First, the basic principle of the centrifugal pump
The pump works by centrifugal force. Look at the figure below, when the pump chamber is full of water, the impeller is driven by the motor, fast rotation, and water because of centrifugal force, from the center of the impeller to fly around, at this time the center of the impeller to form a vacuum, the outside liquid will flow to the low-pressure impeller because of the pressure difference, so that the external liquid will be pumped away. The fluid that builds up at the outlet creates a high pressure that can eventually be vented from the bilge to the side valves at high altitudes.
Second, why do pumps need self-priming devices?
From the above explanation, it should be seen that when the impeller rotates, only “textured” things can be thrown from the center to the surrounding, so that an obvious vacuum is formed in the center. Obviously, if the pump cavity is filled with air, no matter how fast it rotates, a vacuum cannot be formed. Therefore, before the centrifugal pump works, the pump chamber must be filled with liquid in advance, which is the origin of the self-priming device.
Three, what are the forms of self-priming devices?
I’ve seen two common self-priming devices:
The first is to use compressed air to vacuum the pump shell to divert water; Before starting the pump, during the work of the self-priming device, the valve on the outlet side should be kept closed, or a spring-acting back pressure valve should be installed. When the outlet pressure reaches about 60% of the normal working pressure, it will open automatically. The vacuum suction must be connected to the highest point on the suction side of the pump and placed in the suction pipe to form a gas chamber. Usually followed by reduced pressure of compressed air, the use of the Venturi tube effect, so that the pump shell produced a vacuum, filled with water, this device allows the centrifugal pump to dry for a short time.
Another kind of self-priming device is a vacuum pump, such as the following emergency fire pump self-priming device, this disadvantage is that the pump structure becomes complex, the volume increases, and every time you start to ensure that the above tank is full of water.
Four, a self-priming pump
Self-priming pump, before starting without irrigation, with the help of its own device characteristics can achieve the effect of inlet pipeline exhaust irrigation. Common-used self-priming pump refers to the external mixing centrifugal self-priming pump, according to the impeller rotation to make the liquid flow to the vortex shell, so that the pump body forms a vacuum so that the one-way valve of the self-priming pump will be opened, the liquid is sucked in from the inlet, and the liquid is transported out through the impeller channel, this is the working principle of the self-priming pump. The difference between an ordinary centrifugal pump and a centrifugal pump
1) Before starting, a certain amount of starting circulating water should be injected into the pump body when the self-priming centrifugal pump is started for the first time; Each time the centrifugal pump starts, it is necessary to fill the inlet pipe and the pump body with water or to pump the inlet pipe with auxiliary devices.
2) on the device, the self-priming centrifugal pump is only installed at the lower end of the intake pipe with a filter screen and no bottom valve; And the centrifugal pump in the inlet pipe must be installed at the bottom of the valve or at the exit with a pumping device.
3) when running, the same performance parameters of the pump, the general self-priming centrifugal pump is lower than the centrifugal pump efficiency, and cavitation performance is poor.
The self-priming pump is divided into the following categories according to the principle of action:
1. Gas-liquid mixture type (including internal and external mixing type);
2. Water ring wheel type;
Jet flow (including liquid jet and gas jet).
Gas-liquid hybrid self-priming pump working process: due to the special structure of the self-priming pump body, pump after stalling, there is a certain amount of water in the pump body, the pump starts again due to the impeller rotation, suction pipe fully mixing air and water, and into the gas water separation chamber, the upper part of the gas water separation chamber gas overflow, the lower the water back impeller, mixed with the surplus air inlet line again, until all the pumps and suction tube of gas discharge, complete the imbibition, and normal pumping.
Water ring wheel type self-priming pump is the water ring wheel and pumps impeller combined in a shell, with the aid of water ring wheel gas discharge, self-priming. When the pump works normally, the channel between the water ring wheel and the pump impeller can be cut off through the valve, and the liquid in the water ring wheel can be discharged.
Jet self-priming pump, by centrifugal pump and jet pump (or ejector) combination, relying on the injection device, at the nozzle caused by a vacuum to achieve suction.