NBSIR 78-1491

TESTS RELATING TO A PLASTIC GAS

PIPE/METAL COUPLING PULL-OUT

FAILURE IN LAWRENCE, KANSAS

Samuel D. Toner

Product Safety Technology Division

Center for Consumer Product Technology

May 1978

This is a final report

Report to Pipeline Accident Division

National Transportation Safety Board

Washington, D.C. 20591

U.S. DEPARTMENT OF COMMERCE, Jusnita M. Krsp§, Secretary

Dr. Sidney Harman, Under Secrstery

Jordan J. Baruch, Assistant Sscretsry for Science end Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambles, Director

TESTS RELATING TO A PLASTIC GAS PIPE/METAL COUPLING

PULL-OUT FAILURE IN LAWRENCE, KANSAS

The Pipeline Accident Division, National Transportation Safety Board,

requested that the Product Safety Technology Division conduct a series of

tests on a polyethylene natural gas main. Of particular interest was an evaluation of the axial stress required, under various conditions of tests,

to pull sections of the plastic pipe out of a metal transition coupling. The

results of these tests would subsequently be analyzed by the Pipeline

Accident Division to determine whether current regulations governing

installation of gas piping systems should be revised to reduce the accident

potential of those systems involving combinations of plastic and metal

components.

On December 15, 1977, an explosion and fire occurred near 747

Massachusetts Street, Lawrence, Kansas, resulting in two fatalities. The cause of the explosion was reported to have been due to a gas leak resulting

when a section of polyethylene gas main pulled out of a metal coupling. The

2-inch (5-cm) plastic pipe had been installed by the insertion renewal method

into a section of an existing 3-inch (7.6-cm) IPS metal gas main. The

section of plastic pipe in question, had been attached to an adjacent section

of the 3-inch (7.6-cm) metal main by means of a standard Dresser Style 90

compression coupling.

The Pipeline Accident Division had requested the Kansas Public Service

Company, Inc., 73 Massachusetts Street, Lawrence, Kansas, to submit for

tests, the coupling assembly, the section of plastic pipe that had pulled out

of the coupling, and an additional 20 feet (6.1 metres) of plastic pipe from

the same installation. These materials, as well as some other items for use

in this investigation, were delivered in January 1978 to the Structures,

Materials and Safety Division of the National Bureau of Standards, by a representative of the Kansas Public Serice Company, and were subsequently

received in this laboratory on February 24, 1978.

Materials

Among the items received from the accident site were the following:

1 . The section of 2-inch (5-cm) plastic pipe containing a metal insert

in one end, which was identified as having been attached to the

metal coupling. The pipe had a Kansas Public Service Company

identification tag attached to it. A legend imprinted on the pipe

wall, which was partially decipherable, read as follows: "IPS SDR11

DUPONT ALDYL

1

A (..?..) T0307733".

2. The end of the metal gas main into which the plastic pipe had been

inserted. This section was about 11.75 inches (29 cm) in length,

and had been attached to the plastic pipe by means of pipe wrap tape

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at a point approximately 13-15 inches (33-33 cm) from the pipe end

containing the insert.

3. The Dresser coupling into which the plastic pipe had been inserted.

The coupling was still attached to a section of the original metal

gas main by means of a short length of 2-in (5-cm) metal pipe. The

nut on the plastic pipe end of the coupling was loose, and on removal the presence of a metal gasket retainer and a rubber gasket

was noted. The rubber gasket bore the following identifying legend:

"Dresser 2 ID No. 11D0237 GRADE 27 M- 13837 -10- AZ HC". 'The gasket,

which was 0.75 inch (1.9 cm) in length, exhibited evidence of

permanent compressive deformation on its outer circumference at

approximately 0.19 - 0,25 inch (0.48 - 0.64 cm) from the outer face,

i.e., in the area where the gasket appeared to have been in contact

with the outer rim of the coupling barrel

.

4. Six pieces of 2-inch (5-cm) plastic pipe, each about 3 feet (0.9 m)

in length, which had been removed from inside the metal gas main.

These sections bore the following legend: "ASTM D25 1 3 2" IPS SDR11

DUPONT ALDYL

k

A PE2306 T0307J33". This indicated that the pipe

conformed to the requirements of the American Society for Testing

and Materials (ASTM) Method D25 1 3 , Standard Specification for

Thermoplastic Gas Pressure Pipe, Tubing, and Fittings for outside

diameter controlled, nominal 2-inch (5-cm) diameter iron pipe size,

and had a Standard Dimension Ratio of 11 (the quotient of the

nominal O.D. divided by the minimum wall thickness). The legend

also indicated it was manufactured from E.I. DuPont de Nemours and

Company Aldyl KA medium density polyethylene pipe compound conforming

to ASTM Grade PE;^and was rated at a hydrostatic design stress of

600 psi (414 x 10

4 Pascals).

In addition to the above items from the accident site the following were

also received for use in the individual tests: a new Dresser Style 90

coupling complete with gaskets, gasket retainers and nuts; a 5-inch-long 0 3-

cm) straight metal pipe insert; and several additional rubber gaskets. All

of the rubber gaskets contained a metal bead which had been partially

embedded around the circumference of the inner end of the gasket prior to

vulcanization. The metal bead consisted of four pieces, each approximately

1.75 - 1.88 inches (4.4 - 4.8 cm) in length, of tightly spiraled, 0.125-inchdiameter (0.3-cm) coils of metal wire. All of the new gaskets, including the

two in the coupling, were marked as follows: "Dresser 2 I.D. GASKET No.

11D0237 GRADE 27 OLD No. 7164-27 HF".

The Pipeline Accident Division provided, in addition, a slip-proof pipe

grip assembly for use in attaching one end of the plastic pipe specimen to

the load cell of the Universal Testing Machine to be used in the pull tests;

a special wrench for torquing the compression nut on the coupling; and a special locking-type Dresser Style 90 coupling for use in sealing off one end

of a pipe specimen when being pressure tested to failure.
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on of Failed Section of Pipe

This section of pipe had been identified as that which was attached to

the coupling at the accident site. Figure 1 is a photograph of the pipe

section. The overall length was of the order of 21.5 inches (0.55 m) . The

pipe was permanently bowed to the extent that when a straight edge was laid

across the ends of the pipe, on the inside of the bow, there was a gap of

about 0.3 inch (0.76 cm) between the straight edge and the outer pipe wall,

at the midpoint of the length. Similar bowing was observed in the other

sections of pipe that had been received for tests, and was consistent with

the fact that the pipe had been coiled after its manufacture.

Approximately 8 inches (20 cm) of the left end of the pipe, as viewed i; the figure, had been inserted into the original metal gas main. Some of the

pipe wrap tape used to attach it to the metal pipe is visible. The right enc of the pipe had been inserted into the coupling. This end contained a 5-incl

long (12.7 cm) straight metal pipe stiffener. The end of the pipe had not

been cut square, and gave the appearance of having been cut with a carpenter

saw or hacksaw. As a result, when the metal stiffener had been inserted

until the outer end was flush with the end of the pipe on the inside of the

bow, the opposite side of the stiffener protruded about 0.19 - 0.25 inch

(0.48 - 0.64 cm) beyond the end of the plastic pipe. Visual examination

indicated that at the time of installation about 2.8 inches (7.3 cm) of the

pipe had been inserted into the gasket and coupling barrel.

The original plastic pipe was tan in color; however, that portion

between the original metal gas main and a point near the end that had been

inserted into the coupling, was discolored. The discoloration was

perceptibly darker in the region between the metal main and the apparent

initial coupling point, roughly defined by the location of the pieces of tape

adhering to the pipe, as shown in figure 1. The discoloration was not

particularly uniform, appeared to be greater on what was believed to be the

top of the pipe in its installed configuration, and did not appear to occur

in areas protected by the pipe tape and/or its adhesive. The other sections

of pipe submitted for tests had been installed in the metal gas main and

exhibited no evidence of discoloration. The discoloration appeared to be

strictly a surface phenomenum, and probably was caused by the presence of

some unknown constituent in the surrounding soil.

Of particular interest were a series of scratch-like marks in the outer

wall of the pipe that were essentially parallel to one another, perpendicular

to the length of the pipe, and located in the area along the inside of the

bow in the pipe. These marks can be seen in figure 2. The mark indicated by

the arrow on the left was about 2.8 inches (7.3 cm), and that on the right,

about 1.25 inches (3.2 cm) from the end of the pipe, as measured along the

inside of the bow to the point where the metal stiffener was flush with the

end of the pipe. The actual orientation of this section of pipe with respect

to the installed configuration had not been indicated, however, a considerable amount of debris was adhering to a portion of the inner wall of

the metal insert and the adjoining inner wall of the plastic pipe,
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approximately opposite the descriptive legend imprinted on the pipe.

