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Polyvinylchloride (PVC) pipe reliability and failure modes
Rehabdity Engineering 13 (1985) 11-21
Polyvinyichloride (PVC) Pipe Reliability and Failure Modes Margaret Mock Carroll Safety Engineering Department, Sandla National Labs, Albuquerque, New Mexico 87185, USA (Received 16 November 1984)
ABSTRACT Research on plastic pipe failures revealed that ptpe and fittmgs often had manuJacturing flaws and that many common mstallatton praetwes could cause significant damage to the pipmg To determine effects of failure of pressurized PVC pipe, tests were conducted to charaetertze fadure modes and consequences.
1
INTRODUCTION
The industrial use of plastic pipe began in the mid-1940s and has grown extensively since then. The increase m use has been due primarily to economics--plastic pipe (61n diameter or less) costs less to purchase, transport and install than metallic pipe of the same size and service. 1 Plastic pipes are lightwexght and reqmre substantmlly less labour and equipment to install than metalhc pipe. Plastic pipe does not corrode or require cathodic protection as metallic pipe does, and it can often be used where there ~s a reactwity or compatibility problem with standard iron or steel pipe. In the 1960s, with increased research and development of petrochemical products, plastic pipe materials and installation techniques A versxon of thxs paper was presented at the 8th Advances in Rehabihty Technology Symposmm--ARTS '84, 25-27 April 1984, Umverslty of Bradford, UK, and is reproduced by kind permission of the orgamsers_ 11
Rehabthty Engmeermg 0143-8174/85/$03 30 © Elsevier Applied Science Pubhshers Ltd, England, 1985 Printed in Great Britain
12
Margaret Mock Carroll
improved and the piping became even more widely used. In 1966, 12 ')o of the new (natural) gas d~stnbutlon installations (in the United States) were plastic, in 1978, 73 °o of new installations were plastic J This study will be narrowed to one variety of plastic pipe, polyvlnylchloride (PVC), because of its c o m m o n and extensive industrial, commercial and residential apphcat~ons. Some of the advantages of using plastic pipe have been cited above, however, there are certain disadvantages which must also be considered. PVC pipe is essentially brittle in nature, and It iS relatively easily damaged by mechanical means such as being struck by tools or other matermls, being weakened or fractured by gouging, abrasion or rough handhng such as c o m m o n l y occurs when pipe sections are thrown or walked on by workers_ Excesswe heat, welding slag, sunlight, and certain chemicals adversely affect it 2 There xs also some concern about manufacturing processes which can affect product quality and integrity 5
2
MANUFACTURERS' RECOMMENDATIONS
F O R USE
Although the use of PVC pipe IS increasingly common, what lS not c o m m o n is the knowledge that PVC pipe is not stated to universal apphcatlon. Manufacturers' recommendations for use are often unknown by the end user, and are sometimes hard to determine The consensus standards offer little help in assuring proper selection, installation or use In most manufacturers' catalogs, there is a notation that PVC piping is not recommended for use with compressed air or gases, although plastic pipe is used in most new natural gas systems Research of manufacturers' catalogs shows that many include the warning that PVC is not resistant to ketones, and aromatic and chlorinated hydrocarbons_ There are also many other chemicals which are not compatible with PVC, and this list varies with each pipe manufacturer. PVC piping IS ultraviolet sensitive and when exposed to sunlight for a period of time it becomes noticeably more brittle than unexposed piping 3.4 Extremes of hot or cold also weaken PVC The temperature parameters are listed in each catalog, and temperature cycling is discouraged by most manufacturers Vibration and surging effects are also deleterious to PVC piping. It has been found that repeated surges below the rated burst strength of the pipe can lead to failures. 6
PolywnylchlorMe ( PVC) ptpe hazards
3
13
STANDARD ORGANIZATION RECOMMENDATIONS
The American National Standards Institute (ANSI) recommends that plastic pipe be used for water and non-flammable chemicals, and for air serwce where experience or tests have demonstrated that the plastic pipe ~s suitable for the service conditions, and the pressure and temperature are within the manfacturer's recommendations. The standard further recommends that until allowable stress values are estabhshed for these matermls, pressure should be limited to 150 psi and temperature to 140 °F for polyvmylchloride (PVC), and 120°F at 150psi for polyethylenefl
4
S P E C I F I C C H A R A C T E R I S T I C S : P I P E VS F I T T I N G S
