into the plastic and soften it. Therefore,

it is highly possible that ballooning occurred during the initial

pressure testing of the pipe assembly, even though this may have

occurred up to several days after installation. Under such

circumstances, immediate rupture of the pipe might have been prevented because: 1) the test pressure was too low, with respect

to the degree of softening of the pipe; or 2) the pipe wrap

exerted sufficient counterpressure to limit the degree of

ballooning. Although the ballooning was the obvious result of poor

assembly techniques, there seemed to be no direct connection

between it and the general swelling of the remainder of the wrapped

area presumed to have been due to the components of the wrap or wrap primer. Although the general swelling of the upper half of the

pipe could have been caused by solvent vapors from the pipe cement,

which usually consists of about 75 percent solvent, this possibility

was essentially ruled out after visual inspection of the inner

surface of the pipe, and examination of cross sections of the pipe

wall. These inspections revealed that both the inner surface and

the pipe wall were light green in all areas opposite the brown-stained

areas on the outer surface of the pipe, and that those areas of

the inner surface, opposite of the light green areas on the outer

surface, were unchanged in color from the natural light grey color

of the pipe. This indicated staining from the outside of the pipe,

i.e., due to solvent from the pipe wrap primer. Whereas, the solvent used in the pipe cement, excess of which was present within

the socket fitting and along the bottom of the short, cracked section

of the pipe from the butt weld within the socket to the wrapped

area of the pipe, caused no discoloration of the inner wall of the

pipe.

Properties of the Pipe

The only requirements in CS 237~6l , that were applicable to the

size of the sample and could be checked on the finished pipe,

involved dimensional requirements and chemical composition of the

basic pipe compound. The pipe was found to comply with the dimensions and tolerances set forth in CS 237-61. Qualitative infrared

spectrograph i c analysis indicated that the pipe compound was based

on a PVC homopolymer and probably contained an aliphatic hydrocarbon, elastomeric polymer, such as po lybutad i ene , to act as an impact modifier. This analysis indicated that the pipe was fabricated from the stated Type 2, Grade 1 PVC based compound.
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Although not required in CS 237~6l , the pipe was subjected to

flattening tests for resistance to cracking, and to an extrusion

quality test for PVC pipe, both of which are tests included in

current plastic pipe specifications, issued either as ASTM Standards

or as NBS Voluntary Product Standards. Test specimens passed the

current requirements for the flattening tests, but failed the

extrusion quality tests. The former test requires that the pipe

specimen exhibit no splitting, cracking or related defects, when

compressed to kO% of its actual outside diameter. The latter test

is currently used to determine whether the extruded pipe compound

Is properly fused, by immersing specimens in anhydrous acetone for

20 minutes at room temperature. Although its use is limited to

certain types of PVC pipe only, it includes the type of compound

used in this pipe. Properly fused PVC homopolymer pipe compound

will show no evidence of splitting, cracking, flaking, or the like on completion of this test. However, when specimens of this pipe were subjected to this test, the central portion, representing

about one-third of the total wall thickness, was rapidly disintegrating within 6 to 8 minutes, followed by further gross cracking

and splitting of the material comprising the remaining inner and

outer portions of the pipe wall. To the best of our knowledge,

Including Information In the latest ASTM version of this test,

there Is, as yet, Insufficient data to provide a correlation between

failure In this test and other physical and chemical properties of

PVC pipe.

Concl us ions

On the basis of this analysis, it Is apparent that the failure

of the pipe was due to a combination of several factors. The pipe

was chemically attacked by one or more components In the pipe wrap

or primer, resulting in swelling of the pipe. The swelling appeared

to have been enhanced by the apparent poor quality of the extruded

material, as Indicated by Its poor resistance to acetone. Separation of the swollen pipe from the metal Insert, particularly along

the top of the pipe, reduced the effectiveness of the insert in

preventing undue strain on the plastic pipe adjacent to the coupling,

such as the obvious tensile stresses created along the upper portion of the pipe by the nearby creep- type deformation of the long

section of pipe. The nature of the stress crack indicated, to a high degree of probability, that embr I tt lement resulted, in part,

from fatlque stresses, such as those due to expansion and contraction of the soli, and to vibrations from overhead traffic, on the

excessively swollen area of the wrapped section of the plastic

pipe, immediately adjacent to the affixed, rigid coupling. In addition, the presence of a small cavity in the Inner wall, at the

small transverse crack, near the middle of the longitudinal crack,

whether caused by an occluded particle, or not, as well as the

cavities produced by the presence of the occluded sand- like particles, near the outer end of the crack, could have contributed to
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crack propagation. In both cases the presence of the cavities

effectively reduced the wall thickness, and general experience

indicates that the consequence would be a significant reduction

in the mechanical strength of the pipe.
Figure 1. Damaged section of pipe, including

attached components of coupling, pipe

wrap, and socket fitting. Point A is

outer end of visible stress crack in the

pipe; point B is outer limit of crack in

pipe wrap tape.