Effect of Mass Transfer On Flow Accelerated
Corrosion Downstream an Orifice
Wael H. Ahmed*, Meamer A. El Nakla, Abdelsalam Alsarkhi, and Mufatiu M. Bello
Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals
Dhahran, Saudi Arabia, *e-mail: ahmedw@kfupm.edu.sa
Abstract—Local flow parameters play an important role in
characterizing the flow accelerated corrosion (FAC) downstream
of sudden area change in power plant piping systems. Accurate
prediction of the highest FAC wear rate locations enables the
mitigation of sudden and catastrophic failures, and the
improvement of the plant capacity factor. The objective of the
present study is to evaluate the effect of the local flow and mass
transfer parameters on flow accelerated corrosion downstream of
an orifice. In the present study, orifice to pipe diameter ratio of
0.5 was investigated numerically by solving the continuity and
momentum equations at Reynolds number of Re = 20,000.
Laboratory experiments, using test sections made of hydrocal
(CaSO4.½H2O), were carried out in order to validate the present
numerical results. The numerical results were compared to the
plants data as well as to the present experiments. The maximum
mass transfer coefficient found to occur at approximately 2-3
pipe diameters downstream of the orifice. This location was also
found to correspond to the location of elevated turbulent kinetic
energy generated within the flow separation vortices downstream
of the orifice. The FAC wear rates are well correlated with the
turbulence kinetic energy and the wall mass transfer. The
current study offered very useful information for FAC engineers
for better preparation of plant inspection scope.
Keywords-FAC; Local Flow Parameters; Mass Transfer;
Restriction Orifice
I.
INTRODUCTION
Flow Accelerated Corrosion (FAC) is a major safety and
reliability issue affecting carbon-steel piping in nuclear and
fossil power plants. This degradation mechanism results in
wear and thinning of large areas of piping and fittings that can
lead to sudden and sometimes to catastrophic failures, as well
as a huge economic loss. FAC is a process caused by the
flowing water or wet steam damaging or thinning the
protective oxide layer of piping components. The FAC process
can be described by two mechanisms: the first mechanism is
the soluble iron production (Fe2+) at the oxide/water interface,
while the second mechanism is the transfer of the corrosion
products to the bulk flow across the diffusion boundary layer.
Although the FAC is characterize by a general reduction in the
pipe wall thickness for a given piping component, it is
frequently occurs over a limited area within this component
due to the local high area of turbulence. The rate of the metal
wall loss due to FAC depends on a complex interaction of
several parameters such as material composition, water
chemistry, and hydrodynamic.
Failures due to FAC degradation have been reported at
several power plants around the world since 1981 [1].
However, a close attention to the FAC damage did not start
before the severe elbow rapture downstream of a tee occurred
at Surry Unit 2 power plant (USA) in 1989, which caused four
fatalities and extensive plant damage and resulted in a plant
shutdown. In 1999, an extensive steam leakage from the
rupture of the shell side of a feed-water heater at the Point
Beach power plant (USA) was reported by Yurmanov and
Rakhmanov [2]. In 2004, a fatal pipe rupture downstream of an
orifice in the condensate system due to FAC occurred in the
Mihama nuclear power plant Unit 3 (Japan) [2]. More recently,
the pipe failure downstream of a control valve at Iatan fossil
power plant in 2007 resulted in two fatalities and a huge capital
of plant loss as reported by Moore [3].
The recent study by Ahmed [4] indicated that a significant
research has been conducted on investigating the effect of fluid
chemical properties on flow accelerated corrosion (FAC) in
power plants. However, the hydrodynamic effects of single and
two-phase flows on FAC have not been thoroughly
investigated. In order to determine the effect of the proximity
between two components on the FAC wear rate, Ahmed [4] has
investigated 211 inspection data for 90o carbon steel elbows
from several nuclear power plants. The effect of the velocity as
well as the distance between the elbows and the upstream
components was discussed. Based on the analyzed trends
obtained from the inspection data, the author indicated a
significant increase in the wear rate of approximately 70% that
was identified to be due to the proximity.
The repeated inspections in both fossil and nuclear power
plants systems have shown that piping components located
downstream of flow singularities, such as sudden expansion or
contractions, orifices, valves, tees and elbows are most
susceptible to FAC damage. This is due to the severe changes
in flow direction as well as the development of secondary flow
instabilities downstream of these singularities [4]. Moreover, in
two-phase flows, the significant phase redistributions
downstream of these singularities may aggravate the problem.
Therefore, it is important to identify the main flow and
geometrical parameters require in characterizing FAC damage
downstream of pipe fittings. These parameters are: the
geometrical configuration of the components, piping
orientation, and the flow turbulence structure which will affect
the surface shear stress and mass transfer coefficients.
For single phase flow, the secondary vortices and/or flow
separation downstream of pipe fittings considered to be
important parameters need to be analyzed and modeled while
predicting the most FAC wear rate location. For example; the
secondary flows in elbows induce a pressure drop along the
elbow wall that can significantly increase the wall mean and
oscillatory shear stresses as discussed by Crawford et al. [6].
