Analysis of failure in boiler industry--a case study.Introduction
For any Boiler industry to generate power, apart from the main equipments, some auxiliary equipments are also needed to support them for the generation of power. The equipments for the processes like fuel supply, air supply, ash removal, water supply are pulverizing mills conveyors, induction fans, forced draught fans, water circulation systems, etc. The pulverizing mill is used for converting the raw lignite into fine powder. This process is termed as pulverization. Pulverization is done in order to improve the utilization of the fuel efficiently. This helps in reducing the unburnt fuel in the boilers. Such pulverizers may lead to some failures. This leads to the reduction in power generation. Each boiler consists of pulverizers for fuel pulverization. If any one of them fails, the fuel supply from that mill will be stopped and it reduces the generation of power. In this article, some of the failures that occur in the pulverizers were identified, the consequences of failures were discussed and proper remedial actions or suggestions were given which will definitely reduce the failures in the pulverizing mills.
Construction Details of Pulverizing Mills
The mill consists of a hollow shaft on which discs with suspended hammers are provided. Spacers are provided to maintain the distance between the discs. The hammer holders are fixed upon the holder's pins that pass through holes in the discs. The hammers in turn are fixed to hammer holder pins. The hammers are made of high wear resisting material such as 13% of manganese steel. The mill housing is of welded structure. The inner surface of the housing is faced with inner plates to protect the walls against adhesive effect of lignite and hammer is about 30mm which increases with wear on the hammers and wear plates. The side walls of housing are provided access for replacing the worn out hammer and holders. The mill rotor is mounted on the double row self adjusting roller bearings. A water jacket assembly is fitted in the bearing housing. Cooling water flows from the free end to motor end through a metal tube and comes out through the hollow space of the rotor, there by cooling the rotor which is subjected to the heat of gas and mixture during the fact. Each mill is provided with emergency button nearby for stopping the mill in case of emergency.
Identification of failures in the industry
During the operation of pulverizing mills, different types of mill break downs, outages occur due to many reasons. Some are inevitable, for example, Failure due to wear and tear and some are accidental, for example failure due to foreign material entry.
Pulverizing mill break downs has a great significance in the operation of a boiler unit as it leads to a considerable drop in steam generation and power generation. Some common failures in pulverizing mills of the industry are also discussed.
(1) Hammer throw off.
(2) Bearing Failure.
(3) Rotor shaft failure.
(4) Wear plate failure.
(5) Shield plate failure.
(6) Over loading of mills.
(7) Knocking of bearings.
Hammer Throw Off
This refers to a failure which occurs due to dislodging of hammers from its position. It is a chain/cascade process and dislodging of one hammer leads to hitting the other hammer and there by the damage is much with in a short time span.
Hammer throws off
Normally, hammer throw off occurs due to entry of foreign material with lignite. When the foreign material hit the hammer it is dislodged from its position and leads to a cascade process of hammer damage. Some times worn out hammers, holders also happen to be the reason for hammer throw off.
Consequences of Hammer Throw:
If the mill is not stopped immediately, hammer throw off can cause much damage to many parts of the mill namely, hammer, bearings, rotor, shafts, insulation plates and leads to imbalance of rotor.
There are two bearings at both the ends of the rotor(one is free and the other is coupling end). Any damage to the bearings will lead to stoppage of mill and needs minor replacement.
Rotor Shaft Failure
This is a major failure in mills. Normally rotor shaft failure occurs due to shearing of the shaft. The failure generally originate from the key hole area of the shaft and propagate to other parts of the shaft and finally the whole shaft is sheared off. Though the rate of occurrence is much less, the failure leads to stoppage of mill for up to 2 days.
Wear Plate Failure
Wear plate is fastened to the base plate or mother plate. Due to failure of fasteners, which fix the wear plate with mother plate, rub with hammers and lead to over loading of mill or hammer throw off.
Shield Plate Failures:
Due to continuous exposure of high temperatures welding joint failure occurs in the plate and that leads to overloading of mills. The temperature in the mill shaft is to be strictly adhered to avoid plate failures.
Overloading of Mills:
This is another type of performance failure which leads to stoppage of mill. Various factors are responsible for overloading of mills. They are worn out hammers, more air ingress and choke in pulverized fuel burner.
Knocking on Bearings"
This is an indication of imbalance of rotor or failure of bearings. The mill has to be stopped and inside of mill are to be inspected to find out the reason for knocking.
