State Monitoring and Fault Diagnosis of Electric Valves

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Introduction

The primary goal of electric valve condition monitoring is to assess whether the equipment is operating correctly and within specified parameters. However, the diagnostic capabilities of the monitoring system are limited, as it can only confirm whether the equipment is operating within expected parameters and provide critical data for further fault diagnosis. Fault diagnosis for electric valves focuses on identifying latent faults during operation, evaluating current performance, monitoring the degradation of components, and forecasting potential failures. In most instances, fault diagnosis does not require the continuous operation of the equipment and is typically carried out offline.

1. Structure and Operating Principles of Electric Valves

Electric valves are operated by electric actuators, which control the valve's opening and closing. These valves primarily consist of drive motors, transmission components, and valve bodies. Due to their compact size, low manufacturing complexity, and cost-effectiveness, electric gate valves are widely used in large-diameter pipelines, especially in nuclear power plants. This paper uses the typical electric gate valve as a case study to examine its fault characteristics and detection methods.

As shown in Figure 1, a typical electric gate valve is powered by a drive motor that generates rotational force. This rotational movement is converted into linear up-and-down motion of the valve stem via the valve stem nut inside the valve body. The electric gate valve uses a gate plate or disc to block or allow fluid flow. The valve stem moves the gate plate vertically, thereby controlling the opening and closing of the fluid passage.

1.1 Closing Stroke Mechanism of Electric Gate Valves

The valve stem thrust curve during the closing stroke of the electric gate valve is shown in Figure 2. When the valve receives a closing signal, the motor is activated, driving the worm gear to rotate. This rotation gradually reduces the force applied to the valve stem until the hammer effect occurs. At this point, the valve stem nut begins to rotate, releasing the tension on the valve stem. Once the valve stem passes through the threaded gap, the force is reduced to zero. The valve stem nut then begins to push the valve stem downward. As the valve stem moves, it passes through the T-slot of the valve disc and the packing, ultimately seating the valve disc against the valve seat.

1.2 Opening Stroke Mechanism of Electric Gate Valves

The valve stem thrust curve during the opening stroke of the electric gate valve is shown in Figure 3. Upon receiving an opening signal, the motor is activated, causing the worm gear to rotate. This reduces its torque on the worm wheel, drive sleeve, and valve stem nut. The friction between the valve stem and the valve stem nut, resulting from the self-locking feature of the valve stem, causes most of the force on the valve stem to remain. When the worm gear engages the hammer ear of the drive sleeve, the drive sleeve rotates, further driving the valve stem nut. This process continues until the compression force on the valve stem is fully released. At this point, the valve stem begins to move upward, and the valve disc finally lifts off the valve seat.

2. Analysis of Typical Failure Modes of Electric Valves

Electric valves may experience various types of failures during operation, which can occur in sequence or simultaneously. Based on their failure modes, the primary failure categories are classified into 13 types:

Aging, deterioration, or insufficient lubricating oil
Torque switch failure
Reduced closing torque switch setpoint
Increased closing torque switch setpoint
Reduced opening torque switch setpoint
Increased opening torque switch setpoint
Decreased open limit switch activation value
Increased open limit switch activation value
Decreased close limit switch activation value
Increased close limit switch activation value
Contaminated valve stems
Excessive packing gland screw torque
Insufficient packing gland screw torque

2.1 Aging, Deterioration, or Insufficient Lubricating Oil

Aging or insufficient lubricating oil can impair the lubrication of the mechanical transmission system, leading to increased wear and potentially causing slippage or failure of the transmission mechanism. This may result in delayed valve action or failure to operate.

Failure characteristic parameters

Increased three-phase current and active power consumption
Extended switching time
Increased valve stem thrust

2.2 Torque Switch Failure

If the torque switch is damaged and the contacts remain closed, the valve will not stop when the electric actuator is engaged. This can lead to motor burnout, damage to the valve stem or stem connector, and may compromise the valve body’s sealing surface. If the contacts are open, the electric actuator will not operate.
Fault characteristic parameters:

Excessive current, power, and thrust during valve operation (either opening or closing)
Failure of the torque switch to engage or reset

2.3 Decreased Closing Torque Switch Setpoint

When friction is high during the valve closing process, the closing torque switch may prevent proper seating of the valve, leading to internal leakage.

Fault characteristic parameters:
• Reduced current and power when the closing torque switch is actuated;
• Decreased seating force;
• Shortened valve closing time.

2.4 Increased Closing Torque Switch Setpoint

Excessive seating force can damage the sealing surface, leading to internal leakage, and may also cause damage to the valve stem and electric actuator.

Fault characteristic parameters:

Increased current and power upon actuation of the closing torque switch
Increased seating force
Prolonged valve closing time

2.5 Decreased Opening Torque Switch Setpoint

If the opening torque switch is activated prematurely during the valve opening process, the valve will not open.

Fault characteristic parameters:
• Reduced current and power during opening torque switch activation
• Reduced tension on the valve stem
• Shortened valve opening time

2.6 Increased Opening Torque Switch Setpoint

Excessive opening torque may damage the valve stem, sealing surface, and electric actuator.

Fault characteristic parameters:
• Increased current and power during opening torque switch activation
• Increased tension on the valve stem
• Prolonged valve opening time

2.7 Decreased Opening Limit Switch Setpoint

When the opening stroke control malfunctions, the valve may fail to open fully, resulting in throttling.

Fault characteristic parameters:
• Reduced valve stroke
• Shortened switching time

2.8 Increased Opening Limit Switch Setpoint

When the open limit switch fails, the valve will be controlled solely by torque protection, which could prevent proper valve opening.

Fault characteristic parameters:
• Increased valve stroke
• Prolonged switching time

2.9 Decreased Close Limit Switch Setpoint

When the closing stroke control malfunctions, the valve cannot fully close, causing internal leakage.

Fault characteristic parameters:
• Reduced valve stroke
• Shortened switching time

2.10 Increased Close Limit Switch Setpoint

When the closing limit switch fails, the valve will be controlled solely by the closing torque switch, which may lead to valve damage.

Fault characteristic parameters:
• Increased valve stroke
• Prolonged switching time

2.11 Poor Valve Stem Cleanliness

Contaminated valve stems can cause the valve to function improperly, increase friction, and accelerate wear.

Fault characteristic parameters:
• Increased three-phase current and power
• Increased friction

2.12 Excessive Packing Gland Screw Torque

As the system temperature increases, the packing may expand, potentially causing the valve stem to seize and malfunction.