Failure Analysis Of A Connecting Rod In A Reciprocating Compressor

Students are required to:

1. Choose one engineering failure or disaster (due to fatigue, impact, overload, resonance, wear…) of your interest 

2. Write a report to detail the failure itself, the findings, and solutions (10%). Prepare and deliver a presentation to your fellow students

Causes of Connecting Rod Failure

The advancement in the development of technology and particularly equipment has raised a lot of concerns when it comes to the reliability, availability, and reduction of risks in equipment. This, therefore, means that the maintenance strategies of machines should be of greatest priority in any industry (Alarbe, 2010, p. 886).  The maintenance helps to cover up for the failures that come along with this system, and it should be in an efficient manner.  Since failures in machines cannot be avoided, a mechanism known as failure analysis is applied to find out and assess the cause of these failures (Hutchings, 2016, p. 433). This study is significant in the following ways;

• it not only helps to locate the cause of a fault but also helps in eliminating them
• it creates a technical knowledge on the assets hence availing it at the maintenance department
• it leads to improvements in the future preventive plans
• Also, failure analysis will bring about positive impacts such as cost reduction and client satisfaction(Hammill, 2015, p. 65).

A reciprocating compressor converts the power from prime movers i.e. diesel engine or a turbine into kinetic energy. It does this by compressing and pressurizing gas and the discharging it into a pressure system. Reciprocating compressors are applied in various industries such as oil refineries, chemical plants, gas pipelines, etc. The compression is done by the connecting rod which transforms the rotary motion of the crankshaft into reciprocating motion of the piston in a cylinder (Hault, 2016, p. 67).

The connecting rod has two ends, thus by the aid of a piston pin one end is connected to the piston while the other end revolves around the crankshaft. It is separated in a way that it makes it possible for it to be clamped around the crankshaft (Memorial, 2012, p. 123).The connecting rod is produced by use of casting materials with excellent mechanical properties such as steel, ductile iron cast, titanium or aluminum. The aluminum or titanium ensures high performance   whereas the iron reduces the cost .in addition, the connecting rod is normally made of nodular graphite cast iron that is frequently taken as a substitute of the cast steel components (Eqe, 2010, p. 321).

 When the engine is in operation, the connecting rod in the reciprocating compressor will be susceptible to compression, tensile and bucking loading. More often, the failure of the connecting rod can be attributed to the damaged or broken connecting rods shank especially when there exists a possibility of being pushed to the side of the crank case hence resulting in failure of the reciprocating compressor. This makes the engine impossible to repair (John R. , 2012, p. 700).

Failures in a con rod are mainly due to fatigue near a physical defect in the connecting rod. Other factors include: cracking, overheating of the engine, failure of connecting rods due to defects, lubrication failure in the bearing due to inaccurate or faulty maintenance, improper tightening and lastly using stressed bolts where not recommended (Lauralice de Campos Franceschini Canale, 2008, p. 266). The diagram below shows an example of a failure of a connecting rod.

Preventive Measures to Avoid Engine Failure

 When a connecting rod is subjected to a repetitive and alternating stress, it will fail at a lower stress that is required to cause fracture (C., 2014, p. 229). These failures that occur after a considerable period of service under conditions of dynamic loading are referred to as fatigue failures. They account for 90 % of all the failures in a connecting rod due to mechanical causes. Fatigue failure can be recognized by inspecting the fracture which will have two distinct surfaces (Jones, 2014, p. 542).i.e. one being  rough as a result of rapid failure and the other smooth due to frequent rubbing of top and bottom surfaces .the stresses are applied in various ways such as;

Torsional – a situation whereby the connecting is twisted and untwisted along its axis. The shaft that is rotating and consequently driving the load will be exposed to torsional vibrations which are visualized in the crankshaft of the engine thereby leading to engine failure (Michigan, 2012, p. 339).

Axial – here the material is subjected to compression or tension along its axis.  Since during power strokes and compression, the connecting rod also gets in compression, and due to forces of inertia that exist in the running gear whenever the direction between inlet strokes and exhaust is changed by the piston, the rod will be put into tension. This will intensify the tension at the bottom end bolts resulting in cyclic stressing and consequently failure of the engine (Nishida, 2014, p. 290).

Bending – this is a situation whereby the inside of the connecting rod gets into compression while the outside gets into tension. It is as a result of thermal stressing or rather the temperature difference on the bottom surface and top surface due to overheating of the engine (Webster, 2011, p. 652).

To analyze the failures in connecting rod of a reciprocating compressor, finite element analysis, and metallographic testing methods are used.

This method is used for analyzing fatigue and in approximating the durability of the component which makes it be of more benefit than other techniques. When this process is integrated with MSC patran software, the different elements of the car can be analyzed from varied aspects such as fatigue thereby reducing the cost and time (Ullman, 2010, p. 775). This method also makes it possible to access the distribution of the strains and stresses over the entire component thus obtaining precise critical points (Hault, 2016, p. 448).

Calculations of Forces

To carry out a failure analysis in a connecting rod of a compressor resulting from fatigue, the below procedure is applied. Assuming the connecting rod in a compressor is under full reverse loading such that the load is applied then removed, and after that applied in the opposite direction to maximum loading (McCall, 2012, p. 992).

