Failure Investigation Of Super Heater Tubes Of Coal-fired Power Plant: Risk Analysis, Theory And Solution

Research and Theory on the Problem

The report mainly discusses about the analysis of the failure that occurred in two adjacent super heater tubes that were made up of CrMo steel. This was a 60 MW thermal power plant that made use of the coal. The major reason lying behind the failure was due to the oxide deposition upon the internal surface of the tube and this deposition was initially responsible for the creation of a significant amount of resistance for the heat transfer process [1]. This in turn was associated with undesirable rise in the temperature of the components. Besides this the report has also been associated with proposing the risk mitigation strategy as well.

Specific Theory -In a power plant the prime importance is provided to the process required for the purpose of generating electricity without the existence of any kind of forced outages. The major concern for the forced outages in the coal fire station is due to eh failure of the super heater tube of the boilers. The major reason lying behind the damage occurring over the time of operation is due to the flowing of the fuel gases over the super heater tubes. This type of damage is often termed as the fireside damage or corrosion. This type of damage is generally dependent upon the coal quality, as well as on the materials that are being used along with the various operations and the maintenance. It is seen that the tubes interior is generally vulnerable and are primarily dependent upon the water quality that is used for the generating steam of high pressure [2]. So there is an essential need of a steady or continuous flow of steam so as to make sure that the materials of the tube are maintained under the temperature that has been prescribed. In case of its absence there exists the high possibility of shooting-up of the temperature which in turn is responsible for the detrition of the materials as well as the subsequent failure at a fast rate. For such instances the overall efficiency of the plant is entirely dropped. So it is very essential to analyse the failure of the tubes in order to find as solution for taking certain actions that would be helping in avoiding such kind of risks in the future [3].

The possible ways that are to be adopted in order to eliminate the further risks similar to the boiler incident have been listed below:

• The tube wall are to be cleaned at regular intervals in order to avoid similar incidents. The regular checking would be helping a lot in detecting any kind of defects if occurs before any kind of severe consequences. This would be helping a lot in removing all the hindrance or the heterogeneities as well [4].
• One major thing that is to be done is checking the quality of water. This is to be done at regular intervals. The major reason behind doing this is for the purpose of making sure that all the pH and the oxygen level is maintained at specified levels.
• Another problem which might occur is due to the contamination of the water. This mainly occurs due to the leakage in the tubes that might lead to high salt concentration in the water. By regular monitoring it is possible to eliminate this kind of risk as well.
• The strength of the materials are to be checked as well so as to eliminate the risk of the thickening of boundary walls [5].
• Maintenance of the oxide scale so as to make sure that there is smooth heat transfer that would be helping in the elimination of risks related to overheating of the tube walls.
• All this things are to be maintained in order to eliminate the risks related to bulging which mainly occurs due to high hoop stress that might be leading to failure.

The damage of the tubes were visible with naked eyes only. It was observed that both of the tubes were associated with experiencing a temperature of around 540 C and was associated with a stress level of around 100kg/cm_­^{2 ­}at the time of operation. In one of the sample it was observed that there existed fish mouth cracking I one side whereas on the other side of the welding there was a bulge. Besides this there also existed a layer wise corrosion at the open end of the crack [6]. Besides this there existed a drastic decrease in the thickness of the walls as well. There also existed multiple cracks in the inner surface in the longitudinal direction but this type of cracks were entirely absent in the outer surface. So it was observed that the outer surface was blackened that lead to thermal effect whereas the inner surface was entirely deep brown in colour having multiple red spots.

Some of the facts that were identified after analysing the entire incident have been listed below:

• Cr-Mo low alloy steel was used for the purpose of making the component and besides this the composition was very much close to the specification of the polish. Some of the specifications mainly includes the DIN 10CrMo910/T22. Besides this it was also observed that the major chemical concentrations were within the limit that was specified previously.
• It was seen that there existed polygonal fertile grains as well as alloy carbide in the iMicrostructure of the steel tube. At the time of service exposure there occurred the formation of oxide scales in the interior of the tube. During the time when it was observed that the thickness become substantial then it was time when the process of heat transfer was hindered all across the walls of the tube [7]. This initially led to the localized heating. Besides this the formation of the scales along with the growth as well as the subsequent removal due to the loosening from the materials surface occurred as a continuous process. Initially this was responsible for the reduction in the thickness of the tube walls. Despite of all this there was the condition when it entered the non-steady state phenomenon and along with this any kind of future quantification of the data would be associated with misleading of the information related to the high temperature deformation characteristics. In addition to this the heating was associated with the coarsening of the carbidesand was also responsible for propelling of the perception of the new brittle phases along with the boundaries of the grain. This was further responsible for the embrittlement [8].
• Facing excessive amount of stress as well as high temperature the sliding of the grain boundaries initially promoted the formation of void all along the boundaries and initially at the latter stage they were interconnected with each other leading to the weakening of the structure[9].
• Besides this there was a drastic change in the dimensions of the tube near the fracture and it looked like the materials of the tube got softened at an extreme rate. The evaluation as associated with further confirming of the material softening and this evaluation was generally done upon the bulk hardness that was observed near the location which failed. The values that were obtained was seen to be exorbitantly law when compared with the materials that were investigated. Additionally the earlier effect of the formation of the scale, and besides the softening was also responsible for effecting the flow of the materials along with the creation of the bulge.
• The inter-granular cracking along with the subsequent overloading of the ductile fracture under excessive amount of hoop stress that was beyond the yield point of the tube material initially lead to the failure at the time of exploitation of the service [10].

The most possible ways by which this kind of failures can be eliminated is by proper monitoring of the entire process. Everything needs to be monitored in a proper way along with which proper materials are also to be used of the purpose of creating a proper tube of the boilers which are essential for the purpose of eliminating this kind of the problems. Besides this there is also a need of proper cleaning as well so as to eliminate the formation of oxides as well as the subsequent spilling from inside the tubes wall. Besides this good quality water should also be used so as to prevent thickening of the walls [11]. Creep and softening of the materials is another problem that was responsible for the failure. Oxide scale management would be helping in the smoothing of the heat transfer process initially helping in the elimination of local overheating. Besides the steam pressure is to be decreased at a substantial rate so as to decrease the stress occurring due to high temperature creep. This would be helping in preventing the softening of the materials as well.

References

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