engineering failure analysis-investigation on the failure of air compressor

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Investigation on the failure of air compressor

S. Sivaprasad a, N. Narasaiah a,*, S.K. Das a, G. Das a, S. Tarafder a, K.K. Gupta b, R.N. Ghosh a

a Materials Science and Technology Division, National Metallurgical Laboratory (Council of Scientific and Industrial Research), Jamshedpur 831 007, Indiab Analytical Chemistry Centre, Jamshedpur 831 007, India

a r t i c l e i n f o

Article history:

Received 25 April 2009

Accepted 27 April 2009

Available online 5 May 2009

Keywords:

Air compressor

Impeller

Fatigue

a b s t r a c t

The cause for the failure of an air compressor has been investigated. It was found that apre-existing fatigue crack was present at the root of the impeller blade. Transients and

unsteady operation of the equipment prior to the accident are thought to have grown

the fatigue crack to its critical size, thereby causing an imbalance in the impeller rotation

and leading to failure.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Air compressor is one of the vital equipments in a plant that is used for the production of liquid air and other liquid gases.

The first step in liquefying the air in a liquid gas plant is compression of air by the multistage highly efficient air compressor.Any failure in the compressor seriously jeopardises operation of the plant. One such air compressor catastrophically failed in

a liquid gas producing plant and the industry wanted to know the reasons for the failure. The failed air compressor had a

total of six compressor stages mounted on three shafts being driven by a bull gear and the major damage was restricted

to stage 1 and stage 2 compressors.

On discussion at site, it was noted that prior to the failure, the upstream turbine fed by the compressor tripped. On check-

ing for obvious malfunctions, the turbine was re-loaded. The turbine tripped again twice in quick succession after about 2.5 h

of operation. It was decided then to wait for a thorough instrumentation check on the turbine prior to commencement of

operations. The compressor was therefore being idled under no-load, awaiting clearance from the turbine side. While the

compressor was idling, it suddenly failed without any prior detectable signatures. At the time of failure, minor fires had

engulfed the bearing areas because of oil leakage.

2. Visual inspection

The extent of damage could be determined only after opening the casings. Visual inspection of the damaged components

of the compressor parts was made and the following observations were noted:

the inlet contour vanes of the stage 1 were dented;

impellers of stages 1 and 2 had been sheared off their shaft and abraded along their periphery;

tie rods of both stage 1 and stage 2 were broken;

the nose cone of both stages 1 and 2 were dented, as would happen if they are displaced while in service, with their

fastening bolts broken;

1350-6307/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2009.04.016

* Corresponding author. Tel.: +91 657 2345187; fax: +91 657 2345213.

E-mail address: [email protected] (N. Narasaiah).

Engineering Failure Analysis 17 (2010) 150–157

Contents lists available at ScienceDirect

Engineering Failure Analysis

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a te / e n g f a i l a n a l

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the contour ring of stage 1 and impeller eye labyrinth of stage 1 were deeply scoured;

gas and oil seals of bearings of the 1–2 shaft were broken;

the teeth of the bull gear and 1–2 stage rotor and the thrust collar of the bull gear were damaged.

The damage was more severe in the stage 1 impeller. The nose cone of stage 1 impeller was severely dented and de-

formed, and the locator pin of stage 1 nose cone was missing, but that of stage 2 was intact. Stage 1 impeller was abraded

to greater depth, and its blades were deeply dented and deformed. The bearing on the stage 1 side suffered greater damage

than that on the stage 2 side. Some of these failed components are shown in Fig. 1. Schematic diagram of impeller and its

assembly with other parts of compressor unit is given in Fig. 2

A detailed examination of the stage 1 impeller revealed that a long crack (approximately 70 mm in length) had formed at

the base of one the blades. The crack had penetrated through to the leading edge of the blade. The edges of many of the

blades had become severely abraded as would happen if the blades touched the contour ring while in motion at high speed.

Some of the blades were deformed, being bent in the direction of rotation.