There is a difference between the strength of piping and fittings of the same size and schedule. (As an example, A N S I / A S T M Schedule 80 1.5-m PVC pipe is rated at 540 psi, while standard 1.5-m PVC glued-joint fittings are rated at 500 psi, and the threaded fittings at 240 psi.) It is obvious that the inevitable will happen: a system designed using the nominal rating of the pipe (540 psi) may in reality be safe to use at a maximum pressure of only 240 ps~ reflecting the rating of its weakest component. This difference in strength among the glued fittings, threaded fittings and pipe is not well known even among designers and installers
5
I N S T A L L A T I O N OF PIPE
Because of its brittle nature, PVC pipe Is relatively easy to damage during transportation, installation and use. If the p~pe is thrown onto or impacted by other objects, it can be damaged in a manner which may not be evident Dragging across concrete or rough metal can abrade and gouge the pipe, weakening it. During installation it may be damaged by factors c o m m o n on ajobslte: other p~pe, tools, equipment and personnel. Even pipe placed m a protective chase or trench is subject to damage from personnel using it as a step, or striking it with tools or equipment. If pipe supporting clamps are installed so that they compress the pipe or 'dig into' its surface, damage will result, possibly causing its failure. 2
Margaret Motk Carroll
14
6
INSTALLATION OF FITTINGS
To obtain their rated strength m a system, the glued fittings and the pipe to which they attach must be properly prepared for the glue and then mated correctly The quality of the glued joint depends on the expertise and thoroughness of the installers. Most manufacturers recommend using a strap type wrench on PVC threaded fittings because the fittings can easily be damaged by the toothed jaws of an ordinary pipe wrench During installation of PVC pipe at one location, a substantial number of threaded fittings broke as they were installed with conventional wrenches 2
7
PVC PIPE F A I L U R E E X P E R I E N C E
Because of its frangible nature, when PVC pipe fails under gas load, the mode is 'catastrophic'. The piping tends to break into sharp-edged, rather energetic fragments, spreading over a fairly large area. In one instance, a 1.5-in pipe (rated for 500 psi)was pressurized to 300 psi inside a 40-in deep trench covered with 2 x 4 in steel grating, scattering fragments through the pipe trench 65 ft in one direction and 72 ft in another. PVC debris was found over an area of approximately 6000ft 2 in th~s lncldent.: A number of incidents of PVC failure, without recognized damage to the material, have been reported. One such incident involved a hquld nitrogen delivery system where the pipe was run outdoors The failure has been attributed to ultraviolet embrittlement of the pipe over several years" exposure a Temperature extremes and temperature cycling are also implicated Jn this case. A second incident of failure involved a new installation for gaseous sulphur hexafluorIde, designed for 500 psi use. The system was proof tested at 650 psi and was being used at 300 psi when it failed, apparently from defects (voids) in an elbow fitting When the elbow failed, it induced failure in the adjacent pipe runs. 2 The SF 6 was not chemically interactive with the PVC By contrast, metallic piping material would probably have split and vented its contents Three British workers 8 reported on the 'unpredictable brittleness' of PVC pipe, which they say has been accepted because of the many other advantages the pipe has offered, and they suggest that in light of 'more severe legislation and h~gher penalties, it is not acceptable to offset such a
PolywnylchlorMe (PVC) ptpe hazards
15
potential hazard m this way'. These workers, who are managers and technical officers in an industrial research and development agency, state that property differences, particularly impact resistance, do exist between p~pes of nominally ~dent~cal material. They attribute the difference to variations of the c o m p o u n d i n g additions made to the PVC.
8
PVC P I P E T E S T I N G
In order to define failure modes, tests of PVC pipe and fittings were conducted by Sandla National Laboratories (SNL). For the tests, 2-in diameter schedule 801 pipe* wtth glued fittings was selected Six test setups were assembled to simulate a piping system which had accidentally exploded at an S N L facxlity where tt was employed in a gaseous sulphur hexaflouride system. The test pipe, rated at 470 psi, was pressurized w~th dry a~r to 750 ps~ In order to assure high-speed camera tracking, a crack was lnmated m the pressurized pipe by firing a small Reynolds RP-2 exploding bndgewlre detonator, which contains 6 4 m g of exploswes. The detonator was attached to the pipe with epoxy resin. Neither the firing of the detonator nor the epoxy resin produced any damage on an identical but unpressurtzed specimen of the pipe. Crack initiation, growth, and subsequent explosion of the p~pe were observed by means of high-speed photography recorded by a camera set-up (F~gs 1-3) A video camera was used to capture real-t~me movement; a high speed Nova camera, running at 5000 frames per second, was used for overall coverage; and a H y c a m 1/4 frame camera, running at 36000 frames per second, provided a closeup of the test pipe.
8.1 Results Analysis of the two high-speed films yielded an average crack propagation rate of approximately 616 m/s, or a b o u t 2020 ft/s_ The velocity of the pipe fragments was calculated from the measurements at approximately 38m/s or about 125 ft/s After the test, fragments of pipe and fittings were recovered from 0-6 to 36 m (about 2 to 118 ft) away from the test site The fragments recovered * Pipe schedule number = 1000 x P/SE, where P is the operating pressure (lb/m 2) and SE = allowable stress range x joint efficiency
16
~targaret Mo~ k ('at rol/
Fig. 1. Typical fragments of shattered PVC pipe F~rst number 1s indicative of the test sequence, second number is the d~stance from the point of failure to where the fragment was recovered Sandla National Labs Photo PVC Pipe Fadure Tests
Polyvmylchloride (PVC) ptpe hazards
17
~J
416.7
Fig. 2. The top photo m the sequence shows the initiation of reduced fadure in PVC ptpe Each successive photo shows the failure growth Sandla National Labs Photos--PVC Ptpe Fadure Tests weighed from 4.05 to 41-85g (0-14 to 1-46 ounces). The fragments are typically sharp-edged, usually elongated in shape 9 8.2 Other observations At the point of failure, the pipe sections moved apart and away from the m~tial fracture site. This movement caused other pipe secttons to rupture due to compression even t h o u g h they were not a part of the original fadure In actual installations, pieces of the fractured pipe could strike
18
Mal gatet ~1o~k Carroll
Fig. 3 Thlsshowsthelnducedfaflurema PVCplpeatawell-advancedstage Notesharp edges as also evident in Fig l Sandla National Labs Photo - PVC Pipe Failure Tests o t h e r pressurized pipe with sufficient force to cause it to fracture This a p p e a r s to be the m e c h a n i s m by w h i c h r u p t u r i n g o f pipe at o t h e r than the site o f origin t o o k place in an earlier accidental explosion o f piping. 2
9
UNIFORMITY
AND RELIABILITY
V a r i o u s r e p o r t s have established t h a t u n p r e d i c t a b l e failures in P V C piping o c c u r in a wide variety o f a p p l i c a t i o n s and c i r c u m s t a n c e s z2'5' 1o It
Polyvmylchlortde (PVC) pipe hazards
19
has been suggested that differences m strength in apparently identical pipe may result from the variation in molecular structure of the material. Another known source for weakening PVC pipe and fittings is voids m the material. Reports of incidents have indicated a very large proportion of failures may be due to these voids, which are more often found in fittings than in lengths of piping. Perhaps this ~s because their structure ~s more complex than that of straight piping. 2'5 The voids may be due to gas being entrained in PVC material or to foreign matter, such as sand, being incorporated 5 during manufacture.2 In our study, the voids found measured ~6 to ¼1n diameter. In some fittings and pipe sections, several voids were found close together. It is obwous that the strength of such pipes and fittings is predictably below their rated value. Typically, these voids are not vmible on the surface of the pipe or fittings. When the p~pe or fittings are cut, some voids are visible on the reside surfaces, others appear m the cross-section. In actual circumstances where PVC p~pe is used, ~t is not inspected in this manner. One would expect such examination as a sampling technique to assure quahty and reliability, or after a failure, to determine causes or modes. In general practice, however, products such as piping are accepted for use on the assumption that the product rating for strength is indicative of the actual strength of the product. In light of the numerous instances of unexpected fadure in PVC pipe and fittings, one is led to question whether the standards for quahty and rehabd~ty are being met by manufacturers If standards are being met, then are they insufficient? Are the quality controls incorporated by the manufacturers less stringent than industry ~s accustomed to working with? It is clearly an area to which the plastics industry and standards orgamzations should address themselves. 10
SUMMARY AND CONCLUSIONS
