|NDT.net January 2003, Vol. 8 No.01|
Many utility systems in the world have power plants operating with fossil fuel. In the thermal power plants maximum requirements of fuel is a coal. The handling of this fuel is a great job. To handle the fuel i.e. coal, each power station is equipped with a coal handling plant. Maintenance of Critical Equipments for Coal Handling Plants (CHP) of Thermal Power Stations is typical job. The failures of these equipments have led to high maintenance and operation costs. Generally Non Destructive Testing (NDT) techniques adopted in the residual life assessment of power plant components like Boilers, Headers, Steam lines, Turbines, Feed water Heaters and Condensers. The reason for inspection depends on the component and its effect on plant operation. But one of the main systems of thermal power plant is coal-handling system. No such efforts are carried out to assess the life of coal handling plant component.
The concept of NDT, discussed in this paper for Coal Handling Plant is to offer significant benefits. Guidelines for implementation of NDT in CHP are also discussed in this paper.
To maintain an efficiently operating unit and avoid failure of critical equipment, it is necessary to maintain the critical parts of that equipment. There are varieties of critical equipments components in Coal Handling Plants. These components require routine inspection to ensure their integrity. The purpose of the inspection is to identify any degradation in the integrity of the systems during their service life and to provide an early warning in order that remedial action can be taken before failure occurs. Conventional methods in CHP for identifying the presence of such damage are often " visual" methods. For each of these components, there can be different types of flaws and damage. This may include cracks, pitting, material degradation, etc. Because of this combination of component types and defect types, several types of NDT methods have to be implemented in CHP.
A careful selection of methods is necessary for effective NDT. It is essential to identify the critical areas where failures are likely to occur and select suitable NDT techniques for detection of such failures. Based on design criticality, past experience and previous failure information, suitable approach in inspection methodologies can be adopted.
The basic layout of Coal Handling Plant is shown by block diagram. (See Fig. No 1) the coal is unloaded at various unloading station and transported by conveyors to crushing and screening plant via transfer house. After crushing required quantity of coal is transported to bunker via transfer house and remaining coal is stored in stockyard. This coal is reclaimed as per requirement. From the bunker the coal flows through coal mills to boiler furnace. The main aim of CHP to maintain level of coal in bunkers for smooth coal supply to boiler
There are different streams for transporting of coal. For caring out preventive maintenance schedule one of the stream kept under shutdown. If at the same time breakdown occurs in a machine in other stream, which interrupt the coal supply to boilers. Due this loss of generation will occur.
A plant, which supply of coal to boilers having capacity of 750 tones per hour failed to fulfill need will loss generation of 0.6 MU for one hour. This cost 1.20 Core of Rupees.
Working conditions in CHP are dusty, dirty and often wet conditions. So there is a constant quest to improve machine uptime and avoid unplanned maintenance. The NDT will help to choose proper shutdown of stream to avoid further problems.
The main objectives of NDT for CHP should be as follows.
The steps for implementension of “In Service NDT Inspection” are given below.
5.1 Detection Of Critical Area/Equipments: -
In CHP the equipments which performance effects directly on plant operation and plant performance are known as Critical Equipment. It is important to identify the critical equipment of CHP. Crushers, Wagon Tippler, Bunker Feeding Conveyors and Feeders are critical equipments. To maintain an efficiently operating unit and avoid failure of critical equipment, it is necessary to maintain the critical parts of that equipment. So it is also important to identify the critical parts. The parts directly affect the performance of equipment performance and operations are known as critical parts of equipments.
5.1.1 Crushers: -
In CHP crusher work on principle of combination of impact and attrition crushing. In this type of crushing first coal is break due to impact and further scrub between two hard surfaces to get desired coal size. Some crushers are work only on principle of impact crushing. Generally these crushers are used before final crushers.
The output size of coal affects the performance of CHP. Naturally these two hard surfaces of crusher are critical parts. One of these surfaces are known as grinding plates and other may known as rings, hammers etc. The linkage between crusher rotor and drive assembly are also critical parts. As failure of these linkages will stop crushing.
5.1.2 Wagon Tipplers: -
In CHP generally there are two types of wagon tippler. They are known as rota type and rotary type. The main difference between these tipplers is that rotary type tippler is having floating barrel  and rota type tippler turns between two bearings.
The drive linkages are undergoing of cyclic loading and failure of these linkage stop the equipment operation. Due to this unloading of coal cars affects, which drop the performance of CHP.
5.1.3 Bunker - Feeding Conveyors: -
CHP are having number of conveyors but bunker-feeding conveyors are playing vital role. The main aim of each CHP is to maintain bunker levels for smooth coal supply to boilers. As these conveyors feeds the bunker their performance affects CHP performance. The drive linkages consist of gearbox and couplings. Failure of any part of the linkage will stop operation of feeding bunker level. So these parts are critical parts of bunker feeding conveyors. The conveyor pulleys are also critical parts.
