Automatic control of high pressure water descaling system

0 Preface
In the production process of hot-rolled steel, the furnace pig iron oxide scale on the surface of the billet is one of the main reasons for affecting the surface quality of the steel. Due to its existence, various product defects such as pits, pitting, and iron oxide intrusion are formed on the surface of the steel. . In order to solve this problem, a variety of descaling methods have been adopted at home and abroad. In contrast, the high-pressure water descaling technology has the advantages of a wide range of steel grades, high removal rate and low comprehensive cost. It has been applied in hot descaling and cold descaling, and has become the mainstream of today's descaling methods. In September 2008, the hot-rolled strip steel plant of Jingtang Iron and Steel Co., Ltd. (referred to as Jingtang Iron and Steel) invested a set of high-pressure water descaling device, which was put into use in December of that year. The scale works reliably and the descaling effect is good.

1 The mechanism of high-pressure water descaling
After the billet is released from the heating furnace, the iron oxide scale covered on its surface is rapidly cooled, and the iron oxide scale generated in the furnace presents network cracks. Under the injection of high-pressure water, the surface of the iron oxide scale is locally quenched, resulting in a large shrinkage, so that the cracks of the iron oxide scale expand and partially warp. After the impact of the high-pressure water flow, the dynamic pressure of the high-pressure water in the crack becomes the static pressure of the fluid and is driven into the bottom of the iron oxide scale, so that the iron oxide scale is peeled off from the surface of the billet, and the purpose of removing the iron oxide scale is achieved.

According to the above mechanism, when designing descaling equipment, special consideration should be given to the influence of factors such as rolling speed, rolling temperature, water flow rate of the nozzle, and water flow pressure at the nozzle. Furthermore, the chemical composition of the scale and the composition of the position layer are closely related to the raw material composition of the steel material, the heating temperature, the heating time, the atmosphere conditions in the furnace, and the rolling process. For carbon steel, the surface layer of iron oxide scale is Fe2O3, the middle layer is Fe3O4, and the inner layer is FeO. At present, there are still some problems in the design and application of the high-pressure water descaling system. In order to summarize and improve the descaling effect of high-pressure water, some main problems of the system are discussed in this paper for the reference of relevant design and production management personnel.

2 Selection and installation of nozzles
2.1 Selection of Nozzle
The descaling effect of high-pressure water depends to a large extent on the structure of the nozzle and the shape of the nozzle. There are three basic requirements for descaling nozzles:
(1) The water jet should be wide and flat, forming a water jet like a sharp knife.
(2) The distribution of the blowing force of the water flow along the width of the water flow should be as uniform as possible. For the iron oxide scale produced by ordinary carbon steel in the furnace, the uniform striking force is expected to be 2 × 10 ~ 2.5 × 10 Pa. High pressure water pressure is generally 16 ~ 28MPa.
(3) The material of the nozzle is required to be wear-resistant
According to the calculation, when the water pressure reaches 16MPa, the water velocity at the nozzle mouth can reach about 144m/s. Therefore, if there is no wear-resistant material, the nozzle must be replaced frequently, which increases the downtime and affects the production efficiency.
According to the test, the nozzle with rectangular section is not suitable for high-pressure water descaling, because the impact force of the nozzle is large on the edge, and the impact force in the middle is small. The test data and production practice show that the oval nozzle section is good.
Various types of rolled steel have their suitable high pressure water consumption (jet strength). For example, on a hot-rolled strip production line, the general high-pressure water consumption of carbon steel and low-alloy steel is: 605Lt rolling stock, 350Lt on the blooming stand, and 225Lt on the finishing stand. Different spray strengths are required for different steel grades and different materials.
The water consumption Q (L/S) of a single nozzle can be calculated as follows:

In the formula: μ: flow coefficient, the outer cylindrical nozzle μ= 0.82 (according to different nozzle shapes, its value is between 0.8 and 1)
g : Gravitational acceleration (9.8m/s)
p : working pressure (m water column)