PVC pipe should be used only for low pressure liquid systems where the liqmd is at or near ambient temperature. PVC pipe, because of its brittle nature, should be used only m u n m a n n e d areas or where it is isolated from personnel. It should be otherwise protected so that it is not subjected to ultraviolet light, temperature cycling or damage after installation. Heat (particularly welding and cutting) should be avoided m the ~mmediate vicimty of the piping. PVC pipe should not be used where ~t would be subject to vibration. The effects of certain chemicals on PVC should be carefully considered, both where the pipe is a carrying m e d m m for the
20
Margaret Mock Carroll
chemical and where the pipe m a y be exposed to the chemicals externally. PVC pipe is desirable for many reasons but should, nonetheless, be used with forethought. Manufacturers' recommendations for selection, use, and installation should be actively sought out and carefully followed. G o o d transportation, Installation, on-site inspection, and maintenance practices, as well as proper design criteria must be utilized when dealing with PVC pipe and its fittings Because of its brittle nature and catastrophic failure mode, if there is a question a b o u t PVC piping being used in a particular situation, it should not be used, another alternative should be sought. REFERENCES 1 National Transportation Safety Board, Pipeline lndustry's practice using plastic pipe in gas plpehne facilities and the resulting safety factors, DOTOPS-75/07 and MTB Leak Report, in An Analysts oj Accident Data From Plastic Pipe Natural Gas Distribution System, US Department of Transportation, Washington, DC, 19 September 1980, p_ l_ 2 Sandla National Laboratories Internal Report, PVC pipe accident, 1980 3 Pantex Internal Accident Investigation Report, Mason and Hanger--Sllas Mason Co., Inc, Amarillo, Texas, November 1979 4 Walker, R P , Jr The effects of UV aging on PVC pipe, Proceedings oJ the International Conference on Underground Plastw Pipe, 1981, pp 437 and 446 5 Tonler, Samuel D Failure Analysts oJ Polyvmylchlorlde Natural Gas Piping, US Bureau of Standards, Washington, DC, 1974 6 Vlnson, Herbert W. Response of PVC pipe to large, repetitive pressure surges, Proceedmgs of International Conference on Underground Plastic Pipe, 1981, pp. 485 and 487. 7 ANSI B31 1-1973, Part III, 105 3 A, Power piping 8 Cowley, W E, Dent, N D and Morris, R H_ The brittle failure PVC h ned glass reinforced plastic pipelines, Chemlstry and Industo'(3 June 1978), p 365 9 Padllla, R PVC pipe testing data, Sandla National Laboratories Internal Report, July 1981 10 Kirby, P C Surge and fatigue in unplastlclzed PVC sewer rising mains, Plastws and Rubber Materials and Apphcatlons, May 1980, p 79
BIBLI O G R A P H Y ANSI/ASTM F412-77b, Standard definmons of terms relating to plastic plpmg systems, 1977
Polyvmylchlomde (PVC) plpe hazards
21
ASTM D 2517-73, Standard specification for reinforced epoxy resin gas pressure pipe and fittings, 1973 (reapproved 1979). ANSI/ASTM 2513-78ES, Standard specification for thermoplastic gas pressure pipe, tubing & fittings, 1978 Shannon, R W E and Wells, A A. Brittle crack propagation in gas filled plpehnes, The Queen's University of Belfast, Belfast, Ireland, Int. J. Fracture, 10 (4) (1974), pp. 471-86 Anon Long-life safety of PVC water pipe, Modern Plastics (November 1974), pp 41-3. Carroll, M M_ et al, Sandia National Laboratories Internal report on PVC pipe fadure, 1981 Schrock, B Jay (Ed) International Conference on Underground Plastw Pipe, American Society of Civil Engineers, New York, 1981 Proceedings of the 4th International Conference on Plastw Pipes, University of Sussex, Falmer, Brighton, 28-30 March 1979_ Plastic & Rubber Institute, London. Benjamin, P A review of current practice Llndner, H. Applications and apphcational limits of plastic plpework systems for industrial installations_ Berry, D B S. Standard testing of pipes and examination of failures Taylor, G Warren, Jr, Slip lining of municipal sewer mains with HDPE piping in the US International Standards Organization, Use of PVC pipe & fittings, ISO/TC 138 (2)(3), 1978.
