E-Journal of Advanced Maintenance Vol.8-1 (2016) 1-12
Japan Society of Maintenology

Measurement of Flow Accelerated Corrosion Rate at an Elbow Pipe andCombination Effect of an Upstream Orifice
Yoichi UTANOHARA1,*, Koichi KAMAHORI1†, Akira NAKAMURA1 and Michio
MURASE1
1

Institute of Nuclear Safety System, Inc., 64 Sata, Mihama-Cho, Mikata-Gun, Fukui 919-1205, Japan

ABSTRACT
Flow accelerated corrosion (FAC) rates at an elbow pipe with the diameter of D = 50 mm were measured byusing corrosion sensors made of carbon steel. In addition, FAC rates at the elbow with an upstream orifice were
measured to investigate the enhancement of FAC due to pipe geometries. The diameter ratio of the orifice was
0.5. The water temperature was 150 °C, the mean cross-sectional velocity was 4.98 m/s, the dissolved oxygen
concentration was under 0.2 μg/kg, and pH was nearly neutral (about 7.0) at room temperature. The FAC rate
was smaller at the intrados of the elbow pipe than at other circumferential locations due to low flow velocity
near the inner surface. The ratio of the maximum FAC rate at the elbow pipe to the FAC rate in the straight pipe
was about 3.7 without the upstream orifice and about 15.4 with the upstream orifice. This showed that theorifice significantly affected the FAC rate at the elbow pipe because of the short distance of 2.65D between theorifice and the elbow pipe.
KEYWORDS

ARTICLE INFORMATION

flow accelerated corrosion, elbow pipe, wall thinning rate, electric
resistance method, orifice

Article history:
Received 18 January 2016
Accepted 30 March 2016

1.

Introduction

Flow accelerated corrosion (FAC) is an important issue for aging management of fossil andnuclear power plants. FAC occurs at the pipe geometry where flow is strongly disturbed such as
downstream from an orifice and at an elbow. FAC causes thinning of the pipe wall which occasionallyleads to a pipe rupture accident. Thinning of pipe wall thickness had been managed based on
non-destructive inspection of pipe wall thickness and evaluation of remaining lifetime following eachelectric utility’s own guideline. The Japan Society of Mechanical Engineers published its guideline onpipe wall thinning management in 2005 [1]. Since then, the management method for pipe wall
thinning has been unified under this guideline in Japan.
For prediction of FAC rates, it is important to understand factors influencing FAC. Generally,
influencing factors are the water temperature, pH, dissolved oxygen concentration, material
composition and fluid dynamics factor [2]. FAC studies from the viewpoint of the fluid dynamics
factor (which the authors focus on) have mainly been evaluated by using the mean cross-sectional
velocity [3][4], but recently the relationship between FAC and a local flow field has been reported[5]-[9]. The essential fluid dynamics factor influencing FAC is recognized to be the mass transfer
coefficient near a pipe wall, and recent studies have been conducted based on this concept. To
validate prediction methods for FAC rates, FAC data are needed. There are many power plant data but
uncertainties for operation conditions and water chemistries are generally large. Therefore, the authors
have measured FAC rates under well-controlled conditions in a test loop for the region downstream
from an orifice and numerical simulations for the flow field in the experiments have been carried out[10]-[12]. The goal of these studies has been to develop a numerical simulation method by which
effects of the local flow field on FAC can be evaluated.
In this study, the authors focused on FAC rates at an elbow pipe, because there are many elbows*

Corresponding author, E-mail: utanohara@inss.co.jp
The Kansai Electric Power Co., Inc., 13-8 Goichi, Mihama-cho, Mikata-gun, Fukui 919-1141, Japan†

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

1

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipein a power plant but FAC rates at an elbow pipe have not been directly measured in experiments,
although the distribution of the mass transfer coefficient was measured experimentally [6] and wall
thinning data in a power plant were examined [7]. The authors measured FAC rates at an elbow pipewith the diameter of 50 mm. Furthermore, the enhancement of FAC rate due to combinations of pipegeometries was investigated. Generally, if a certain pipe geometry exists upstream, the FAC rate at
the downstream pipe geometry increases [13]. Hence, the FAC rates were compared at the elbow pipewith and without an orifice (the diameter ratio of 0.5) upstream from the elbow pipe. Experimental
conditions were the water temperature of 150 °C and the mean cross-sectional velocity of 5.0 m/s.