3 Automatic control of descaling system
3.1 Design two sets of descaling systems according to the rolling descaling process
Compared with a system, the overlapping descaling flow is reduced, the pressure fluctuation at the descaling point and the overlapping descaling times are reduced, and the descaling quality and uniform cooling are ensured;
3.2 The speed regulation of the coupling is used to realize the "supply on demand" of the centrifugal pump
Reduce operating costs
Avoid closed-point pressure and overheating of the centrifugal pump, and improve the reliability of system operation;
Reduce the continuous peak pressure of the accumulator and reduce the high-pressure risk investment of the system;
Reduce the high-speed and high-pressure operation time of the centrifugal pump, and improve the service life of the pressure-resistant components of the high-pressure system;
Avoid the top of the accumulator, improve the reliability of the system operation; improve the production efficiency
3.3 The rough rolling pump station adopts two arrangements of accumulator and pump station
The water supply pressure source is close to the descaling point, and the pressure drop is less, which is beneficial to the quality of descaling;
The accumulator is close to the descaling point to reduce centrifugal pump load and pipeline impact;
Two pressure sources supply water for the three descaling points, and the system pressure is balanced.
3.4 FSB system uses centrifugal pump to directly supply water
The descaling point pressure is constant without fluctuation; the descaling quality is good;
The system pipe network has no shock and vibration, and the operation reliability is high;
It adopts two descaling processes of intermediate billet full descaling and intermediate billet let first to meet the descaling needs of different steel grades.
3.5 The control system of the roughing and finishing rolling pump station and the non-production shutdown operations such as descaling and roll changing of the rolling line can realize the centrifugal pump water supply according to the needs of rolling and descaling, and the pressure at the descaling point can be adjusted to 22MPa to realize the non-production shutdown period , The pressure from the pump station to the descaling point is about 1.5MPa, which is beneficial to the safety of operators in the rolling mill area.
 

4 Electronic control instrument
4.1 Basic automation part
The two pump stations are equipped with two sets of PLC control systems, using Siemens PLC S7-317, equipped with multiple ET-200M workstations, for high-pressure water descaling pump, high-pressure motor, hydraulic coupling, hydraulic station, combined oil station, descaling Machine, descaling valve, accumulator and other equipment realize remote control and signal acquisition. According to the process requirements of the descaling system and the particularity of the descaling equipment, a safe and reliable operation program is compiled to ensure the safe, reliable, automatic and energy-saving operation of the descaling system. .
4.2 Human-machine interface part of industrial computer
Each pumping station is equipped with two sets of industrial computers and a touch screen, which are respectively located in the control room, on-site operation console, and underground descaling pumping station.
 

4.3 The industrial computer adopts Siemens 6AG4011 industrial computer and WCC monitoring software to monitor the operation of each part of the high-pressure water descaling system through data collection. System fault alarm screen, fault and accident records, trend screen.
 

4.4 Instrument part:
The primary instrument uses the German HYDAC pressure transmitter and temperature sensor to provide signal feedback to the pressure measuring points and temperature measuring points of the descaling pump, high-pressure motor, hydraulic coupling, hydraulic station, combined oil station and other equipment, and descaling the high-pressure water. The main parameters of the system are monitored in real time on the man-machine interface.
 

4.5 Communication between descaling pump station and rolling line computer
(1) The computer communication content of the descaling pump station supplying the rolling line
Information on the start-up conditions of the descaling pumping station
Information about the normal operation of the descaling pumping station
Descale pump station provides pressure information
The descaling pump station does not need high pressure water for a short time such as roller change
Descaling Pump Station Alarms and Incident Downtime Information
Normal shutdown information of descaling pumping station
(2) Information provided by the rolling line computer to the descaling pump station
Injection valve opening and closing status information
Descale system pressure setpoint information
No need for high pressure water for a short time such as roller change
HSB descaling information
(3) Communication network: Ethernet or PROFIBUS-DP
(4) Chain control content of descaling pump station and rolling line equipment
l Centrifugal pump startup conditions All injection valves are closed;
l When the water level of the accumulator reaches the lower accident water level, all injection valves are automatically closed;

