Improvement of the Steam Turbine Maintenance of Mae Moh coal-fired power plant

Kiatisak Teowkul

Steam Turbine Maintenance Section, Mae Moh Power Plant Maintenance Division Electricity Generating Authority of Thailand (EGAT)

800 Moo 6 Mae Moh District, Lampang, Thailand 52220 Tel 66-54-253540 Fax 66-54-253548 Email: kiatisak.t@egat.co.th

Abstract Mae Moh Power Plant consists of ten

power plants located in Mae Moh, Lampang, Thailand. The plants exploits coal which produces total capacity of 2400 MW. It therefore is known as an important and constant plant in the north and of the nation. Not only is it a low-cost power plant, but also an essential organization in community development in Lampang. Having been established for over than 30 years, Mae Moh Power Plant has set a structure for plant’s equipment maintenance. Minor inspection is carried out every two years while major overhaul is every eight years. Critical equipment inspection is carried out during the process of major overhaul such as steam boiler, steam turbine, components like turbine valve and turbine lubricant oil system. The significant original engineering inspection is examining without destroying non destructive testing. The process consists of dye penetration testing, magnetic particle testing and ultra sonic testing. The inspection for low pressure turbine rotor is normally consisted of cleaning up, visual inspection, and magnetic particle testing, which probably are not enough. The steam turbine maintenance section has developed inspection process by adding glass beading to clean up rotor surface and phase array testing. After improvement of maintenance process, cracks of two blade roots and grooves were found during the process of low pressure turbine inspection of Mae Moh Power Plant Unit 8 in 2004. After replacing new blades and lathing cracks by grinding, the parts were reassembled. As a result, the Unit 8 was able to efficiently and safely generate with the full capacity of 300 MW. Considering a critical loss in case of blade rupture, low pressure turbine may be severely damaged that can not be continually operated. The calculated loss from such condition is 225 million baht (7.5 million USD). As a consequence, the Electricity Generating Authority of Thailand (EGAT) will lose 1,100 million baht (36.7 million USD) from 55 days of power generating shutdown due to the low pressure turbine repair work

Introduction

Mae Moh Thermal Power Plants Unit 8-13 are 300 MW plants that use Lignite from Mae Moh

Mine. The Mae Moh Plant has a number of significant instruments such as steam boiler, steam turbine, flue gas de-sulfurization plant, etc. Steam turbine is one of the most important equipments. There are three set of turbines: high pressure turbine (HP Turbine), intermediate pressure turbine (IP Turbine), and low pressure turbine (LP Turbine)The minor and major overhauls are needed every two years and eight years respectively. Life assessment is needed when the generating reach a period of time such as 80,000 hours. The process is to calculate the time remaining for safe and efficient generating. In the assessment of the plant unit 9 (during the major overhaul & life assessment from September 2 – November 2006), we had improved the inspection method by glass beading for cleaning the equipment surfaces and checking the critical surface by using non destructive testing such as PT, UT, MT and Replication, then many cracks of the components of steam turbine were found on end face of blade root and groove. Therefore appropriate solution is required to continuingly prevent severe damage and loss in the future.

Mae Moh Power Plant’s Steam Turbine

The steam turbine shown in Figure 1 is tandem compound 3-casing, double flow exhaust, reaction, reheat, condensing type. It consists of three minor sets of turbine. First casing is barrel type high pressure steam turbine (HP Turbine), which was designed to fit immediate load change. Steam will be directly sent to reaction blade via two main stop valves and four control valves. The second casing is intermediated pressure turbine (IP Turbine) which was designed in double flow structure including reheat stop valve and intercept control valve. The last casing is double flow low pressure turbine (LP Turbine).

Figure 1: Mae Moh Steam Turbine

Figure 2: Turbine Assembly

HP Turbine HP. Turbine is designed in barrel type to

operate in different conditions such as immediate start or immediate load change due to alteration of high steam pressure. The designed casing does not have cut on both axis and radius. As a result, a balance turn in HP Turbine occurs. The design also includes the prevention of metal stress in high temperature.HP Turbine is designed to control by throttle governing system when steam goes through reaction blades via two main stop valves and four control valves.

IP Turbine IP Turbine was designed to be double flow connecting to stop valve from reheater and interceptor valve. It is to control steam flowing between reheater and IP Turbine. IP casing is horizontal type divided to inner casing and outer casings. Steam pipe directly jointed with outer casing was designed to be modifiable by using special L-shape ring.

LP Turbine Like IP Turbine, LP Turbine is designed in double flow feature. LP Turbine joints with IP Turbine via crossover pipes. LP casing is designed in separate horizontal parts consisting of three shells. The inner casing no.2 is hold first row guide blade, which is supported in inner casing no.1. The middle shell is in the outmost one. The blade ring, holds last stage guide blade, adheres to four points of middle shell on beside low pressure turbine support.