Also, orifices and valves promote turbulence close to the wall
in the downstream pipe and thus enhance the rate of mass
transfer at the wall [5]. These mechanisms have been identified
as the governing factors responsible for FAC as explained by
Chen et al. [6].
In summary, the pipe downstream of an orifice is found to
be one of the locations where aggressive FAC occurs.
Therefore, the main objective of the present study is to
characterize FAC downstream of an orifice in order to identify
the location of the highest FAC wear rate. The effect of local
flow and mass transfer parameters on FAC wear rate is
evaluated. The findings will enable the mitigation of sudden
and catastrophic failures due to FAC and consequently improve
the plant capacity factor.
of Class 2M. The measured wear is calculated by measuring
the difference between the actual scanned surface and a CAD
model representing the new pipe without corrosion. Wear
measurements were obtained by scanning the cut pipe with a
measurement accuracy of ±.037 mm. KUBE software was used
to laser measurement capturing and GEOMAGIC studio
software was used for data processing for each test section.
After the cut-test section was scanned, the point cloud data was
optimized by reducing the data noise, over lapping triangular
mesh and overhanging data. Then data was merged into
polygons and converted into one stretched water-polygon
structure. It should be noted that no data modification or
smoothing operation carried out in order to keep the original
data trend. After the data imported in to GEOMAGIC studio,
reference CAD geometry was created to represent the new
pipe. The wear data were obtained for segments of 10mm strips
along the pipe as shown in “Fig. 3”. This method is
representing similar reduction of inspection data in power
plants. The average wear is calculated for the 10mm strip and
carried out over the length of the test section to determine the
variation of wear along the pipe axis.
6
7
II.
EXPERIMENTAL FACILITY
Experiments are performed in a flow loop shown in “Fig.
1” that is designed to accommodate different test section
geometries as well as running single and two-phase flow test
conditions. Water is supplied from a 50 Liter reservoir through
a centrifugal pump driven by a variable speed electric motor. In
the present condition, the air line is shut off and only water is
allowed through the test section. The flow rates are controlled
by controlling the pump rotational speed in addition to a gate
valve located on the water flow line. The water flow rate is
measured using a turbine flow meter, and the temperature is
measured using thermocouples at various locations along the
flow loop. Experiments were performed using a 1-inch
diameter straight tubing at a Reynolds number of 20,000. A
straight section of approximately 75 diameters is installed
upstream of the straight test section to ensure fully developed
inlet flow conditions. An additional straight section of 100
diameters is installed downstream of the straight test section.
The test section downstream of the orifice is made of
hyrdical (CaSO4.½H2O), as shown in “Fig. 2”, in order to
obtain wall wear patterns in a reasonable test time. This
technique applied and tested before by Poulson, [7] and the
dissolution of the wall material depends on the mass transfer of
hydrocal from wall into the bulk flow and used to simulate
FAC wear in carbon steel piping components. Although the
changes to the surface occurring from the mass transfer of the
hydrocal to the flow may not be exactly the same as that would
occur in carbon steel piping systems in power plants, the wear
pattern developed from hydrocal is expected to be reasonably
similar to that generated over a longer period of time in carbon
steel piping component. The overall mass transfer over the
entire hydrocal test section surface is determined by measuring
the electrical conductivity, using EU Tech-PC300 meter with
an accuracy of ±1%, of the circulating water within the flow
loop.
The wear measurements were obtained using FARO-Axis
CMM with Laser Scanner D100 attached to laser power source
5
10
Air
11
9
8
12
13
Water
4
3
1
2
1
2
3
4
5
6,7
Water tank
Centrifugal pump
Turbine flow meter
Rotameter
Air-water mixer (two-phase experi
Pressure gages
Figure 1.
8,9
10
11
12
13
Void fraction meters
Hydrocal test section
Air-water Separator
Conductivity probe
Thermometer
Schematic diagram of the flow loop
Figure 2. Hydrocal test section
data obtained in the literature for moderate and high Reynolds
numbers at power plant conditions can lead to significant
errors.
mm
Figure 3. Surface measurments using FARO-Axis CMM with Laser Scanner
III.
FAC RATE AND MASS TRANSFER
The FAC process in carbon steel piping is described by four
steps. In the first process, metal oxidation occurs at metal/oxide
interface in oxygen-free water and explained by the following
reactions:
Fe + 2H2O →Fe2+ + 2OH- + H2
Fe2++ 2OH-↔Fe(OH)2
3Fe + 4H2O →Fe3O4 + 4H2
A. Modeling Mass Transfer downstream an orifice
Once the relationship between mass transfer and FAC wear
rate is established, the computational model for MTC
downstream of an orifice will be the objective of this section.
Fully developed turbulent pipe flow is assumed in order to
determine the mass transfer coefficients profiles downstream of
the orifice. ANSI specifications of orifice were used to construct
the geometrical model. Since the experimental condition in the
present study is carried out for straight pipe section fabricated
from hydrocal downstream of an orifice. The Solution is
obtained for k-ε turbulent flow model in conjunction with the
species transport equations using FLUENT CFD code.
The velocity field of the incompressible viscous flow is
obtained using the Reynolds averaged governing equations as
follows:



u i
0
xi
 u i


 0 
 x i


Momentum equation:
The first process involves the solubility of the ferrous
species through the porous oxide layer into the main water
flow. This transport across the oxide layer is controlled by the
concentration diffusion. The second step is described by the
dissolution of magnetite at oxide/water interface as explained
by the following reaction:
1/3Fe3O4 +(2-b)H+ +1/3H2↔Fe(OH)b(2-b)+ + (4/3-b)H2O)

where Fe(OH)b(2-b)+ represents the different iron ferrous
species b=(0,1,2,3)
In the last step, a diffusion process takes place where the
ferrous irons transfer into the bulk flowing water across the
diffusion boundary layer. In this process, the species migrated
from the metal/oxide interface and the species dissolved at the
oxide/water interface diffuse rapidly into the flowing water. In
this case, the concentration of ferrous iron in the bulk water is
very low compared to the concentration at the oxide/water
interface.
It can be noticed that FAC mechanism involved convective
mass transfer of the ferrous ions in the water. Over a limited
length of piping component, FAC rate is considered as direct
function of the mass flux of ferrous ions and can be calculated
from the convective mass transfer coefficient (MTC) in the
flowing water. Then, FAC rate is calculated from the MTC
and the difference between the concentration of ferrous ions at
the oxide/water interface (Cw) and the concentration of ferrous
in the bulk of water (Cb) as:
FAC rate = MTC( Cw – Cb)

It should be also noted that most of the experimental studies
and the correlations developed for MTC were carried out under
low flow rates conditions compared with common operating
conditions in power generation industry. Therefore, the MTC
uj

ui
P
  1  u i


 u i u j 


x j
x i x j  Re x j


Species mass transport equation for a steady process with no
chemical reaction is:
.(  vYi )  .J i  S i

where J i is the diffusion flux of species i, and arises due
to concentration gradient and S i is the source term. The
calculation of the local MTC is obtained similar to El-Gammal
et al. [8] as:
MTC ( z ) 
 DSL c / n | w
(cw  cb )