Examination of the failed parts clearly showed that the primary failure had occurred in the stage 1 impeller. Since stage 2

was mounted on the other side of the same shaft, it also suffered damages. It was therefore decided to focus the investigation

on stage 1 impeller and its immediate neighbourhood.

3. Experiments and results

A portion of the impeller blade containing crack at one end, a broken tie rod and a broken nose-cone bolt, all collected

from the stage 1 impeller were brought to the laboratory for further investigation. In the laboratory, the chemistry of theimpeller blade material and tie rods, microstructure of each of these components and hardness measurements were made.

The fractured surfaces of all these failed parts were examined in scanning electron microscope (SEM). The results of the

investigations are discussed below.

Fig. 1. Some of the failed components from stage 1 compressor: (a) impeller blade, (b) magnified view of impeller showing presence of crack (cracklocations are shown by arrow marks), (c) nose cap, (d) nose-cap bolt and (e) tie rod.

S. Sivaprasad et al./ Engineering Failure Analysis 17 (2010) 150–157 151


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3.1. Chemical analysis

The chemical analysis of the collected materials was carried out using the energy dispersive X-ray analysis. The results

showed that the impeller blade belongs to X4CrNi13-4 grade steel and the tie rod material was found to be 2%Ni–2%Cr

low alloy steel. The chemical composition of the blade and the tie rod materials are shown in Table 1.

3.2. Hardness survey

Hardness measurements of blade, tie rod and nose-cone bolt materials were made using 30 kg load. Hardness of the var-

ious parts of the failed components is given in Table 2.

Nose cone

Contour vane

Nose cone bolt

Tie rod nut

Tie rod

Connecting shaft

Impeller blade

Impeller

Contour ring

Fig. 2. Schematic diagram of impeller and its assembly with other parts of compressor unit.

Table 1

The chemical composition (in wt.%) of the blade and the tie rod materials.

Material Fe Cr Mn Ni Mo Si

Blade 82.47 13.03 0.42 3.51 0.31 0.25

Tie rod 94.99 1.86 0.30 2.10 0.49 0.25

Table 2

The hardness of the various parts of the failed components.

Material Vickers hardness, HV30

Impeller blade 285, 280, 283, 287, 284

Tie rod 354, 351, 355, 352, 352

Nose-cone bolt 258, 258, 252, 259, 249

152 S. Sivaprasad et al. / Engineering Failure Analysis 17 (2010) 150–157


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3.3. Microstructural study

Microstructure of the failed parts was analysed by optical as well as by scanning electron microscope. The microstructure

of the blade material is given in Fig. 3. The microstructure shows that the steel possesses a coarse martensitic structure.

Although the specification [1] of the material and the inspection certificate mentions that tempering treatment is required,

the same was not clearly evident from the microstructure. The microstructure does not resemble a tempered structure as

documented in literature [2].

The microstructure of the tie rod is observed to be tempered bainite (Fig. 4) that is typical of hardened and tempered

Cr–Ni steel.

3.4. Fractography

The fracture surfaces of the failed components were studied under scanning electron microscope. Fig. 5 shows a montage

of the fracture surface of the impeller blade. The fracture surface had distinct four regions, marked as A, B, C and D in Fig. 5.

Detailed viewof regions A, B, C and D are given in Fig. 6. Region A shows ductile striations typical of fatigue surface. Region B

is a rubbed area, where the two faces of crack has rubbed against each other during operation. Region D is the skin repre-

senting the remaining ligament prior to fracture, and shows dimpled shear fracture features.

Fig. 3. Microstructure of blade.

Fig. 4. Microstructure of tie rod.



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In Fig. 5, a location has been marked as C, which appears to be a probable region of initiation of the fatigue crack. Details at

location C are presented in Fig. 6.

The fracture surface of broken tie rod is shown in Fig. 7. The fracture surface shows ductile failure, which is indicative of

overload failure under tensile load. The shallow dimples point to high strain rate of deformation as during impact loading.

