The self-priming lift of a self-priming pump depends on factors such as the clearance between the impeller inlet and the casing (front seal clearance), the pump's rotational speed, and the liquid level in the separation chamber. A smaller front seal clearance results in a greater self-priming lift; this clearance is typically set between 0.3 and 0.5 mm. If the clearance increases, not only does the self-priming lift decrease, but the pump's head and efficiency also drop. The self-priming lift increases with the impeller's peripheral speed ($u_2$); however, once the maximum self-priming lift is reached, further increases in rotational speed do not raise the lift further-they merely shorten the time required for self-priming. Conversely, if the rotational speed decreases, the self-priming lift also decreases. Provided other conditions remain constant, the self-priming lift increases with the height of the stored water (up to the optimal storage level of the separation chamber). To facilitate effective air-water mixing within the pump, the impeller should have fewer blades-thereby increasing the blade pitch-and a semi-open impeller (or one with wide flow channels) is preferable, as this allows the recirculating water to penetrate deeply into the impeller's blade passages.
Most self-priming pumps are paired with internal combustion engines and mounted on portable carts, making them suitable for field operations.