where n is the normal vector to the wall surface and DSL is
the diffusive coefficient of the solid species.
Grid independence tests were performed by increasing the
number of control volumes from 373,164 to 725,886. The
effect of mesh refinement, on the variation of the velocity and
mass transfer coefficient, found to be negligible beyond
725,886 grid points. The grid independence test resulted in a
maximum difference of less than 1% in the mass transfer
coefficient as the number of finite volumes increased from
725,886 to 853,240. Fully developed turbulent velocity profile
is selected as the entrance condition for the inlet pipe.
Assumption such as no-slip condition at the walls, steady,
viscous, incompressible liquid and fully turbulent with constant
transport properties are also used in the present analysis. The
mass concentration of the mixture species along the walls are
adjusted to unity. Also, the solubility (Cw) of species in water is
set to 0.275g/100g as specified by the manufacturer’s
properties table for hydrocal-X21 [9].
RESULTS AND DISCUSSIONS
Z/D≅ 2
Figure 5. Normalized Turbulent Kinetic Energy Distribution downstream the
orifice
1.40E-09
B. Experimental and Simulation Results
The surface wear morphology for the hydrocal test section
after 60 min running time is shown in “Fig. 5”. The figure
shows a maximum wear at approximately Z/D≅ 2 downstream
the orifice. This is found to be consistent with the practical data
shown in “Fig.4”.
The axial distributions of the average mass transfer
coefficient (MTC) and the wear rate are shown in “Fig. 6”. The
distribution of the MTC is found to provide a strong indication
of high FAC wear rate. This can be explained as the MTC
relates the diffusive surface species flux and the concentration
driving force given in “Eq. (5)”. “Fig. 6” clearly shows that
MTC increases steeply downstream of the orifice reaching its
maximum value at Z/D ≅ 2, which is located in the area with
high circulation within the separating vortices.
Measured Wear Rate (mm/year) x 10
[1]
1
[7]
0
0
1
2
3
4
5
6
7
8
L/D
8.00E-10
2
6.00E-10
1.5
4.00E-10
1
2.00E-10
0.5
0
2
4
6
8
10
12
REFERENCES
[5]
[6]
2
2.5
The authors appreciate the support of the Deanship of
Scientific Research at KFUPM for their financial support under
the (Grant No. IN090038)
[4]
3
Experimental Data
ACKNOWLEDGMENT
Reheat Drain System
4
1.00E-09
Figure 6. Normalized Turbulent Kinetic Energy Distribution downstream the
orifice
[3]
Reheat Supply System
3
Z/D
Moisture Separator Drain System
5
CFD Data
0
Condensate System
6
1.20E-09
0.00E+00
[2]
7
3.5
Plaster Pipe Wear (mm)
A. Power Plants inspection data
Ultrasonic techniques (UT) measurements are commonly
used to determine the wall thinning measurement in nearly all
power plants and to provide more accurate data for measuring
the remaining wall thickness in piping system. In the present
study, 132 inspection data collected from 5 nuclear power
plants and 3 fossil power plants for piping downstream an
orifice were analyzed. The data of very high and low values of
wear are compared to adjacent inspection readings in order to
remove data outliers. Once the data set for each inspection
location is verified, the wear is identified at each band along
the pipe axis. The measured wear data at different location
from the orifice were presented for different piping systems as
shown in “Fig. 4”. Although, the data collected at different
station show wide scatter in the measured wear, however, it is
clearly indicating that the maximum wear for different piping
system is located between 2-3 pipe diameters downstream of
the orifice.
MTC (m/s)
IV.
[8]
Figure 4. Measured wear rate downstream orifces at different power plants
[9]
Kanster, W., Erve, M., Henzel, N., and Stellwag, B., “Calculation code
for erosion corrosion induced wall thinning in piping system”, Nuclear
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Yurmanov, V., Rakhmanov, A., 2009, “International Atomic Energy
Agency, Workshop on Erosion-Corrosion, 21–23 April 2009, Moscow,
Russian Federation.
Moore, F.E., 2008. Welding and Repair Technology for Power Plants,
18th Int. EPRI Conference.
Ahmed, W.H. " Evaluation of the Proximity Effect on Flow Accelerated
Corrosion ", Annals of Nuclear Energy, Vol. 37, pp. 598-605, 2010.
Poulson B. (1999) Wear 233-235, 497-504
Crawford, N.M., Cunningham, G., Spence, S.W.T. (2007) Proc. of the
Institution of Mechanical Engineers, Part E, Journal of Process
Mechanical Engineering, Vol. 221, no. 2, pp. 77-88
Poulson, B., “Mass Transfer from Rough Surfaces”, Corrosion Science,
vol. 30, No. 6/7, pp. 743-746 1990.
El-Gammal M., Mazhar H., Cotton J.S., Shefski C., Pietralik J., Ching
C. Y., 2009. The hydrodynamic effects of single-phase flow on flow
accelerated corrosion in a 90-degree elbow”, Nuclear Engineering and
Design, vol. 240, 6, pp. 1589-159, 2010.
USG Corporations, "Material Safety Data Sheet for Hydrocal X-21”
MSDS#52-100-087, pp. 1 – 9.