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Failure analysis for grey cast iron water pipes Makar, J. M.
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Failure Analysis for Grey Cast Iron Water Pipes
Jon Makar, Ph. D.
Urban Infrastructure Rehabilitation Institute for Research in Construction
National Research Council Canada 1500 Montreal Road, Ottawa, Ontario
K1A 0R6 Canada
Résumé
La plupart des opérateurs de réseau de distribution d’eau sont familiers avec les dégradations typiques des conduites en fonte grise : bris circulaires, fissurations de la cloche, corrosion autour des regards et fissures longitudinales. Si les forces qui sont à l’origine de ces dégradations sont connues, en revanche, les processus métallurgiques qui interviennent entre ces forces et les dégradations visibles ne sont pas bien compris. Une façon de comprendre ces procédés consiste à analyser les dégradations des conduites brisées. Les résultats des analyses effectuées sur 35 conduites de fonte grise de 4”, 6” et 12” qui ont été extraites seront présentées. Cette étude a montré que, souvent, les bris circulaires et les fissurations des cloches ne se produisent pas en une seule fois. Il arrive que la conduite se fissure en partie mais reste en service pendant un certain temps avant que n’ait lieu la rupture définitive. On discutera des causes possibles à l’origine de ce comportement ainsi que des répercussions sur la gestion du réseau de distribution d’eau.
Introduction
A large water utility may have 300 or more failures a year in its distribution system and a smaller number of more serious breaks in its large diameter transmission lines1. These failures are expensive to repair and often disrupt water supply and create property damage. The large number of failures experienced each year mean that the failure modes typical of grey cast iron pipes are well known to the water industry. These modes include circumferential breaks, bell splits, longitudinal splitting and corrosion through holes. Although the precise ratio of the failures modes in a given water system depends on the local soil conditions, external loading and pipe diameter, circumferential breaks are the most common failure mode in small diameter pipes, with longitudinal splitting and corrosion ‘through holes’ being more common in the larger diameter pipes.
The forces that produce pipe failures are also well known2. Some of the most important effects include corrosion, internal pressure, frost loading in northern climates, truck loading, thermal stresses due to differences in ground and water temperature, bending due to poor bedding and the forces produced by expansive clays. However, the connection between these forces and the failure modes seen by a water utility’s employees as they repair pipe breakages are not as well understood. The typical sequence of a pipe failure has often been viewed as:
• local stresses exceed the strength of the material;
• the pipe breaks; and
• emergency repairs are required.
Recent work by the Institute for Research in Construction has indicated that the failure process is somewhat more complicated than this picture3. In many older pipes the cast iron itself plays a role in the failure. More importantly, there is evidence that a pipe failure does not happen all at once in circumferential breaks and bell splits. Instead, the failure may take place in multiple stages, raising the possibility of finding damaged pipes before they completely fail, rather than reacting to them after the failure has taken place.
Although failure analysis is not in common use in the water industry, it can be a useful management tool in the right circumstances, providing useful information for decisions on whether to repair, rehabilitate or replace a damaged pipeline. This paper provides background information on failures and failure analysis and on the failure process in grey cast iron pipes. It presents the latest results of the Institute’s pipe failure research and finishes by discussing how failure analysis can be applied by the water industry as a management tool.
Failures and Failure Analysis
In the broadest sense, a water pipe has failed if it can no longer carry it’s intended flow from one end to the other without losses in pressure, volume or quality. Under this broad definition, problems with joint leakage, contamination, pipe fracture or tuberculation can all produce pipe failures. However, the most problematic and expensive pipe failures tend to be those associated with pipe fractures. Although the costs from a small diameter pipe failure are usually confined to the cost of the repair itself, the direct costs associated with a large diameter failure can be much higher. While the most obvious cost is still that of the repair itself, other direct costs include lost water/revenues, property damage and the possible liability due to injury or death associated with the pipe failure.