Assuming that this debris collected in the bottom of the pipe while it was

still in the ground, then the markings shown in figure 2 would have been

along the side of the pipe. There were ten visible marks on the pipe,

separated from each other by an average distance of about 0.19 inch (0.48

cm). The two outer marks indicated by the arrows were approximately 1.25

inches (3.2 cm) in length, while the eight intermediate marks were of the

order of 0.75 inch (1.9 cm) in length. Microscopic examination of these

marks indicated that in proceeding from left to right in figure 2, the first

five and the last three marks were due to gouging of the plastic and cold

flow of the plastic toward the end of the pipe. It is believed that these

eight marks were caused by the pipe being thrust against a metal component of

the coupling, either the inner edge of the nut, or, more probably, the metal

gasket retainer. The sixth mark showed evidence of having been caused by

indentation of the plastic by the metal bead on the gasket, 'while the seventh

mark consisted only of a slight depression of the surface, with no unusual

features. No similar marking was observed anywhere else on the outer

circumference of the pipe wall on that portion believed to have been

originally inserted into’ the coupling. The surface of the plastic within the

first five scratches, from the left, was definitely discolored in the same manner as the surrounding areas of the pipe wall. The remaining marks were

in an area in which the surface discoloration was somewhat mottled in

appearance, and it could not be determined whether the surface within these

marks was also discolored.

The previously noted discoloration of the outer pipe wall may also be

observed in figure 2. Discoloration extended about 0.4 in (1.0 cm) to the

right of the last parallel mark. The edge of the discolored area was sharply

defined. Since there was no evidence to indicate that the discoloration of

the pipe was due to the gasket, this suggests that the outer face of the

gasket would have been at this location. The distance from the edge of the

discolored area to the end of the pipe ranged from 0.6 - 0.8 inch (1.6 -2.1

cm), as a result of the angle at which the pipe had been cut. It could be

further assumed that approximately 2 in (5 cm) of the pipe originally

inserted into the coupling, had pulled out and been exposed to the conditions

causing the discoloring, for some time prior to the accident.

The deep, slightly curved scratch located in the area containing the

parallel marks appears to be due to some type of mechanical damage unrelated

to that which caused these marks.

Physical Measurements

The wall thickness of the pipe averaged about 0.23 inch (0.58 cm), was

found to be consistently uniform, and well within the allowable dimensional

tolerances of 0.216 to 0.242 inch (0.549 to 0.615 cm) specified in ASTM

D2513.

All of the sections of pipe were somewhat flattened and out-of-round.

Outside diameter measurements made with a vernier caliper from the inside to

the outside of the bow in the pipe were consistently smaller than those made
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perpendicular to this dimension. However, the average values were sufficient

to indicate that the pipe probably met the required nominal dimension of

2.375 inches (6.032 cm) at the time of manufacture. It should be noted that

for roundable plastic pipe, the specified tolerances for out-of-round are required to be met only at the point of manufacture. This variation in the

pipe diameter was attributable in part to the fact that it had been coiled.

Outside diameter measurements were made on the section of pipe that had

pulled out of the coupling at the accident site, in the area where the gasket

and compression nut appeared to have been originally located. These

measurements did not exhibit as great a difference between the observed

minimum and maximum values as occurred in other sections of the pipe.

Therefore, it appeared that the compressive forces exerted on the pipe when

first installed had had the effect of partially rounding the pipe. However,

it could not be determined whether these forces had resulted in any permanent

compressive set in the plastic, primarily because of the tendency for the

pipe to be out-of-round.