2.

Experimental Method

2.1. Test loop
The test loop is shown in Fig. 1. The experimental method was almost the same as in previous
studies [10]-[12]; however, the test section was modified as described in Sec. 2.2. The recirculation
pump was equipped downstream from the test section to avoid pump-induced disturbance effects on
the test section. Dissolved oxygen concentration of the feed water was decreased by using thedegasification unit before heating. Dissolved oxygen concentration was normally under 0.2 μg/kg
during experiment. The pH was not controlled and was nearly neutral at room temperature with noadditives. Iron concentration was kept constant at about 0.03 μg/g during the experiment by using thedemineralizer. The loop temperature was automatically controlled by the heater power and cooling
unit equipped in a small branch pipe downstream from the test section. Under the steady-state
operation, cooling was needed to keep the water temperature constant due to Joule heating of therecirculation pump.
During the experiment, the pressure in the hot water tank, the water temperature and flow rate in
the loop were measured. Dissolved oxygen concentration, conductivity and pH were measured at
atmospheric pressure and room temperature by using the sampling system. The measurement
instruments for water chemistries are listed in Table 1 [12].
Safety valve

P

Nitrogen gas
cylinderVacuumpump

Demineralizer

Hot watertank( 2 MPa)

Degasification

Flow stabilizer

Feed water

Water chemistry
control sys.
Hydrazine (N2H4)
Sulfuric acid
(H2SO4)

HeaterCoolerF

Depressurization
Valve

Recirculation pump
(Inverter control)

DO: Dissolved oxygen
C : Conductivity

C

DO

pH

Drain

T

Test section(D = 50 mm)

CoolerMeasurements ofwater chemistry

Sampling sys.

Fig. 1. Test loop for measurements of FAC rate
Table 1 Specifications of measurement instruments

Item measured

Manufacturer

Instrument model

Dissolved oxygen

Nikkiso7115-22

Conductivity
pH
Iron concentration

NikkisoHoriba
Hach

9792
D-51
Pocket colorimeter II

2

MethodMembranepolarography
Glass electrodeTPTZ

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe2.2. Test section
The test section for FAC rate measurement is shown in Fig. 2. The test section was made ofstainless steel (SUS304) pipe which had an inner diameter of D = 50mm. The corrosion sensors weremade of carbon steel plate (STPT42: Ni, 0.02 wt. %; Cr, 0.04 wt. %; Mo, 0.01 wt. %) and implantedon the pipe inner surface. Twenty corrosion sensors were installed: four at 9.25D upstream from theentrance of the elbow in the straight pipe, four at the center of the elbow pipe, four each at 2.68D and
3.68D downstream, and two each at 4.68D and 5.68D downstream from the exit of the elbow. It
should be noted that the elbow pipe was bending in the horizontal plane, namely, the upstream pipe,
the elbow pipe and the downstream pipe were placed in the horizontal plane.
The detailed geometry of the elbow pipe used in the experiment is shown in Fig. 3. Its geometry
was different from that of normally used elbow types, because of the difficulty to install the corrosion
sensors on the wall surface. In the previous studies [10]-[12], the inner surface of the test section wassmoothed using a boring machine to remove undesirable bumps between the corrosion sensors and thepipe wall and to get a tight fit. However, such machining cannot be applied on the elbow surface.
Hence, a straight part of 54 mm was sandwiched between two 45-degree elbows with the curvatureradius of Re = 76.2 mm to install corrosion sensors. The equivalent curvature radius of the elbow pipe
was Rep = 114.4 mm.
In the case of the combination of the elbow pipe with the upstream orifice, the orifice plate wasplaced 2.65D upstream from the entrance of the elbow pipe. The profile of the orifice plate with thediameter ratio of β = 0.5 is shown in Fig. 4. The same orifice plate as that in the previous studies[10]-[12] was used to compare the FAC rates.
The layout of corrosion sensors as viewed from upstream is shown in Fig. 5. Four corrosion
sensors were installed in the circumferential direction at the center of the elbow pipe. The directions
were the upper, the lower, the extrados and the intrados of the elbow pipe. The corrosion sensors were
insulated from the stainless steel pipe by a covering resin and were implanted on the pipe innersurface so that one surface of the plate was exposed to the fluid (hot water) as shown in Fig. 6.