5.1.4 Feeders: -
The performance of feeders affects the efficiency of CHP. The feeders used in CHP are Apron Feeder, Vibrating Feeders, Roller Screens and Vibrating Screen Feeders etc. Generally vibrating feeders, which are used, are of electromagnetic type. The springs and coils and suspension rods are the critical parts. Weak coil springs that are not generating sufficient accelerating forces can also cause low speed and reduce the performance. In vibrating screen feeder have critical part like beam and its members, drive linkages etc. Apron Feeders is sturdy machine the main critical parts are pans, chain and rollers. The roller screens have critical parts in drive linkages.
Depending upon the actual operating and environmental condition, material properties of CHP equipments degrade as a function of service life due to one or more of the time dependent material damage mechanism such as fatigue, corrosion, erosion etc. See Table No 1
|Component||Type of defect||Affecting factor||Reasons|
|Transfer Chute Liners, Grinding jib of crushers.||Reduction in thickness due to wearing of surface||Continuous coal flow||Friction between coal and component|
|Transfer Chute Liners, Grinding jib of crushers.||Development of cracks, holes||Impact of coal||Crack generated from the holes for fixing of bolts|
|Transfer Chute Liners, Grinding jib of crushers||Pitting||Corrosive component of coal||The wet coal when flows through then chances are more.|
|Conveyor structures||Reduction in thickness due to wearing of surface and pitting||Corrosive component of coal||The acumination of coal on structures|
|Conveyor structures||Catastrophic fracture failure||Cyclic Loading||A result of manufacturing fabrication defects or localized damage in service,|
|Crusher Rotors, Motor shafts, Suspension Bars, Arms||Development of cracks||Impact of coal||Due to internal flaw|
|Bearings||Development of cracks in the races||Improper loading,||Due to internal flaw|
|Conveyor pulleys||Due to End disc failure||Cyclic loading||Failure of the weld between the hub and the end disc in welded-in hub designs.|
|Drive foundations||Bolt failure, Frame failure||Cyclic loading||A result of manufacturing fabrication defects or localized damage in service,|
|Conveyor pulleys||Failure of locking assembly||Cyclic loading||Failure of locking bolts|
|Wire ropes of Aerial Ropeway||Due damage in strut||Cyclic loading||Due to weak area of strut|
6.1 Fatigue Failures: -
Repeated cycling of the load causes metal fatigue. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe.
Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength. This can result in an unexpected  and catastrophic failure in use.
Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure.
6.2 Stress Corrosion Cracking: -
Stress corrosion cracking is an insidious  type of failure as it can occur without an externally applied load or at loads significantly below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Pitting is commonly associated with stress corrosion cracking phenomena.
Aluminum and stainless steel are well known for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion cracking in the right environment. Structures subjected to fluctuating service loads are vulnerable to fatigue damage
6.3 In Service Examination: -
Periodic in service examination is necessary for critical components. In non-destructive testing, crack detection plays an important role. Various NDE techniques like Ultrasonic thickness gauging, Ultrasonic flaw detection, Penetrant testing, Flourescent magnetic particle testing, can be used. (See Table No 2) Specialized techniques such as Digital Radiography; Acoustic Impact Techniques can be used for in service examination. Metallurgical tests such as in-situ metal-o-graph using replica  method, in-situ chemical analysis by metal spectroscope / X-ray flourescence method, in-situ hardness measurement can be carried out.
|Thickness||Ultrasonic  thickness gage  like “ Nova TG2”||Surface||Transfer Chute Liners, Grinding jib of crushers, Conveyor structures etc.|
|Thickness||Digital Radiography ||Surface||Transfer Chute Liners, Grinding jib of crushers, Conveyor structures etc.|
|Flaw||Ultrasonic  Flaw Detector  like “Quantam TE”||Surface/inside||Crusher Rotors; Jack shaft, Grinding jibs; Suspension Bars, Arms, Sprocket Tiers Drive Foundations etc.|
|Flaw||Acoustic Impact  Technique||Surface/inside||Crusher Rotors, Bearings|
|Flaw||Magnetic  Flaw Detector  like “MD20 Wire Rope Tester”||Surface/inside||Wire ropes of Aerial Ropeway|
|Flaw||Weld regions by x-ray diffraction||Surface/inside||Drive Foundations|
|Thickness||Ultrasonic  thickness gage||Surface||High pressure pipe line of hydraulic system of Wagon Tippler|
By applying in service inspection using NDT for Critical Equipment of CHP will allows:
For applying these methods of inspection will require a substantial investment. The return on this investment will be dependent on the effectiveness of its implementation and the commitment of all personnel
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