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Application of direct - supply steam in thermal power plant in vacuum treatment of molten steel


Wei Zijie Bai Jie Zhang Hongxing Yuan Yanjie
(CITIC Heavy Industry Technology Research Center, Luoyang 471039)

Summary:This paper describes the use of temperature reduction decompression device, the introduction of power plant direct steam applied to the vacuum treatment system, to solve the enterprise development and energy conservation targets, environmental protection between the contradictions.
Key words: Vacuum reduction and vacuum steam treatment

  1. statement of problem

In order to meet the growing demand for production capacity, in 2007, CITIC Heavy Industries to build a new 80t electric arc furnace, 150t LF / VD (VCD) two sets of online refining furnace, 200t ~ 600t ingot vacuum pouring tank six, with the new 400kg / h, 500kg / h steam jet vacuum pump two. Plus the original steelmaking equipment supporting 250kg / h, 300kg / h two steam jet vacuum pump, a total of superheated steam maximum flow 40t / h.
Company boiler room two original 10t coal-fired boiler, the steam produced enough to supply the original 250kg / h, 300kg / h steam jet pump use. With 400kg / h, 500kg / h steam jet pump construction, steam demand increases, by the energy-saving emission reduction targets and environmental protection policy factors, the new steam equipment is constrained.
Second, the vacuum treatment of molten steel on the steam quality and flow requirements
1, steam use point (four vacuum jet pump dedicated steam drum) steam quality requirements:
(1) steam pressure: 0.87MPa ± 0.02Mpa;
(2) steam temperature: the pressure under the superheat (20 ~ 30 ℃) ± 2 ℃;
(3) steam use time for the random use, four pumps for the random combination of start, up to the number of pumps used at the same time three;
(4) the use of two adjacent steam is not continuous, the use of interval time of 0.5 ~ 72h;
(5) The steam flow rate is jumped up according to the starting sequence of the vacuum pump. At the end of the processing, the maximum flow rate is about 12t / h or so to 3 ~ 4t / h or "0"
2, steam flow design:
(1) the use of steam flow of the vacuum pump see: (Table 1)


Vacuum pump

Minimum flow rate (t / h)

Maximum flow rate (t / h)

250kg/h

 9

12

300kg/h

3

10.5

400kg/h

2.5

9

500kg/h

2.65

11

Table 1: The vacuum pump using steam flow
(2) flow design:
Maximum flow: according to any three pumps at the same time work, according to 35 ~ 40t / h design;
Minimum flow: 400kg / h vacuum pump alone work the minimum flow calculation, according to 2.5t / h design;
Fourth, the steam supply solution
To ensure the progress of the project, the multi discussion, demonstration, a total of the following five sets of solutions. See: (Table 2))


Steam Solutions

advantage

Disadvantages

1

20t coal-fired boiler

Investment, operating costs lower

Subject to energy-saving emission reduction targets and environmental protection restrictions

2

80t electric arc furnace waste heat boiler

Waste heat utilization

Steam production is limited, waste heat boiler investment

3

20t gas boiler

Low investment, strong adaptability, good mobility

Steam production costs are higher

4

Power plant direct steam

The steam supply is reliable and the steam cost is 15% lower than that of the coal-fired boiler.

Pipeline length, need to solve the pipeline temperature drop, hydrophobic, water hammer and other issues

5

Co - combustion steam generation system

Knot simple, investment savings, thermal efficiency up to 98%

The steam cost is high, producing steam containing 6.9% water-insoluble gas CO2.

