Modes Of Failure In Cast Iron Pipes - Causes And Impacts

Longitudinal bending failures in cast iron pipes

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Failure mode refers to the way in which cast iron pipes fail as opposed to the mechanism that brings about the failure. The modes of failure are determined by the size of the diameter of the pipe where small-sized pipes are found to be more prone to longitudinal bending failures due to the low pressure of water and small moments of inertia (Dawson, 2014). Larger pipe diameters have greater moments of inertia thereby producing longitudinal cracking and shearing at the bell of the pipe. The modes of failure include:

Bell splitting: Most common in pipes of small diameters in which the joints were initially made by sealing a rope the spigot of one pipe to the bell of another. Molten lead is then poured onto the joint to reinforce the seal. Molten lead was substituted with leadite, which was found to be having a higher thermal coefficient than lead being a non-metallic compound (Frankel, 2009). Leadite can cause splitting under very cold temperatures hence resulting in a failure of the pipe. 

Blowout holes: Corrosion causes most of the mechanical failures in pipes. Corrosion itself or in collaboration with an internal pressure of water can result in a failure of the pipe. Under such a case, corrosion pitting takes place until the walls of the pipe have thinned out to the extent that the pressure of the water blows out the remaining walls of the pipe (R.D.J.M. Steenbergen, 2013). A small or a large hole may result from this corrosion failure depending on the nature of localization of the corrosion process.

Circumferential cracking: Most occurs in pipes with diameters less than 380 mm and is caused by bending forces, which are exerted on the pipe (Giorgio Lollino, 2014). The failure resulting from the bending forces occur in a way that resembles twig snapping, and the failure crack goes on all over the circumference of the pipe. The failure may result from movements of soil that yield tensile forces on the pipe culminating in basic tensile failures.

Spiral Cracking: Occurs in pipes with diameters ranging between 380mm and 500 mm. Starts as a crack of the pipe in a circumferential manner, which then proceeds all through the length of the pipe in a spiral manner. The failure is associated with surges in the pressure of the water and can be linked to bending forces and the internal pressure in the pipe (Mohamed Ouessar, 2017).

Longitudinal Cracking: Mainly confined to large diameters and could be attributed to crushing forces acting on the pipe, internal pressure or even compressive forces acting on the surface of the pipe. Any of the mentioned loading on the pipe can result in a longitudinal crack, which once initiated spreads to the whole length of the pipe (Buchman, 2015). In other cases, the crack may be found to form on the opposite sides of the pipe, which finally results, into the removal of the top section of the pipe resulting into a hole that could be as long as the pipe.

Different types of cracks and shearing in cast iron pipes

Bell shearing: Pipes with large diameters fail by having a section of the pipe shear off which may result from compressive forces that would force the spigot of the pipe into the bell of the preceding pipe along the line of pipes. Bell shearing is more likely to be caused by bending forces. A crack may be produced by compressive loading and spread down the length of the pipe (Mohamed Ouessar, 2017).

Cast iron mainly experiences localized corrosion as the primary corrosion degradation. Localized corrosion refers to the process in which a metal is selectively removed by corrosion in small regions or zones on the surface of the metal, which is in contact with the corrosive environment, which is in most cases a liquid (Look, 2014). It takes place when tiny local sites are under attacks at relatively higher rates than the remaining surface of the metal. The corrosion rate in cast iron depends on the microstructural features of the various classes of the cast iron. Such features include pearlite, ferrite, graphite flakes (Mambretti & C. A. Brebbia, 2014). The level of existence of the some of the phases in the microstructure influences the level of corrosion. Localized corrosion occurs when corrosion works in conjunction with other destructive processes such as erosion, stress, fatigue and any other forms of chemical attacks on the metal. This type of corrosion is more destructive than any of the other destructive processes individually. Chloride stress corrosion, heat exchanger tube denting, caustic stress corrosion, pitting, wastage and intergranular attack corrosion are some of the different forms of corrosion (Buchman, 2015). Localized corrosion may lead to failures of the pipe as well as complete dissolution.

The Tammann model can be used in the incorporation of localized corrosion in the structural analysis of the corrosion losses. The model deploys mathematics in establishing the rate of diffusion of oxygen through the corroded layers that are formed on copper (Mikael Braestrup, Jan B. Andersen, Lars Wahl Andersen, Mads B. Bryndum & Niels-J Rishøj Nielsen, 2009). The model was further refined by Booth by introducing a pit or corrosion depth growth law. The law is as follows c (t) =At^B in which t is the time while A and B are constants. The model is usable in the estimation of pit initiation and early pit growth in pipes and as well forms the basis for much modeling of losses resulting from long-term atmospheric corrosion (Mounir Bouassida, 2017).

The performance of water pipes is affected changes in the seasonal climates and the changes in the volume of clay soils. These changes are responsible for the failures experienced in buried pipes. Still, failure of pipes is influenced by seasonal climates with an annual pipe failure pattern showing that the rates of failures of pipes are at peak during the times when the ground temperature is below the normal temperature. The rates of failures of pipes increase to a great extent when the pipes are laid in environments that are perceived to be aggressive for example in reactive clay (Council, 2017). Most of the failures in such environments are mostly experienced during the dry summer months, and during years that experience prolongs periods of dryness. Still, the distribution of failure rates of pipes in sandy soils and gravel is fairly random meaning that the failure can occur at any time of the year without necessarily being under significant influence from the seasonal climatic changes (David Crichton, 2009). Maximum failure occurs during the times of maximum soil moisture deficit.

