art3A10.11342FS1061830913090027

ISSN 1061�8309, Russian Journal of Nondestructive Testing, 2013, Vol. 49, No. 9, pp. 538–542. © Pleiades Publishing, Ltd., 2013.Original Russian Text © V.V. Artamonov, V.P. Artamonov, 2013, published in Defektoskopiya, 2013, Vol. 49, No. 9, pp. 56–61.

538

Gas�main pipelines are intended for transporting gas over large distances. It is necessary to create largepressures in order that gas can move in pipes in a given direction. This pressure is created at compressionstations by gas turbine complexes consisting of a gas�turbine plant and centrifugal pump gun.

The rotor blades of the gas turbine operate under very stringent conditions (high temperatures, inertialand aerodynamic loads, and corrosion action of combustion products) and are crucial and expensiveassemblies of gas turbines. Among parts of the flow�through section of the turbine, the rotor blades havethe shortest service lives. In this connection, one of the urgent lines of repair work by the gas�transportindustry is the renewal of blades that have used up the manufacturer’s specified service life. The repairwork, including the removal of damaged thinnings, welding or weld deposition of new thinnings, thermaltreatment, finishing, and diagnostics by the gravimetric method, is performed at a repair depot. An addi�tional service life equal, as a rule, to the overhaul life of the turbine is assigned to the repaired blades. Whenthe turbine has used its economic lifetime, it is tested for industrial safety. In this case, an expert organi�zation has to solve the complex technical problem of evaluating the capabilities and optimizing the life�time of a turbine with rotor blades that had used up their service lives and were renewed. An importantsource of technical information on the serviceability of repaired blades is the analysis of causes of theirdamage.

In 2011 the emergency shut down of the ГTK�10�4 gas turbine, whose operating time was 120000 h attime of the breakdown, occurred at one of compressor stations of the Urengoi–Chelyabinsk–Petrovsk

main gas pipeline.1 The cause of the forced stop of the turbine was the breakdown of one of the rotor blades

(Fig. 1) of the high�pressure turbine (HPT). The set of these blades was renewed after 40000 operatinghours at a repair enterprise and again installed on the turbine. The set of the renewed rotor blades hadoperated for 900 h at the time of breakdown.

The breakdown of the HPT blade led to the deformation of the rotor and guiding blades (Fig. 2) of thelow�pressure turbine (LPT) and, due to the unbalance and beats of the HPT rotor, grinding of the bladesof the axial�flow compressor with thinning wear (Fig. 3).

The fracture pattern of the HPT rotor blades has some special features. First, the breakaway of theblade feather (see Fig. 1) took place at a height of 40 mm from the tail flange. The most probable crackpropagation zone is the blade base, since the inertial load and bending moment caused by the action of gasdynamic forces are at maximum here. Secondly, 9 of the 89 survived blades of the renewed set had damage

1 The name of the Line Production Agency (LPA), the serial number of the unit, and the company that renewed the blades arenot given due to considerations of confidentiality.

Diagnostics of the Causes of the Operational Destructionof Rotor Blades of Gas Turbines

V. V. Artamonova and V. P. Artamonovb

aLenorgenergogas Specialized Agency, St. Petersburg, Russiae�mail: [email protected]

bToraigyrov Pavlodar State University, Pavlodar, Kazakhstane�mail: [email protected] October 22, 2012

Abstract—The results of technical diagnosis of the destruction of renewed rotor blades of a stationarygas turbine plant are described. Visual, fractographical, and metallographical inspection methods wereused to determine the causes of the destruction. The causes of the operational destruction were deter�mined. Practical recommendations for preventing similar destructions were formulated.

Keywords: compressor station, rotor blade, restoring repair, economic life, fatigue crack, operatingtime

DOI: 10.1134/S1061830913090027

GENERAL ASPECTSOF NONDESTRUCTIVE INSPECTION

RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING Vol. 49 No. 9 2013

DIAGNOSTICS OF THE CAUSES OF THE OPERATIONAL DESTRUCTION 539

in the form of a fatigue crack on the exit edge of the blade (Fig. 4). Like the plane of the fracture of thefailed rotor blade, the cracks are located at a height of 30–40 mm from the tail flange of the blade.

The material of the HPT rotor blades is XH65BMTЮ alloy. The outward appearance of the fractureof the destroyed rotor blade has features that are related to the XH65BMTЮ alloy oxidation mechanismat an operating temperature of 670°C.

Under the long�term temperature exposure in the long�term crack propagation zone, the fracture sur�face is covered by a compact fire scale of chromium and nickel oxides, which reflects light well and hasanthracite brilliance (in contrast to magnetite, which is formed on oxidable fractures of steel parts and hasa black color). Under short�term temperature exposure, an oxide film is formed in the break zone, whosethickness is comparable with the light wavelength (about 550 nm). This oxide film refracts the light that isreflected by the metallic base. In short�term crack�growth stop zones, the fire scale thickness is equal to1.5–2 light wavelengths and these zones can be observed as dark transverse strips (see Fig. 1). In addition,the light long�term crack propagation zone has dulled (due to the thick fire scale) grains and the dark�yel�low break zone has relief contrast grains. The crack nucleation center was located on the exit edge. Thefractographical picture of the failure plane (see Fig. 1) unambiguously indicates that the destruction prop�agated in accordance with the fatigue mechanism.

As the crack propagated, the effective (i.e., not yet failed) cross�section of the blade decreased. As theincreasing stress in the unfailed cross�section of the blade reached a certain value, the fatigue mechanismchanged to quasifragile and the breakage occurred.

