Introduction: The high-pressure control valve of Turbine No. 1 at Zhanjiang Power Plant No. 2 failed to open, significantly impacting the safe and stable operation of the unit. After disassembling and inspecting the valve, it was discovered that the high-pressure governor valve seat had sunk by 10mm, causing the valve disc guide shoulder to move out of the guide groove, which prevented proper steam regulation. Given the limited time available for unit repair and the difficulty in restoring the sunken seat, a decision was made to use surfacing to increase the height of the guide shoulder, thereby restoring the original function of the high-pressure main steam valve.
1. Repair Plan for the High-Pressure Main Steam Valve
1.1 Working Mechanism of the Valve Disc Guide Shoulder
The turbine at Unit 2 is a N300-16.7 / 537 / 537-3 type from East Turbine Factory. The high-pressure main steam valve consists of a main steam valve and two regulating valves, with the high-pressure regulating valve responsible for controlling the pressure in the intake cylinder. During operation, oil motive force acts as a mechanical lifting device, allowing the valve disc guide shoulder to move up and down along the guide groove, thus controlling the valve's opening. The regulating valve handles high-temperature steam at 16.7MPa and 537°C. The guide shoulder is mainly subjected to thermal stress and some circumferential shear stress. The diameter of the valve disc and seat is 170mm, with the guide shoulder dimensions of 55mm × 30mm × 10mm (height × width × thickness). The disc material is 20Cr3MoWVA alloy steel, and its surface has undergone nitriding treatment to enhance resistance to cavitation erosion.
1.2 Selection of Welding Materials and Equipment
Considering the working conditions of the regulating valve, the guide shoulder must maintain sufficient high-temperature strength and wear resistance. Due to the limited availability of materials during the repair, TIG-R34 (12Cr2MoWVTIB) welding wire, similar to the base material, was selected. The welding equipment used was Lincoln V300-1, along with an argon arc welding kit. Temperature monitoring was done using a U.S. MX2 infrared thermometer.
2. Analysis of Welding Performance
According to the carbon equivalent formula, the 20Cr3MoWVA material contains a high amount of carbon and alloy elements, making it prone to hardening in the weld and heat-affected zones. When the rigidity of the weldment and joint stress are high, cold cracks may occur. Additionally, after nitriding, the surface hardness of the valve disc reaches HV900, making it more susceptible to cracking during welding.
3. Surfacing Process
3.1 Process Route: Cleaning before polishing - preheating - surfacing - post-weld heat treatment - machining after welding.
3.2 Preparation for Welding: First, the nitrided layer was thoroughly removed using an angle grinder within a 20mm range, with a grinding depth exceeding 0.4mm. The hardness of the polished area was measured to ensure it met HB185–321 requirements. According to JB4730-94 standards, the polished surface was inspected for cracks, slag, and other defects, meeting level I quality. The valve plate was then cleaned with acetone, ensuring no water or oil contamination within 50mm of the welding area. Sandpaper was used to remove oil and rust from the TIG wire and surrounding areas.
3.3 Welding Parameters and Process: Preheating was conducted using oxyacetylene flame to 350–400°C, with temperature measured using a thermometer. The non-welding parts were covered with asbestos cloth to prevent arc burns. Welding parameters included current I = 80–90A, argon flow rate 8–10L/min, DC positive polarity, voltage 10–15V, welding speed 30–45mm/min, swing width ≤10mm, and weld thickness ≤4mm. Interlayer temperature was maintained at 300–400°C. Any defects found were immediately removed with an angle grinder. To ensure machining allowance, the diameter was increased by 5mm, reduced by 5mm, and the pile height was set to 13mm. The other side was surfaced after completion. After welding, the part was wrapped in aluminum silicate refractory cotton for slow cooling to 100–150°C, followed by local heat treatment at 640–660°C for 30 minutes. Heating and cooling rates were controlled below 300°C/h.
4. Post-Weld Inspection
After completing the surfacing and mechanical processing, the valve disc underwent penetration testing and ultrasonic testing. No cracks, slag, or porosity were found, meeting JB4730-94 standard level I. Hardness testing showed an average value of HB241, close to the original base material, satisfying the performance requirements.
5. Conclusion
After nearly one year of operation following the welding repair of the high-pressure main steam valve on Unit 1, the unit was dismantled for overhaul. The surfacing on the valve disc shoulder had run for 5,616 hours without any issues. The welding process using TIG-R34 argon arc welding wire, preheated to 350–400°C and post-weld heat-treated at 640–650°C for 30 minutes, proved to be both reasonable and feasible. Not only did it restore normal steam regulation, but it also saved tens of thousands of yuan in costs.
The demand for high-performance films is driving innovation in China's functional cast film machine industry. These machines, also known as functional cast film lines or extrusion cast film machines, go beyond producing simple films. They create specialized plastic films with tailored properties like enhanced barrier protection for food packaging, improved mechanical strength for construction materials, or even specific surface treatments.
This category connects you with China's leading functional cast film machine suppliers and manufacturers. Explore a wide range of machines designed to meet your specific production needs, unlocking a world of possibilities for diverse industries like packaging, agriculture, construction, and healthcare. Discover how functional cast film machines can revolutionize your film production with innovative functionalities and propel your business to the next level.
Functional Cast Film Machine,Functional Cast Film Line,Extrusion Cast Film Machine,Cast Film Extrusion Machine
Baijia Mechanical Equipment (Huizhou) Co., Ltd. , https://www.castfilmmachine.com