Large Diameter High-temperature and High-pressure Integral Forged Globe Valves

The structural characteristics, working principle and performance of integrally forged globe valves for large-scale thermal power boiler units are introduced; the use and maintenance of forged globe valves are discussed and materials for main parts are selected.
 
1. Overview
In recent years, with the mass production and operation of high-parameter, large-scale units and supercritical thermal power units, the requirements for generating units are getting higher and higher. The globe valve is one of the important supporting devices to ensure the safe and economical operation of the unit. The function of the main water supply globe valve is to continuously deliver unsaturated water to the boiler feed water pipeline to ensure the normal operation of the boiler. Since the main water supply globe valve operates under high temperature and high pressure, an integral forged large-diameter globe valve is designed according to the requirements of the working condition system.
 
2. Working principles
A globe valve is composed of a valve body, a large valve disc, a small valve disc, a valve bonnet, a pressure equalizing ring, a quarter ring and a valve stem. the valve disc adopts a combined structure of a large valve disc and a small valve disc due to the large nominal diameter of the globe valve to reduce the opening and closing torque of the valve. When the valve is opened, the small valve disc is lifted by the valve stem first, and the small valve disc has a small stress area and is easy to open. In the lifting process of the small valve disc, the pressure difference between the two ends of the valve gradually decreases; as the valve stem continues to rise, it drives the opening of the large valve disc. This structure reduces the damage to the valve stem and disc caused by the medium at the moment of opening and closing, avoids the disadvantage of large opening and closing torque of the large-diameter globe valve, and reduces the torque and manufacturing cost of the actuator. To ensure the reliability, flexibility and universality of the connection between the small valve disc and the valve stem, and to facilitate assembly and disassembly, the small valve disc is connected by a T-shaped slot.
 

1. Valve bodies 2. Large valve discs 3. Small valve discs 4. Valve stems 5. Bonnets 6. Self-sealing packing 7. Pressure equalizing rings 8. Quarter rings 9. Pressure plates
Figure 1 Main water supply globe valves
 
3. Main technical parameters
Nominal size: 200mm
Nominal pressure: 32MPa
Design temperature: 250℃
Pipe specification: 223 × 12. 7 mm
Applicable media: water and steam
Connection form: butt welding
Drive mode: bevel gear transmission
 
4. Design
The design of the globe valve adopts relevant standards such as ASME B16. 34 and GB/T12221. Since the pressure-temperature rating of the globe valve conforms to the special level of ASME B16.34, the pressure corresponding to 32MPa is Class 2000 by interpolation method. The connection between the valve body and the bonnet adopts an internal pressure self-tightening sealing structure. The higher the internal pressure of this structure, the better the sealing performance of the bonnet becomes. Affected by structural factors, the globe valve has great flow resistance and high-pressure drop. The material of the sealing surface of the valve disc and the valve seat must not only be corrosion resistant and erosion resistant, the hardness at high temperatures also needs to be ensured. Therefore, the sealing surface is surfacing with hard alloy (stellte No. 6), enhancing its wear resistance and scratch resistance, and improving the service life of the sealing surface. The globe valve drives the valve stem to move up and down through the bevel gear transmission to realize the opening and closing of the valve. In the fully open state, the disc is fully lifted and slightly higher than the center line of the outlet passage, so that the pressure drop of the valve is small. According to the requirements of ASME B16.34, the wall thickness of the valve body is selected as 50mm by interpolation method. The minimum wall thickness of the valve body neck is 75mm. After the size of each component is determined, check the strength of the top of the valve body, valve stem and valve bonnet.
 

Figure 2 Minimum wall thickness of the valve body neck
　　
5. Materials
The valve body is integrally forged from a rough steel ingot, and the inner cavity parts of the valve are made of forgings or sheets. The valve body adopts the form of free forging on large-scale forging equipment. After upsetting, drawing, punching, expanding, shoulder pressing, indentation, chamfering and straightening, the shape of the forging is close to a rectangle. The forged valve body is assembled with other parts after rough machining, overall heat treatment, elimination of heat treatment stress, ultrasonic flaw detection of the whole valve body, finishing and overall coloring inspection.
 
The valve stem is made from 38CrMoAlA nitrided alloy steel, which has good oxidation and mechanical properties. After nitriding treatment, it has a good surface hardness of 1 100 to 1 200HV, and has high fatigue strength and high-temperature allowable stress. The diameter of the valve stem is reduced to make the overall structure of the globe valve compact. See Table 1 for materials of the main parts of the globe valve.
 
Table 1 Materials of main parts

　
6. Structural features
(1) The large valve disc and the small valve disc are matched with the inner cavity of the valve body and the inner hole of the large valve disc for easy guidance. It can avoid the disc from being pushed to one side by the fluid when the valve is opened and closed under high-pressure difference, resulting in failure of the seal between the disc and the valve seat, and also avoiding the bending of the valve stem under extreme conditions. At the same time, the valve disc is guided in the valve body, which can maintain the coaxiality of the valve disc sealing surface and the valve seat. The gap between the valve body and the valve disc is relatively big due to the use of the guiding function, so that the valve disc can be lifted and lowered flexibly.
(2) Since the small disc opens first when the valve is opened, the opening torque of the valve is reduced, which not only reduces the torque of the bevel gear transmission, reduces the cost of the transmission, but also reduces the wear of the sealing surface.
(3) Stellite alloy is surfacing at the contact between the middle cavity of the valve body and the self-sealing packing to prevent the inner wall of the middle cavity of the valve body from being worn under high temperature and strong corrosion of the self-sealing packing, which can effectively ensure the sealing of the middle cavity.
(4) The stem’s sealing adopts flexible graphite braided packing and flexible graphite ring, in which the first and bottom rings are made of flexible graphite braided packing, and the middle ring packing is made of flexible graphite ring to prevent graphite from being squeezed into the cavity of the valve body after being compressed. When the packing is installed, the packing should be preloaded one by one to maintain a good sealing performance of the packing.
(5) A guide block is installed between the valve stem and the column to ensure that the valve stem only moves up and down without rotating, reducing wear on the packing and ensuring the sealing of the packing.
 
7. Use and maintenance
The globe valve should be installed vertically, and the arrow marked on the valve body is the actual flow direction of the medium. There must be room for operation and maintenance at the installation site. The valve bonnet should not be wrapped with an insulation layer, so that the packing part can be cooled naturally as much as possible to ensure the safety and reliability of the packing seal. When the valve leaves the factory, anti-rust paint is applied to the groove connecting the valve body and the pipeline. Before welding with the pipeline, the anti-rust paint, oil and dust should be cleaned. Do not damage the groove of the valve body. Take protective measures for the bevel gear transmission of the valve to prevent damage and misoperation. Prevent the dust, chemical substances or humid gas in the atmosphere from corroding the valve stem thread and the transmission device, making its opening and closing fail. Working areas such as threads, bearings or gears should be lubricated regularly.
 
8. Conclusion
The sealing structure of the opening and closing parts of the large-diameter high-temperature and high-pressure integral forging globe valve is reliable, and the small disc opens first, which greatly reduces the opening and closing force and the cost of the actuator. After various inspections, the performance is good, and no leakage can be achieved under long-term working conditions, which improves the service life of the valve and plays a positive role in stabilizing product quality and ensuring the safe operation of supporting facilities.
 

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