Principles related to self-priming pumps, selection criteria, and installation considerations

Thread Content

Problems related to self-priming pumps are often encountered. I have some information on self-priming pumps that I’ve compiled, and I’d like to share it with everyone! Self-priming cycle: Sump Filling, Pumping Stopped. SELF-PRIMING PUMPS always retain an adequate amount of liquid in the priming chamber and within the pump casing, which is necessary to create a vacuum inside the casing so that liquid can be drawn in through the suction inlet. When a Pump Start-Up self-priming pump begins to operate, the liquid inside the pump chamber is continuously drawn out of the suction inlet by the impeller, thereby removing air from there. At this point, what the impeller discharges is a mixture of liquid and gas; the start-up chamber (or pump cavity) at the impeller’s discharge outlet serves to separate the gas from the liquid. The gas separated at the outlet is discharged further toward the exit, while the liquid flows into the pump chamber through the return holes. As the air in the inlet pipe is gradually expelled, the liquid at the end of the inlet pipe is slowly forced into the pump chamber by atmospheric pressure. As repeated pumping cycles continue until all the air inside the pipe is expelled, the liquid in the tank will fill the pump inlet and the pump chamber. At this point, the pump begins to discharge liquid to the outlet normally. At this point, the Yongfeng self-priming pump, once it has completed self-priming, will operate in the same manner as a regular standard centrifugal pump. Sump Empty, Pump Stopped: After the pump stops, the liquid located before the check valve behind the pump will flow back into the tank through the pump chamber, while sufficient liquid remains inside the pump chamber for self-priming. Unless it needs to be primed during the first operation or when the internal liquid evaporates due to drying, self-priming pumps are completely automatic pumps. Selection data: The performance curve of a typical self-priming pump includes not only the pump’s performance but also the curve showing the priming time. When selecting a pump model based on flow rate, head, and speed, the self-priming time can be determined directly. Just as performance changes with rotational speed and impeller diameter, the self-priming time also varies with these two parameters. The self-priming height, specific gravity, and inlet pipe diameter directly affect the self-priming time. Static Suction Lift: The higher the static suction lift, the more air needs to be removed, and consequently the suction time also increases. If a pump equipped with a 13\" impeller requires 18 seconds for priming at an elevation of 10 FEET (3m), it needs 30 seconds for priming at an elevation of 15 FEET (4.6m). Specific gravity: The higher the specific gravity of the medium, the longer the self-priming time required. Just as listed earlier, the time required to pump a medium with a specific gravity of 1.4 up to a height of 10 FT (3 m) is the same as the time required to pump water (with a specific gravity of 1.0) up to a height of 14 FT (4.3 m), which is 26 seconds. (But when the specific gravity is less than 1.0, it is calculated as 1.0.) Since the actual maximum self-priming height is 20 FT (6.1 m), when the specific gravity is greater than 1.0, the actual maximum self-priming height can be obtained by dividing this limit of 20 FT (6.1 m) by the specific gravity. Example: 20FT (6.1m) ÷ 1.4 = 14.3FT (4.4m). Suction pipe diameter: The principle of self-priming is to expel the air from within the suction pipe in order to achieve self-priming capability. The self-priming curve is actually based on the self-priming height in the inlet pipe and the air present in the horizontal pipe over the first 3 FT (1 m) ahead of the pump inlet. If calculated based on the diagram: with a self-priming height of 10 FT (3 m) and a horizontal pipe length in front of the pump inlet of 3 FT (1 m), the amount of air contained inside is 1150 in3. If the horizontal pipe length in front of the pump inlet is increased to 9 FT, the amount of air inside will increase by 500 in3. Therefore, the self-priming time will become 1650/1150×18 seconds = 26 seconds, which is 8 seconds more than the standard self-priming time. Precautions during installation: To ensure the proper operation of self-priming pumps, the following points should be taken into account: l Place the pump as close as possible to the water source. l Keep the total length of the suction pipe as short as possible, within 25 FT (7.6m). l Use a suction pipeline of the same size as the pump’s inlet flange. l No bottom valve should be installed on the suction pipeline. l To ensure smooth water intake at the suction port, install the suction inlet at a position at least 1.5D above the bottom of the container. l Since air cannot be easily carried away by the liquid at the outlet, install a drainage valve and drainage pipeline at the highest point of the outlet pipeline.