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ENSURE FACILITY ROBUSTNESS BY ESTABLISHING OPTIMAL

TECHNOLOGY OF MULTI STAGE AXIAL COMPRESSOR

By

Angga Yudistira P* Ilham Bimaskara **

Synopsis :

Blower is one of main facility in the ironmaking blast furnace system, as blast furnace using

combustion to generate molten iron. There are 2 main components of blower, driving

equipment and driven equipment. Driving equipment (motor) converts electrical energy to

mechanical energy (rotation), and driven equipment (compressor) converts rotation into

pressure and flowrate of air needed for blast furnace process.

Blower supplies pressurised air needed for combustion process at blast furnace. Pressurised

air is generated by multi-stage axial compressor with high mass flow rate. Axial compresor

core component such as : rotor, stator, casing and coupling are critical for its purposes.

As operating time passes on, blower needs to be overhauled regula rly to make sure the

facility condition is reliable. This paper present theoritical, experiment and problem solving

for core component of axial compressor, such as: stator and rotor gap measurement, detecting

defect at the blade using NDT methods, casing bolt tightness, and coupling torque

management. While overhaul, each critical component may faces several problem. When

measuring the gap of rotor and stator blade, some gap may below or above standard. When

check the defect which can be ignored or should be repaired immediately. And regarding bolt

tightening that the bolt can be over tighten, under tighten or tighten by wrong methods.

Keywords : blower, axial compressor, overhaul

* Mechanical Maintenance Engineer, PT Krakatau Posco, Cilegon, Indonesia

** Electrical Maintenance Engineer, PT Krakatau Posco, Cilegon, Indonesia

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1. Introduction

Fig.1 Schematic Diagram for Blast Furnace

When materials is transported (4) and charged through top bunker. Air from atmosphere

is filtered and compressed (1). Compressed air is heated (2) up to 1250ºC. Hot mixed gas

created by combustion at point 2 dissolves supply materials by heating (3). Through chemical

reaction molten iron and slag was ejected through tapping hole. The gas flew upwards and

turns the turbine to generate power. After that gas stored in holder (5) for combustion in hot

stove.

Blower (point 1) supplied compressed air needed for combustion process consisted of

several main equipment (Fig. 2)

Fig 2. Configuration of blower

First component is exciter (EX) is a part of driving equipment. Synchronous motor (SM)

rotating speed is 3000 rpm and served as prime mover for Axial compressor (blower) can’t

self starting, therefore require trigger or excitation to run the field in the stator to the

rotational speed of the stator rotary field.

1 2

3

4

5

6

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13-stage axial compressor (blower) is the driven equipment and selected to supply high

flowrate of pressurised air. And turning device is installed at the discharge side of blower to

rotate at low speed (33 rpm) to prevent bending because of inertia when blower is starting or

stopping. Driving and driven equipment is connected by solid coupling flange type with 14

reamer bolt as tightening agent.

Moving on to axial compressor (blower). An axial compressor is one in which the flow

enters the compressor in an axial direction. The axial compressor compresses its working

fluids by first accelerating the fluids and then diffusing it to obtain the pressure. The flow is

accelerated by a row of rotating airfoils (blades) called the rotor, and then diffused in a row

of stationary blades called stator. A combination of rotor followed by a stator make-up a

stage in compressor, as ilustrated at Fig.3

Fig. 3. Configuration of axial compressor

As time passes on equipment will be degraded over time. To make sure reliability of

blower, regular overhaul need to be conducted. This core equipment must be overhauled with

right methods and technology.

2. Stator and Rotor Blade Defect Inspection

In an axial compressor, air passes through one stage to the next, each stage raising

pressure slightly. The length of the blades, and the annulus area, this is the area between the

shaft and shroud, decreases throughout the length of the compressor. This reduction in flow

area compensates for the increase in fluid density as it is compressed, permitting a constant

axial velocity. Looking at stator blades, the blade design is curved to make the differences of

area at the leading and trailing edge that will be resulting to differences in pressure. Rotor

blade designed to convert rotation of the motor into velocity of air.