5 Rough rolling and descaling pump station control
5.1 Working principle
Install a fluid coupling between the centrifugal pump and the motor. The motor runs at a constant speed, and the rotational speed of the centrifugal pump-discharge pressure can be adjusted by changing the position of the spoon tube of the coupling-transmitting torque. When the accumulator pressure or water level reaches the set upper limit, the hydraulic actuator controls the spoon tube to go down and discharge the working oil in the rotor of the coupling. When the energy pressure or water level reaches the set lower limit, the hydraulic actuator controls the scoop tube to go up, the working oil in the coupling rotor is full, the coupling pump wheel and turbine rotate synchronously under the action of hydraulic power, and the centrifugal pump runs at high speed and high pressure load, so as to realize The centrifugal pump supplies water according to the rolling-descaling water consumption, just like the working system of the plunger pump-accumulator descaling system. It is directly related to whether it can meet the requirements of the rolling-descaling process and the number of times the centrifugal pump is up and down.
5.2 Centrifugal Pump Control
(1) Automatic start conditions:
The suction valve of the descaling pump is open and the outlet valve is closed;
The circulation valve is closed;
The injection valve is closed;
The fluid coupling is in a low speed state;
Water inlet pressure≥0.25MPa
Hydraulic station system pressure ≥ 6MPa
Lubricating oil pressure ≥0.12MPa
Coupling working oil pressure≥0.12MPa
(2) Alarm conditions:
When the water inlet pressure is less than or equal to 0.15MPa, the outlet valve, low liquid level valve and injection valve are automatically closed;
Lubricating oil pressure≤0.07MPa;
The working oil pressure of the coupling is less than or equal to 0.08MPa;
The temperature of each bearing of pump, coupling and motor is ≥70°C
Motor stator temperature ≥95°C
Pump body temperature≥60°C
The oil level of the oil tank in the lubricating oil station and the working lubricating oil station reaches the lower alarm oil level
(3) Trip condition: delay () seconds
Water inlet pressure≤0.08MPa
Lubricating oil pressure≤0.05MPa
Coupling working oil pressure≤0.06MPa
The temperature of each bearing of pump, coupling and motor is ≥80°C
Motor stator temperature≥105°C
Pump body temperature≥70°C
The oil level of the oil tank in the lubricating oil station and the working lubricating oil station reaches the lower accident oil level
5.3 System safety operation control
(1) The low-pressure water supply accident cuts off the water to prevent the control of "burning the pump" and "running gas"
When the low-pressure water supply pressure is lower than 0.12MPa, the centrifugal pump will automatically run at a reduced speed, the pump outlet valve will be automatically closed, and the injection of the low-level valve will be automatically closed; if the pressure is lower than 0.08MPa, the centrifugal pump will trip and stop running;
(2) Prevent high-pressure water from entering the low-pressure pipe network control
Measure:
Anti-centrifugal pump reversal: the pump outlet check valve is closed by hydraulic power, and the pressure area is large and the closing force is large, which is easy to close tightly; after the standby pump stops, the pump outlet hydraulic valve is automatically closed; in addition, there is a pressure sensor to detect the low-pressure pipe When the pressure exceeds 0.8MPa, the centrifugal pump will automatically run at a reduced speed, the pump outlet valve will automatically close, and the low liquid level valve injection will automatically close; three insurances.
(3) Prevent accumulator from "running out of gas" and "gas tank inflow" control
Measures to prevent accumulator from "gassing":
The lower-controlled low-level valve is used to ensure quick closing and good air-tightness; if the water level of the accumulator falls to the second accident water level if it is not closed tightly, the injection valve of the rolling line is automatically closed, and double insurance is provided.
Measures to prevent the "gas tank from entering water" in energy storage:
When the gas capacity of the accumulator is small, the water level will rise, causing the roof to rise, damage the float of the liquid level gauge, and the high-pressure water will enter the gas tank, affecting the effective volume of the accumulator. Using the upper accident water level control, when the water level of the accumulator reaches the upper accident water level, the pressure has not yet reached the requirement, indicating that the gas in the accumulator is less, and at the same time, the centrifugal pump automatically slows down and stops the water supply to ensure the safety of the system.
5.4 Main and auxiliary pump control
The two working centrifugal pumps are set with the main and auxiliary pump working system. The purpose is to reduce the number of times of speed up and down of the centrifugal pump, reduce the operating cost and improve the service life of the centrifugal pump under the flow conditions of the rolling and descaling process.
The main pump increases or decreases speed according to the accumulator liquid level or pressure signal.
The auxiliary pump increases speed according to the descaling signal of the rolling line, and decelerates according to the accumulator liquid level or pressure signal.