Polyvinyichloride (PVC) Pipe Reliability and Failure Modes Margaret Mock Carroll Safety Engineering Department, Sandla National Labs, Albuquerque, New Mexico 87185, USA (Received 16 November 1984)
ABSTRACT Research on plastic pipe failures revealed that ptpe and fittmgs often had manuJacturing flaws and that many common mstallatton praetwes could cause significant damage to the pipmg To determine effects of failure of pressurized PVC pipe, tests were conducted to charaetertze fadure modes and consequences.
1
INTRODUCTION
The industrial use of plastic pipe began in the mid-1940s and has grown extensively since then. The increase m use has been due primarily to economics--plastic pipe (61n diameter or less) costs less to purchase, transport and install than metallic pipe of the same size and service. 1 Plastic pipes are lightwexght and reqmre substantmlly less labour and equipment to install than metalhc pipe. Plastic pipe does not corrode or require cathodic protection as metallic pipe does, and it can often be used where there ~s a reactwity or compatibility problem with standard iron or steel pipe. In the 1960s, with increased research and development of petrochemical products, plastic pipe materials and installation techniques A versxon of thxs paper was presented at the 8th Advances in Rehabihty Technology Symposmm--ARTS '84, 25-27 April 1984, Umverslty of Bradford, UK, and is reproduced by kind permission of the orgamsers_ 11
Rehabthty Engmeermg 0143-8174/85/$03 30 © Elsevier Applied Science Pubhshers Ltd, England, 1985 Printed in Great Britain
12
Margaret Mock Carroll
improved and the piping became even more widely used. In 1966, 12 ')o of the new (natural) gas d~stnbutlon installations (in the United States) were plastic, in 1978, 73 °o of new installations were plastic J This study will be narrowed to one variety of plastic pipe, polyvlnylchloride (PVC), because of its c o m m o n and extensive industrial, commercial and residential apphcat~ons. Some of the advantages of using plastic pipe have been cited above, however, there are certain disadvantages which must also be considered. PVC pipe is essentially brittle in nature, and It iS relatively easily damaged by mechanical means such as being struck by tools or other matermls, being weakened or fractured by gouging, abrasion or rough handhng such as c o m m o n l y occurs when pipe sections are thrown or walked on by workers_ Excesswe heat, welding slag, sunlight, and certain chemicals adversely affect it 2 There xs also some concern about manufacturing processes which can affect product quality and integrity 5
2
MANUFACTURERS' RECOMMENDATIONS
F O R USE
Although the use of PVC pipe IS increasingly common, what lS not c o m m o n is the knowledge that PVC pipe is not stated to universal apphcatlon. Manufacturers' recommendations for use are often unknown by the end user, and are sometimes hard to determine The consensus standards offer little help in assuring proper selection, installation or use In most manufacturers' catalogs, there is a notation that PVC piping is not recommended for use with compressed air or gases, although plastic pipe is used in most new natural gas systems Research of manufacturers' catalogs shows that many include the warning that PVC is not resistant to ketones, and aromatic and chlorinated hydrocarbons_ There are also many other chemicals which are not compatible with PVC, and this list varies with each pipe manufacturer. PVC piping IS ultraviolet sensitive and when exposed to sunlight for a period of time it becomes noticeably more brittle than unexposed piping 3.4 Extremes of hot or cold also weaken PVC The temperature parameters are listed in each catalog, and temperature cycling is discouraged by most manufacturers Vibration and surging effects are also deleterious to PVC piping. It has been found that repeated surges below the rated burst strength of the pipe can lead to failures. 6
PolywnylchlorMe ( PVC) ptpe hazards
3
13
STANDARD ORGANIZATION RECOMMENDATIONS
The American National Standards Institute (ANSI) recommends that plastic pipe be used for water and non-flammable chemicals, and for air serwce where experience or tests have demonstrated that the plastic pipe ~s suitable for the service conditions, and the pressure and temperature are within the manfacturer's recommendations. The standard further recommends that until allowable stress values are estabhshed for these matermls, pressure should be limited to 150 psi and temperature to 140 °F for polyvmylchloride (PVC), and 120°F at 150psi for polyethylenefl
4
S P E C I F I C C H A R A C T E R I S T I C S : P I P E VS F I T T I N G S
There is a difference between the strength of piping and fittings of the same size and schedule. (As an example, A N S I / A S T M Schedule 80 1.5-m PVC pipe is rated at 540 psi, while standard 1.5-m PVC glued-joint fittings are rated at 500 psi, and the threaded fittings at 240 psi.) It is obvious that the inevitable will happen: a system designed using the nominal rating of the pipe (540 psi) may in reality be safe to use at a maximum pressure of only 240 ps~ reflecting the rating of its weakest component. This difference in strength among the glued fittings, threaded fittings and pipe is not well known even among designers and installers