In conclusion, air goes through suction of the axial compressor. After that, it goes through

inlet guide vane to be directed at right angle of rotor blade 1st stage. At rotor blade 1st stage,

velocity of air is increased and pressure is slightly increased because of rotation power from

motor. And then, air goes through 1st stage of stator blade where air with high velocity goes

Stage

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to diffuser design of stator blade. That means bigger area, so the velocity of air is decreases

but pressure is increases. After that, air goes through 2nd stage rotor blade to increase its

velocity and goes through 2nd stage of stator blade to increase the pressure and so on until air

reach 13th stage and get desired pressure. It can be simplified on Fig.4

Fig 4. Schematic diagram of airflow through axial compressor.

Another defining factor is crack. Crack must be detected early, due to stress concentration

effect that can lead to blade failure if not prevented, as ilustrated on fig.5

Fig 5. Schematic diagram of crack

Stress concentration increase stress that endured by materials exponentially. As stated on

equation below

........ (1)

Where : = Stress (Pa)

a = distance parallel to force (m)

b = distance perpendicular to force (m)

When crack involved. The distance perpendicular to force (b) is almost 0, that means the

stress will be divided by 0 and the stress will be infinite. Crack will goes on longer and longer.

The longer it gets the faster crack will spread. Based on this factor, early detection must be

done. One of the methods to detect surface crack is using non-destructive testing (NDT)

Several methods is available, but penetrant testing was chosen to be conducted at rotor

blade, for the sake of efficiency and convenience. Penetrant test is one of the non-destructive

testing that based upon capillarity principle, where low surface tension liquid penetrates into

clean and dry surface-breaking discontinuities.

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Fig.6 Step for penetrant test Fig.7 Penetrant test at compressor rotor

Specimen must be cleaned before test can be conducted (1). Apply penetrant liquid to the

surface of specimen (2). Wait for 10 minutes and clean excess penetrant (3). Apply developer

and wait for more or less 10 minutes, the result should visible (4) and can be seen in Fig.7

As viewed from fig.7 at bottom right picture, there is dent at rotor blade. If dent like that

is found, judgement must be made. If the problem like this deemed plenty enough or blade

defect is big enough. Blade must be changed and opened. Thorough Inspection must be

conducted at joint and end point. If the joint, shaft, and blade condition is good. The

inspection of crack and defect is finished, and rotor component can be used again. In case of

big defect, related component must be changed. If component is changed, do not forget to

conduct balancing test, usually weight is different than existing one. If balancing test is not

conducted, vibration will be high and will make other blade defected/broken.

For stator blade, Magnetic Particle Inspection (MPI) is conducted. Stator blade can be

disassembled one by one from casing. MPI test basically use magnetic field to detect defect.

Magnetic flux will be different in case of discontinuity (defect). Ferrous particle sprinkled

and will be attracted to flux leakage area and form indication, see fig.8

Fig 8. Schematic diagram of MPI

Stator blade disassembled from the casing one by one and tested with MPI methods with

magnetic yoke, use this step.

1) Cleaning surface of the blade from contaminant

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2) Spray check point using white paint, because ferrous particle color is same as stator blade

and it will be hard to see the result .

3) Spray ferrous magnetic particle to inspection point

4) Apply magnetic yoke, and rotate about 90 degrees near inspection point to detect vertical

and horizontal discontinuities, because discontinuity only detected if the discontinuity is

perpendicular to magnetic field lines

5) Make judgement after that make sure blade is cleaned from white paint and ready for re-

assembly process.

Conduct this inspection one by one for every stator blade installed. As stated above, rotate

magnetic yoke 90 degrees to make sure discontinuity horizontal and vertical is detected. This

is one of the most time consuming works and time should be managed properly. Same as

rotor, thorough inspection must be conducted at joint and end point of the blade.

3. Manage Gap Between Rotor and Casing

Based on configuration of the axial compressor (refer to Fig.3). Stator blade is connected

to the casing, while rotor blade connected to the rotor shaft. There have to be some gap

between stator blade and rotor shaft, or rotor blade and casing to accomodate vibration and

thermal expansion when compressor is operated.