• First, the geometry will be imported, and the boundary conditions of maximum loading introduced. After that the fatigue tool is inserted
• to generate alternating stress cycles, the rod is under full reversed loading is specified
• After that, the von-misses will be applied and determined by comparing fatigue material data as a stress-life fatigue analysis(John, 2014, p. 446).
• Then the modification factor is defined as shown in the figure above
• Using the solve command, stress and fatigue calculations are performed
• the sensitivity of life on loading is then calculated  and the minimum and maximum alternating stress determined
• Finally, a graph of safety for design life is plotted(Acton, 2012, p. 530).

To prevent failures coming from the various stresses and forces experienced in the connecting rod of a reciprocating compressor, the weight of the rod should be reduced .this is aided by using a light weight connecting rod made of high forged steel strength which is based on middle carbon micro alloyed steel and increased portion of the vanadium content (Edward Ghali, 2007, p. 183). The design of the connecting rod should also incorporate the use of longer fillet radius to reduce the concentration of the stress. Furthermore, a simulation software for the assessment of fatigue life should be applied. Besides, grinding of the edges of the rod to a smooth radius and balancing of the connecting rod assemblies to equal weights also helps to reduce these stresses (Hutchings, 2016, p. 294).

Oil starvation between the bearings and journal results to premature bearing failure. The big ends of connecting rod in a reciprocating compressor are made of an alloy of aluminum with a 6% of tin that hardens the intergranular pools (Memorial, 2012, p. 372). During high temperatures, the tin melts and later solidify in the crankshaft journal providing temporary lubrication .this deposits are not able to tolerate embeddability I.e.  They cannot absorb foreign particles. The hard ferrous particles thus get deposited as debris and blacken the surface of the bearing (Michigan, 2012, p. 527).

To prevent these from being experienced, there are some suggestions that should be put into place; for example eliminating the central oil groove in the bearing, this will not only reduce the effects of reduction in effectiveness of bearing posed by the two inner edges but also increase the ratio in highly loaded bearings (Sridhar, 2011, p. 334).

Also, repositioning of the oil supply hole of the crankpin will improve the thickness of the oil film. The oil film pressure at regions of high hydrodynamic pressure is greater than the oil supply hole pressure and relocating the supply hole will ensure that there is continuous oil supply to the small end of the connecting rod (Hammill, 2015, p. 226).

Joints Fail due to displacement and fracture.  When a cap screw with low yield strength for the forces is applied to the reciprocating compressor, the crew will stretch. Lessening the load results to a loose nut which vibrates off the bolt. Alternatively using mismatched joint components will result in failure and consequently connecting rod of a reciprocating compressor gets lose (Nishida, 2014, p. 550). 

Conclusion

Reciprocating compressors involves encountering of long-term cyclic loads, and to prevent future failures occurring from losing nuts, fastener components of high tensile should be used which will include heat treatment after the fabrication process.  Also, when steel is under high load conditions, it should be avoided as it makes it vulnerable to fatigue failure (Webster, 2011, p. 442).

• Failure due to Physical Defects in the Connecting Rod

Physical defects such as fracture in the connecting rod of a reciprocating compressor may lead to the detachment of the connecting rod cap from the connecting rod (William, 2015, p. 618).  These fractures may come as a result of corrosion pit on the contact surface of the bearing shell which eventually propagates rapidly due to high cycle fatigue. This subsequently makes the cap to fail in overload conditions and breaks resulting in the fracture (John, 2014, p. 311). 

To prevent future failures emanating from fracture, frequent maintenance of original parts is necessary, and it is recommended to regularly inspect the connecting rods. A dye penetration tests should be conducted on the serrated area (McCall, 2012, p. 30).

The calculations of forces are calculated by considering the engine cycle as a link mechanism and then plotted using statistical software on a chart. Normal forces, axial forces and their resultant are obtained per degree of rotation (Ullman, 2010, p. 447).  These forces are taken at the two ends one at a time while the other kept constant, and after that the system is balanced.  Certain software’s help to carry out the calculations and plotting, i.e. Hyper Mesh software and Minitab 16 statistical software (C., 2014, p. 370).  

Conclusion

In this report, the failure analysis of connecting rod in a reciprocating compressor is discussed (Hattangadi, Failure Prevention of Plant and Machinery, 2014). The various causes of failures of connecting rods in a reciprocating compressor that consequently lead to engine failure are detailed. Besides, the preventive measures and solutions are also discussed. For instance, failures due to lubrication of the bearings can be prevented by eliminating the central oil groove in the bearing thus reducing   the effects of lowered effectiveness of bearing posed by the two inner edges but also increase the ratio in highly loaded bearings (Hutchings, 2016, p. 66).the oil supply hole can also be repositioned   improving the thickness of the oil film. Failures in connecting rod as a result of improper tightening can be reduced by using fastener components of high tensile which will involve heat treatment after the fabrication process (James, 2016). On the failures that come from physical defects of the connecting rod, they can avoid by regularly inspecting the connecting rods and performing dye penetration tests on the serrated areas (McCall, 2012, p. 348). Finally, the failures that emanate from fatigue imposed by the various torsional and axial forces on the connecting rod can be prevented by reducing the weight of the connecting rod. This is aided by using a light weight connecting rod made of high steel strength which is based on middle carbon micro alloyed steel and increased portion of the vanadium content. Also, simulation software for the assessment of fatigue life should be applied (Alarbe, 2010, p. 99).  

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