5
I N S T A L L A T I O N OF PIPE
Because of its brittle nature, PVC pipe Is relatively easy to damage during transportation, installation and use. If the p~pe is thrown onto or impacted by other objects, it can be damaged in a manner which may not be evident Dragging across concrete or rough metal can abrade and gouge the pipe, weakening it. During installation it may be damaged by factors c o m m o n on ajobslte: other p~pe, tools, equipment and personnel. Even pipe placed m a protective chase or trench is subject to damage from personnel using it as a step, or striking it with tools or equipment. If pipe supporting clamps are installed so that they compress the pipe or 'dig into' its surface, damage will result, possibly causing its failure. 2
Margaret Motk Carroll
14
6
INSTALLATION OF FITTINGS
To obtain their rated strength m a system, the glued fittings and the pipe to which they attach must be properly prepared for the glue and then mated correctly The quality of the glued joint depends on the expertise and thoroughness of the installers. Most manufacturers recommend using a strap type wrench on PVC threaded fittings because the fittings can easily be damaged by the toothed jaws of an ordinary pipe wrench During installation of PVC pipe at one location, a substantial number of threaded fittings broke as they were installed with conventional wrenches 2
7
PVC PIPE F A I L U R E E X P E R I E N C E
Because of its frangible nature, when PVC pipe fails under gas load, the mode is 'catastrophic'. The piping tends to break into sharp-edged, rather energetic fragments, spreading over a fairly large area. In one instance, a 1.5-in pipe (rated for 500 psi)was pressurized to 300 psi inside a 40-in deep trench covered with 2 x 4 in steel grating, scattering fragments through the pipe trench 65 ft in one direction and 72 ft in another. PVC debris was found over an area of approximately 6000ft 2 in th~s lncldent.: A number of incidents of PVC failure, without recognized damage to the material, have been reported. One such incident involved a hquld nitrogen delivery system where the pipe was run outdoors The failure has been attributed to ultraviolet embrittlement of the pipe over several years" exposure a Temperature extremes and temperature cycling are also implicated Jn this case. A second incident of failure involved a new installation for gaseous sulphur hexafluorIde, designed for 500 psi use. The system was proof tested at 650 psi and was being used at 300 psi when it failed, apparently from defects (voids) in an elbow fitting When the elbow failed, it induced failure in the adjacent pipe runs. 2 The SF 6 was not chemically interactive with the PVC By contrast, metallic piping material would probably have split and vented its contents Three British workers 8 reported on the 'unpredictable brittleness' of PVC pipe, which they say has been accepted because of the many other advantages the pipe has offered, and they suggest that in light of 'more severe legislation and h~gher penalties, it is not acceptable to offset such a
PolywnylchlorMe (PVC) ptpe hazards
15
potential hazard m this way'. These workers, who are managers and technical officers in an industrial research and development agency, state that property differences, particularly impact resistance, do exist between p~pes of nominally ~dent~cal material. They attribute the difference to variations of the c o m p o u n d i n g additions made to the PVC.
8
PVC P I P E T E S T I N G
In order to define failure modes, tests of PVC pipe and fittings were conducted by Sandla National Laboratories (SNL). For the tests, 2-in diameter schedule 801 pipe* wtth glued fittings was selected Six test setups were assembled to simulate a piping system which had accidentally exploded at an S N L facxlity where tt was employed in a gaseous sulphur hexaflouride system. The test pipe, rated at 470 psi, was pressurized w~th dry a~r to 750 ps~ In order to assure high-speed camera tracking, a crack was lnmated m the pressurized pipe by firing a small Reynolds RP-2 exploding bndgewlre detonator, which contains 6 4 m g of exploswes. The detonator was attached to the pipe with epoxy resin. Neither the firing of the detonator nor the epoxy resin produced any damage on an identical but unpressurtzed specimen of the pipe. Crack initiation, growth, and subsequent explosion of the p~pe were observed by means of high-speed photography recorded by a camera set-up (F~gs 1-3) A video camera was used to capture real-t~me movement; a high speed Nova camera, running at 5000 frames per second, was used for overall coverage; and a H y c a m 1/4 frame camera, running at 36000 frames per second, provided a closeup of the test pipe.