Entrance ofan elbow pipe

Corrosion sensors
upstream from an elbow

180

132.6

φ50

Exit ofan elbow pipe

Flow
150

83.8

650

Corrosion sensors
at an elbow

Orifice position
50 50 50 50

Corrosion sensors
downstream from an elbow
350

Fig. 2. Test section for measurements of FAC rate, overhead view (unit: mm)

3

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe

132.6

Entrance ofthe elbow

Flow
direction
Re = 76.2

Exit ofthe elbow

Orificeposition

83.8

Fig. 3. Detailed geometry of the elbow pipe (unit: mm)

Inner diameterφ50
β = 0.5

5

φ25

φ70

Fig. 4. Orifice plate (unit: mm)

upper

View fromthe upstream

Verticaldirection

intrados

extrados

lower
Fig. 5. Layout of sensors for measuring FAC rates
stainless steel
Carbon steel
STPT42flow

d=0.5mm
L = 20mm

Corrosion sensor

insulating resin
exposed plate

Fig. 6. Corrosion sensors

4S

w = 3mm

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe2.3. Measurement method for FAC rate
The FAC rate was measured by the electric resistance method. In the previous studies [10]-[12],a reference plate of the same material and same size as the exposed plate was used to compensate forthe temperature change of resistance, but the reference plate was not used in this study. It is expectedthat failures of corrosion sensors would be reduced by simplifying the electrical-measuring circuit.
The electric resistance of the sensor plate R is expressed by equation (1):R  e

L
L
 e
S
d w

(1)

where ρe, S, L, d and w are the electric resistivity, cross-sectional area, length, thickness and width of
the sensor plate, respectively. While the simplified corrosion sensor has the advantage describedabove, the measured data are subject to more influence by temperature fluctuation because ρe depends
on temperature. As described in Sec. 2.4, the temperature change during the experiment was within
about ±0.1 °C, then ρe can be treated as a constant value in this study. Hence, the thickness of thesensor plate d can be measured from the value of the electric resistance R and then the FAC rate of thesensor plate can be obtained from the gradient of the time history of metal loss, as shown in Sec. 3.1.

2.4. Measurement system for electric resistance
The measurement system for the electric resistance is shown in Fig. 7. This system consisted ofcorrosion sensors, a data acquisition system and a PC. The data acquisition system included 11 sets ofdata loggers (CorrLog, manufactured by CorrOcean), allowing measurement of the electric resistanceof the corrosion sensors. There were 20 corrosion sensors in the test section (Fig. 2), but there only 11
data logger sets. In case of failures of corrosion sensors as mentioned in Sec. 2.3, the number ofcorrosion sensors was more than that of data loggers. Therefore, only 11 corrosion sensors wereselected for measurement considering the necessity for understanding the FAC characteristics of theelbow. The monitoring software MultiTrend (CorrOcean), which manages the measurement data log,
controls the measurement system and provides online monitoring, was used for data acquisition of themeasured electric resistance values.

