1.0 Abstract: -

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.

2.0 Introduction: -

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.

3.0 Importance of NDT for CHP: -

Fig 1:

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.

4.0 Objectives of NDT: -

The main objectives of NDT for CHP should be as follows.


1. Establishing method for planning the requirement of long lead items based on NDT. And selection of stream for shutdown purpose based on NDT.
2. Prediction of impending failures of critical plant components like Crusher Rotors, Conveyor Pulley Shafts, in real time resulting in enhanced safety, operational reliability, availability, and maintainability.

5.0 In Service NDT Inspection Implementations: -

The steps for implementension of “In Service NDT Inspection” are given below.


1. Detection of critical area/Equipments,
2. Defect Identification
3. In-Situ Examination

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 [1] 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.

6.0 Defect Identification: -

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
Table 1:

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 [2] 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 [3] 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 [4] method, in-situ chemical analysis by metal spectroscope / X-ray flourescence method, in-situ hardness measurement can be carried out.

Measurement		Equipment
Parameter	Instrumentation	Positions	Description
Thickness	Ultrasonic [5] thickness gage [6] like “ Nova TG2”	Surface	Transfer Chute Liners, Grinding jib of crushers, Conveyor structures etc.
Thickness	Digital Radiography [7]	Surface	Transfer Chute Liners, Grinding jib of crushers, Conveyor structures etc.
Flaw	Ultrasonic [8] Flaw Detector [6] like “Quantam TE”	Surface/inside	Crusher Rotors; Jack shaft, Grinding jibs; Suspension Bars, Arms, Sprocket Tiers Drive Foundations etc.
Flaw	Acoustic Impact [9] Technique	Surface/inside	Crusher Rotors, Bearings
Flaw	Magnetic [10] Flaw Detector [11] 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 [12] thickness gage	Surface	High pressure pipe line of hydraulic system of Wagon Tippler
Table 2:

7.0 Conclusion: -

By applying in service inspection using NDT for Critical Equipment of CHP will allows:


• To plan shutdown before severe damage occurs and reduce reactive maintenance practice.
• Cause of failure can be analyzed
• Will benefits in Critical Equipment availability, optimized use of resources, reduced downtime, which increased availability of CHP

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

Reference:

1. A technical article on “ A Case Study-Failure of Motor Shaft Of Rotary Wagon Tippler -Reasons and Remedy” by Makarand Joshi at web site www.plant-maintenance.com/articles/RotaryWagonTipplerCaseStudy.pdf
2. Technical article at website www.metelsengineer.com/CA-fatiuge.htm of Metallurgical Consultants.
3. Technical article at website www.metelsengineer.com/CA-scc.htm of Metallurgical Consultants.
4. Technical article at web site www.ndt.net/v05n07.htm on “Role of NDE in Residual Life Assessment of Power Plant Components” by Sundaram Baskaran Industrial Quality Concepts Chennai, India published in NDT.net - July 2000, Vol. 5 No. 07
5. Technical article at web site www.mac-ndt.com/moreaboutndt.html “Flaw Detection & Thickness Measuring Using Ultrasonic Systems” of Magnetic Analysis Corporation, 535 South 4^th Avenue Mount Vernon, NY USA 10550-4499 Telephone (914) 699-9450 Fax (914) 699-9837
6. Catalog of NDT Systems, Inc. 17811 Georgetown Lane, Huntington Beach, CA. 92647 Phone (714) -893-2438 * Fax (714) -897-3840 for their products at web site www.ndtsystems.com/Products/products.html
7. Technical article at web site www.ndt.net/ut1097.htm on “Wall Thickness Determination In Digital Radiography” by T. Wawrzinek, U. Zscherpel, C. Bellon presented at the NDT Conference and Exhibition of the French NDT Society (COFREND), SEP 22-26 '97, Nantes.
8. Technical article at web site www.ndt.net/article/wcndt00/papers/idn437 on “Ultrasonic Crack Detection Trials” by Mr. Tor Harry Fauske, Statoil, Norway Co-author: Dr. ing. Peer Dalberg, CorrOcean ASA, Norway Co-author: Mr. Arnfinn Hansen, CorrOcean ASA, Norway, presented at 15th World Conference on NDT (15-21Oct 2000) in Rome.
9. Technical article at web site www.ndt.net/article/wcndt96/papers/idn232 “Nde Of Helicopter Composite Rotor Blades Using Acoustic Impact Technique” by Deepak Bansal and C. R. L. Murthy Indian Institute of Science, Bangalore 560 012, India presented at 14^th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996
10. Technical article at web site www.ndt.net/article/wcndt00/papers/idn281 “NDT of steel ropes with Magnetic Flaw detectors: Documentation and interpretation of test Results” by O.Gronau DMT, Bochum, Germany S.Belitsky, V.Sukhorukov INTRON PLUS, Moscow, Russia, presented at 15^th World Conference on NDT (15-21Oct 2000) in Rome.
11. Catalog of Zawada NDT , ul.Tatarkiewicza 8, PL- 41-819 Zabrze, Poland Phone: 48 32 271 6231, Fax: 48 32 275 2167 for their products at web site www.webmedia.pl/zndt
12. Technical article on “New ASTM Standard for Pipeline Inspection” by Ed Ginzel published in NDTnet- April 1999, Vol. 4 at web site www.ndt.net/v04n04.htm

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