Test Procedures and Results

Initially, the Pipeline Accident Division had requested that a series of

nine tests be conducted to determine what effects differences in installation

procedures, subsequent environmental exposure conditions, and testing speeds

used in laboratory pull-out tests might have on the amount of axial stress

required to pull the plastic pipe out of a standard Dresser coupling. These

tests, A through H below, were prescribed in a Notice of Inspection obtained

through the United States District Court for the District of Kansas, on January 5, 1978. The tests were to be conducted using pipe specimens taken

from the three-foot (0.9 m) sections of the plastic gas main, using the new

Dresser coupling and new gaskets, supplied to this laboratory. In addition,

Test A was to be repeated using the failed pipe specimen (shown in figure 2)

and the original coupling from the accident site. This test was designated

as test A-2. Subsequently, after these tests were completed and a preliminary analysis of the data completed, two additional tests, following

the procedures of test A, were conducted as requested by the Pipeline

Accident Division after their receiving concurrence from the Kansas Public

Service Company. These tests were designated as A-3 and A-4.

In accordance with the ASTM recommended practices (ASTM Standard Method

D618) for conditioning of plastic test specimens, the plastic pipe was conditioned for a minimum of 40 hours at 73.4° + 3.6°F (23° + 2°C) prior to

testing. In those cases where the test procedures required additional

conditioning at lower temperature prior to the initiation of a test, the same minimum conditioning time was also used as a criterion for the establishment

of thermal equilibrium.

The pipe test specimens were approximately eighteen inches (46 cm) long.

One end of each specimen was cut with a hacksaw, providing a bias cut in

which the length of the specimen was about 0.25 inch (0.64 cm) shorter on the

outer curved surface than on the inner curved surface, to simulate the type

of installation procedure that had been used at the accident site. When the

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straight metal insert was placed into this end of the pipe until its outer

end was flush with the end of the plastic pipe along the inner curved

surface, the edge of the insert protruded about 0.25 inch (0.64 cm) beyond

the end of the plastic pipe along the outer curved surface.

A special test fixture was fabricated as a means of attaching the nut on one end of the Dresser coupling to the crosshead of the testing machine.

This fixture was also designed to allow the complete test assembly to be

internally pressurized when conducting tests A and B. In conducting the pull

tests, the bias-cut end of the specimen was inserted into the coupling in

such a manner that the outer face of the rubber gasket was at the desired

insert length of the pipe. The gasket retainer and nut were then attached to

the upper end of the coupling and the nut torqued to 150 foot-pounds (204

Newton-meter) just prior to initiation of the test. A grooved metal insert

was placed in the other end of the pipe specimen, and a circular clamp

attached to the outside of the pipe to prevent the specimen from slipping. A fitting on the outer end of the metal insert was used to attach the specimen

to the load cell of the testing machine.

In the results described below the torque relaxation, TR in ft-lb (N-m),

is given in terms of a decrease in the initially applied torque of 150 ft-lb

(204 N-m). The percent TR was computed as follows: % = 100(150 - TR)/150.

In all cases, the maximum pull-out force occurred just prior to failure.

Test A

The pipe specimen was inserted three inches (7.6 cm) into the. coupling

and the nut was torqued. Thec-specimen was immediately pressurized with

nitrogen to 50 psig (3.4 x ICr Pascals), held at that pressure for fifteen

minutes, the amount of torque relaxation measured, and the specimen

immediately tested to pull-out failure, while still pressurized, at a crosshead speed of 2 inches per minute (5 cm/min). This test was performed

four times as follows:

A-1 Using a pipe specimen with the new coupling and new rubber gasket.

A-2 Using the pipe section that had failed (shown in figure 2) and the

associated coupling barrel , gasket, gasket retainer, and nut into

which it had been inserted at the accident site. The coupling

barrel was removed from the nut and gasket by which it had been

attached to the metal pipe, and was attached to the testing machine

crosshead using a gasket, gasket retainer and nut from the new

Dresser coupling.

A-3 Using a pipe specimen with the same coupling assembly used in Test

A-2.

A-4 Same as Test A-2 except that a new rubber gasket was used in the

coupling from the accident site.
Results:

A-1 The torque relaxation was 70 ft-lb (95 N-m) , or 46.7*. The initial

pull-out force was 515 lb (234 kg), which then leveled off at about

700 lb (318 kg), and then increased to a maximum of 825 lb (374 kg).

A-2 The torque relaxation was 0%. The initial pull-out force was 70 It (32 kg), which then leveled off at about 180 lb (81 kg), and then

increased to 300 lb (136 kg).

A-3 The torque relaxation was 0%. The initial pull-out force was 240 lb

(109 kg), which then leveled off at an average of about 310 lb (141

kg), and then increased to 390 lb (177 kg).

