【Abstract】In view of the requirement that the pumps of the important plant of pressurized water reactor nuclear power plants are flooded under accident conditions, and the pump unit can still operate normally, a bearing waterproof submergence protection mechanism is studied, and the mechanism is proved to be safe and reliable through test verification. of.
I. IntroductionThe important plant water pump of the pressurized water reactor nuclear power plant is a nuclear safety three-stage pump, which serves the heat exchanger of the equipment cooling water system, and is used to transfer the heat in the equipment cooling water system heat exchanger to the final heat sink——the sea . It is required that under accident conditions, when the pump room is flooded and the water level does not exceed the dust-proof plate at the drive end of the important plant water pump, the important plant water pump can still operate normally. If the bearing parts of the ordinary centrifugal pump are submerged by water, water will immediately enter the bearing cavity, causing the grease to fail, causing the water pump to fail to operate normally due to failure or damage.2. Pump unit installationThe important plant water pump is located in the pit of the combined plant (P X) of the power plant, and the foundation installation elevation is -20.00m. Each nuclear reactor is divided into two series, A and B, each series is equipped with 2 pumps, which suck water from the final heat sink (seawater) through the water intake structure, and the inlet pressure of the pump is 3.5-20m. The piping layout and installation position of the pump on site are shown in Figure 1 and Figure 2.3. Institutional design research1. Pump structureImportant plant water pumps are vertical, single-stage, single-suction, cantilever, radial split and double volute structure. The working medium is sucked in from the bottom of the pump and discharged horizontally. The pump is mainly composed of pump body, pump cover, impeller, pump shaft and bearing. The pump and the motor are connected by a diaphragm coupling. The shaft seal is a packing seal. The bearing is composed of a cylindrical roller bearing (non-driving end) and Paired angular contact ball bearings (drive end) combination, bearing lubrication is grease lubrication.2. Bearing protection mechanism(1) Design principle During the operation of the pump, an axial force acts on the rotor, which will pull the rotor to move axially. When designing the pump, it is necessary to try to eliminate or balance this axial force in order to make the pump work normally. A back-blade method is often used in the design to balance the axial force of the rotor. The principle of the back-blade balancing axial force is shown in Figure 3.The formula for calculating the balanced axial force of the back blade is as follows:F1=(/8)[(s+t)2/s2－1](Ae－Ah)(Re2－Rh2)/2In the formula, the radius of Re back blade, the unit is mm;Rh impeller hub radius, in mm;t The width of the dorsal blade, in mm;s The gap between the rear cover plate and the shell wall, in mm;Ae takes Re radius as the area of the circle, the unit is mm2;Ah takes the radius of Rh as the area of the circle, and the unit is mm2.It can be seen from the above formula that when other parameters are constant, the larger the value of t, the smaller the gap (st) between the back blade and the shell wall, the larger the diameter of the back blade, and the greater the ability of the back blade to balance the axial force. powerful. The back vane can not only balance the axial force, but also reduce the pressure of the liquid in front of the shaft seal, and has the function of preventing impurities from entering the shaft seal.(2) Design research According to the working principle of the back blade and the function of the back blade to prevent impurities from entering the shaft seal, assuming a certain external pressure, adjust the diameter of the back blade and the gap between the back blade and the shell wall, whether the back blade can Completely prevent the external medium liquid from entering the cavity. In this regard, a sealed power wheel and a test device were designed for test verification. The test device is shown in Figure 4. The sealed power wheel is installed at the lowest end of the test device and is connected to the transmission shaft of the device through a key. The uppermost end of the device is equipped with a motor. The motor drives the transmission shaft to rotate through the coupling, and finally makes the sealed power wheel rotate; all in the device is relatively static There are O-rings for sealing between the parts.(3) The test verifies that a level gauge contact plate is installed in the cavity of the isolation cavity cover on the back of the sealed power wheel. When water enters the cavity of the isolation cavity cover, the indicator light connected to the level gauge contact plate will be on, and the indicator light will be on. The number represents the level of water entering the cavity; at the same time, a scale is marked on the support tube, which can intuitively reflect the depth of water flooding of the sealed power wheel when the indicator light is on.During the test, the starter motor slowly put the test device into the pool with the sealed power wheel rotating, and observe the indicator light while sinking the device. When the indicator light is on, record the depth of the device inserted into the water at this time, as shown in Figure 5. ; By adjusting the diameter of the sealed power wheel, the gap value between the sealed power wheel blades and the isolation cavity cover, several sets of sealed power wheel diameters, the gap between the sealed power wheel blades and the dust cover, the insertion depth, and the indication are obtained. The data of the number of lights is shown in the table below.(4) Test verification conclusions It can be drawn from the test verification results that if the diameter of the sealed power wheel and the gap between the sealed power wheel blades and the shell are properly matched, when the water is flooded to a certain level, the pump shaft drives the sealed power wheel to rotate, sealing power The centrifugal blades on the back of the wheel do work on the water and under the action of centrifugal force, a high pressure area is formed at the outer edge of the sealed power wheel, which can prevent water from entering the bearing cavity.Four, product applicationTests have verified that the sealed power wheel has a sealing effect on water energy, and the sealed power wheel is applied to the structural design of the bearing parts of important plant water pumps, as shown in Figure 6.The sealed power wheel is designed with centrifugal blades, which are axially positioned through the sleeve and the shaft shoulder, and are connected by a key and rotate with the shaft; an O-ring is installed between the sealed power wheel and the pump shaft to realize the axial seal of the bearing cavity; The cavity cover and bearing gland are designed with radial seals. In order to ensure that a low negative pressure zone is not formed at the bearing cavity of the non-driving end during operation, which will cause grease loss and affect bearing lubrication, a balance pipe is installed on the isolation cavity cover, and the balance pipe communicates with the isolation cavity. The end of the balance pipe is higher than the submerged liquid level and the opening is downward or horizontal to ensure that the normal operation of the pump or the flooding of the pump room can ensure the pressure balance at the bearing cavity of the non-driving end without affecting the normal operation of the pump equipment.V. ConclusionThe bearing waterproof submergence protection mechanism has the characteristics of simple structure, safety and reliability, and can ensure that under accident conditions, that is, when the pump room is flooded and the water level floods the important plant water pump bearing components, the important plant water pump bearing will not fail and still It can operate normally, provide cooling water for the heat exchanger of the equipment cooling water system, and export the heat transmitted by the equipment cooling water system to ensure the safety of nuclear power plants.
Post time: 21-09-21