Created on:2021-07-16 09:03

Principle of hydraulic system_ Fault analysis and improvement

Principle of hydraulic system_ Fault analysis and improvement

(1) Hydraulic system for the package

The sld-140 horizontal continuous casting unit is fully hydraulic driven, and the inter ladle to ladle hydraulic transmission is adopted. The schematic diagram of the hydraulic control system is shown in Figure Q. the system uses the superposition integrated control mode and has two-stage pressure control. Before the tundish contacts the mold, the rated working pressure of the system p = 7.0Mpa is used to operate the tundish; When the tundish is in contact with the mold, the pressure is changed immediately, and the system pressure is reduced to P2 = 1.5 ~ 2.0MPa. When the electromagnet 1dt of solenoid valve 2 is powered on, the pressure oil flows through solenoid valve 5 → superimposed one-way valve 3 → solenoid valve 2 → superimposed one-way speed regulating valve 1 → shunt and flow collecting valve 8. After the bypass adjustment of precision speed regulating valve 6 and 7, it respectively enters into the rodless cavity of the two cylinders to push the hydraulic cylinder forward accurately and synchronously, thus driving the tundish forward quickly and starting the tundish alignment; When the tundish is close to the mold, the electromagnet 3DT of the solenoid valve 5 is energized, and the pressure oil flows through the solenoid valve 5 → the superposition pressure reducing valve 4, and the working pressure is reduced to the set pressure. Due to the cut-off effect of the one-way valve 3, the pressure oil continues to move forward and enters the rod less cavity of the hydraulic cylinder. Due to the hydraulic resistance effect of the pressure reducing valve, the hydraulic cylinder moves forward slowly, so as to drive the tundish up slowly and close to the mold, Complete the process of tundish to tundish; When electromagnet 2DT of solenoid valve 2 is powered on and electromagnet 3DT of solenoid valve 5 is powered off, the pressure oil enters the rod cavity of two cylinders through solenoid valve 5 → superposition check valve → solenoid valve 2. Push the hydraulic cylinder back, so as to drive the tundish back and complete the tundish back process.

Q.jpg

(2) System fault analysis

After the system is put into use, it is found that the system works abnormally. When the set pressure P2 = 1.5MPa, the speed of tundish to tundish forward (3DT power-off state) is very slow, which can not meet the design requirements; When 3DT is in decompression state, tundish forward speed is normal. After tracking analysis, it is found that the root cause of the system fault is the superposition pressure reducing valve.

Figure R (a) shows the structural diagram of the superimposed pressure reducing valve. Its working principle is: the pressure oil enters through hole a, flows out through orifice a, and the hydraulic pressure P in chamber a acts on the feedback piston 1 through channel 2 and hole C, making the piston 1 move to the left. Due to the interaction of force and reaction force, the main spool 2 moves to the right and compresses the regulating spring 3. Under the action of spring force, the main spool 2 is in a balanced position, so that the orifice (1) has a certain opening, the pressure oil is throttled by the orifice (1), and the working pressure is reduced from P1 to P2.

R.jpg

The stress analysis of the main valve core of the pressure reducing valve is carried out, as shown in Fig. R (b). Ignoring the influence of friction and hydrodynamic force, when the main valve core balance is stable:

FA =fk (4-5)

Where FA -- load pressure feedback force, FA = P2a;

FK -- set spring force, FK=( Δ X+X)K。

The mathematical model of pressure reducing valve is obtained

(4-6)

 

Where P2 -- pressure of load chamber,

K - spring stiffness;

X -- spring preload;

Δ X -- displacement of main valve core;

A -- sectional area of feedback piston.

The mathematical transformation of equation (4-6) is carried out

Δ X=(p2A)/K-X (4-7)

It can be seen from equation (4-7) that:

① With the increase of P2, Δ When x increases, the main valve core moves to the right;

② If K becomes smaller, Δ When x increases, the main valve core moves to the right.

The following conclusions can be drawn.

① When P2 is higher and K is lower, Δ X is larger when Δ X1≤ Δ X≤ Δ When X2, the main valve core 2 moves to the right, the throttle port ① is closed, the pressure oil is cut off from the channel a ′ → a, and the oil flow is blocked. That is to say, if the pressure setting is moderate and the spring stiffness is suitable, that is to say, the pressure reducing valve is suitable for the system requirements and adjusted properly, then the pressure reducing valve has unidirectionality: reducing pressure from a to a 'and stopping pressure from a to a'.

② When x is larger and K is larger, 0 ≤ Δ X< Δ When x1, the main valve core moves to the right, but it is not enough to close the throttle port ①, and the channel a ′ → a still communicates, that is, the pressure setting is too high, and the spring stiffness is not suitable, so that the one-way function of the pressure reducing valve is lost, that is, the pressure is reduced from a ′ → a ′, and the pressure is returned from a ′ → a.

③ When x is small and P2 is high and K is small, then Δ X is so big that Δ X> Δ When X2, the main valve core moves to the right, and the throttle port (1) is closed, so that the a'cavity communicates with the t-cavity, and the system overflows. That is to say, when the set pressure is very low, the spring stiffness is also small, and when the load pressure is very high, the one-way performance of the pressure reducing valve is also lost, that is, the pressure is reduced from a to a ′, while when a ′ → a returns, a and t communicate and overflow, which plays the role of pressure impact protection.

From the above analysis, it can be seen that the unidirectionality of the pressure reducing valve changes with the change of working conditions, so it is not suitable to highlight its unidirectionality in the system.

As shown in the system schematic diagram Q, when the solenoid valve 5 is in normal state, the pressure oil (P = 7.0Mpa) flows back from chamber a to the pressure reducing valve through the solenoid valve 5 and the check valve 3. Because the set pressure (P = 1.5MPa) of the pressure reducing valve is low, that is, the set spring preload is small, and the system pressure is high, the chamber a of the pressure reducing valve communicates with chamber T, the system is in overflow state, and the pressure oil bypasses the oil tank, As a result, the pressure oil flow into the hydraulic cylinder is not enough, resulting in the hydraulic cylinder moving slowly.

(3) System improvement

There are some defects in the design of the system, so a one-way valve must be added to channel a behind the pressure reducing valve to make up for the deficiency that the one-way function of the pressure reducing valve changes with the change of working conditions. In this system, the speed of tundish movement is not high and stable, and there is not too much pressure impact. With the one-way valve, even if the pressure impact protection performance of pressure reducing valve disappears, the normal operation of the system will not be affected.

In the improved system, a superimposed double check valve is used to replace the superimposed check valve 3 in the original system. After improvement, the system has been running normally.

If there is no superimposed double check valve, the double hydraulic control check valve can be used to remove its hydraulic control function and transform into a double check valve, which is installed on the system.

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