A-4 The torque relaxation v/as 0%. The initial pull-out force was 370 lb

(168 kg). The force then steadily decreased to 125 lb (57 kg), and

then increased to 670 lb (304 kg).

After test A-4 was completed, reexamination of the pipe showed the

presence of five new scratch-like marks similar to those shown in figure 2.

These marks are shown in figure 3. The mark on the far left appeared to

slightly overlap the end of one of the original marks, and was at the

approximate original position of the gasket retainer prior to the start of

this pull test. The distance between the first and fifth marks was of the

order of 0.94 in (2.4 cm). The wall of the pipe was slightly abraded in the area containing five of the original marks closest to the end of the pipe.

An area between the two sets of marks, approximately 0.25 in (0.63 cm) in

width, and extending for a' distance of about 2.5 in (6.4 cm) from the end of

the pipe had also been abraded by the metal bead on the gasket.

Test B

The special Dresser Style 90 coupling designed specifically for

attaching metal pipe to plastic pipe was hermetically sealed at the metal

pipe end, and attached to the pipe specimen into which a flanged lock insert

designed for use with this coupling had been placed. The bias-cut end of the

specimen v/as inserted three inches (7.6 cm) into the standard Dresser

coupling used in the pipe pull-out tests and the nut torqued to 150 ft-lb

(204 N-m). The pipe was pressurized to 50 psig (3.4 x 1 0

J Pa) and maintained

at that pressure for a period of 1 hour. The pressure was then increased,

initially^by 20 psig (13.8 Pa), then slowly at a rate of approximately 5 psig

(3.4 x 10 Pa) per minute until the specimen pulled out of the standard

coupling.

Results:

Approximately 17 minutes after the increase in pressure v/as begun, and

while the pipe was pressurized at 150 psig (10.3 x 10 J Pa), the first

perceptible movement of the pipe out of the coupling was observed. The test was continued and about 20 minutes later, or about 37 minutes after the first
8

increase in pressure, the pipe failed by coming out of the coupling under the

force created by a pressure of 250 psig (17.2 x 10 J Pa).

Test C

The pipe specimen was inserted 3 inches (7.6 cm) into the coupling, the

nut torqued to 150 ft-lb (204 N-m), the torque relaxation measured after

fifteen minutes, the nut retorqued to 150 ft-lb (204 N-m), and the specimen

immediately pulled at a crosshead speed of 1 in/min (2.5 cm/min) to failure.

Results:

The torque relaxation was 40 ft-lb (54 N-m), or 26 . 7 %. The initial

pull-out force was 590 lb (268 kg), then the force leveled off at about 1120

lb (508 kg), and then increased to 1200 lb (545 kg).

Test D

The pipe specimen was inserted 1 inch (2.5 cm) into the coupling, the

nut torqued to 150 ft-lb (204 N-m), the torque relaxation measured after 15

minutes, the nut retorqued, and the test begun.

Two tests were conducted on separate specimens as follows:

D-1 crosshead speed of 0.2 in/min (0.5 cm/min)

D-2 crosshead speed of 1 in/min (2.5 cm/min)

Results:

D-1 The torque relaxation was 28 ft-lb (38 N-m), or 18.7%. The initial

pull-out force was 625 lb (284 kg), and the force steadily increased

to a maximum of 905 lb (411 kg).

D-2 The torque relaxation was 3^ ft-lb (46 N-m), or 22.7%. The initial

pull-out force was 380 lb (172 kg), and then steadily increased to

1105 lb (502 kg)

.

Test E

The pipe was inserted 1 inch (2.5 cm) into the coupling, the nut torqued

to 150 ft-lb (204 N-m), the torque relaxation measured after 15 minutes, and

the nut retorqued. The entire test assembly was cooled to 32°F (0°C) for

approximately 65 hours, then tested immediately at a crosshead speed of 0.2

in/min (0.5 cm/min).