Indirect and social costs can add to the total bill from pipe failures. Some examples include the loss of public confidence in the water utility, lost production in industries such as breweries that are dependent on a constant water supply, accelerated damage to nearby infrastructure assets, contaminant intrusion into the water system, and the effects of loss of water supplies to institutions such as day care centres, schools and hospitals. The effects are often more difficult to quantify, but they add substantially to the real cost of a major pipe failure. Such indirect costs often result even from a small diameter distribution system failure. The scale is smaller, but the same effects can occur in a local area.
The direct and indirect costs provide a strong incentive for water utilities to develop an understanding of why they are having pipe failures and of how to prevent them. While the cost of rehabilitating a pipe may be greater than the cost of an individual repair, the direct, indirect and social costs of pipe failure will often mean that rehabilitation or pipe replacement is a better solution to a problematic water line than simply continuing to repair it. However, the high costs of rehabilitation and replacement mean that water utility managers need to be able to prioritise specific areas of their systems for renewal. Priority setting has often been based on the past
failure history of the local pipe section, with sections that reach a certain number of failures per kilometre (or mile) per year being considered for renewal. However, failure analysis can in certain cases provide an alternative method of condition assessment that can assist in making renewal decisions.
Failure analysis is a systematic investigation into the causes of the failure of an object, whether a pipe, brake drum or oil tank. Its practice requires knowledge of the material properties of the object that failed and of the external environment and forces that affect that object. There are essentially three reasons why failure analyses are conducted:
• for research purposes;
• for system management, in order to determine whether similar components are likely to fail and what preventative measures are required; and
• to establish legal responsibility in the case of a failure.
The same basic principles apply in each case. The site where the failure began should be found, the condition of the failed object at that site should be determined, and the cause of the failure identified. Management decisions require a further step, in that it becomes necessary to determine how frequently the causes of the failure will occur in other objects similar to the one being investigated. The application of these basic principles to the water industry are discussed in more detail below.
The research aspects of failure analysis are important for identifying the basic failure process. Once the failure process is known, engineers can use the information to conduct similar analyses on other systems. In addition, other research can be performed to find ways to detect the first indications of a failure or to prevent it from happening at all.
The Failure Process in Grey Cast Iron Pipes
Casting Flaws
Failures in grey cast iron pipes take place when the forces on the pipe exceed the strength of the pipe material. Corrosion will accelerate this process by reducing the pipe’s cross-section, but failures can still occur when no significant amounts of corrosion are present if the load on the pipe becomes high enough or the pipe has inherent casting flaws. Casting flaws are most likely to be a problem in the oldest, pit cast pipes, while simple corrosion pitting or graphitisation can produce failures in any cast iron pipe. In addition, pit cast pipe is inherently weaker than the more modern spun cast pipes4 because they have much larger graphite flakes than spun cast pipes5. These flakes act as crack initiator sites and produce a lower fracture toughness and mechanical strength. The early pit cast pipes also have other problems including porosity5, excessively large graphite flakes6 and inclusions5.
Large graphite flakes are typically found in the large diameter pipes, where slow cooling times promoted flake growth. Flakes more than 2 mm thick have been reported6. Porosity is a more serious problem. Pores form during casting when air bubbles are trapped in the metal as it cools. Particularly poorly cast pipes may be riddled with pores. These pipes will have very poor mechanical strengths, being in some cases so weak that the pipe metal fell apart as it was being
prepared for mechanical testing7. It is also possible that inclusions in the form of undissolved constituents such as ferrosilicon (used to add silicon to the molten cast iron) will be present. Each of these defects have similar effects on a pipe, reducing the overall thickness of the pipe wall and lowering the pipe’s strength.
Mechanical Strength
While locating corrosion pits, inclusions, pores and large graphite flakes can help to identify the causes of a failure, knowledge of the actual mechanical strength of the pipe is essential as it will enable the investigator to determine whether the pipe was likely to have failed due to the forces it should be routinely expected to encounter or whether some extraordinary load was responsible for the failure. It should not be assumed that a pipe will have the same mechanical strength as was required by the pipe standards that were prevalent at the time of manufacture. Recent mechanical tests on pit and spun cast pipes show a wide variation of tensile strengths and fracture toughnesses4. Measured pit cast pipe strengths ranged from 33 MPa to 231 MPa, while the values for spun cast grey iron pipes ranged from 58 MPa to 302 MPa. Multiple samples from the same pipe are likely to show different values, depending on the exact amount and distribution of graphite flakes and casting flaws in the pipe.
