1. Using forged steel parts instead of cast steel parts for the bearing seat of the super-large valve
The bearing seat is the most critical force-bearing part of the valve, and almost all the hydraulic thrust of the valve is borne and transmitted through the bearing seat. Steel castings are generally recommended for the bearing seat due to their large size and irregular shape. However, large steel castings are prone to defects such as shrinkage porosity, cold insulation and even cracks and are difficult to repair. Considering that the valve is subject to complex forces under high-flow dynamic water, after adjusting the size and shape of the bearing seat several times, forged steel parts are adopted instead of cast steel parts to avoid leakage which may be caused by steel castings. Through the strength test for the valve body, the delivery test of the valve and inspection, there is no leakage in the bearing seat in all tests.
 
1.2 Realization process and method of zero leakage of forward and reverse sealing
If there are no special requirements, the requirements of one-way positive sealing only need to be met for the butterfly valve due to its structure. The forward seal is easy to achieve, and the hydraulic force helps to increase the specific pressure of the seal and improve the sealing performance. When the butterfly valve is reversely sealed, it is often difficult to achieve the expected ideal effect. For the butterfly valve with large diameters, it is extremely difficult to ensure reverse sealing. According to the special requirements of forward and reverse double seals under working conditions, the forward seal test pressure is set to be 1.1 times the design pressure, and the reverse seal test pressure is set to be 0.6 times the design pressure. To ensure the reverse sealing, the design plan mainly considers the following aspects:

(1) Special design of the cross-sectional size and shape of the sealing seat

Relying on the design results of the super large-diameter valve with forward sealing and the repeated comparison and calculation of FEA finite element analysis for the sealing specific pressure, the contour line of the sealing seat is changed from the overall arc shape to a special tri-polyline arc transition shape, and the tri-polyline arc transition shape can greatly reduce the deformation of the valve body and the influence of hydraulic thrust on the sealing performance.
 

(2) Controlling the section size of the sealing ring and the amount of pre-compression

The section size of the sealing ring is optimized from 30 mm × 20 mm to 50 mm × 30 mm, and the pre-compression is adjusted to not less than 1.5 mm through the finite element contact stress analysis results. When the sealing width increases, the specific pressure of the seal must be reduced, which not only meets the forward and reverse requirements, but also greatly prolongs the service life of the rubber sealing ring.
 
(3) Reasonably controlling the rigidity of the key parts such as valve bodies, discs and shafts
The influence of the rigidity of the key parts on the valve performance is congenital. When the valve requires reverse sealing, the rigidity of the key parts and the overall rigidity of the assembly need to be moderately strengthened.
 
(4) Adopting the patented technology of the large-scale valve axial positioning structure
The overall rigidity of the valve is improved, which is conducive to the realization of two-way sealing.
 
(5) The fully closed position adopts multiple adjustable precise positioning technology. There are three fully closed mechanical limit devices in the pipeline, gear box and electric device.
 
(6) Self-locking transmission actuators
The Rotork gearbox has good self-locking, and the disc does not shift under the action of water pressure or even disturbance.
 
(7) High precision with the transmission system and the machining accuracy of the sealing surface: According to the research on the sealing mechanism, the dimensional accuracy and surface roughness of the two-way sealing valve are required to be improved by at least grades 1 to 2.
 
1.3 The influence of the flat pressure of the bypass valve in the system on the opening and closing torque of the main valve
The service valve of DN5000 (the main valve) is provided with two completely symmetrical bypass pipes and bypass valves. Each channel is equipped with one fixed cone valve of DN500 and one ball valve of DN500. The fixed cone valve of DN500 controls the opening and closing, and the ball valve of DN500 is the service valve for fixing cone valves. When the water conveyance tunnel is in normal operation, the service valve of DN5000 is in the fully open position; when the water conveyance tunnel needs to be inspected and drained in sections, the still water is closed. After the overhaul, the butterfly valve of DN5000 is not opened directly. Two bypass cone valves are filled with water and opened by still water.
 
The butterfly valve of DN5000 is used as a service valve, and its opening and closing torque is great; especially when there is pressure water on one side of the pipeline and no water on the other side, the hydrostatic torque has a great adverse effect on the opening and closing torque. In the design of the oversized butterfly valve, manual electric operation is adopted for the execution structure, and it is extremely beneficial to the main valve to add a bypass valve to reduce the pressure before opening. On the one hand, the torque in the opening and closing of the valve is greatly reduced by the measures of pressure reduce before and after the valve; on the other hand, the influence of the hydrostatic torque under the condition of no water on one side is eliminated, thereby greatly improving the force condition of the butterfly plate, valve shaft and shaft seat, reducing the wear of the transmission system and prolonging the service life of the valve.