   Table 2: Steam supply solutions
As can be seen from the above analysis, Option 4: The use of power plant steam direct supply is the best solution.
Local thermal power plant has two 165MW pumping cogeneration unit, the design pumping capacity of 1. 47Mpa, 274 ℃, steam
Steam flow 335 × 2 = 670t / h, stand-alone maximum pumping capacity of up to 400t / h. This steam is used for urban heating, in the urban heating off-season, in order to ensure two 165MW normal power generation, using a 75NW turbine generator as a regulator of steam load. Power plant steam can fully meet the needs of our company 40t / h, the steam pipeline to the temperature and decompression station, and then by the temperature and pressure decompression device to adjust the temperature and pressure, the supply of steam jet pump, the use of power plant direct steam costs comparable Coal boilers are 15% lower.
Fifth, the power plant direct steam process:
Steam from the power plant exports to the company entrance, steam pipe length of 1200 meters (buried), diameter D630 × 9. From the company's entrance to the vacuum treatment of molten steel vacuum relief device, the steam pipe length of 1100 meters (overhead), diameter D426 × 9. Steam reduced by decompression, from the two branches to 250kg / h, 300kg / h vacuum pump and 400kg / h, 500kg / h vacuum pump four steam points, each pipe length of about 150 ~ 200m. According to this, the steam into the pipeline to reach the point of use, according to the different flow rate, the use of steam time interval and the use of steam points using the parameters, need to take the temperature, decompression or decompression to ensure that the steam point of steam requirements.
Six, the design of decompression station:
Because the temperature and pressure reducing device on the steam quality requirements, and the control system is difficult, my company through the domestic and foreign programs of the full argument, the final selection of cost-effective integrated advantage of Hangzhou Meiya Power Equipment Co., Ltd. temperature and pressure decompression equipment.
In order to ensure the stability and reliability of the steam source, the design of the temperature reduction and decompression device adopts a parallel design. When the cooling and decompression device fails, the equipment will be used in the process of vacuum treatment. Can alarm and automatically switch quickly to start another standby cooling and decompression device to ensure the safety of the system, stable and reliable. The conditions of the temperature and pressure relief device and control system put forward a very high demand. Mainly have the following difficulties: 1, the overall adjustment range of the device. General temperature and pressure reducing device flow adjustment range of 1:10, and the device requires a range of 2.5 ~ 40t / h, plus a steam from the heating network in the steam peak and low trough pressure, the temperature parameters will fluctuate , The actual adjustment range of 1:20 or so; 2, the load of the ladder jump type change. Due to the specificity of the load (vacuum pump), the actual steam flow will produce a maximum of 10t / h mutation (three pumps with steam volume at the same time change), the change time is only 2 ~ 3S, valves and control systems must adapt to this condition, Fast tracking stability device outlet pressure, temperature; 3, the leakage level requirements. Vacuum pump is intermittent operation, in all the pump to stop the steam at the moment, the steam flow has been close to the shut-off valve is completely closed about 1 minute time, this time must be buffered by the control valve, which 4, the device in the actual use of the process must be continuous operation, because once the failure caused by steam out of control will lead to the vacuum pump can not run properly so that smelting failure will result in hundreds of thousands or even million yuan of raw materials and equipment loss. So the working state of the device suddenly fails, the backup device must immediately switch enabled, and the switching process must ensure that the parameters are smooth, does not affect the normal operation with the steam side; 5, fully automated operation, the system to achieve unattended. The control system is controlled by the straight-stroke electric actuator and PLC, and can realize the operation of the remote (vacuum control room) and the local (decompression and decompression station). The temperature and temperature can be continuously adjusted continuously. Equipment main parameters:
Import steam maximum flow: Q = 40t / h
Import steam pressure: ~ 1.4MPa
Imported steam temperature: ~ 270 ℃
Outlet steam flow: Q = 2.5 ~ 40t / h
Outlet steam pressure: 0.87 MPa
Outlet steam temperature: the pressure under the saturation temperature + superheat (20 ~ 30) ℃
Outlet pressure error: ± 0.02MPa
Outlet steam temperature error: ± 2 ℃
To achieve the above requirements, we and Hangzhou Meiya Power Equipment Co., Ltd. R & D personnel after several exchanges, demonstration, analysis of domestic and foreign advanced cases, and made a bold innovation design. And finally determine the device using the following configuration: 1, the use of pressure relief valve Y948Y-type straight multi-stage sleeve control valve. The valves are designed to be equal in percentage characteristics, with a wide range of good regulation properties, very small leaks and low noise. 2, the temperature reduction system using Venturi type, no moving parts, the effect of good temperature reduction, easy maintenance. Water supply valve with TZAZP single-seat control valve. A wide variety of Kv value options, wide adjustment range, integrated spool stem design and efficient operation and stability, with comparable to the valve low leakage. 3, the temperature reduction and decompression control valves are equipped with intelligent frequency conversion electric actuator, the use of advanced vector frequency technology, in addition to perfect electrical protection, but also has a precise torque protection and stepless speed control, the valve at the same time to 3.5mm / s high speed and 0.5% of the high-precision operation to ensure rapid response to sudden changes in load and precise positioning; 4, water pump water supply automatic pressure regulator system so that the warm water always maintain the appropriate pressure, while eliminating the pump pressure 6, set the electric fast hydrophobic bypass, to ensure that the device is not used again after a long time automatically put into operation quickly; 7, the control system using Siemens S7 (Figure), the use of electric doors for the normal stop and emergency shutdown; -200 high-performance PLC, internal design fault diagnosis alarm, fuzzy control, automatic switching, signal conditioning and other software modules. In addition to the local touch screen monitoring, and then high-speed PROFIBUS bus (1.2Mbps baud rate) connected to the vacuum control room TP270 touch screen to achieve remote operation. 8, the control cabinet main power through the dual power switch to achieve dual power supply, and the control circuit using UPS power supply, with the ability to fight instantaneous power failure, power grid surge to meet the continuous operation requirements. In the actual operation, the device can quickly adapt to the load mutation. In the detection of decompression and cooling failure can quickly respond, timely switching, switching can still ensure the normal use of steam. The above design can give full play to its effectiveness, so that the system to achieve the desired control function, fully meet the stringent requirements of continuous and stable operation.
Seven, the application effect:
Project in 2008 to complete the production has been 5 years, steam pipe network and temperature and pressure relief device running reliable, stable steam supply to meet the expansion of molten steel vacuum degassing needs.
Since the four vacuum jet pump direct use of power plant steam, the deactivation of the original two 10t coal-fired boilers, the company reduced energy-saving emission reduction targets. At the same time, the power plant direct supply of steam than coal-fired boiler steam per ton of 15 yuan per ton, according to my company's existing vacuum degassing steel production calculation, the annual savings of millions of dollars a year to recover the investment in temperature and pressure relief device.