CorrOceandata logger x 12 sets
(CorrLog)
Corrosion sensors

Dataacquisition
systemTest section

RS232CPC

Fig. 7. Measurement system for the electric resistance2.5. Experimental conditions
The FAC experimental conditions are shown in Table 2. The orifice upstream from the elbow
pipe was not installed in Run 1, but it was installed in Run 2. The experimental period was 261 hours
for Run 1 to obtain a thinning thickness of more than about 5 μm for reliable data and 100 hours for
Run 2 because FAC rates were larger than those in Run 1. The mean cross-sectional velocity was
about 5.0 m/s and its change during the experiment was within ±0.3 %. The water temperature wasabout 149 °C and its change was within ±0.1 °C. The water was pressurized to 1.5 MPa to avoid
cavitation in the test loop. The dissolved oxygen concentration was almost always under 0.2 μg/kg.
The pH was not controlled and was nearly neutral at room temperature. The difference of pH at each5

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipesampling measurement was relatively large. Iron concentration increased a little initially and was kept
constant at about 0.03±0.01 μg/g during the experiment.
Table 2. FAC experimental conditions

Run 1
Run 2
25.0 (β = 0.5)
261
100
4.98
4.98
149.2
149.3
0.1
0.2
7.0 (6.2 to 7.5)
7.0 (6.9 to 7.2)
0.01 to 0.04
0.01 to 0.04
Carbon steel plates(STPT42: Ni, 0.02 wt. %; Cr, 0.04 wt. %; Mo, 0.01 wt. %)*a: The average during the corrosion measurement. The numbers in the parentheses are theminimum and the maximum of the measurement values.
Orifice diameter [mm]Experimental period [h]Mean cross-sectional velocity [m/s]Temperature [°C]
Dissolved oxygen [μg/kg]
pH*a
Iron concentration [μg/g]
Corrosion sensor property

3.

Experimental Results

3.1. Time history of metal loss
Time histories of metal loss of corrosion sensors in Run 1 and Run 2 are shown in Fig. 8.
Descriptions in the legend are positions of the corrosion sensors. The origin of the longitudinal
direction is the inlet of the elbow pipe. The metal losses changed with time and gradients became
constant after a certain period of time. In Run 1, the gradients started to decrease after around 50
hours and then became relatively constant. In Run 2, the metal losses were small during the heatingprocess, started to increase after 10-20 hours, and became almost linear after 20-40 hours. Comparedwith Run 2, the time histories in Run 1 had slight variations and they were a kind of noise mainly
caused by change of atmospheric temperature. Since FAC rates in Run 1 were smaller than those of
Run 2, variations became noticeable. The influence of noise can be reduced by time-averaging
procedure. To obtain FAC rates after reaching steady state, the time-averaged increasing rate of themetal loss from 449 to 561 hours in Run 1 and from 50 to 98 hours in Run 2 was used. In Run 2, themetal loss at the elbow pipe was very large, and the experiment was finished before the metal loss at-9.25D in the straight pipe reached about 5 μm.

6Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe
Run 1

-35

-9.25D, extrados-9.25D, intrados
Elbow, extrados
Elbow, lower
Elbow, intrados
Elbow, upper
6.15D, extrados
6.15D, upper
6.15D, intrados
7.15D, extrados
7.15D, intrados-30

Metal loss [μm]

-25
-20
-15
-10

Time period
to derive
FAC rate

-5
0
0

100

200

300
Time [h]

400

500

600

(a) Run 1
Run 2

-50

-9.25D, extrados-9.25D, intrados
Elbow, extrados
Elbow, lower
Elbow, intrados
Elbow, upper
6.15D, extrados
6.15D, upper
6.15D, intrados
7.15D, extrados
7.15D, intrados

Metal loss [μm]

-40
-30

Time period toderive FAC rate

-20
-10
0
0

20

40
60
Time [h]80100(b) Run 2
Fig. 8. Time histories of metal loss of corrosion sensors

3.2. FAC rates at the elbow pipe without the orifice
Figure 9 shows the distribution of FAC rates at the elbow pipe (Run 1, the case without theorifice). The mean cross-sectional velocity was 4.98 m/s. As mentioned in Sec. 3.1, time-averaged
values are plotted in Fig.9. Their standard deviations (not shown in the figure) were 0.3 to 0.5 mm/yand comparable with time-averaged values, but influence of variation were reduced and tendency canbe extracted by time-averaging procedure. The FAC rate was smaller at the intrados of the elbow pipe
than at other circumferential locations. This seemed to be due to the velocity distribution in the elbow.
Generally, secondary flow is induced in the elbow because of the centrifugal force through the elbow
radius and faster flow moves to the extrados. Hence flow velocity near the intrados at the middle partof the elbow is relatively low. The possible reason for the low FAC rate of the intrados at the elbow
was likely related to the low velocity near the inner surface. The FAC rates at the elbow pipe were7