Results:

The torque relaxation was 30 ft-lb (41 N-m), or 20.0%. The initial

pull-out force was 265 lb (120 kg), and then the force steadily increased to

630 lb (286 kg).
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Test F

The pipe specimen was inserted 1 inch (2.5 cm) into the coupling, the

nut torqued to 150 ft-lb (204 N-m) , the torque relaxation measured after IF minutes, and the nut retorqued. The entire test assembly was cooled at 32 F (0°C) for about 64 hours, then allowed to warm up at roan temperature, 73-A°F

(23°C) for 17 hours. The torque relaxation was measured. Then the test was begun using a crosshead speed of 0.2 in/min (0.5 cm/min). When the axial

load reached 650 lbs (295 kg) , the approximate pull-out force measured for

test specimen E, the test was stopped, the entire test assembly removed from

the testing machine and recooled at 32°F (0°C) for 24 hours, the torque

relaxation remeasured, and the specimen then tested to failure at a crosshead

speed of 0.2 in/min (0.5 cm/min) at room temperature.

Results:

The torque relaxation measured 15 minutes after the initial assembly of

the test specimen was 20 ft-lb (27 N-m), or 13- 3% . After the specimen had

been cooled and subsequently allowed to warm at room temperature for 17

hours, the torque relaxation was 0%. The pipe specimen was then stressed to

650 lb (295 kg). After recooling for 24 hours, the measured torque

relaxation was 62 ft-lb (84 N-m), or 41.3%. During the final pull test, the

initial pull-out force was 760 lb (3^5 kg), then the force increased steadily

to a maximum of 1245 lb (565 kg).

Test G

o The pipe specimen was inserted 1 inch (2.5 cm) into the coupling, the

nut torqued to 150 ft-lb (204 N-m), the torque relaxation measured after 1^

minutes, the nut retorqued, and the entire test assembly cooled at 32°F (0°C)

for about 44 hours. The torque relaxation was measured, and the pull test

begun at a crosshead speed of 0.2 in/min (0.5 cm/min). The initial intent of

this test had been to stop the test when the axial stress reached 50 lb (23

kg) above the initial force required to start pulling the pipe out of the

coupling, release the stress, and allow the specimen to relax overnight at

room temperature.- The test was then to be restarted at a crosshead speed of

0.2 in/min (0.5 cm/min), but the testing speed was to be reduced as necessary

to prevent the axial stress from exceeding the initial pull-out force plus 5C

lb (23 kg) limit, until failure occurred. Although these procedures were

followed, two variants occurred. First the initial axial stress built up so rapidly that the testing machine could not be stopped before the force had

reached 80 lb (36 kg) above the initial pull-out force. Secondly, ’when the

test was restarted the following day, the force required to pull the pipe

completely out of the coupling did not exceed the original pull-out force, sc the crosshead speed was maintained at a constant rate of 0.2 in/min (0.5

cm/min) until the test was completed.

Results:

The initial torque relaxation was 24 ft-lb (33 N-m), or 16%. After the

pipe had been cooled the measured torque relaxation was 56 lb (76 N-m), or

3 f . 3% . The initial pull-out force was 1020 lb (463 kg) and the test was
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stopped when the axial stress reached 1100 lb (499 kg). When the test was restarted the initial pull-out force was 300 lb (136 kg), then the force

steadily increased to a maximum of 925 lb (420 kg) over a period of 2 minutes

and 40 seconds.

Test H

The specimen was inserted three inches (7.6 cm) into the coupling, the

nut torqued to 150 ft-lb (204 N-m) , the torque relaxation measured after 15

minutes, the nut retorqued, and the test begun at a testing speed of 0.2

in/min (0.5 cm/min). The original plan had been to tighten the nut, after

the pipe started to pul'l out, until the axial force reached 625 lb (284 kg).

However, retightening was not necessary, since the axial stress built up

rapidly to 625 lb (284 kg). At that point, the test was stopped, the axial

stress removed from the specimen, and the pipe allowed to relax for 8 hours,

prior to testing to failure.

Results:

The torque relaxation was 3^ ft-lb (46 N-m), or 22.7%. The initial

pull-out force was 350 lb (159 kg), and the test was stopped when the force

reached 625 lb (284 kg). After the 8-hour relaxation period, the stress was

reapplied. The initial pull-out force was 350 lb (159 kg), then the force

continued to increase, leveling off at about 650 lb (295 kg), then increased

to a maximum of 775 lb (352 kg).