Analysis of the Fracture Surface
Most information about a failure is contained in the fracture surface, which is the area of metal exposed when the fracture takes place. Analysing grey cast iron pipe failures is particularly complicated because corrosion products form rapidly on the fracture surface once the pipe is broken. However, an examination of the corrosion products themselves can reveal useful information about how failures are taking place. At the time of writing, researchers at the Institute for Research in Construction in Ottawa have examined 27 failed pipes from the Regional Municipality of Ottawa-Carleton (RMOC) and the City of Toronto. These pipes had circumferential, longitudinal, bell split and corrosion through hole failures and ranged in diameter from 100 mm (4”) to 300 mm (12”). The examination showed that certain types of pipe failures from both cities are produced by a multi-stage process.
A visual examination of the fracture surface of a bell split pipe from RMOC found that there were two distinct regions of corrosion on the surface4. The area closest to the bell end of the pipe had a thick (0.4 mm) deposit of orange to orange-grey corrosion products. The area nearest to the point where the crack stopped had a layer of dark red-brown corrosion products that was too thin to be measured using a vernier caliper. The grey graphite flakes that give grey cast iron its name could still be seen through the latter deposit of corrosion products. This examination suggested that there were two distinct fracture events during the failure, with a considerable length of time between them.
A chemical microanalysis was conducted on the sample using a scanning electron microscope’s energy dispersion x-ray analysis system. The microanalysis showed very different chemistry in the two regions of the fracture surface. The thick corrosion region showed strong peaks of aluminum, potassium, sulphur, silicon and calcium in addition to iron, while the thin corrosion region showed only weaker peaks of silicon and calcium. The oxygen that was present in the corrosion products and the carbon in the cast iron do not show up in this type of elemental analysis. While some of the silicon in the pipe is from the metal itself, the remaining elements are from the surrounding soil. The elemental analysis therefore confirms that the fracture surface at the bell end must have been exposed for a considerable length of time to the surrounding soil
in order to build up the chemically complex corrosion products. In contrast, the simpler chemistry of the narrow end of the fracture confirms the visual indications that the fracture surface had not been exposed to the outside environment for long before the pipe was removed from the ground.
Further examination of the pipe showed that there was a corrosion pit at the end of the bell where the crack had started. The joint of the pipe itself was sealed with leadite. This type of seal has been frequently associated with bell splits which are likely due to the differences between the coefficients of thermal expansion of the rigid, non-metallic seal and the metallic pipe. The examination of the pipe therefore suggests the following:
1. A corrosion pit formed at the bell end of the pipe, reducing the pipe’s strength
2. A crack initiated during a particularly cold or hot period, but did not cause the pipe to loose water.
3. At a later time, further cracking occurred, causing the pipe to fail and producing the leak that led to the pipe being replaced.
A similar analysis was conducted on a circumferentially cracked pipe from RMOC. The results again indicated that the pipe had failed in two stages with the failure initiating at a corrosion pit. In this particular case the elemental microanalysis showed similar chemistry in the two different regions of corrosion products with the exception that presence of chlorine was identified only in the thicker region of corrosion products. Chlorine could only have entered the fracture surface from the soil around the pipe, so these results again indicate a multistage process where the second stage fracture surface was not exposed to the soil long enough to become contaminated.
