Shigeru YOSHIKAWA
Fluid Machinery & Systems Company
Since BFPs are used to feed high-temperature/pressure water to boilers, their history is closely related with the improvement toward higher boiler capacities and higher temperatures and pressures.
Fig. 1 Boiler pressure and BFP discharge pressure
| Purpose | Main feed water |
Start-up and
backup
|
|
| Capacity | t/h | 1200 | 730 |
| Discharge pressure | MPa | 38.05 | 37.26 |
| Rotation speed | min−1 | 6000 | 6300 |
| Water temperature | ℃ | 188.4 | 184.1 |
| Drive | Steam turbine |
Motor drive (with
a fluid coupling)
|
|
| Output | kW | 17500 | 12000 |
| Number of units | 2 | 1 | |
Fig. 2 Double casing barrel type BFP for supercritical thermal power plants
Heat recovery steam generators for combined-cycle plants are often structured with three stages (high-middle-, and low-pressure drums), and designed to extract intermediate-pressure feed water from the intermediate stage of the BFP to feed it to the middle-pressure drum. In other words, one unit of BFP can feed middle- and high-pressure water. Total amount of the middle- and high-pressure feed water are sucked from the suction casing. After the amount of the water to be fed to the middle-pressure drum is extracted from the extraction stage, only the amount of the water to be fed to the high-pressure drum is pressurized. For this reason, the specific speeds (Ns) of impeller and diffuser for the stage before the extraction are often different from those after the extraction.
Fig. 3 Structure of a BFP for combined cycle thermal power plants
With the growth of equipment for thermal power generation in capacity and pressure, BFPs have been upsized and become sophisticated. Among the pumps used in a thermal power plant, the BFP uses the most power because it must produce high pressures required by the boiler. This means that the improvement in BFP efficiency is a critical challenge that must be solved to reduce the environmental load. The impeller used in a BFP is a centrifugal pump with a specific speed (Ns) of approximately 120 to 250 (m3/ min, m, min-1). Generally, the pump efficiency is higher when the specific speed is higher within this range or the flow rate is higher if the specific speed is the same. In a normal case, two BFPs each with a 50% capacity are used as the main feed pumps. If one BFP can provide a 100% capacity, it improves the efficiency through the increase in capacity and enhancement in specific speed as well as helps save space and resources4).In Japan, EBARA has experience in designing, fabricating, and delivering main feed pumps with a specification of 100% capacity per unit for 500 MW and 600 MW supercritical thermal power plants and these pumps are successfully operating. In some countries and areas, a system based on a single main feed pump with a 100% capacity is in actual use in 1000 MW plants. Recently, EBARA also manufactured and delivered a large BFP that satisfied this requirement. Figure 4 shows actually shipped BFP and followings are the outlined specifications of this BFP.
Capacity of 3200 t/h×total pump head of 3800 m× shaft power of 37700 kW×rotation speed of 5000 min-1
Specific speed of approximately 250 (m3/min, m, min-1)
Fig. 4 100% capacity BFP for 1 000 MW supercritical thermal power plants
|
Rated
plant
output
|
Capacity |
Total
pressure
|
Rotation
speed
|
Shaft
power
|
Efficiency |
Number
of unit
|
Power
ratio
|
| MW | t/h | MPa | min−1 | kW | % | Unit | % |
| 500 | 890 | 29.67 | 5500 | 9999 | 82 | 2 | 4.00 |
| 500 | 1630 | 30.1 | 5500 | 17747 | 86 | 1 | 3.55 |
| 600 | 1000 | 30.1 | 5500 | 11157 | 83.5 | 2 | 3.72 |
| 600 | 1860 | 33.2 | 5000 | 22589 | 85.3 | 1 | 3.76 |
| 700 | 1120 | 30.6 | 5500 | 12711.7 | 85 | 2 | 3.63 |
| 1000 | 1650 | 30.5 | 5500 | 18393.3 | 86 | 2 | 3.68 |
| 1050 | 1700 | 31.2 | 6000 | 19279.5 | 85.5 | 2 | 3.67 |
Recent years have seen increased introductions of renewable energies such as solar and wind power. Renewable energy is expected to continue to spread as one of the measures against global warming because it does not use fossil fuel and therefore emits no carbon dioxide when used for power generation. However, the power output based on solar and wind power significantly depends on the meteorological conditions such as the weather and wind conditions, resulting in the drawback that it is difficult to stably operate electric power systems based on renewable energy. In order to cope with this issue, thermal power plants are increasingly required to provide more flexible power system operation with a higher level of supply-and-demand adjustment capability. Specifically, they are required to improve the load change rate, minimize the minimum load factor, and shorten the start-up time.
Fig. 5 BFP structure that has incorporated measures for increasing the robustness
| No | Degradation | Causes | Measures for increasing the robustness |
| ① |
Wall thickness reduction of
discharge nozzle
|
Erosion caused by high velocity and/or uneven flow
|
Overlaying austenitic stainless steel onto the inner surface
|
| ② |
Erosion of high-differential-pressure part of the inner volute
|
Degraded seal performance of metal touch seal or self compressed gasket associated with increased frequencies of start and stop actions
|
Using an auxiliary O-ring in combination |
| ③ | Incipient cavitation |
Increased duration of low flow operation associated with the adoption of DSS, etc.
|
Replacing the first stage impeller with new one designed for low flow regarding with inlet
configuration
|
| ④ |
Cracks at the portion to which
auxiliary piping is attached
|
Impact of pulsation caused by increased duration of low flow operation
|
Modifying the nozzle stub so that it will be a more rigid structure
|
| ⑤ | Vibration caused by misalignment |
Changes in nozzle load associated with increased
frequencies of start and stop actions
|
Installing an stabilizing device |
| ⑥ | Increased bearing vibration |
Impact of pulsation caused by increased duration of low flow operation
|
Using a full circular bearing housing |
| ⑦ | Damaged bearing metal |
Wire-wool damage of 13Cr steel shaft caused by foreign particles in lube oil
|
Overlaying carbon steel to the main shaft journal
|
| ⑧ |
Occurring of abruptly changing
vibration
|
Torque locking caused by poor sliding of the gear-coupling tooth surface
|
Upgrading to a flexible disk coupling |
| ⑨ |
Fretting corrosion on the fitting
portion of the thrust disk
|
Decreased torque of the shaft-end nut. Loosened fixed disk caused by secular distortion of the disk contact surface
|
Using a shaft-end nut of a locking sleeve type Using a long hub type thrust disk
|
| ⑩ |
Degraded mechanical seal for inter stage bleed off
|
No maintenance for a long time because it is installed on the suction cover and not required
when the inner-volute rotor disassembly
|
Discontinuing the use of the bleed off pipe and the mechanical seal as the structure for extraction from the discharge cover
|
The above number is the part that shows the number of the in Fig. 5.
Fig. 6 BFP with an inducer
Fig. 2 Double casing barrel type BFP for supercritical thermal power plants
Fig. 7 Disassembled inner volute and the rotor to be taken out
図8 フルカートリッジ構造，輪切り型BFP Fig. 8 Full-cartridge, sectional-type BFP
Fig. 9 Boiler feed pump outline drawing (with oil supply unit)
Fig. 10 Self-lubricating bearing
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