If the gap too big compressor efficiency will decreased and have a chance of leakage. If

the gap is too small, when axial compressor is operated and coupled with motor, vibration

and thermal expansion of the material will occur. If there is no room to accomodate such

displacement, the blade will touch with corresponding pair and the blade will break. In

manual book, there are standard for the gap and there are 4 points that must be inspected.

Upper (0o), lower (180o), right (90o), and left (270o). With view from suction to discharge

side and degree of rotation clockwise.

To measure upper gap and lower gap, tools needed is lead wire and thickness gauge.

Open maker standard of the gap and check maximum value, for example the maximum value

is 2.42 mm. Lead wire must be bigger than that gap (choose 3mm).

To measure upper gap there are several step need to be conducted

1) Tape lead wire to the top of rotor blade and at the stator blade upper casing

2) Lift upper casing and place it to the top of rotor blade until surface of upper casing touch

lower casing. So lead wire will be pinched and leave a dent

3) Lift upper casing and move it to designated area

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4) Take out lead wire tapped at the blade, measure the dent using thickness gauge and write

it in the checksheet

For detailed visual can be seen on Fig.9 below

Fig.9 Measurement step for upper gap Fig.10 Measurement methods for

left and right gap.

For lower gap the measurement step is generally the same. Rather than upper casing is

lifted, rotor is lifted and fitted to bottom casing instead. For left and right gap is quite simple.

Measurement using tappered gauge and blue coloring as indicator of the thickness, viewed on

Fig.10 above.

This inspection must be conducted 2 times. On disassembly process and on assembly

process after action has been taken. There are several case of problem regarding this gap.

First, if right or left gap is too big or too small in one side or 2 side. Casing must be moved to

the right or to the left using chainblock or hydraulic jack, while centering guide adjusted with

added or removed liner. Install dial gauge to measure movement of the casing. In case of

upper or lower gap is too big or too small, shimplate on the casing needed to be added or

removed to adjust the gap. Basically, we can only adjust casing position.

4. Casing Bolt Tightness Management

Axial compressor used in blower plant can be split horizontally and divided into 2 parts:

upper casing and lower casing. This casing divided into 3 segment : suction, center, and

discharge. Upper and lower section connected by hexagonal head bolt for suction and several

reamer, stud bolt for center and discharge side. The tightening load of the bolt must not be

too low, excessive or not equally distributed among the bolts.

For suction casing, where temperature and pressure is low, hand tightening method is

applied with no quantitive indicator used. Center casing is where stator blade positioned. At

this point, pressure and temperature will gradually increase. To prevent leakage, tightening of

1 2

3 4

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the bolt must be precise, so hydraulic bolt tensioner is used. Discharge casing pressure and

temperature is the highest among the three. Up to 4 Bar and 200oC. Same as center casing,

hydraulic bolt tensioner is used.

When bolt is tightened, tightening load generated after tightening process is a tension load

on the bolt and a compression on the joint to prevent any movements between them. As,

ilustrated on Fig.11

Fig. 11 Schematic presentation of tightening load in bolted joint

As viewed on Fig.12 below, order of tightening is used to make sure load is distributed

equally among the bolt. tightening process conducted crossed, by turns left and right. Reamer

bolt tightened first to act as an anchor for the casing. Tightening process conducted 2 times,

first tightening use 75% of recommended pressure (50 MPa). Second tightening use full

pressure (78.5 MPa) with margin of error 5%.

Fig.12 Casing bolt composition and order of tightening

As the name suggest, hydraulic bolt tensioner use hydraulic pressure as a method of

tightening. Tightening bolt with hydraulic bolt tensioner consists in applying a traction load

directly to the bolt to extend it and then to turn down the nut until it is contacting with the

surface of the structure. With such a tightening mean, there is almost no torsion stress in the

bolt after the tightening operation is completed (Monville et al, 2016)

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When hydraulic bolt tensioner is installed to the bolt and pressure is applied.

Hydraulic pressure give force to the bolt. Bolt have no other means to move except elongated

following the movement of tensioner. While bolt is elongated, nut can be tightened easily.

When pressure is released, bolt want to come back to original form but held by nut tightly, so

tension of the bolt remain and bolt is tight.

Fig.13 Schematic drawing of how hydraulic bolt tensioner works

5. Coupling Bolt Tightness and Alignment

After core component of axial compressor is inspected, now it’s time to make sure

connector between driving engine (motor) and driven engine (compressor) is connected

smoothly without problem in the future. Severe misalignment between motor and compressor

will generate excess vibration. If vibration is too high, small gap between stator and rotor

blade (see chapter.2) can’t accomodate it. It makes stator and rotor touch each other while on

operation and the result will be catastrophic. A standard for misalignment was made, and

measured misalignment must be within standard (radial or axial). At first, we must measure

misalignment between motor and compressor using dial gauge and installed as viewed on

Fig.14

Fig.14 Misalignment measurement

With the methods above, we can measure misalignment on axial and radial on 90°, 180°

and 270°. When misalignment is above standard. Alignment proccess must be done. To align

motor and compressor, one of the equipment must be fixed (compressor) and the other can be

1. Installation 2. Pressure appied, bolt

elongated (elastic area)

3. Nut rotated while elongated 4. Pressure relase, load remained

Radial

measurement

Axial

measurement

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moved (motor). So, basically we move the motor up and down to make sure motor and

compressor align perfectly.

To align the coupling, we followed general alignment formula

Based on this formula, there are several variable that need to be measured. Support

distance means the distance between motor base to the motor coupling face. Coupling

diameter is clear. Axial and radial total indicator reading (TIR) means axial and radial total

indicator reading by dial gauge. Measured variable can be seen in Fig.15

Fig.15 Measurement variable

For example on point 1, dial gauge for radial and axial showing -0.59 mm and -0.2 mm

respectively. It can be calculated like this:

Based on this calculation we can conclude that on point 1, shimplate that must be

removed is 0.365 mm, but because plate with thickness of 0.015 is quite rare, we round it off

to 0.35 mm. After this process is complete, we can tighten the bolt.

Bolt for coupling is a bit special, because it use fitting with tolerance. So let’s start

discuss about fitting a bit. The International Organisation for Standardization (ISO) system

splits the three main categories into several individual fits based on the allowable limits for

hole and shaft size. Each fit is allocated a code, made up of a number and a letter. In using

this standard, capital letter always refer to the hole; lowercase are always used for the shaft.

Fig.16 Type of fit

Formula : 𝑺𝒖𝒑𝒑𝒐𝒓𝒕 𝑫𝒊𝒔𝒕𝒂𝒏𝒄𝒆

𝑪𝒐𝒖𝒑𝒍𝒊𝒏𝒈 𝑫𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝒙 𝑨𝒙𝒊𝒂𝒍 𝑻𝑰𝑹 ±

𝑹𝒂𝒅𝒊𝒂𝒍 𝑻𝑰𝑹

𝟐

Point 1 : 𝟓𝟎𝟎

𝟓𝟕𝟎 𝒙 𝟎.𝟎𝟖

𝟎 .𝟓𝟗

𝟐 = 0.365 = 0.35

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There are preferred fits that can be divided into 3 types: clearance, interference and

transitional fit. Below are the description of preferred fits using basic hole system. Based on

ANSI B4.2-1978

Type of Fit Description Symbol

Clearance

Loose running fit: for wide commercial tolerances or allowances on external members

H11/c11

Free running fit: not for use where accuracy is essential, but good for large temperature

variations, high running speeds, or heavy journal pressures

H9/d9

Close running fit: for running on accurate machines and for accurate location at

moderate speeds and journal pressures

H8/f7

Sliding fit: where parts are not intended to run freely but must move and turn freely and locate accurately

H7/g6

Locational clearance fit: provides snug fit

for location of stationary parts, but can be freely assembled and disassembled

H7/h6

Transition

Locational transition fit: for accurate

location, a compromise between clearance and interference

H7/k6

Locational transition fit: for more accurate location where greater interference is permissible

