There are mainly four structures for the nuclear grade gate valve, wedge elastic single plate gate valves, wedge double plate gate valves, parallel double plate gate valves with spring preload and parallel double plate gate valves with top block expansion. The DN of the gate valve
in NI is generally above 80mm. Forging must be adopted for the gate valve body of the nuclear level 1 gate valve. Castings are only allowed for the gate valve body of the nuclear level 2 and 3. However, the quality of the castings is not easy to control and guarantee, forgings are often used. In order to prevent the leakage of the medium, the gate valve usually adopts double-layer packing with a discharge pipe, and is equipped with a disc spring pre-tightening device to prevent the packing from loosening. There are two kinds of driving methods, namely, manual and electric. The electric transmission device used for electric gate valves should consider the influence of the rotational inertia of the motor on the closing force. It is best to use a motor with braking to prevent overload. There are two connection forms between the valve body and the bonnet, that is, flange bolts and pressure self-sealing, but the flange bolt is more commonly used, because this structure is conducive to adding a lip seal weld, which is more reliable.
The four gate valves have different structures and performances. The wedge elastic single plate gate valve has the characteristics of simple sealing pairs, being solid and reliable, but there are strict requirements on the matching angle error between the plate and the sealing surface of the valve body, so it is widely used in the main circuit system. The wedge double plate gate valve has a usual structure in thermal power plants. Since the angle of the wedge double plates can be adjusted by itself, the sealing is more reliable and the maintenance is more convenient. The parallel double plate gate valve with spring preload has the advantage that the load of the plate will not increase sharply when the plate is closed. However, it will bring spring force to keep the plate from the valve seat when it is opened and closed, and the sealing surface is worn. At the same time, there are many auxiliary parts for the plate sealing pair, which makes it inconvenient to assemble. The parallel double plate gate valve with jacking blocks uses the method of staggering the two plates along the inclined plane by the jacking block to make the gate valve well closed, and the sealing is relatively reliable. However, there are also problems with the number of parts for plate sealing pairs.
In addition to the above four structures, there are two gate valves without stuffing boxes. One is a hydraulically driven gate valve reported abroad, which uses its pressure water to push the piston to open or close the valve. Since the piston is directly connected to the plate through the valve stem, there is no packing seal, which can avoid external leakage. However, the sealing between the piston and the cylinder is difficult to be leak-proof. How to maintain the stability of the pressure. Measures needs to be taken in the control system. The other is a fully enclosed electric gate valve successfully designed and developed in China. The valve uses a special screen-closed motor, and the plate is opened and closed through the inner planetary reduction mechanism that is immersed in water. Rolling screw pairs are adopted. There is no filler, so the energy consumption is reduced. It has a good effect on ensuring safe operation and simplifying maintenance. The disadvantages of these two gate valves are that the structure is more complicated and the cost is high.
Types and applications of the main gate valve
1) Welded hydraulic double plate parallel gate valve has nominal pressure of PN17.5MPa, working temperature 315℃ and nominal diameter 350 to 400mm.
2) Electric wedge double-disc gate valves are mainly used in the primary circuit of light water coolant, with a nominal pressure of PN45.0MPa, temperature 500°C, and nominal diameter DN500mm.
3) The electric wedge double-disc gate valve is used in the high-power graphite moderated reactor nuclear power plant, with a nominal pressure of PN10.0MPa, nominal diameter DN800mm, and an operating temperature 290°C.
4) The welded electric elastic plate gate valves are used for steam and process water pipelines of the steam turbine unit, with a nominal pressure of PN2.5MPa, an operating temperature of 200°C, and a nominal diameter of 100 to 800mm.
5) The double gate parallel gate valve with diversion holes used for the refueling machine of the high-power graphite moderated boiling water reactor nuclear power plant has a nominal pressure of PN8.0MPa. The valve can only be opened or closed when the pressure drops to less than and equal to 1.0MPa.
6) Elastic plate gate valves with frozen solid packing are used for fast neutron reactor nuclear power plant.
7) Internal pressure self-sealing bonnet wedge double-disc gate valves are used for water-hydraulic reactor units, with a nominal pressure of PN16.0MPa and a nominal diameter of DN500mm.
8) The flange of wedge double disc gate valves with butterfly spring on the stroke part is connected by bolts and welding is adopted.