Turbine Governing System The steam turbine is controlled by digital

electro-hydraulic governor. The steam controlling valve will be directed by electro-hydraulic actuator, which receives analog signal from a digital electronic controller together with controls by using motive fluid supply system. Electro-hydraulic controlling system is not only accurate, but also efficiently control adjustment of the turbine’s speed in case of increasing or decreasing of the load or immediate no load or runback.

Methodology for Nondestructive Testing

Nondestructive testing

Nondestructive testing (NDT), also called nondestructive examination (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). In fact methods from the medical field have often been adapted for industrial use, as was the case with phased array ultrasonics(PA) and computed radiography. While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively.

The need for NDT

It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture. During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods.

Methods and techniques

NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.

Dye penetrant inspection (DPI),

also called liquid penetrant inspection (LPI), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic-particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components. DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - fluorescent or nonfluorescent (visible). Fluorescent penetrants contain two or more dyes that fluoresce when excited by ultraviolet (UV-A) radiation. Since FPI is performed in a darkened environment, and the excited dyes emit brilliant yellow-green light that contrasts strongly against the dark background, this material is more sensitive to small defects.When selecting a sensitivity level one must consider many factors, including the environment under which the test will be performed, the surface finish of the specimen, and the size of defects sought.

Magnetic particle testing or Magnetic particle inspection

Magnetic particle inspection processes are non-destructive methods for the detection of defects in ferrous materials. They make use of an externally applied magnetic field or DC current through the material, and the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material. The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance. The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. The suspension is

sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles.This inspection can be applied to raw material in a steel mill (billets or slabs), in the early stages of manufacturing (forgings, castings), or most commonly to machined parts before they are put into service. Ultra sonic testing (UT)

In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. The technique is also commonly used to determine the thickness of the test object.Phased Array (PA) ultrasonics is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing, originally pioneered by Albert Macovski of Stanford University.When applied to steel the PA image shows a slice that may reveal defects hidden inside a structure or weld.

The PA probe is comprised of many small ultrasonic elements, each of which can be pulsed individually. By varying the timing, for instance by pulsing the elements one by one in sequence along a row, a pattern of interference is set up that results in a beam at a set angle. In other words, the beam can be steered electronically. The beam is swept like a search-light through the tissue or object being examined, and the data from multiple beams are put together to make a visual image showing a slice through the object. There are two methods of receiving the ultrasound waveform, reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection.

Hardness test Hardness is the property of a material that

enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting.

Measurement of Hardness: Hardness is not an intrinsic material

property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure. Hardness of materials has probably long been assessed by resistance to scratching or cutting.There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers, and Rockwell. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry. Replica or Replication or Metallography

Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructual constituents. After preparation, the sample can easily be analyzed using optical or electron microscopy. Preparing Metallographic Specimens Metallographic specimens are typically "mounted" using a hot compression thermosetting resin, such as a phenolic resin, or with a cast resin, such as an epoxy resin. Mounting a specimen provides a safe and ergonomic way by which to hold a sample during the grinding and polishing operations.After mounting, the specimen is wet ground to reveal the surface of the metal. The specimen is successively ground with finer and finer grades of silicon carbide paper to remove damage from sectioning and then from each grinding step. After grinding the specimen, polishing is performed. Typically, a specimen is polished with a slurry of alumina, silica, or diamond on a napless cloth to produce a scratch-free mirror finish, free from smear, drag, or pull-outs and with minimal deformation remaining from the preparation process. After polishing, certain micro-structural constituents can be seen with the microscope.

Steam Turbine Major Overhaul

Mae Moh Power Plant’s inspection process includes examining steam turbine equipment which consists of high pressure turbine, intermediate pressure turbine, and low pressure turbine. During the process, those parts dealing with high pressure and temperature such as HP rotor, HP inner casing, and etc, will need to be removed and inspection. The process includes cleaning, and NDT (Non destructive testing,) which consists of: 1) Dye penetrant testing ( PT) to inspect damage of metal surface 2) Magnetic particle testing(MT) to inspect crack within the metal 3) Visual inspection for ordinary inspection.

During the process non destructive testing, the surface to be inspected will need to be cleaned. In the previous time, surface scrubbing by manual

labor could only clean roughly. A dirty surface could produce an inaccurate result.

Development of Rotor Surface Cleaning

In the previous time, profound inspection

was not experienced because the recommended process of maintenance from manufacturer was strictly followed. However, after having used turbine for more than 20 years plus several years of overhaul experience, the staff team has found that the features of damage have varied. The ordinary inspection probably is not enough or cannot find the damage in the future. To produce a more efficient maintenance work, better cleaning process are developed. Glass beading has been included to surface cleaning process. Sand blasting technique also helps remove the dregs gathered during the generating. Moreover, it obviously reduces metal surface cleaning time.

Life Assessment Evaluation During the major overhaul, EGAT has

included life assessment process in order to evaluate the life of major equipments such as HP Rotor, IP Rotor, LP Rotor, Inner casing, and welding joint on different positions. This is to inspect the remaining life of the critical equipment in order to plan a long-term equipment development. As Mae Moh Power Plant is a lowest-cost plant, it is possible to expand the life of the plant to another 15 years. The development of major equipments such as replacing new set of LP turbine is also one of the life expansion projects.

Non Destructive Testing and Life Assessment of Mae Moh Power Plant

The life assessment was carried out in the

major overhaul of Mae Moh power plant unit 9 during September 26 – November 18, 2006 due to long period of operation. Critical components or equipments needed to be inspected by NDT method in order to estimate whether the components were able to perform until the plant’s expiration.

Steam turbine and its components were investigated by steam turbine engineering and maintenance section including with section of material testing during maintenance. The inspection extents are as follows:

Equipment Component

Location of Inspection

NDT Inspection Method

HP Rotor Whole Surface, Fillet Radius

MT,UT Volumetric Electrode Polarization Replication ,Hardness,VI

IP Rotor Whole Surface, MT,UT

Fillet Radius

Volumetric Electrode Poralization Replication ,Hardness,VI

LP Rotor Whole Surface, Fillet Radius, Groove Blade

MT, Hardness,VI

The NDT Result of Low Pressure Turbine Rotor (LP Rotor) We found that primary crack inspection on

the surface of end face of all blade roots and grooves of stage L-0, L-1, L-2 on gov.side and gen.side using magnetic partical testing(MT) technique, following cracks were: Gov.Side Blades and Grooves Inspection

At Stage L-1, 71 Grooves of crack were found, 150 mm. Longest, 24 mm. deepest (100% inspected) 7 blades were additionally found on blade root. Blade Root

48-mm. crack was found on blade no.18. one side steeple on steam outlet.

Figure3 : Crack on Blade Root LP Turbine Stage L-1 and on Rotor Grooves LP Turbine According to the inspection, it can be concluded that the crack found in LP rotor grooves was due to stress corrosion cracking (SCC). The metallography of micro structure shows trace of corrosion around the crack. Many cracks were found both stage L-1 on gov. side and gen. side.

Crack on Stage L-1 was found by NDT - 71 grooves of crack were found on gov. side. After lathing the cracks at steeple 1, the deepest crack found was 12 mm. The length in the grooves was 48 mm. at grooves No.19 on steam inlet and at steeple 3 and over than 15 mm. which exceeded maximum allowable crack depth (4 mm.) determined by OEM. As a result, LP Rotor is expired when its remaining life is calculated.

Result and Discussion

Applying the NDT process in inspecting

critical equipment makes equipment damage analysis obviously accurate as well as steam turbine inspection of Mae Moh Power Plant Unit 8-11. EGAT started the inspection by using NDT and obviously found damages of LP rotor blades and LP Rotor grooves for example, in power plants unit

8 and 9. As soon as EGAT is aware of the result of the accurate inspection, the damage crisis is appropriately resolved. Non destructive testing also concretely supports time reduction plan of steam turbine major overhaul. Additionally, non destructive testing (NDT) encourages continually operation of relevant rotor inspection and overhaul which includes: replacing seal fin of steam turbine rotor/ inner casing and balancing of HP-IP rotor.

Conclusion

EGAT has developed steam turbine

inspection process by adding surface cleaning (Glass beading) and a number of significant inspection. Additionally required test method such as phase array ultrasonic testing makes EGAT able to accurately inspect the damages as well as to save more time in running the process without having to remove equipment or take them outside. Moreover, magnetic particle testing creates an obvious crack inspection of LP rotor and rotor groove like Mae Moh Power Plant Unit 8-9. In case that no damage is found and the equipments are composed back to their positions, a severe damage from blade crack may occur.

Considering a critical loss from blade rupture case, low pressure turbine may be severely damaged that can not be continually operated. The calculated loss from such condition is 225 million baht (7.5 million USD). As a consequence, the Electricity Generating Authority of Thailand (EGAT) will lose 1100 million baht (36.7 million USD) from 55 days of power generating shutdown due to the low pressure turbine repair work. For solving this trouble Mae Moh Power Plant therefore develops new overhaul process by using non destructive testing for inspection technique, which is one of the means to increase overhaul efficiency.

References

1.Steam turbine maintenance section, Mae Moh Power Plant (2006) Inspection Summary report of Mae Moh Power Plant, Unit 9. Lampang: Steam turbine maintenance section. 2.Power Plant Maintenance Department (2006). Primary Inspection Report, Mae Moh Power Plant Unit 9, Lampang: plant maintenance department. 3.Maintenance instruction Manual of Mae Moh Power Plant, Japan, 1997. 4. Operation Instruction Manual of Mae Moh Power Plant, Japan, 1997 5. Wikimedia foundation (2008), Nondestructive testing retrieved 13 Mar 2008 from http://en.wikipedia.org/wiki/Nondestructive_testing