H7/n6

Interference

Locational interference fit: for parts requiring rigidity and alignment with prime

accuracy of location but without special bore pressure requirements

H7/p6

Medium drive fit: for ordinary steel parts or shrink fits on light sections, the tightest fit

usable with cast iron

H7/s6

Force fit: suitable for parts that can be

highly stressed or for shrink fits where the heavy pressing forces required are impractical

H7/u6

After alignment is complete, correct tightening of a bolt means making the best use of the

bolt’s elastic properties. The tightening process exerts an axial pre- load tension on the bolt.

This tension load is of course equal and opposite to the compression force applied on the

assembled components while in elastic zone. So it means, to work well bolt must behave like

a spring. Bolts are most often made of steel. Like most metals, steel is elastic, at least as long

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as the strain (elongation) does not exceed the elastic limit beyond which permanent

deformation occurs. Within the elastic limit, a metal part such as a bolt follows Hooke’s law,

that is to say that the strain (elongation) is proportional to the stress (load), as shown on the

Fig.17 below.

Fig.17 Graph for stress vs strain Fig 18 Elongation measurement of the bolt

While the strain (elongation) is still within elastic limit the bolt will come back to original

size when stress is removed. When the bolt come back to original size, it create tension forces

on the bolt because of the nut that prevented bolt back to original size. As a reaction forces

the joint experience compression forces and seal the joint tightly. Based on this theory,

tightening (steess applied) of the bolt must be controlled to make sure it is still within elastic

range. Or control elongation of the bolt, if stress measurement is not applicable.

Bolt tightening methods on the coupling is different than casing bolt tightening. While

casing bolt measure the force applied by hydraulic tensioner, coupling tightening use more

traditional methods. Tightening conducted using hand wrench (hit with hammer) and length

of the bolt is measured before (L1) and after tightening (L2) to represent elongation of the

bolt. length differences (ΔL) between L2 and L1 must be within standard.

While this methods is much simpler, it has several disadvantages. It generates damage on

the joint surfaces caused by compression of the nut to the joint, as can be seen in fig.19

Fig.19 Effect of hand tightening to joint surfaces Fig.20 Parasite torsion stress

Another weakness of tightening using wrench is appearance of “parasite torsion stress”.

It means while bolt is tightened using rotation, not all of this energy is converted into tension

Before

tightening

After

tightening

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stress. Parasite torsion stress made tightening takes more effort than it actually is, as

illustrated on Fig.20

One of other weakness is the difficulty to untighten the bolt. It is often much more

difficult to unscrew a torqued bolt than it was to screw it on in the first place. Damage to the

contact surfaces, corrosion problems, impose higher torque loads, which can cause damage to

various parts of the assembly. Tightening of coupling bolt supposedly only have one chance

to do it right and mistakes can be catasthrophic and takes a lot of time to solve.

6. Management sensor attached at Axial Compressor

Sensor is attached to protect and detect the state of Axial Compressor during operation.

Several variable (vibration, position, temperature) can be detected by sensor .

Fig.21 Aplication of sensor mounting at the shaft

Vibration sensor uses electromagnetic eddy current technology to sense the distance

between the probe tip and rotating machine shaft. Eddy Current efects injected sinusoidal

wave high frequency aleternating current as carrier wave. For this type of displacement

sensor is called “Non conatct pick-up”, so sensor not directly contact with object (rotating

machine shaft) and not causes vibration and friction with the object. The vibration of rotating

Shaft will make the distance of axis to vibration sensor installed at a fixed point will always

changeable and consequently change the energy of the Eddy Current effect which modulates

the carrier wave.For illustration wokring procces of sensor can see by figure 22.

Fig.22 illustration how sensor working

Before the rotor blade and bearing is removed, the sensor must first be removed. This is to

avoid sensor damage. Proper methode for released and reinstall of sensors is an important

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factor in management of vibration sensors during overhaul.