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipelarger than those upstream and downstream from the elbow pipe and the FAC rates downstream fromthe elbow pipe decreased along the flow direction. The tendency of low FAC rate at the intrados side
continued downstream from the elbow. These results indicated that the velocity distribution, low near
the intrados and high near the extrados, was still retained downstream. The ratios of the maximum
FAC rate at each x/D to the FAC rate in the straight pipe (x/D = -9.25) were respectively about 3.7,
2.8 and 2.7 for x/D = 1.74, 6.15 and 7.15.
0.6

upper
extrados
lower
intrados

FAC rate [mm/y]

0.5
0.4
0.3
0.2

at the elbow

upstreamfrom the elbow

0.1

downstream
from the elbow

0.0
-10

-8

-6

-4

-2

0

2

4

6

8

10

Distance from entrance of elbow [x/D]Fig. 9. FAC rates at the elbow pipe without orifice (Run 1)

3.3. FAC rates at the elbow pipe downstream from the orifice
The FAC rates of the elbow pipe downstream from the orifice with the diameter ratio of β =
0.5 (Run 2) were measured under the conditions of the same measuring positions and velocity of
about 5.0 m/s as those in Run 1 (described in Sec.3.1), and the results are shown in Fig. 10. Standard
deviations (not shown in the figure) were 0.15 to 0.38 mm/y and smaller than time-averaged values,
particularly at the elbow in Fig. 10. Overall, FAC rates increased compared with Run 1, the casewithout the upstream orifice. The FAC rate was smaller at the intrados of the elbow pipe than at othercircumferential locations like that in Run 1, but the circumferential difference of FAC rates was small
at x/D = 6.15 and 7.15 unlike Run 1. The ratios of the maximum FAC rate at each x/D to the FAC ratein the straight pipe (x/D = -9.25) were respectively about 15.4, 2.8 and 1.9 for x/D = 1.74, 6.15 and7.15. The comparison with those in Run 1 (3.7, 2.8 and 2.7) showed that the orifice largely affected
the FAC rate at the elbow pipe (x/D = 1.74) but did not affect the FAC rate so much at x/D = 6.15 and
7.15.

8

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe
4.5

upper
extrados
lower
intrados

4.0

FAC rate [mm/y]

3.5
3.0

at the elbow

2.5
2.0

downstream
from the elbow

1.5
upstreamfrom the elbow

1.0
0.5
0.0
-10

-8

-6

-4

-2

0

2

4

6

8

10

Distance from entrance of elbow [x/D]Fig. 10. FAC rates at the elbow pipe downstream from the orifice (Run 2)

4. Discussion
Figure 11 compares FAC rates in Run 2 with those downstream from the single orifice without
the elbow [11]. The locations of corrosion sensors with the single orifice were 1, 2, 3 and 4D
downstream from the orifice outlet. This comparison showed that the FAC rate at 2.65D downstream
from the orifice (x/D = 0 in Fig. 11) was large and the effect of the upstream orifice was strong at theentrance of the elbow pipe. The maximum increase in the FAC rate due to the elbow pipe from thesingle orifice case was about 1.2 mm/y (= 4.2 mm/y – 3.0 mm/y) and was about 3.4 times that in Run
1 of the single elbow case (0.48 mm/y – 0.13 mm/y = 0.35 mm/y, Fig. 9). This showed that the
combination of an orifice and an elbow pipe with the short distance of 2.65D promoted the FAC rate
at the elbow pipe.
To understand the influence of the upstream orifice, the measured and predicted orifice flow
fields [10] are shown in Figs. 12 and 13. In the case of the single orifice, the reattachment point wasaround 2.5D downstream from the orifice. Flow structures such as the velocity distribution did notdepend on the mean cross-sectional flow velocity Uave when normalized by Uave. From Fig. 13, theorifice jet continued around 3D downstream from the orifice. Hence, in the present study of the orificeand elbow combinations, the distance 2.65D downstream from the orifice was just behind thereattachment point and the orifice jet probably reached the elbow extrados. This jet flow mightenhance the FAC rate at the elbow. It is expected that FAC rates at the elbow pipe with the distanceover 4D between the orifice and the elbow pipe may be mitigated, because the FAC rate at 4D wasabout 40 % of that at 1D to 3D downstream from the orifice, and also the orifice jet in Fig. 13
disappeared at 4D downstream.
According to the above discussion, the orifice and elbow were combined at the most effective
location. Kastner et al. [13] proposed a formula of combination effect and an upstream component A
with geometry factor kc,A affects a downstream component B with geometry factor kc,B. The increaseof downstream geometry factor is described as follow:

kc,A  kc,A exp C  z / D  .

(2)Here, z is distance between component A and B, and C is constant (= 0.231). The total geometry
factor of downstream component k’c,B iskc, B  k c,B  k c,A .

9(3)

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe

The influence of upstream component becomes the maximum when z/D = 0 and k’c,B equals the sumof kc,A and kc,B. In the present study, the sum of FAC rates of the single orifice (about 3.0 mm/y) andthe elbow in Run 1 (about 0.5 mm/y) was comparable with the FAC rate of the downstream elbow inRun 2 (about 4.0 mm/y). In this way, the results of this study agreed with the formula proposed by
Kastner et al. [13].
The discussion here is based on the previous results of the single orifice case [10]. To understandthe combination effect of the upstream orifice and downstream elbow, numerical simulation of thiscombination is needed and it remains as future work.
4.5
Orificeoutlet4.0

FAC rate [mm/y]

3.5
Run 2, upper
Run 2, extrados
Run 2, lower
Run 2, intrados
Utanohara et al. [11]

3.0
2.5
2.0
1.5
1.0
0.5
0.0
-10

-8

-6

-4

-2

0

2

4

6

8

10

Distance from entrance of elbow [x/D]Fig. 11. Comparison of FAC rates in Run 2 with those downstream from single orificeFig. 12. Axial velocity profiles near the wall (y = 1 mm from the wall) downstream from the single orifice
[10]

10

Y. Utanohara, et al./Measurement of Flow Accelerated Corrosion Rate at Elbow Pipe

Fig. 13. Time-averaged velocity distribution of the orifice flow (Uave = 0.453 m/s) predicted by large eddy
simulation [10], tick marks are placed every 1D distance5.

Conclusions

In this study, the authors measured FAC rates at an elbow pipe. In addition, to understand theenhancement of FAC rate by the combination of pipe geometries, FAC rates with and without an
orifice (respectively Run 2 and Run 1) upstream from the elbow pipe were compared with each other.
The following conclusions were obtained.
1) In Run 1 and Run 2, FAC rates at the elbow pipe were larger than those upstream anddownstream from the elbow pipe. The FAC rate was smaller at the intrados of the elbow pipe
than at other circumferential locations. This might be due to the low velocity near the intrados.
FAC rates downstream from the elbow pipe decreased along the flow direction.
2) In Run 1 (without the upstream orifice), the ratio of the maximum FAC rate at the elbow pipe to
the FAC rate in the straight pipe was about 3.7.
3) In Run 2, when the orifice was 2.65D upstream from the elbow pipe, the ratio of the maximum
FAC rate at the elbow pipe to the FAC rate in the straight pipe was 15.4. The comparison of thevalue of 15.4 with the value of 3.7 in Run 1 showed that the orifice largely affected the FAC rate
at the elbow pipe.
4) The comparison of FAC rates in Run 2 with those downstream from the single orifice without the
elbow showed that the effect of the upstream orifice was strong at the entrance of the elbow pipe
because the FAC rate at 2.65D downstream from the orifice was still large. The maximum
increase in the FAC rate due to the elbow pipe of about 1.2 mm/y in Run 2 was about 3.4 times
that in Run 1 (0.35 mm/y) and this showed that the combination of an orifice and an elbow pipewith a short distance promoted the FAC rate at the elbow pipe.

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