Thermal Contraction

• A request was made to attempt to measure the degree of thermal

contraction that the pipe might undergo when exposed to low temperatures, by

means of a simple, practical test. A section of the pipe was measured using

a vernier caliper, accurate to 0^001 ingh (0.025 mm), at 73.4°F (23°C), then

cooled for about 53 hours at -13UF (-25 C), and remeasured. The coefficient

of thermal expansion computed for the change in gauge length at a aT of

86.4°F (48 C) was 8.9 x 10 J in/in/°F (1.60 x 10 cm/cm/ C). The pipe was

allowed to equilibrate to 73.4 F (23 C) and a second test conducted. In

this case, the pipe specimen was cooled at -16.6°F (-27°C) for about ^52

hours . q

The coefficient of gxpansion computed for this test was 9.1 x 10

D in/in/u

F (1.64 x 10” cm/crnC). These rpsults compared favorably with the

manufacturer ’ s reported value of 9 x 10~ J in/in/°F, obtained by dilatometric

measurements.

Discussion of Pull-Out Tests

A comparative evaluation of laboratory data obtained from single

specimens tested under a variety of conditions is a difficult task, since the

results may not be a true statistical representation of the sample.

However, the data obtained in these tests seem to indicate seme trends.
The force required to initiate pipe pull-out did not seem to fall

into a pattern that could be related to the pre-test procedures used

in installing the pipe into the coupling. This variability may have

been related to the fact that the pipe specimens were bowed, and

that in some cases these forces may have been affected by initial

straightening of the pipe due to the applied axial stress. In

conducting tests on pipe that has been coiled, the use of shorter

test specimens, in which the effect of the bowing would be lessened,

might lead to more consistent values.

The tests conducted on unpressurized specimens at 0.2 in/min (0.5

cm/min) seemed to provide data more consistent with that obtained

using test procedure A, than did those tests conducted at 1 in/min

(2.5 cm/min).

The variability in torque relaxation after the initial tightening of

the coupling nut seems to indicate that retorquing, after an established relaxation period, would be good practice in field

installations.

In every case, the maximum force achieved in the pull-out tests

occurred when less than 1 inch (2.5 cm) of the pipe specimen still

remained in the gasket. However, visual examination of each

specimen immediately after completion of a test showed no evidence

of plastic deformation, or cold flow, that would have resulted in ar effective increase in the pipe outside diameter. Neither was there

any evidence of deformation of the pipe wall attributable to the

metal bead on the gasket.
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^ j 5, l) tH. OF COMM.

glflUOGRAPHIC DATA

SHEET

1. IMIHI.K A i ION OK KI I'OK I NO.

NBS I R78- 1 491

2. («>v ’t Ai i i ssion

No.

«Jll | l.l. AND SUM IT I I.E tests relating to a plastic gas pipe/metal

COUPLING PULL-OUT FAILURE IN LAWRENCE, KANSAS

3. Recipient’s Accession No.

5. Publication Date May 1978

6. Performing Organization Code

763

ALTMOR(S)

Samuel D. Toner

8. Performing Organ. Report No.

'97 PI- R FORMING ORGANIZAT ION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No. 3120412

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address ('S.'reef, City, State, ZIP)

Pipeline Accident Division

National Transportation Safety Board

Washington, D.C. 20591

13. Type of Report St Period

Covered

Final

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOT ES

16. AHSTRAC I (A 200-word or less factual summary of most significant information, if document includes a signi ficant bibliography or literature survey, mention it here.)

A fatal explosion due to a natural gas leak occurred when a polyethylene gas

main pulled out of a metal coupling, apparently as a result of thermal contraction

of the plastic pipe. This report is based on an evaluation of the axial stresses

required, under various conditions of test, to pull sections of plastic pipe, from

the accident site, out of metal compression couplings. The objective of these

tests was to provide data to assist the Pipeline Accident Division in determining

the adequacy of current regulations governing installation of gas piping systems

involving combinations of plastic and metal components.

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 of gas pipe; gas pipe failure; plastic gas pipe; polyethylene gas pipe

18. AVAILABILITY [ j Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

KX ^ or Official Distribution. Do Not Release to NTIS

UNCLASSIFIED 13

Cl J Order Front Sup. of Doc., U.S. Government Printing Office Washington, D.G. 20102, SD Cat. No. C 11

20. SECURITY CLASS

(THIS PAGE)

22. Price

[ J Order !• rom National Technical Information Service (NT IS1 Springfield, Virginia 22151 UNCLASSIFIED

U5COMM.DC 2S042-P74