Factors affecting the performance of self-priming pumps

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Self-priming centrifugal pumps utilize the principle of air pressure, with the water storage chamber enabling one-time priming for continuous self-priming. So, what are the main factors that affect the performance of self-priming centrifugal pumps? It is mainly reflected in the following six aspects. 1. Determination of the liquid storage volume and height: The liquid storage volume refers to the volume of the pump chamber that can hold liquid after the pump stops operating; in other words, it is the volume of the pump chamber below the lowest point of the pump’s suction inlet. This volume consists mainly of the discharge chamber and part of the gas-liquid separation chamber. The liquid storage volume should be no less than half of the pump’s designed flow rate in seconds, which results in an increased priming time or even makes priming impossible ； This makes the pump bulky. In addition to having sufficient liquid storage volume, there must also be a certain liquid storage height; this height is the distance from the lowest point of pump suction to the center of the impeller, and it is usually taken to be approximately equal to the radius of the impeller. 2. Determination of the volume of the gas-liquid separation chamber and the height of the pump outlet. Gas-liquid separation takes place in the area outside the pump’s discharge chamber; the larger this volume, the better the separation of gas and liquid, and the faster it occurs. However, once the volume of the gas-liquid separation chamber reaches a certain level, further increases in volume do not yield significant improvements. On the contrary, it can make the pump more bulky. Therefore, there is an optimal volume for the gas-liquid separation chamber. Based on existing experience, this value is equal to or slightly greater than the volume reserved for storing liquid. While ensuring an appropriate volume for the gas-liquid separation chamber, the height from the pump outlet to the centerline of the impeller can be determined by considering the volumes of the liquid storage area and the gas-liquid separation chamber. 3. Determination of the liquid flow velocity in the discharge chamber: The liquid flow velocity in the discharge chamber of centrifugal self-priming pumps is lower than that in ordinary centrifugal pumps, being approximately 80-90% of the liquid flow velocity in the discharge chamber of regular centrifugal pumps. 4. Determination of the gap between the septum tongue in the discharge chamber and the impeller: For centrifugal self-priming pumps, the gap between the septum tongue and the impeller has a significant impact on the self-priming capability. The smaller the gap between the septum tongue in the upper discharge chamber and the impeller, the shorter the self-priming time; this gap is usually determined through external mixing ； Internal mixing extraction. If it is designed with two volutes, the gap between the lower discharge chamber lip and the impeller can generally be determined using the standard methods for determining such gaps in conventional centrifugal pumps; that is, the lower lip is located on the base circle of the volute. 5. Turning of the impeller rear cover plate: For external mixing centrifugal self-priming pumps, when the speed of the outer surface of the impeller is low, the self-priming performance is poor. The method of turning the impeller rear cover plate can be used to improve self-priming capacity, as it facilitates the mixing of liquid and gas during the self-priming process. The amount of turning cannot be too large, otherwise it will significantly reduce the pump’s head and efficiency. After the diameter of the rear impeller cover is cut, reinforcement is added at the upper partition to prevent liquid containing bubbles from circulating at the cut area of the rear cover, thereby avoiding an impact on the self-priming effect. 6. Area of the return hole: For internally mixing centrifugal self-priming pumps, the area of the return hole has a significant impact on the self-priming capability. A larger area allows more liquid to return to the inlet of the impeller through the return hole, resulting in a shorter self-priming time; however, it also reduces the achievable vacuum level ； When the area of the return hole is small, the self-priming time is long, and a higher vacuum level can be achieved during self-priming. Taking all of the above into account in the pump industry, when determining the area of the return hole, it is necessary to take into consideration both the requirements regarding self-priming time and self-priming height.