Before the sensor is released, the first time to record the initial condition of the sensor before

the overhaul there is the distance between the sensor with the shaft; Output voltage at the

position of distance between the sensor with the shaft.

If this is not done, then the possibility of sensor damage and readings of the readable

vibration values are inaccurate. If there is no known distance between the sensor and the shaft

at initial conditions, it will be very difficult to determine the initial distance settings between

the sensor and the shaft.

And for adjustment measuring vibration sensor in accordance with the instruction the

manufacturer of the vibration monitoring device. For adjustment vibration sensor, we can

following 2 methode for setting gapping the tip sensor with the shaft,there is:

1) Mechanical methode with spacer or thickness gauge, there is by rotating the locknut of

sensor until distance between sensor and shaft which is determined.

2) Electrical methode with adjustment and rotating the locknut sensor and then DC voltage

check.

So to adjust the distance between the sensor and the shaft can be done with two methode as

above, by looking at the conditions of how application of the sensor installation. To get

accurate results,both ways can combined.

For monitoring temperature of bearing used temperature sensor “Thermocouple K-type”.

Thermocouple is comparised of at least two metal conductors joined together to form two

junction and then produce “Thermo-Electric”effect. Where the heat gradient of a different

metal conductors will produce an electric potential (voltage). “The electrical potential in the

juncture-points of two dissimilar metals when there is a heat difference between the joints”.

This was the thermoelectric effect and is known as the Seebeck Effect.

During overhaul conducted, temperature sensor attached at the bearing cover must be remove.

To remove teh sensor by turning locking bolt (to hold the sensor position unchanged) and

then the sensor install released on the cover bearing.

Some keypoint management of sensor during overhaul conducted,there is :

a) The availability of spare part sensor becomes critical factor when overhaul is conducted.

Sensor damage during overhaul will stretch overhaul schedule that have been made due to

unavailability of spare parts.

b) Proper removal and re-installation procedure can minimize errors and also sensor damage.

c) Thorough inspection (position, bolting, sealing, cable sensor) before bearing cover

installed.

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7. Conclusion

Overhaul is conducted to maintain reliability of axial compressor. But with wrong

methods, result will not be fully optimized or even contrarily with reliability. Every works

for axial compressor has specific methods and usually maker of the axial compressor made

the guide book about possible methods and user have to choose what’s more suitable to them.

This process should be “one-time and get it right” type of works, where every little mistakes

could lead to big loss of time. For example, when coupling bolt is tightened but coupling face

on the axial compressor is not corresponding to coupling face on the motor. It almost

impossible to untighten the bolt and we forced to use destructive methods, e.g boring,

grinding, etc.

References

1. ASTM E1417. 2016, Standard Practice for Liquid Penetrant Testing, ASTM International,

West Conshohocken, PA

2. ASTM E709-15. 2015, Standard Guide for Magnetic Particle Testing, ASTM

International, West Conshohocken, PA

3. Boyce P Meherwan. 2002, Gas Turbine Engineering Handbook 2nd Ed ition, Gulf

Proffesional, USA, Page 164-192

4. Budynas G Richard, Nisbett J Keith. 2011, Shigley’s Mechanical Engineering Design 9th

Edition, Mc Graw-Hill, NY, USA, Page 395 - 398

5. Cengel A Yunus, Boles A Michael. 2004, Thermodynamics: An Engineering Approach

5th edition, Mc-Graw Hill, NY, USA

6. Hibbeler C Russel. 2004, Mechanics of Materials 6th Edition, Mc-Graw Hill, USA, Page

507-521

7. MES. 2010, Instruction Manual for Blower Proper, Japan

8. Monville Jean-Michel. 2016, Optimal Tightening Process of Bolted Joint. MZ Intelligent

System, France

9. NASA. 2015, Axial Compressor, Nancy Hall, USA

10. SIEMENS AG, Additional Documents_REV_0_EN_20130508_122944, Installation

Manual PROXPAC XL Proximity Tranducer Assembly, page D-160-161/230

11. SKF. 2001, Bolt Tightening Handbook, SKF Montigny le Bretonneux, France, Page 13-

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