I. Survey：

The 100MW unit of Beijing Jingfeng Thermal Power Co., Ltd boiler feed water system mainly consists of three 6kV/1600kW water feed pumps and high-low adjusting valve, high-pressure heaters and other equipments. The system configuration is shown as below. Water pump transfers water of deaerator finally into boil-bag by high-pressure heater. Usually, the system adjusts the opening of gate according to load, achieving the purpose of water level stability. The system has the following questions:

1.      The pump runs in constant speed with great throttle loss, a high pressure on outlet,  serious damage on pipe, low efficiency and a waste of energy.

2.      When the flow is decreasing and the opening is down, increasing pressure differential makes low safety and a big loss in pressure, causing more energy consumption.

3.      Long term opening in 40~70% accelerates wear and tear of the vavle, leading to bad valve control features.

4.      The high pressure of pipe net threatens sealing of the system seriously, leading to valve leakage.

5.      Short service time, large routine maintenance and high cost in maintenance cause a tremendous waste of resources.

Using the variable frequency control technology is one of the important methods to solve the problem. Controlling feed pump by VFD can achieve the flow control in variable load. It solves the problem and improves the reliability of the system; furthermore, it decreases the throttle loss, reduces the wear and tear of the valve and damage on seal of the pipe, makes the equipment work longer, reduces the maintenance，improves the economic benefit of the system，saves energy and provides a good way to reduce power consumption of plant.

II. The selection for system solutions

As the system is in mode of two work one spare, so we consider to use one in direct on line, one in variable frequency or both in variable frequency. The original H-Q curve shown in Figure 1. The two pumps curve combine into n. On load of 100 MW, the pipe network characteristic is rA’. The work point is A when the valve is fully open. Using valve control, the available point is A when the opening is decreasing. It means the boiler system only needs 11.0Mpa in 100MW load, flow 385m³. The active power of two pumps is PA’. It is corresponding area with“A’-385-0-11.0”, and the “A- A’-11.0-14.0” is consumed by valve throttle. The operation conditions of 90MW and 70MW are the same as 100MW. From figure 1, less flow, more throttle loss and lower efficiency.

If the system adopts mode of one in direct on line, one in variable frequency, with valve full open, H-Q curve of variable frequency pump is shown n0 . In 100MW load, characteristic is rA’，the direct on line pump curve is n0, actual pressure is 11.0Mpa, the effective displacement is 270m³/h corresponding to O. After another pump runs in variable frequency, the speed is down. Its curve is nA ’. Work point is A’. Active power of pump is PA’. There is no throttle loss any more. But when the load is 70MW, flow 260 m³/h is less than rated flow 270m³/h. The variable frequency pump can not meet control demand. At this moment, stop line frequency pump and make variable frequency pump meet the system requirements. But one pump operation greatly reduces the safety of the system，it is infeasible.

In mode of direct on line &variable frequency operation, throttle adjustment must be used when load is down. Then, the variable frequency pump should make outlet pressure equal to pipe network. The output flow is 0, the operation curve is nC'. The whole efficiency is reduced; and this mode causes system complex, makes against for peak adjustment. Long term running at full load will influence the lifetime of equipment. Therefore, this mode is not suitable for boiler water pump system.

For the above reasons, the system should adopt the two pumps in variable frequency.

III. System Solutions

By researching and analysing the problem in the original system, we design the reform in order to “ensure safe system, reliable structure, best cost-effective program”,

    The system adopts mode of one drive one. The main electrical principle of system is shown in figure.

IV. Economic Benefit Analysis

1. Field data

1）  Generator Capacity：100MW

2）  Number of feed-water pump：3（Two work and one spare）

3）  Data of pump：

4）  Data of motor：

5）  The parameter statistics in different loads

6）  Cost price：0.232yuan/kW?h

7）  Annual working hours：8061h

2. The annual power consumption in direct on line state：

Pd：Motor power；d：Motor efficiency；U：Input Voltage；cosφ：Power factor。

Formula：P_d =1.732×U×I×cosφ…  ①

Form ①，cosφ=0.87。

The percentage of average running time＝The average running time/24

①The actual power consumption is shown in the below table.

Cd：Annual power consumption；T：Running time of year；δ：The percentage of single-load run time.                  

The formula of annual power consumption：Cd= T×∑（Pd×δ）　…②

Cd =8061×（2712.31×25%＋2531.49×41.7%＋2441.08×33.3%）=20527817 kW?h

Therefore, the total power of two pumps in direct on line is 20527817 kW?h.

3. The annual power consumption in variable frequency state：

1. The pressure of feed pump：

Resistance of pipe net＝Outlet pressure of pump-Inlet pressure of gate

The pressure in the valve full opening=The outlet pressure of gate＋Resistance of pipe net

Work pressure of feed pump= The pressure in the valve full opening-Inlet pressure of pump

According to above, we could know the work pressure of feed pump.

2. The pump characteristics in different loads：

3. The power consumption of feed pump in variable frequency：

Pd’： Shaft Power of motor； P：Shaft Power of pump；Motor efficiency；f：Actual efficiency of VFD；Pump efficiency；Q：Output flow；H：Pressure；λ：Characteristics

Pd’= P   …③

The relationship between motor efficiency and β is shown as figure .

Shaft Power of pump：
Motor power：
we know：
Line side power consumption: