By Jon Makar, Ph.D., P. ENG
Prone to FailEngineering
The water industry faces unique problems in operating its pipeline systems. In most branches of engineering, a single structural failure is a matter for grave concern. In a water distribution system, ...
The water industry faces unique problems in operating its pipeline systems. In most branches of engineering, a single structural failure is a matter for grave concern. In a water distribution system, such problems are a matter of course, with some larger cities experiencing 300 or more failures a year of small diameter (150 – 300 mm) mains. Water utilities simply attempt to manage the damage in order to minimize their total costs, rather than trying to prevent the damage entirely.
The utilities’ response to the problem is driven by the fact that the cost of repairing small diameter failures is usually significantly cheaper than replacing or rehabilitating them. It is only when multiple failures occur (or are predicted to occur) within a single kilometre of pipe that replacing it becomes economically reasonable. Secondly, the commodity transported in water pipes is environmentally benign compared to other commodities such as oil and natural gas. A failed water pipe will not pollute the surrounding environment, burn or explode. As a result, although preventative measures such as cathodic protection or remote field inspection can reduce the risk of failure, simply repairing the breaks and choosing the most cost effective time to replace the line may be the best response.
The situation changes when large diameter (450 mm+) transmission line pipes fail. These failures are rare, but their consequences are so severe that the water utility is better off in taking strong proactive preventative measures. Transmission mains are vital lifelines for the cities they supply, providing an essential service. In addition, the damage produced by a large diameter main failure in a downtown core can run into millions of dollars and has the potential to inflict difficult-to-measure social costs. The recent failure of a large diameter grey cast iron main in Boston produced over US $10 million in damage when it flooded the basement of the city’s main public library and damaged a collection of historical documents.
The National Research Council Canada (NRC) Institute for Research in Construction (IRC) has investigated failures in both transmission and distribution systems, and particularly in grey cast iron pipes. Grey cast iron is the most common piping material in most North American cities and generally the most prone to failure. The NRC study has given us a clearer understanding of the causes of these pipe failures.
Corrosion is well known in the industry as a problem for all diameters of grey cast iron pipes. Failures due to corrosion penetrating the pipe wall become more common as the diameter of the pipe increases. In addition to the pipes that fail directly due to corrosion penetrating the pipe wall, approximately 90% of the other failures examined by NRC were associated with corrosion pits or graphitisation (the process where the iron is leached out of the pipe, leaving behind a weak graphite flake matrix and the appearance of an intact pipe).
Another significant cause of failures is casting defects. They often take the form of air pockets left in the metal of the pipe after it was cast, but they can be inclusions in the pipe metal, changes in pipe wall thickness, or the results of poor heat treatment. Both corrosion and casting defects will cause structural weakening of the pipe, leading to failure under design loading conditions. Although rare, it is also possible that failures can occur in structurally sound pipes. Inadequate original design may be the cause of this type of failure, but such problems are most likely to occur in the initial stages of a pipeline’s operation. Unanticipated increases in traffic, soil or other loads are the most likely causes of breakage in sound pipes.
The action of environmental forces on pipes also produces a number of different mechanical failure modes in addition to corrosion penetration. Typically around 80% of small diameter pipes (300 mm or less) fail in bending by circumferential cracking. Tensile failure due to ground forces and cracks at the bell of the pipe due to differences between the thermal coefficient of expansion of the pipe and the joint sealer (bell splitting) may also occur. Medium diameter pipes (450 mm – 500 mm) sometimes fail by spiral splitting, with an initial circumferential crack that then moves down and around the length of the pipe. Larger pipes (600 mm or more) will often fail by longitudinal splitting or the shearing off of a bell section of the pipe.
When these failure processes have been considered in the past, it has been assumed that pipes fail in a single episode. Recent work by NRC, however, has indicated that many circumferential failures and at least some bell splits occur in a series of events, with the pipe cracking part way through and then stopping, leaving the pipe wall still partially intact. The faulty pipe may even be identified and removed from service before final damage occurs . This work means that it may be possible to refine evaluation techniques such as leak detection or remote field eddy current inspection to improve the chances of finding partially intact pipes and repairing them before a complete failure has taken place. The research has also provided new information to help decipher the full failure process for these pipes.
A second part of NRC’s work has involved examining past failure analyses by consultants and water utilities. This review has suggested that clear, broadly accepted procedures need to be developed to ensure that accurate and comprehensive failure analyses are carried out after a large diameter main failure.
Some past work has missed essential parts of the analysis process, either because certain tests were not being performed, or because incomplete samples were delivered to the consultants who were conducting the analysis. In addition, some utilities appear to have assumed after reading the results of a failure analysis that, since the material of their pipe was shown to be in good condition and without corrosion, they needed simply to replace the pipe. However, a structural analysis should be considered in these cases as it may be necessary to strengthen the pipeline or remove the forces that caused the failure in order to prevent future problems.
The National Research Council is developing failure analysis guidance for grey cast iron pipe. The work will include guidelines on what decisions should be made before a failure has taken place (which pipe failures warrant a full failure analysis, who should do the work, what type of expertise needs to be available), the on-site work as the failure is repaired (types of information to be recorded, the use of cameras and videotapes, types of samples to be taken), the laboratory tests that should be performed, and the decisions that need to be made at the end of the process. The guidelines are intended to allow water utilities and consulting engineers to use the knowledge NRC has acquired about the failure process to make better decisions about their pipe systems. CCE
Jon Makar, Ph.D., P.Eng., is a research officer with the Institute for Research in Construction at the National Research Centre of Canada in Ottawa.
More information about NRC’s failure analysis work an be obtained by contacting him at (613) 993-3797, e-mail firstname.lastname@example.org, or by visiting http://www.nrc.ca/irc/uir/bu/ condition/failure.html