The microstructure of the failed rotor blade is austenitic and the grain value is no. 4 according to theGOST (State standard) 5639. Oxides (pollution by them corresponded to mark 4 of the GOST 1778 scale)and carbonitrides were found in the metal microstructure. Micropores occurred in the body and at the

Fig. 2. Damage to LPT rotor blades.

Break zone FatigueBreakdown center

failure zone

Fig. 1. The failure plane of an HPT rotor blade.

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V. V. ARTAMONOV, V. P. ARTAMONOV

grain borders and the microdamage corresponds to mark 4 of the VTI scale. About 20% of austenite grainshad a fringe consisting of small sigma�phase subgrains (Fig. 5).

The above microstructure of the failed rotor blade gives grounds to state that it had operated signifi�cantly longer, but not 900 h. However, the microstructure state could not be the immediate cause of themass propagation of fatigue cracks, since micropores of creepage did not merge in chains on grain borders,microcracks were not formed, the sigma�phase propagation is comparatively weak, and there are nodefects, such as intercrystalline corrosion in surface layers of the blade metal and, in significant quantities,a gamma–dashphase. A blade with these microstructure could operate for several thousands of hours.Hence, transformation of the blade material microstructure in the operation process could not be thecause of the breakdown.

Judging by the microstructure, the restorative repair technology did not include reconstructive thermaltreatment, which would lead to the dissolution of secondary phases in austenite and in healing micropores.

The most probable cause of the damage is the existence of surface defects, which the blade had beforethe operation started. In the process of repair or transportation defects such as scratches, hairlines, or so�called nicks could occur on the exit blade at a height of 30–40 mm from the tail. These defects would actas stress concentrators and decrease the endurance limit of the blades. However, we failed to detect suchdefects on the failed blade and to document them, since they had a microscopic depth, which is compa�rable with thickness of the fire scale that propagated during use of the blades or these defects were maskedby propagated fatigue cracks.

Fig. 4. One of HPT rotor blades with the fatigue crack.

Fig. 3. Damage to the rotor blades of the axial�flow compressor.


DIAGNOSTICS OF THE CAUSES OF THE OPERATIONAL DESTRUCTION 541

This assumption was confirmed during the 2012 repair campaign, when another set of rotor blades,which were renewed by a technology similar to the repair technology of the set that failed in 2011, waschecked at the same compressor station during an industrial�safety examination. One of blades of the sethad a number of well�defined scratches on the polished back and exit edge, the largest of which were

located at a height of 30–40 mm from the tail flange (Fig. 6a).2

The portion of the surface of the defective blade, which is marked in Fig. 6a by a dotted curve, is shownin Fig. 6b with 100 × amplification. The surface is polished up to a purity of Rz 1 µm. There are transversescratches on the polished surface with depths of about 10–20 µm on the back of the blade feather and 20–50 µm on the exit edge. The scratches were made by a comparatively coarse tool, such as an abrasive wheelwith granularity Rz 10 µm at the grinding stage (one can see microtears, traces of scuffing and cutting onthe thin section), and then they were partially polished by a fine abrasive (transverse hairlines in Fig. 6b).

Thus, it is appropriate to state that causes for the destruction of the HPT rotor blade of the ГTK�10�4turbine were not distant abrasive scratches, which were made during the repair. The scratches acted asstress concentrators. As a result, a deformation center arose on the exit edge of the blade, which trans�formed into a fatigue crack during the operation of the turbine, finally resulting in the operational destruc�tion of the blade. The above cause was the primary one. However, there is one more accompanying cause,namely, a low production culture. Let us explain this thesis in detail.

As authors’ long experience during the inspection and technical diagnostics of power equipment hasshown [1, 2], drawbacks in organizational–technical events also often lead to the operational destructionof equipment, in addition to clearly technical reasons. It would be natural to assume that the set ofrenewed blades arrived from the repair enterprise at the compressor station and was accompanied by adocument, e.g., a quality certificate, which would state that the renewal of the blades was fulfilled quali�

2 Scratch is a term in accordance with the GOST 23505–79 Abrasive Machining. Terms and Definitions.

(a) (b)

Fig. 6. Scratches on the surface of a renewed blade.

100 µm 10 µm

Fig. 5. Microstructure of a failed rotor blade.

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V. V. ARTAMONOV, V. P. ARTAMONOV

tatively and that the repairing factory guaranteed trouble�free blade operation within a specified operationperiod. However, this document was not found at the compressor station. In any case, i.e., whether or notsuch a document existed, the corresponding services of the LPA or the compressor station should performan incoming inspection of the set of renewed blades and execute this inspection results in the form of acorresponding document, which would permit the installation of the set of renewed blades on the turbine.However, this document was not found at the station either. If the incoming inspection would have beenperformed, a visual inspection, i.e., one without recourse to any complex equipment and ultramoderninspection methods, could detect blades with abrasive scratches and reject them, thus preventing the oper�ational destruction with the subsequent emergency shut�down of the equipment for an unscheduledrepair.

REFERENCES1. Artamonov, V.V. and Artamonov, V.P., Optimizatsiya kontrolya i tekhnicheskoi diagnostiki teploenergeticheskogo

oborudovaniya (Optimization of Control and Technical Diagnostics of Heat�and�Power Engineering Equip�ment), St. Petersburg: Nauka, 2009.

2. Artamonov, V.V., Mikrostrukturnyi monitoring energooborudovaniya (Microstructure Monitoring of EnergyEquipment), St. Petersburg: Nauka, 2010.

Translated by N. Pakhomova

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