Water line failure
Though the possibility of failure is remote, the consequences of the failure are very serious leading to mill shaft(rotor shaft) failure.
The methodology used in this article is Failure Mode and Effect Analysis(FMEA). It is used to identify the potential failure modes for a product or process, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.
Types of FMEA
(1) Functional FMEA: This type of FMEA assumes a failure and then identifies how that failure could occur. The functional approach is typically used when individual items cannot be identified or a complex system exists.
(2) Hardware FMEA: The hardware approach investigates smaller portion of the system, such as sub assemblies and individual components. The hardware approach generally involves a bottom up analysis in which the effects of possible failure modes of a subsystem, assembly, component, part, etc. on the entire system are identified.
Terminologies In FMEA
Resolution: Decide on an appropriate system level at which FMEA is performed.
Focus: The FMEA may be identified to determine the effects of failure modes on individual areas such as safety, mission & success or repair cost.
Potential Mode: These are the ways the system or component might fail.
Potential Effect: The consequence of failure mode may be on the operation, function or status of an item. Failure effects are usually classified according to how the entire system is compacted.
Failure Causes: The physical or chemical process, design defects, Part misapplication, quality defects or other processes that are the basic reason for failure or which initiate the physical processes by which deterioration proceeds to failure.
Severity: The consequences of a failure as a result of particular failure mode, the severity can be ranked from 1 to 10 based on its effects. A table shown below rank the severity based on the effect.
Occurrence: The probability that a problem may occur again & again is occurrence. The table given below deplicts ranking for the occurrence.
Detection: The probability that an operator or maintenance crew cab discover the failure by diagnostic action. The ranking table for detection is given below.
Analysis Procedure Followed in FMEA
(1) Assemble the Team.
(2) Establish the ground rules.
(3) Gather and review relevant information.
(4) Identify the items or processes to be analyzed.
(5) Identify the functions, failures, effects, causes and controls for each item or process to be analyzed.
(6) Evaluate the risk associated with the issues identified by the analysis.
(7) Prioritize and assign corrective actions.
(8) Perform corrective actions and re-evaluate risk.
(9) Distribute, review and update the analysis as appropriate.
Methods of FMEA
Most analyses of this type also include some method to assess the risk associated with the issues identified during the analysis and to prioritize corrective actions. Two common methods include Risk Priority Numbers (RPNs) & Criticality Analysis. The most frequently used method is the Risk Priority numbers. The steps are given below.
(1) To use the Risk Priority number (RPN) method.
(2) Rate the severity of each effect of failure.
(3) Rate the likelihood of occurrence for each cause of failure.
(4) Rate the likelihood of prior detection for each cause of failure.
(5) Calculate the RPN by obtaining the product of the three ratings.
RPN = Severity x Occurrence x Detection
The RPN can then be used to compare issues within the analysis and to prioritize problems for corrective action. The risk assessment method is commonly associated with Failure Mode and Effect Analysis(FMEA). The Failure modes, Effects and Analysis procedure is a tool that has been adapted in many different ways for many different purposes. It can contribute to improved designs for products and processes, resulting in higher reliability, better quality, increased safety, enhanced customer satisfaction and reduced costs. The tool can also be used to establish and optimize maintenance plans for repairable systems and contribute to control plans and other quality assurance procedures.
[FIGURE 1 OMITTED]
After collecting the datas for 6 months from July to December and the total equipment outages (in hrs) are tabulated below.
Results and Discussions
The following suggestions were made after analyzing the failures in the boiler industry.
(1) Magnetic separators must be installed at intake side of the mill to avoid the entry of foreign particles into the mills. This reduces the problem of hammer throw off.
(2) The conveyor belt speed must be reduced from 9.3m/sec to an optimum speed of 4.2m/sec in order to separate foreign materials from lignite. Because slower speed increases the contact of lignite with magnetic separator.
(3) Width of the belt over the conveyor should be increased from 900mm to 15600mm as increased width of the belt ensures greater contact of lignite with magnetic separator.
(4) Inspection of bearings must be done at regular intervals to avoid bearing knocking at free and coupling end of the mill. Also proper lubrication must be done to avoid damages due to friction.
(5) The cooling water line must be coated with paint to avoid the corrosion of the pipe lines. Also the replacement of cooling pipe lines at regular intervals of time must be carried out.
(6) The shaft material must be surface hardened to increase the fatigue strength. This must be done to avoid the fatigue failure.
