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Research of the cavitation performance of the condensate pump
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2013 IOP Conf. Ser.: Mater. Sci. Eng. 52 062012
(http://iopscience.iop.org/1757-899X/52/6/062012)
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Research of the cavitation performance of the condensate
pump
H F Li, Z B Pan, M H He, K Ji, W C Zhou and S M Min
KSB Shanghai Pump Co., Ltd., 1400 Jiangchuan Rd., Shanghai, 200000, China,
E-mail: shlihaifeng@gmail.com
Abstract. Condensate pump is an important part of power plant circulation systems, which is
used to pump condensate water. Because the condensate water pressure is very low, the first
impeller of the condensate pump must have a good cavitation performance. Numerical
simulation was employed to study the first impeller cavitation performance. The first impeller
was set in the condensate pump barrel, and the double suction casing was kept, the parts after
the double suction casing was simplified as tube. The simplicity can guarantee the inlet and
outlet conditions of the impeller. Based on the RANS and SST
k
turbulence model, CFD
software was used to simulate the condensate pump at different working conditions. The
numerical simulation shows that cavitation occurred at the suction side of the blades closing to
the leading edge. The cavitation performance of the impeller was predicted based on the
numerical calculation. Comparing with the experimental results, the numerical simulation
result is smaller than that of the experiment in small flux, and the cavitation performance trend
is agreed with that of the experiments.
1. Introduction
Condensate pump is an important part in power plant circulation system, which is used to pump
condensate water from condenser to the deaerator. After doing work, the superheated steam is cooled
to condensate water. Because the internal pressure in the condenser is low and the condensate water is
nearly in saturation temperature, cavitation is early occurred in the first stage of the condensate
pump[1-3]. When cavitation occurred, the air dissolved out from the condensate water and mixed with
the water vapor to form the mixed bubbles. The bubbles will quickly condense and collapse when it
flow to high pressure area. During this process, the complex change of the mass、momentum and
energy occur between the bubbles and liquid. Noise、vibration and other physical and chemical
phenomena also occur. The cavitation may damage the flow passages [4-6]. When the condensate
pump cavitation occurs, the pump efficiency will decrease, and what’s more may lead the whole
thermal power generator units working unnormal. It is important to study the cavitation to improve the
condensate pump anti-cavitation ability. Experiments and numerical analysis are the may
methods to study the cavitation. But for large unit, the cavitation experiments are much cost
and may not do cavitation experiments because the experimental method limits.
With the development of the computational fluid dynamics(CFD), CFD technology has been used
to study the condensate pump phenomena. Feng carried out numerical simulation of the cavitation
occurred in the first stage impeller of the condensate pump. He get the cavitation occurring position
and analyzed the volume fraction of the liquid and gas within the impeller. Because the physical
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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model is so simple that it can not simulate the actual working conditions of the first stage impeller,
further study is need to validate his simulation.
Because the pressure of the inlet flow is very low, the condensate pump usually was designed as
cylinder type. The first stage was set at the bottom of the cylinder to avoid cavitation. The typical type
of the condensate pump is as shown in figure 1. The flow at the bottom of the cylinder is very
turbulent because of the complex structure of the first stage and its accessories. The first stage of is
double suction impeller, the inlet conditions of the up and down are different, so the first stage
impeller cavitation is very complex. If feng’s model was used, it can not to simulate the actual inlet
and outlet conditions, so it can not to correctly predict the condensate pump cavitation performance.
Figure 1. The structure of a condensate pump.
In order to correctly analyze the condensate pump cavitation performance, it is important to
consider the actual inlet and outlet conditions of the first stage impeller. In this passage, a type of
condensate pump NLT500-570 with four stage impellers was studied. The pump was simplified to fit
CFD calculations, first of all, the performance of the first stage double suction impeller was predicted,
then using two phase model to simulate the cavitation occurred in the double suction impeller, at last,
the condensate pump cavitation performance was predicted.
2. Performance prediction
NLT500-570 condensate pump is an important product of Shanghai KSB pump Co.,Ltd.. It has 5
stages; the first stage is double-suction impeller. The double suction impeller has 5 blades. The
parameters of the condensate pump are: the designed rotate speed is 1480rpm, the optimum flux is
2250m3/h, the designed head is 87m and efficiency is 83%. The condensate pump(simplified) and the
first stage impeller and blades is shown in figure 2.
Figure 2. Physical model (simplified).
In order to simulate the first stage real working conditions, the calculation area includes the inlet
barrel parts, the rotating impeller parts and the double-suction volute outlet parts. Tetrahedral mesh
scheme was adopted to mesh the three parts respectively. During the meshing process, the grids were
refined in the area where the geometry changes sharply. Figure 3 shows the meshed grid of the
calculation area.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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In performance numerical prediction, it must to study the mesh sensitivity. Table 1 is the mesh
densities used in numerical prediction. The results of mesh sensitivity studies are showed in figure
4. From the figure, it can be found that the good compromise between the results accuracy and the
resource requirements is the mesh density 13.8×106. As seen, increasing the mesh density beyond
13.8×106 does not result in a noticeable head increase.
Figure 3. Numerical analysis grids.
Table 1. Mesh densities used in numerical prediction.
No.
1
2
3
4
5
Grid density
5E6
8E6
10.7E6
13.8E6
15E6
Figure 4. Grid sensitivity test.
Using the tested numerical scheme, the flow within the condensate pump was simulated and the
performance of the first stage impeller was predicted. Figure 5 shows the impeller efficiency variation
as a function of flux and Figure 6 shows the impeller head variation as a function of flux. In the figure,
“Exp” means experimental results and “Num” means numerical simulation results. From the figure, it
can be found that the numerical simulation results agree well with the experimental results, this
implies that the simplification of the condensate pump and numerical simulation are correct in
predicting the first stage impeller.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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Figure 5. Pump head variation as a function of flux.
