Analysis and treatment of electric gate valve startup failure

1 Overview
The electric gate valve is an isolation device for the containment spray system (EAS) and the pit of a nuclear power plant. In the re-spraying stage of the LOCA condition of a nuclear power plant, the electric gate valve is opened, and the spray pipeline draws water from the containment pit to provide cooling water for re-spraying, so that the pressure and temperature in the containment are reduced to an acceptable level to ensure the integrity of the containment. When the EAS enters the re-spraying stage, the electric gate valve (EAS013-014VB) is opened, and the spray pipeline draws water from the containment pit for recirculation spraying. The recirculation spraying can last for several days to several months. The availability of the electric gate valve determines whether the re-spraying cycle can proceed normally. When the electric gate valve fails to operate normally, the EAS will be unavailable.

2 Fault Analysis
2.1 Working Conditions
The electric gate valve is an electric remote transmission parallel plate (with wedge) gate valve. The electric device is connected to the valve through the remote transmission mechanism. There are three 90 turns in the middle of the remote transmission mechanism (Figure 1). During the commissioning of the nuclear power plant, the torque switch of the electric device was activated when the valve was opened and closed, and the valve could not be opened and closed normally.
Specifically, the torque rod of the torque measurement component of the electric device frequently jumped, triggering the torque switch action, and the electric device stopped. When the set torque of the electric device was increased, the fault was alleviated, but it was extremely unstable and sometimes the valve could not be operated normally.

2.2 Detection Analysis
According to the analysis of the fault phenomenon, the reason why the electric device could not open and close the valve normally is related to the valve opening and closing torque, the efficiency of the remote transmission mechanism and the performance of the electric device (Figure 2).

When the torque wrench was used to directly operate the valve on site, the opening and closing torque was not greater than the designed opening and closing torque. When the remote transmission device was started with a low-speed electric device, the valve could be opened and closed normally. Prove that the valve switching torque does not exceed the design value. In Figure 1, add a torque measuring device at position ① to measure the input torque of the electric device to the remote transmission mechanism, and add a magnetic powder brake (simulating the valve switching torque) and a torque measuring device at position ② to measure the output torque of the remote transmission mechanism. The ratio of the output torque to the input torque is the transmission efficiency of the remote transmission mechanism. After measurement, the transmission efficiency of the remote transmission mechanism exceeds the default transmission efficiency, proving that the transmission efficiency of the remote transmission mechanism meets the design requirements. Use a torque test bench dedicated to the electric device to check its output torque, and the output torque of the electric device meets the design requirements. Through analysis and testing, the performance of the valve, remote transmission mechanism and electric device meets the working conditions. In order to determine whether the coordination of the valve, remote transmission mechanism and electric device is the cause of the fault, a detection device is connected to the power and control circuit of the electric device. At the beginning of closing the valve, it is detected that the motor of the electric device has three current peaks, and the corresponding closing direction torque switch cuts off the power supply three times (Figure 3). When the valve is fully closed, the electric device operates normally. When the valve is in the middle state, the electric device can be detected to start 2-3 times regardless of whether the valve is opened or closed.

2.3 Theoretical analysis
When the valve is fully closed, the medium and static friction will cause the valve to open with too much torque. In order to avoid the electric device from failing to open the valve normally in the fully closed position, the electric device shields the torque switch through the stroke switch when opening the valve in the fully closed position, so that the torque switch fails and the valve opens normally. Therefore, when the valve is opened in the fully closed position, no multiple motor starts are detected. When the valve is in other states, the torque switch is not shielded, so the motor starts multiple times when the electric device starts.

Through qualitative analysis of the current signal, it is found that the time interval between the second start of the electric device motor is very short, and the start signal of the electric device has not disappeared at this time, so the electric device starts again until the valve is driven to open or the start signal disappears. When the set torque of the electric device is increased, the torque switch should not be triggered at this time, so increasing the set torque can alleviate the fault.

Since the torque switch of the electric device is activated at startup, it means that the additional torque is large at this time, which exceeds the set torque of the electric device, causing the torque protection to be activated and the electric device cannot operate normally. Rotational torque M = (. Moment of inertia, angular acceleration). Through quantitative analysis of the motor current, it is found that the motor startup time of the electric device is s level, resulting in a very large angular acceleration. When using a low-speed electric device, the valve can be operated normally because of its low speed and slow startup time. Through analysis, it is known that the electric gate valve has a high speed and a short startup time. Under the action of inertia, the remote transmission mechanism has a large additional torque at startup, which makes the torque required by the electric device at startup larger, exceeding the set torque. 

3 Improvement To solve the startup failure of the electric gate valve, it can only be solved by reducing the additional torque at startup or eliminating the jump of the torque switch through the equipment of the electric device. 

(1) Reduce the inertia of the remote transmission mechanism. The inertia of the remote transmission mechanism is related to the shape, mass distribution and position of the rotating shaft of the remote transmission mechanism. It is an inherent characteristic. To change the inherent characteristic, the remote transmission mechanism needs to be redesigned. The relevant design scheme is restricted by the spatial arrangement of the transmission torque. 

(2) Add a torque shielding device to the fully open position of the electric device. Similar to the torque shielding of the fully closed position of the electric device, adding a torque shielding device to the fully closed position can ensure that the valve is normally started at the fully open and fully closed positions, and the valve is a fully open and fully closed valve. There is no intermediate position in the normal state. 

(3) Reduce the speed of the electric device. When the valve opening stroke and opening time remain unchanged, the speed of the electric device is reduced by adopting the form of double-headed thread on the valve stem.