Causes and impacts of localized corrosion in cast iron pipes

The patterns of failures of pipes are linked to the shrinkage of clays, which results in an extra load imposed on the buried pipes (Jie Han, 2011). On the same note, pipes with smaller diameters are more prone to circumferential failures, and the rate of failure is directly linked to the age of the pipe. This means old pipes are more prone to failures than newer pipes. The relationship between the failure of buried pipes and ground movement result from the expansive nature of the clay soils in which they are laid. Swelling reactive clays produce high stresses, which can cause significant damage to the pipelines. Such damages can be in the form of ruptures or cracks either in a longitudinal or circumferential direction (Garr M. Jones, 2011). The rate and intensity of failure are even higher in areas with reactive clay soils and experienced low annual rainfall.

Most of the failures in pipes are experienced in summer and early autumn months where evaporation and temperature are at maximum while rainfall is lowest. The reactive properties of clay are responsible for the differential longitudinal bending action in the pipes. Soils with higher plasticity indices exhibit higher pipe failures. This can be explained as those reactive soils have a significant role when it comes to rupturing in water pipes (Dawson, 2014).

When the date of installation, current thickness and a nominal thickness of the mains are known, the rate of degradation can be calculated. Schlick’s failure criterion is used in estimating the minimum thickness that is required to sustain the combined internal and external loading that the main will be subjected to. Schlick’s failure criterion is as shown: .

The current thickness of the pipe is obtained through direct measurement using the Echologics method, and a linear extrapolation is done using the measured thickness, the installation date and the nominal thickness (Limited, 2012). Calculation of the minimum required thickness to support the given load is done, and hence the Failure thickness is predicted. The loads that need to be supported include external pressure from the traffic loads and soil and internal pressures from the water columns.

Schlick proposed an empirical failure criterion to be used in the analysis of rigid pipes with large diameters and experiencing resistance. In this failure criterion, it is shown that the failure of a cast iron pipe as a rigid structural component is controlled by parabolic interaction curves, which can be illustrated in terms of normalized loading as well as water pressure by an expression (Buchman, 2015). In the expression, the rigid structural component is under the combined influence of an external load, wc, and internal pressure, p.  where P_c is the internal pressure that results into failure in the absence of external loading and W_c the external load which is required to cause failure in the absence of internal pressure. The method is commonly referred to as the combined loading analysis.

Maximum pressure is estimated using Barlow’s formula, P=2St/D where P is the pressure, S is the stress; t is the wall thickness in m and D the outside diameter of the pipe in m.

P=2St/D

600= (2*S*5/1000)/900/1000

600=0.01S/0.9=0.0111S

600=0.0111S

S=600/0.0111=54054.05MPa

Factor of safety=Strength/Stress

=100/54054.05=0.00185

Q5. Pipe Stiffness factor,

For a deflection of 7.5% PS=30/7.5=4

For a deflection of 5%, PS=30/5=6

For a deflection of 3%, PS=30/3=10

Q6. The mean and standard deviation in the predicted lifetime based on the Schlick’s failure criterion

=0.463

X	x-	(x-)2
0.62	0.157	0.024649
0.40	-0.063	0.003969
0.49	0.027	0.000729
0.49	0.027	0.000729
0.53	0.067	0.004489
0.57	0.107	0.011449
0.54	0.077	0.005929
0.52	0.052	0.002704
0.53	0.067	0.004489
0.48	0.012	0.000144

Variance=(x-) ²/10

=0.054791/10=0.0054791

Standard deviation= sq. root of variance

=

• Failure is only caused by corrosion
• The corrosion defects have the same shapes and size

References

Buchman, A. L. (2015). Nutritional Care of the Patient with Gastrointestinal Disease. London: CRC Press.

Council, N. R. (2017). Drinking Water Distribution Systems: Assessing and Reducing Risks. New York: National Academies Press.

David Crichton, F. N. (2009). Adapting Buildings and Cities for Climate Change. Kansas: Routledge.

Dawson, A. (2014). Water in Road Structures: Movement, Drainage & Effects. London: Springer Science & Business Media.

Folkman, M. S. (2010). BURIED PIPE DESIGN 3/E. Msnchester: McGraw Hill Professional.

Frankel, M. L. (2009). Facility Piping Systems Handbook: For Industrial, Commercial, and Healthcare Facilities. New York: McGraw Hill Professiona.

Garr M. Jones, P. D. (2011). Pumping Station Design: Revised 3rd Edition. New York: Butterworth-Heinemann.

Giorgio Lollino, D. G. (2014). Engineering Geology for Society and Territory - Volume 6: Applied Geology for Major Engineering Projects. Chicago: Springer.

Jie Han, D. E. (2011). Geo-Frontiers 2011: Advances in Geotechnical Engineering. New York: American Society of Civil Engineers.

Limited, S. A. (2012). Pipelines: Gas and Liquid Petroleum, Part 1. Sydney: Standards Australia & New Zealand.

Look, B. G. (2014). Handbook of Geotechnical Investigation and Design Tables: Second Edition. London: CRC Press.

Mikael Braestrup, Jan B. Andersen, Lars Wahl Andersen, Mads B. Bryndum & Niels-J Rishøj Nielsen. (2009). Design and Installation of Marine Pipelines. London: John Wiley & Sons.

Mohamed Ouessar, D. G. (2017). Water and Land Security in Drylands: Response to Climate Change. Oxford: Springer.

Mounir Bouassida, M. A. (2017). Ground Improvement and Earth Structures: Proceedings of the 1st GeoMEast International Congress and Exhibition, Egypt 2017 on Sustainable Civil Infrastructures. London: Springer.

Mambretti & C. A. Brebbia, (2014). Urban Water II. Chicago: WIT Press.