Figure 6. Pump efficiency variation as a function of flux.
3. Cavitation prediction
Based on the performance simulation scheme, two-phase flow model was adopted and different inlet
pressure was set to simulate the cavitation experiments. But when the two-phase flow model was used
and the inlet pressure was set to 1 atmosphere pressure, the numerical simulation can not be
convergence. The results analysis showed that cavitation occurred in the second-stage and the
blending duct area. Figure 7 shows the water vapor volume fraction in the condensate pump. From the
figure, it can be found that when the inlet pressure is 1 atmosphere pressure, the cavitation occurred in
the outlet part. Because the second stage impeller was not considered, the simplified physical model
may not meet the cavitation simulation requirements.
Figure 7. Water vapor volume fraction in the condensate pump.
In order to carry out cavitation simulation, the physical model was simplified for the second time.
Keeping the inlet part and the impeller parts do not change, the double-volute outlet part was re-
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
4
simplified. The double-volute was reserved to keep the outlet conditions of the double impeller do not
change. The secondary stages volutes are simplified as straight pipe. Figure 8 shows the simplified
outlet parts. After this simplification, the condensate pump performance was predicted using the same
numerical scheme. The predicted performance is shown in figure 9 and figure 10. In the figures,
“Num” means the first simplification results, “Num2” means the second simplification and “Exp” is
the experimental results. From the figures, it can be found that the second simplification do not cause
significant difference when used in numerical simulation. The second simplification was used to
simulate the condensate cavitation performance.
Figure 8. The second simplification of outlet part.
Figure 9. Impeller head variation as a function of flux.
Figure 10. Impeller efficiency variation as a function of flux.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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Under the optimum working conditions, the condensate pump cavitation performance was
simulated. The simulation shows that when the inlet pressure is below some extent, the cavitation
occurred in the area of the suction side close to the leading edge. Figure 11 shows the water vapor
volume fraction and the pressure of the blades. It can be found that at the suction side close to the
leading edge, the pressure is low, and then in this area, water vapor area appears. The water vapor
volume fraction distribution is similar to the pressure distribution. This shows that the cavitation is
closely related to the pressure.
Figure 11. Water vapor volume fraction and pressure distribution of the blades.
Figure 12. Water vapor volume fraction in the condensate pump.
Figure 12 shows the water vapor fraction distribution in the simplified model. It can be found that
the water vapor volume fraction in the condensate pump is nearly zero just except the impeller area.
This means that the simplified model can be used to simulate the cavitation simulation.
Based on the simplified model and numerical scheme, the inlet pressure was decreased to model
the cavitation process under the optimum working conditions. The head variation as a function of
cavitation is shown in figure 13. From the figure it can be found that the impeller head changes with
the cavitation decreasing. When the cavitation is below a certain value, the impeller head decrease to
be below 97% impeller head when working under normal atmosphere pressure. The certain value is be
deemed as the impeller cavitation. From the figure, it can be conclude that the cavitation of the
impeller under the optimum working conditions is about 6.81m.
Figure 14 shows the evolution of the water vapor in the impeller with the inlet pressure changing.
With the inlet pressure decreasing, the water vapor volume increase. The pump performance drop
sharply when the water vapor increases to some extent to block the flow passage of the impeller when
the inlet pressure decrease.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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Using the same procedure, the cavitation of the impeller was obtained when the condensate pump
working under different flux. Figure 15 gives the impeller cavitation variation as a function of flux. In
the figure, “Exp” means experimental results and “Num” means numerical simulation results. From
the figure, it can be found that the cavitation obtained by the numerical prediction agrees well with
that of the experimental results. It can also be found that the impeller cavitation is small at low flux,
this means that the pump anti-cavitation capacity is good when it working in small flux. With the flux
increasing, the impeller cavitation increasing significantly, this implies that the pump anti-cavitation
capacity needs to be improved. Because the condensate pump working in a wide flux range, the barrel
height must be high enough t needs to ensure the water in the barrel must be higher than the impeller
cavitation.
Figure 13. Head variation as a function of cavitation.
Figure 14. Water vapor in the impeller.
Figure 15. Impeller cavitation as a function of flux.
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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4. Conclusions
Based on the CFD technology, the flow within the condensate pump was simulated and analyzed. The
performance of the first stage impeller was predicted and compared with that of the experimental
results. Then the condensate pump was re-simplified to carry out cavitation numerical calculation. The
water vapor evolution and the cavitation performance of the impeller were predicted.
The pump performance and cavitation performance predicted by the numerical simulation agree
well with that of the experiments. It is necessary to carry out some simplification of the condensate
pump to meet the numerical simulation.
References
[1] Fen J H, Yang C X, Han B H and Wei P R 2008 Journal of Lanzhou university of technology
34(5) 55-59
[2] Li S H 2006 Thermal power generation 7 70-71
[3] Chen X and Tan H 2008 Electrical Machinery Technology 6 42-44
[4] Wu C S, He S L, Zhu D X, et al 2006 Journal of Hydrodynamics, Ser B 18 (3) 217-222
[5] Dupont P 2003 International Journal of Rotating Machinery 9 163-170
[6] Rhee S H, Kawamura T and Li H Y 2005 Journal of Fluids Engineering 127 986-994
6th International Conference on Pumps and Fans with Compressors and Wind Turbines IOP Publishing
IOP Conf. Series: Materials Science and Engineering 52 (2013) 062012 doi:10.1088/1757-899X/52/6/062012
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