Essential Guide to Gate Valve Failures

Overview

Gate valves are commonly used in industrial systems. They are valued for their pressure resistance, temperature tolerance (both high and low), corrosion resistance (with various materials), bidirectional sealing, wear resistance, and long lifespan. These features make gate valves crucial for controlling the media flow, ensuring system stability and control.

Types of Gate Valves

Gate valves come in different types, such as wedge single gate, wedge double gate, parallel single gate, and parallel double gate. Choosing the right type depends on the specific working conditions. Proper installation, commissioning, and maintenance are necessary to prevent accidents.

Common Failures

Stem Fracture or Deformation

Fracture

Stem fractures often occur at the root of the upper and lower threads due to stress concentration. This happens when the operating conditions are much different from the design parameters. For example, in some power plants, the stem of electric gate valves has broken because of over-tightening and excessive torque during adjustment.

Deformation

Stem bending usually occurs due to improper commissioning of electric valves. This includes excessive closing torque without travel protection, which can lead to significant damage.

Leakage

External Leakage

Valve bodies are typically made of castings, which may have defects like porosity and cracks. These defects can cause external leakage. According to a power plant's 2007 statistics, 37% of external leakage incidents were due to casting defects. Other causes include failures in the stem, middle flange, and end flange seals.

Internal Leakage

Internal leakage often happens due to foreign objects in the sealing surface, especially in new valves. Proper inspection and thorough pipe flushing before operation can reduce such incidents.

Gate Failures

Sticking

Gate sticking occurs when the gate gets stuck to the guide rail inside the valve body, often due to poor design or manufacturing. If the gate is forced up while stuck, it can lead to fractures or deformations in its T-slot.

Cracking

Gate cracking, especially at the bottom, mainly happens in parallel plate structures when the gate bottom directly hits the valve body. This is usually due to improper travel adjustment of electric valves.

Sealing Surface Cracks

Cracks in the sealing surface are often due to improper processing of alloy steel materials. Selecting the suitable materials and controlling welding process parameters can prevent this.

Analysis and Countermeasures

Design and Manufacturing

• End Limit Device: Implement end limit devices on the stem to prevent over-closing of the valve.
• Strengthening the Stem: For large-diameter electric valves, increase the stem diameter and improve the alignment of the upper and lower brackets.
• Bypass Design: Address abnormal pressure rise in the middle chamber by designing internal or external bypasses.
• Thread Longevity: Increase the number of threads and ensure adequate lubrication and regular maintenance.
• Surface Treatment: Ensure proper surface treatment of the stem to prevent rust and corrosion.

Installation and Commissioning

• Bypass Installation: Add bypass devices upstream and downstream of large-diameter high-pressure valves to reduce pressure difference during opening and closing.
• Safety Valves: Install safety valves in the middle chamber to control abnormal pressure rise.
• Proper Flushing: Thoroughly flush pipelines to remove debris that may damage the sealing surface.
• Professional Commissioning: Ensure that professionals commission electric gate valves to avoid over-travel and excessive torque.

Operation and Maintenance

• Torque Enhancement: Use gear mechanisms or impact handwheels for valves requiring large operating torques.
• Periodic Tightening: Periodically tighten pre-tightening bolts to prevent leakage due to pressure fluctuations.
• Regular Lubrication and Testing: Regularly lubricate and test the operation of essential valves to avoid rust and blockage.

Conclusion

Ensuring the safety and reliability of gate valves requires a comprehensive approach, including design, manufacturing, selection, installation, commissioning, and maintenance. Proper measures can prevent major accidents, ensure safe operation, extend equipment life, and reduce maintenance and production costs, improving corporate efficiency.

FAQs

1. What causes stem fractures in gate valves?
   • Stem fractures are often due to stress concentration at the thread roots, especially under conditions deviating from design parameters.
2. How can external leakage in gate valves be prevented?
   • External leakage can be minimized by addressing casting defects and ensuring proper maintenance of seals and packing.
3. What measures can be taken to prevent gate sticking?
   • Ensuring proper design and manufacturing of the guide rails and avoiding excessive closing torque can prevent gate sticking.
4. Why is it important to flush pipelines before operating new valves?
   • Flushing removes debris that could damage the sealing surface and cause internal leakage.
5. How can abnormal pressure rise in the valve's middle chamber be controlled?
   • Installing safety valves and designing internal or external bypasses can effectively control abnormal pressure rise.