8.1 Results Analysis of the two high-speed films yielded an average crack propagation rate of approximately 616 m/s, or a b o u t 2020 ft/s_ The velocity of the pipe fragments was calculated from the measurements at approximately 38m/s or about 125 ft/s After the test, fragments of pipe and fittings were recovered from 0-6 to 36 m (about 2 to 118 ft) away from the test site The fragments recovered * Pipe schedule number = 1000 x P/SE, where P is the operating pressure (lb/m 2) and SE = allowable stress range x joint efficiency
16
~targaret Mo~ k ('at rol/
Fig. 1. Typical fragments of shattered PVC pipe F~rst number 1s indicative of the test sequence, second number is the d~stance from the point of failure to where the fragment was recovered Sandla National Labs Photo PVC Pipe Fadure Tests
Polyvmylchloride (PVC) ptpe hazards
17
~J
416.7
Fig. 2. The top photo m the sequence shows the initiation of reduced fadure in PVC ptpe Each successive photo shows the failure growth Sandla National Labs Photos--PVC Ptpe Fadure Tests weighed from 4.05 to 41-85g (0-14 to 1-46 ounces). The fragments are typically sharp-edged, usually elongated in shape 9 8.2 Other observations At the point of failure, the pipe sections moved apart and away from the m~tial fracture site. This movement caused other pipe secttons to rupture due to compression even t h o u g h they were not a part of the original fadure In actual installations, pieces of the fractured pipe could strike
18
Mal gatet ~1o~k Carroll
Fig. 3 Thlsshowsthelnducedfaflurema PVCplpeatawell-advancedstage Notesharp edges as also evident in Fig l Sandla National Labs Photo - PVC Pipe Failure Tests o t h e r pressurized pipe with sufficient force to cause it to fracture This a p p e a r s to be the m e c h a n i s m by w h i c h r u p t u r i n g o f pipe at o t h e r than the site o f origin t o o k place in an earlier accidental explosion o f piping. 2
9
UNIFORMITY
AND RELIABILITY
V a r i o u s r e p o r t s have established t h a t u n p r e d i c t a b l e failures in P V C piping o c c u r in a wide variety o f a p p l i c a t i o n s and c i r c u m s t a n c e s z2'5' 1o It
Polyvmylchlortde (PVC) pipe hazards
19
has been suggested that differences m strength in apparently identical pipe may result from the variation in molecular structure of the material. Another known source for weakening PVC pipe and fittings is voids m the material. Reports of incidents have indicated a very large proportion of failures may be due to these voids, which are more often found in fittings than in lengths of piping. Perhaps this ~s because their structure ~s more complex than that of straight piping. 2'5 The voids may be due to gas being entrained in PVC material or to foreign matter, such as sand, being incorporated 5 during manufacture.2 In our study, the voids found measured ~6 to ¼1n diameter. In some fittings and pipe sections, several voids were found close together. It is obwous that the strength of such pipes and fittings is predictably below their rated value. Typically, these voids are not vmible on the surface of the pipe or fittings. When the p~pe or fittings are cut, some voids are visible on the reside surfaces, others appear m the cross-section. In actual circumstances where PVC p~pe is used, ~t is not inspected in this manner. One would expect such examination as a sampling technique to assure quahty and reliability, or after a failure, to determine causes or modes. In general practice, however, products such as piping are accepted for use on the assumption that the product rating for strength is indicative of the actual strength of the product. In light of the numerous instances of unexpected fadure in PVC pipe and fittings, one is led to question whether the standards for quahty and rehabd~ty are being met by manufacturers If standards are being met, then are they insufficient? Are the quality controls incorporated by the manufacturers less stringent than industry ~s accustomed to working with? It is clearly an area to which the plastics industry and standards orgamzations should address themselves. 10
SUMMARY AND CONCLUSIONS