(7) The shaft key material must be replaced with C55 Mn75 whose tensile strength is 700-850 N/[m.sup.2], where as the existing one is made of C50 and its tensile strength is 650 N/[mm.sup.2].
(8) Preventive maintenance schedule is to be modified and its frequency must be increased.
(9) The hollow shaft must be replaced with splined shaft to avoid shaft puncture.
Gain In Generation Due To The Suggestions.
Mill outages during the six months period:
Hammer throw = 621.20 hours
Bearing Failure = 163.30 hours
Shaft cut = 42.70 hours
Total = 827.20 hours
The stoppage of the mill for an hour would account to a loss of 5 MW in power generation.
Therefore, for 827.20 hours,
Power generation = 827.20 x 5 MW
= 4136 MW
For one month = 689.33 MW
So, generation loss up to 689.33 MW per month can be avoided by following the suggestions mentioned above.
Amount gained in one month = 689.33 x [10.sup.3] x 1.82
= Rs 12, 54,580.60
Amount gained in one year = Rs 1, 50,54,967.20
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(1) C. Sowmya Danalakshmi and (2) G. Mohankumar
(1) Lecturer, Karpagam College of Engineering, Coimbatore-32 Corresponding Author Mail-id: firstname.lastname@example.org
(2) Principal, Park college of Engg. & Tech. Coimbatore.
Table 1: Severity Ranking. EFFECT RANKING Hazardous with & without warming 10,9 Very high 8 High 7 Moderate 6 Low 5,4 Minor 3 Very minor 2 None 1 Table 2: Occurrence Ranking. EFFECT RANKING Very high: Persistent Failures 10,9 High: Frequent failures 8,7 Moderate: Occasional Failures 6,5,4 Low: Relatively few failures 3,2 Remote: Failure is unlikely 1 Table 3: Detection Ranking. EFFECT RANKING Almost impossible 10 Very remote 9 Remote 8 Very low 7 Low 6 Moderate 5 Moderately high 4 High 3 Very High 2 Very High 1 Table 4: Equipment outages (July--December). S.No. Name of the Outage Total Time(in hrs) 1 Preventive Maintenance 80.25 2 FE bearing knocking 127.20 3 Hammer throw 621.20 4 Inspection 15.47 5 CE bearing knocking 36.1 6 E/R 114.40 7 U/T shaft diaphragm change 15.50 8 D/T shaft diaphragm change 27.20 9 Vibration 2.55 10 Miscellaneous 121.55 Table 5: Failure Mode & Effect Analysis for all failures. Component Potential Potential details/ failure effects of Severity Potential functions mode failure causes Hammer Hammer Damage of 5 Dislocation of throw, structures fasteners, hammer Poor &foreign worn out pulverization materials entry, mother plate & hammer clearance, Mother plate rubbing Shaft Shaft Breaking in 1 Fatigue failure shaft, damage 0 to bearing housing Mother plate Mother Reduction in 5 Worn out plate pulverization fasteners & defect mother plate itself Shield plate Shield Air ingress, 4 Welding failure plate overload to & tear of plate defect mills, poor pulverization Main door Main door Air ingress, 2 Locking defect overload to mechanism & mills, poor hinge failure pulverization Housing Housing Damage of 7 Improper damage bearings balancing, coupling & fastener damage Bearing Knocking Vibration & 5 Friction due to at both abnormal improper ends noise lubrication Knocking Vibration & 5 at abnormal coupling noise end Knocking Vibration & 5 at free abnormal end noise Temp in Damage of 2 Imbalance of bearings bearings rotor & bearing defects Cooling Cooling Shaft failure 7 Corrosion of condensate condensate pipes system line punture Component details/ Occurrence Detection RPN Recommended Actions functions Hammer 10 2 100 Increase magnetic separators, check holders & mother plate thoroughly, Shaft 1 8 80 Reduction of disc diameter, higher fatigue material, high strength key material Mother plate 2 7 70 Redesign fasteners Shield plate 1 8 32 After Mill and before mill temperatures maintained in limits Main door 1 8 16 Constant checking locking and hinges Housing 1 5 35 PM frequency can be increased with special weightage, checking of sound of bed bolts Bearing 2 7 70 Check Housing clearance, rotor balancing & hammer 1 7 35 missing 2 7 70 2 5 20 Cooling 1 5 35 Periodic replacement condensate of piping, painting system with corrosive resistant paints