In Fig. 8, macro and micro views of the failed nose-cap bolt is given. The fractographs indicate shear failure of the bolt

through microvoid growth and coalescence.

4. Discussion

From the understanding achieved by the various studies carried out, and from the information collected at site, three pos-

sible scenarios emerge out that are responsible for the failure and consequent damage of the compressor. They are:

(1) Fatigue crack initiates at the root of the impeller blade during operation. The crack grows during operation. This causes

increased vibrations and imbalance, as the edge of the blade is abraded by the contour ring. Under these conditions,

the nose-cap bolt gets loosened, and ultimately results in the nose cap being dislodged. The heavy nose cap severely

dents and damages the impeller. Its impact causes the tie rod to fracture.

C

B

B

D

A

Fig. 5. Montage of fracture surface of blade.



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(2) The nose-cap bolt is loosened during operation. This leads to the nose cap being dislodged, which dents and damages

the impeller blades, and results in fracture of the tie rod under the impact load conditions set up.

(3) The tie rod fractures first. This leads to nose cap being dislodged through consequent fracture of its fastening bolt. The

nose cap dents and damages the impeller blades as it impacts on the impeller.

The scenarios 2 and 3, however, do not explain why a fatigue crack should form at the root of the impeller blade, the pres-

ence of which has been amply illustrated. Since the initiation and extension of a fatigue crack has to occur through a period

of time, such situations would indicate that the fatigue crack does not cause any unnatural vibrations and imbalances in the

rotation of the impeller. This is improbable because of the length of the fatigue crack (about 70 mm) that was observed.

Hence, the scenario 1 is thought to be responsible for the failure and damage of the air compressor.

It is well known that fatigue crack initiate from a point, or a few points, on a component under the action of cyclic stres-

ses, and grows with continued abnormal operation [3]. Generally, it is impossible to prevent the initiation and growth of

fatigue cracks in components. However, preventing the crack growing to a critical size that would lead to catastrophic events

is of paramount importance. In this case, procedures of condition monitoring and shutdown triggering are mechanisms that

Fig. 6. Details of fracture surface of blade.



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are capable of preventing such occurrences. Fatigue crack growth depends upon a number of factors [3–6], and in this case

may be adversely affected by frequent tripping of equipment and unsteady operational conditions. The transients observed

in the work place and frequent outages of downstream equipment when the accident occurred may have been largely

responsible for the fatigue crack growing to critical dimensions and opening up of the crack may be as a result of impeller

touching the contour ring due to the impact by heavy nose cone.

5. Conclusions

Based on evidences collected at the site and investigations carried out in the laboratory, the cause of the failure of the air

compressor may be attributed to the growth of the pre-existing fatigue crack to critical size triggered by the frequent

Fig. 7. The fracture surface of the broken tie rod.

Fig. 8. Macro and micro views of the failed nose-cap bolt.



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tripping of the equipment and unsteady operating conditions just before the accident. The crack opening at this point should

have resulted in the impeller touching the contour ring due to the impact by heavy nose cap.

References

[1] Metals hand book. 10th ed., vol. 10. Metals Park (OH): ASM; 1990. p. 862.

[2] Metals hand book. 8th ed., vol. 7. Metals Park (OH): ASM; 1972. p. 152.

[3] Suresh S. Fatigue of materials. 2nd ed. United Kingdom: University of Cambridge; 1998. p. 141,222.

[4] Herzberg RW. Deformation and fracture mechanics of engineering materials. 4th ed. Singapore: John Wiley & Sons; 1996. p. 542.

[5] Suresh S, Ritchie O. Int Met Rev 1984;29:445.

[6] Gangloff RP. In: Baklund J, Blom AF, Beevers CJ, editors. Fatigue thresholds. Warly (West Midlands): Engineering Materials Advisory Services; 1983.

p. 75.


engineering failure analysis-investigation on the failure of air compressor

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