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Verification Of Metal Seat Ball Valves Design That Meets Harsh Conditions

June 5, 2018

                                                                                                                                                       Author: SINOVAL

Abstract : Based on the metal seat ball valves under harsh working conditions with strong corrosion resistance, high temperature resistance, strong cavitation resistance, and high hardness of medium particles, the technical difficulties and solutions encountered in the analysis of such problems, it is explained that material pairing and special machining technology are for the metal seat ball mesh sealing surface. Then it performs static structural analysis under the impact of various loads by using Solidworks software to establish the three-dimensional model of the valve body and importing the ANSYSworkbench platform. According to the analysis results, it provides a theoretical basis for structural optimization design of the valve body by checking whether the stress and strain meet the strength requirements of the material. 

Ordinary ball valves generally use soft seal seats, such as polytetrafluoroethylene (PTFE), reinforced polytetrafluoroethylene (RPTFE) or other polymer synthetic materials. The use temperature is generally not high, and particles or fibers are not allowed in the medium, otherwise it will affect the service life and it will not be used at all in harsh conditions. 

1.1. Technical difficulties 

(1) High sealing surface hardness: The high hardness particles will be embedded on the ball surface and the valve seat sealing surface when the opening and closing of the valve, thereby destroying the sealing surface and affecting the overall sealing performance. 

(2) High sealing pressure ratio: Using metal material as the sealing surface, the required sealing pressure ratio is much higher than that of the soft sealing. In order to ensure the proper sealing specific pressure ratio in the design, it is necessary to have a high degree of shape tolerance and surface finish of the part. 

(3) High bonding strength of the spray-welded coating: The medium with particles flows quickly in the pipeline, so the sealing surface of the ball must have high hardness and strength, at the same time, it is also required that the sealing material and the substrate for spray welding have a high bonding strength to prevent the spray-welded coating from falling off. 

(4) Self-cleaning function: Some medium has strong adsorption and are easy to stick on the sealing surface. Destruction of the sealing surface during the opening and closing process affects the sealing performance and greatly shortens the service life of the valve.

(5) Difficulties in processing: The Metal seat ball valve sealing surfaces are all made of super-hard material or surface hardening treatment, which is difficult to cutting process, and the accuracy is not guaranteed. Based on the above analysis results, the structural design and processing technology improvements were made on the basis of ordinary ball valves.

Figure (1) Structure diagram of ball valve

1.2. Solutions 

(1) Sealing material pairing: Nickel-base WC for spray-welded on the ball surface and STL alloy for surfacing on valve seat sealing surface. This pairing guarantees the hardness and thickness of the sprayed-welded layer on the surface of the ball and it can meet the application requirements of high temperature, high pressure and frequent opening. Due to the use of different material pairs, and the nickel-base alloy itself has a high wear resistance and good high-temperature resistance, and the seat sealing surface with thick welded STL alloy layer, plus good bite resistance, so it is suitable for conveying medium under the bad working conditions such as strong corrosive, high temperature, slurry with particles, and powder. At the same time, the wear between the sealing surfaces due to frequent opening is also solved. 

(2) Bi-directional sealing: Improve the sealing performance of the valve and provide guarantee for real-time repair. It also ensures the sealing performance when the medium is in reverse flow and prevents the reverse flow of the medium. 

(3) Seat disc spring compensation structure:

Sealing surface using metal materials requires much higher sealing pressure ratio than soft sealing, and it is particularly important to ensure the proper sealing pressure ratio over the structural design. Therefore, a “spring + valve seat” combination valve seat structure is used to provide an initial preloaded seal specific pressure through a high load disc spring. A combination design method in which the disc springs are connected in series solves the linear expansion effect of the metal material at a high temperature, that is, the initial pretightening force provided by the disc spring is satisfied, and at the same time, the excessive friction torque caused by thermal expansion is solved. Not only can meet the sealing performance under low-pressure conditions, but also have a high sealing shear force, so that the valve seat and the ball always close together with “self-cleaning” function. In addition, the design of the scraping blade is added to the front and rear of the valve seat. During the opening and closing process, the valve seat blade scrapes adsorbent of the ball surface to eliminate the destruction of the sealing surface due to the medium entering between the sealing. 

(4) Processing technology: The precision of the parts is critical to the improvement of the reliability of the seal. Therefore, the valve body is processed with special lathes and grinding machines before and after spraying WC.  After the surfacing and CNC machining to the seat, when the body and valve seat meet certain accuracy requirements, the two can be paired and ground.

Figure (2) Body section stress diagram

2. Finite element analysis and verification of ANSYS-based ball valve body 

In the traditional design process, due to the limitation of the calculation method, it can only be estimated based on some strength formulas. This can not fully reflect the stress state of the valve body, nor can it help the designer find the stress concentration position. With the development of computer technology and finite element theory, the finite element analysis can be used to perform numerical simulation in the design. According to the simulation results, by checking whether the maximum stress value meets the strength requirements of the material, it is intuitive to see the stress concentration parts.  As shown in figure (1), the overall structure of the ball valve: The overall structure of the ball valve is composed of a ball body, a valve body, a connecting flange, an opening and closing transmission structure, a supporting structure, and a sealing structure. The valve body is a major part of the ball valve. Whether its design is reasonable or not will directly affect the performance of the entire ball valve. 

3. Conclusion 

By establishing the three-dimensional model of the valve body and importing the finite element analysis of the static structure in the workbench of ANSYS, the stress and deformation distribution of the medium when flowing through the valve body can be well reflected, and the structural design of the valve body can meet the strength requirements. And this process can be made to equest for verification. For the stress-concentrated sealing surface, it is necessary to consider the shape of the structure in the design so that it can well meet the stress and strain requirements. Before investing in trial production at the initial stage of design, the finite element analysis method was used for testing, checking and finding problems, and the design was adjusted and improved in time. It can not only shorten the design cycle, but also save costs, so it has practical guiding significance for the development of new products.

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