Failures in six of the nineteen pipes removed from the City of Toronto’s water system were also due to circumferential cracking. These pipes had all been replaced before being cracked completely through. Typically about 20-25% of the pipe circumference remained intact at the time of removal. While in some cases only a single cracking incident had occurred before the pipe was removed from service, one of the pipes showed evidence that two stages of cracking had occurred before removal from the ground and two pipes showed evidence of three stages of cracking. A total of eleven circumferential failures have now been analysed from the water systems of the City of Toronto and the Regional Municipality of Ottawa-Carleton. All showed evidence of multi-stage cracking. The causes of this multi-stage failure process remain unclear. In many cases the cracks stopped growing when the two crack ends on either side of circumferential break no longer pointed in the same direction. Once this occurred the forces being applied to the pipe were no longer large enough to produce further cracking. However, other fracture surfaces appear to show a smooth surface without clear evidence of an object or event that arrested the crack growth. Further work will be necessary to fully understand this aspect of the failure process.
The remaining thirteen pipes from the City of Toronto showed nine instances of failure by corrosion through holes, two instances of longitudinal cracking and two cases where the only damage observed on the pipe section had been produced during excavation. All of the Toronto pipes had been supplied to the University of Toronto for a separate project7 on mechanical testing. The ratio of the failure modes are not typical of the City of Toronto’s experience – as other cities most of its failures are due to circumferential cracking.
Since the failures that have been analysed have all come from Ontario, it is unclear whether the different soil and climatic conditions in other parts of North America will produce multistage pipe failures. If the same multi-stage process occurs elsewhere, it may be possible to develop new approaches to distribution system management that will allow pipe failures to be prevented rather then being repaired after the fact. The main technical requirement would be the development of new diagnostic techniques to allow detection of the initial signs of failure. Once such technology is developed, it should become possible to deal with damaged pipes through scheduled repairs, rather than on an emergency basis. This change would reduce or eliminate many of the direct and indirect costs associated with pipe failures.
Failure Analysis as a Management Tool
Although further research will be required to determine if most pipe failures are caused by a multistage process, distribution system managers can also use failure analysis as a tool to choose pipes for rehabilitation or replacement. It is, however, important to use it selectively. In many cases an examination of the failures in small diameter distribution piping will not provide useful information, since the analysis will simply show that the pipe failed because of corrosion pitting and graphitisation. Failure analysis is most likely to be useful in cases where there is already some reason to believe that the pipe failure may be connected to material properties. Three instances where a failure analysis or metallurgical examination should be considered are:
• failures of very old pit cast pipes due to the likelihood of porosity and inclusions;
• failures in areas of the distribution system which have anomalously high break rates compared to other regions with pipes of the same age buried in similar soil conditions; and
• failures of large diameter pipes, where it is important to precisely understand the cause of the failure in order to ensure that no further failures will occur.
In each case the basic goal of the analysis is to determine whether the failure is due only to external forces such as corrosion or ground loading or if the pipe material was partially responsible for the failure. Once the cause is known, the manager must then determine if other pipes of the same vintage and with a similar surrounding environment are likely to have the same problems. Such a determination may require further examination of other pipes using coupon extraction.
A failure examination should generally be carried out by engineering personnel who are familiar with the failure analysis process. If an independent consultant is hired, the staff of the water utility must ensure that the particular conditions and problems associated with water main failures are known to the consultant, including the age of the pipes, effect of variations in casting technique and the extent of graphitisation likely to be encountered in the pipe. To conduct a failure examination:
1. Photographs be should taken and written records made throughout the examination. This is particularly important if the results may used to establish responsibility for the failure.
2. The cause of the failure should be identified by examining the fracture surface. In some cases the cause may be obvious, but in others it may be necessary to determine the cause of failure by looking at the branching of the fracture surface. The lines of
two branches form an arrow that points toward the crack initiation site6, although it is necessary to ensure that the damage observed was caused by the failure itself and not the excavation process.
3. The cause of the failure should be characterised as to type and the effect it has on the pipe. Effects may well be cumulative – large graphite flakes, porosity and a corrosion pit may all occur in the same area of a pipe, reducing its effective wall thickness to a small proportion of the nominal value.
4. Samples for mechanical testing should be taken from the pipe wall near the failure. If possible, the samples should be taken so that the tests will produce loads that are in the same directions as the ones that produced the pipe failure (i.e. axial samples for circumferential failures and circumferential samples for longitudinal failures).