6 Finishing rolling and descaling pump station control
6.1 Do not let the head descaling process
The jet valve is always open, and the scale is removed by the centrifugal pump.
When the HMD detects the head of the intermediate billet, the hydraulic actuator of the hydraulic coupling controls the upward movement of the spoon tube, the working oil in the rotor of the coupling is full, the pump wheel and the turbine of the coupling rotate synchronously under the action of hydraulic power, and the centrifugal pump runs at high speed and high pressure. , supply high-pressure water to the descaling point to descale the surface of the intermediate billet; when the HMD detects the tail of the intermediate billet, the hydraulic actuator of the hydraulic coupler controls the downward movement of the spoon tube to discharge the working oil in the rotor of the coupler, the pump wheel of the coupler and the Turbine produces rotational speed difference, centrifugal pump runs at low speed and low pressure with no-load, supplies low-pressure water (1.3MPa) to descaling point, and fills high-pressure pipeline with low-pressure water to prevent pipeline vibration during descaling;
6.2 Let the head descaling process
Jet valve and centrifugal pump speed up and down interlock control descaling
When the HMD detects the head of the intermediate blank, the centrifugal pump will automatically run at a higher speed, the injection valve is closed, and the pump discharges high-pressure water through the pre-filling valve to fill the high-pressure pipeline to prevent pipeline impact, and the injection valve automatically according to the set time Open, let out the head of the intermediate blank, and descale the intermediate blank;
When the HMD detects the tail of the intermediate blank, the centrifugal pump will automatically run at a reduced speed. After 2 seconds, the injection valve will be automatically closed, and the descaling will be completed. Because the centrifugal pump will be decelerated for 2 seconds, the system pressure will be less than 3MPa, and the pre-fill valve Always on, thus avoiding hydraulic shocks due to jet valve closing.

Operation
Since the new high-pressure water descaling system was put into use in December 2008, the equipment has been running normally, has completed the design function, and met the production requirements. The control system can detect the temperature change of the high-voltage motor bearing bush in time according to the on-site situation, and alarm and trip, so as to avoid major equipment accidents, and the application effect is good.

7 Conclusion
(1) Due to the use of the energy storage device, the function of the system to absorb energy is greatly enhanced, and the phenomenon of water hammer impact is eliminated. The supercharging device achieves the effect of supercharging and energy storage. Without adding other equipment, the use of high-pressure water to directly increase the low-pressure gas to high pressure has proven to achieve the desired effect. This also provides another feasible idea for other fluid pressurization.
(2) The suitable nozzle is selected and arranged reasonably, and the system pressure fluctuation is small, which greatly improves the removal rate of the iron oxide scale of the billet.
(3) The temperature drop of the billet is small, which meets the stable needs of billet rolling.
(4) It has been verified that this set of high-pressure water descaling equipment operates stably; improves the surface quality of the product; reduces consumption and saves energy. Facts have proved the feasibility of the new equipment and achieved the expected results.