Designing Nuclear-grade Large-diameter Ultra-long Distance Gate Valves

1. Overview
With the continuous growth of China's energy demand, nuclear power has entered a new stage of both batch development and technological upgrading. However, facing great nuclear power market demand, there is a big difference in the technical level of nuclear-grade large-caliber ultra-long-distance gate valves for nuclear power plants in China compared with foreign countries, especially in the valve design, manufacturing, and test verification technology. Take the fuel transportation channel gate valve as an example. At present, the valve is supplied by an American supplier. Although some domestic valve manufacturers have the design and manufacturing capabilities of similar valves, they have not developed relevant prototypes. To break the ice that the equipment is controlled by others, it is necessary to carry out localization research and development.
 
2. Main technical parameters of the prototype
The fuel transportation channel gate valve is a non-rising stem wedge gate valve with a diameter of DN750. It is a nuclear level 3 and anti-seismic class I valve that must be used to realize fuel transportation. It is equipped with a long-distance anti-seismic class I operating mechanism. The specific parameters are as follows:
Safety level: Nuclear level 3
Type: Non-rising stem gate valves
Nominal size: DN750
Pressure: CL150
Design pressure: 0.443MPa
Design temperature: 60°C
 
Maximum opening/closing pressure difference: 406.8kPa
CAP1400 security level: C
Requirements for Quality Assurance: QSA3
Nuclear Safety Level: SC-3
Seismic Category: I
Specification: ASME BPVC-Ⅲ N
Containment isolation: Yes
Connections of valves and pipelines: flange ends
Working medium: boron water
Body material: Austenitic stainless steel
The service life for action: 600 times
Service life: 60 years, except for wearing parts
Leakage rates for seats: not exceeding 60cm3/h
Driving units: Seismic class I remote operation devices
 
3. Key points of design and development
Through the research on the gate valve of the fuel transportation channel, we can fully grasp its core technology, including the design of the whole machine and key components (a remote operating mechanism), manufacturing technology, performance test verification technology and identification technology of valves (including analysis and testing). Through the research on the gate valve of fuel transportation passage, the difficulties and major technical problems in the process of its development can be solved, and the foundation for the innovation of gate valve of fuel transportation passage can be laid. In previous projects, the speed-increasing mechanism of this type of valve has been soaked in water for a long time, and its sealing and maintenance are difficult. Moreover, the whole valve has no requirement for seismic resistance, and the operation of the valve is inconvenient, time-consuming and labor-intensive. Because of the above problems, the research focus and measures of the fuel transportation channel gate valve are put forward.
 
(1) The valve is permanently immersed in the boron-containing water with a cumulative dose of γ irradiation greater than 3×106Gy, and the water depth is greater than 13m. For the special working conditions of the gate valve of the fuel transportation channel, the valve and its accessories are made of corrosion-resistant materials to prevent rust or corrosion of the parts from affecting the normal operation of the entire equipment. The underwater bearing adopts a stainless bearing, and the parts are all metal parts.
(2) Structural design should meet the requirements of anti-seismic class I of the valve.
The gate valve of the fuel transportation channel is planned to adopt the structure of the non-rising stem wedge gate valve, which reduces the height of the valve and has a compact structure. At the same time, the strength and seismic performance of the valve are increased by means of reinforcement. The rigid shaft is used for ultra-long remote transmission mechanism to bear the weight of the remote transmission mechanism through support, and finally, meet the requirements of seismic class I.
(3) The support design of the valve
The ultra-long remote transmission mechanism of the fuel transportation channel gate valve is proposed to be designed as a structure with supports to support the weight of the overall remote transmission mechanism. The support structure is shown in Figure 1.


Figure 1 Support structure of remote transmission mechanisms

(4) Easy operation and maintenance
The speed-up mechanism of the gate valve of the fuel transportation channel is planned to be set above the water surface, which solves the problem that the speed-up mechanism is soaked in water for a long time, and the parts are prone to corrosion and difficult to be sealed. The speed-up mechanism is above the water surface for easy maintenance and operation. By studying the use of similar valves in domestic pressurized water reactor nuclear power plants, the operability and maintainability of the gate valve seat of the fuel transportation channel are optimized.
(5) The manufacturing technology of large-diameter valves
The fuel transportation channel gate valve needs to go through the processes such as raw material procurement, processing and manufacturing, cleaning and assembly, which are shown in Figure 2.
 

Figure 2 Manufacturing processes of gate valves for fuel transportation channels
 
(6) Ultra-long distance transmission mechanism manufacturing technology
A rigid shaft design is adopted for the remote transmission mechanism, and the rigid shafts are connected by universal joints. While ensuring the requirements of strength, shock resistance, and flexibility, the transmission efficiency of the remote transmission mechanism is improved. The remote transmission mechanism is designed with a support mechanism that can bear all the mass of the remote transmission mechanism. The research and development of the fuel transportation channel gate valve not only solved the existing problems of the fuel transportation channel gate valve in the past, but also obtained the design, manufacture, inspection, test and qualification test of the fuel transportation channel gate valve. This technology breaks the ice of foreign monopoly and has great international market competitiveness.