5. An estimate of the load necessary to create the pipe failure should then be calculated, based on the reduced pipe wall thickness and the mechanical strength measured during testing. This estimate can be compared to the expected in-service loads to determine if the pipe failed under ordinary usage or an unexpectedly high loading was ultimately responsible for the failure.
6. Once the cause of the failure has been identified, it becomes necessary to determine whether other pipes in the system have the same problem. Some forms of non-destructive testing can measure corrosion pit depths and may be useful if the failure was caused primarily by graphitisation. Coupon samples are more likely to be useful if the cause was due to casting flaws.
7. The number of samples that need to be taken and the time period over which they should be taken will depend on the urgency of the analysis and the budget that allocated to the work. As many samples as possible within these constraints should be taken to provide a good statistical basis for the decision. It may be possible to extract samples during regularly scheduled work or unscheduled repairs if an immediate decision is not needed. More immediate decisions and decisions on work for larger diameter pipelines are more likely to require a special sampling program. 8. Once the analysis of the samples is completed, the utility manager should have a good
understanding of the ultimate cause of the failure and of whether similar failures are likely to occur in the future. This information can then be used to assist in determining an appropriate response to the failure.
Conclusion
Although failure analysis has not been commonly used in the water industry, it can be a useful tool for making specific types of decisions about replacing or rehabilitating pipelines. While many pipe breaks will not yield enough useful information to make a full failure analysis or metallurgical examination worthwhile, failures of large diameter pipes should be investigated to determine whether other pipes in the same pipeline are also likely to fail. Failure analysis can also be useful if smaller diameter pipes are showing unexpectedly high failure rates in order to determine if the cause of the failures is environmental or due to problems with the pipes themselves.
While corrosion pitting (graphitisation) is the major cause of pipe failures, other aspects of the failure process need to be investigated to fully understand why pipes fail. In some cases
the pipe metal itself is flawed, rendering the pipe more susceptible to breakage. The mechanical properties of cast iron pipes varies widely, making mechanical testing a better choice than relying on published standards as a means to obtain material strength. Recent work has also shown that small diameter pipes tend to fail in stages, rather than in a single episode. Although further research is needed, this finding raises the possibility of developing new diagnostic tools that will be able to detect pipes that are in process of failing. Such tools would allow water utility managers to prevent failures and their associated costs.
Acknowledgements:
Information on the costs of failures was provided by Dr. Yehuda Kleiner of the Institute for Research on Construction. The pipes studied for this paper were supplied by the Regional Municipality of Ottawa-Carleton and the City of Toronto. Michael Seica and Professor Jeffrey Packer of the Department of Civil Engineering at the University of Toronto provided access to the City of Toronto pipes. The research presented in this paper was funded by the National Research Council Canada.
References:
1. O’Day, D.K., Weiss, R., Chiavari, S., and Blair, D., Water Main Evaluation for Rehabilitation/Replacement, American Water Works Association Research Foundation, Denver, Colorado, 1986.
2. Rajani, B., Zhan, C. and Kuraoka, S., Pipe-soil interaction analysis of jointed water mains., Canadian Geotechnical Journal, Vol. 33, No. 3., 1996.
3. Makar, J.M., A Preliminary Analysis of Failures in Grey Cast Iron Water Pipes, to be published in Engineering Failure Analysis, 1999.
4. Rajani, B., Makar, J. and McDonald, S., Mechanical Properties of Grey Cast Iron Water Mains, paper submitted to the Journal American Water Works Association, 1998.
5. Makar, J.M. and Rajani, B., Grey Cast Iron Water Pipe Metallurgy, submitted to Journal of Infrastructure Systems, 1999.
6. Dickson, J.I., Failure Analysis of a gray iron water pipe (report prepared for Gesmec, Inc. Ottawa, Ontario), Laboratoire de caractérisation microscopique des matériaux, École Polytechnique de Montréal, 1982.
7. Seica, M., et. al., Evaluation and Testing of Cast Iron and Ductile Iron Water Main Samples, Interim Report to the City of Toronto, University of Toronto Department of Civil