PVC pipe should be used only for low pressure liquid systems where the liqmd is at or near ambient temperature. PVC pipe, because of its brittle nature, should be used only m u n m a n n e d areas or where it is isolated from personnel. It should be otherwise protected so that it is not subjected to ultraviolet light, temperature cycling or damage after installation. Heat (particularly welding and cutting) should be avoided m the ~mmediate vicimty of the piping. PVC pipe should not be used where ~t would be subject to vibration. The effects of certain chemicals on PVC should be carefully considered, both where the pipe is a carrying m e d m m for the
20
Margaret Mock Carroll
chemical and where the pipe m a y be exposed to the chemicals externally. PVC pipe is desirable for many reasons but should, nonetheless, be used with forethought. Manufacturers' recommendations for selection, use, and installation should be actively sought out and carefully followed. G o o d transportation, Installation, on-site inspection, and maintenance practices, as well as proper design criteria must be utilized when dealing with PVC pipe and its fittings Because of its brittle nature and catastrophic failure mode, if there is a question a b o u t PVC piping being used in a particular situation, it should not be used, another alternative should be sought. REFERENCES 1 National Transportation Safety Board, Pipeline lndustry's practice using plastic pipe in gas plpehne facilities and the resulting safety factors, DOTOPS-75/07 and MTB Leak Report, in An Analysts oj Accident Data From Plastic Pipe Natural Gas Distribution System, US Department of Transportation, Washington, DC, 19 September 1980, p_ l_ 2 Sandla National Laboratories Internal Report, PVC pipe accident, 1980 3 Pantex Internal Accident Investigation Report, Mason and Hanger--Sllas Mason Co., Inc, Amarillo, Texas, November 1979 4 Walker, R P , Jr The effects of UV aging on PVC pipe, Proceedings oJ the International Conference on Underground Plastw Pipe, 1981, pp 437 and 446 5 Tonler, Samuel D Failure Analysts oJ Polyvmylchlorlde Natural Gas Piping, US Bureau of Standards, Washington, DC, 1974 6 Vlnson, Herbert W. Response of PVC pipe to large, repetitive pressure surges, Proceedmgs of International Conference on Underground Plastic Pipe, 1981, pp. 485 and 487. 7 ANSI B31 1-1973, Part III, 105 3 A, Power piping 8 Cowley, W E, Dent, N D and Morris, R H_ The brittle failure PVC h ned glass reinforced plastic pipelines, Chemlstry and Industo'(3 June 1978), p 365 9 Padllla, R PVC pipe testing data, Sandla National Laboratories Internal Report, July 1981 10 Kirby, P C Surge and fatigue in unplastlclzed PVC sewer rising mains, Plastws and Rubber Materials and Apphcatlons, May 1980, p 79
BIBLI O G R A P H Y ANSI/ASTM F412-77b, Standard definmons of terms relating to plastic plpmg systems, 1977
Polyvmylchlomde (PVC) plpe hazards
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ASTM D 2517-73, Standard specification for reinforced epoxy resin gas pressure pipe and fittings, 1973 (reapproved 1979). ANSI/ASTM 2513-78ES, Standard specification for thermoplastic gas pressure pipe, tubing & fittings, 1978 Shannon, R W E and Wells, A A. Brittle crack propagation in gas filled plpehnes, The Queen's University of Belfast, Belfast, Ireland, Int. J. Fracture, 10 (4) (1974), pp. 471-86 Anon Long-life safety of PVC water pipe, Modern Plastics (November 1974), pp 41-3. Carroll, M M_ et al, Sandia National Laboratories Internal report on PVC pipe fadure, 1981 Schrock, B Jay (Ed) International Conference on Underground Plastw Pipe, American Society of Civil Engineers, New York, 1981 Proceedings of the 4th International Conference on Plastw Pipes, University of Sussex, Falmer, Brighton, 28-30 March 1979_ Plastic & Rubber Institute, London. Benjamin, P A review of current practice Llndner, H. Applications and apphcational limits of plastic plpework systems for industrial installations_ Berry, D B S. Standard testing of pipes and examination of failures Taylor, G Warren, Jr, Slip lining of municipal sewer mains with HDPE piping in the US International Standards Organization, Use of PVC pipe & fittings, ISO/TC 138 (2)(3), 1978.