At what height above the pump inlet should the liquid level be to prevent air from entering?

It should be completely submerged; no matter how little gas is present, the design of the centrifugal pump means that it is not possible to expel the gas.

Are you referring to the vortexes that form at the outlet of the tower or tank, near the pump inlet? The design usually features a cross-shaped anti-rotation device; it’s not clear how effective it is

Hello, I mean, since it was completely submerged, not much water had covered it – because we have a pump here that starts to draw in air when the liquid level drops, but the liquid level still didn’t reach the inlet pipeline of the pump. Thank you

It’s the connection at the pump inlet pipeline where it meets the tank; isn’t the anti-rotation device located at the bottom of the tank? It seems that our pump inlet pipeline doesn’t lead from the bottom of the tank. Thank you

Everything is normal with the pump now that the liquid level has risen

Try to ensure complete submersion; regarding the possible impact of swirls as you mentioned, it might help to install a siphon tank at the pump inlet, which could improve the situation. As for how to calculate this, I don’t know how.

a) The difference between the energy per unit mass of liquid at the pump inlet (the point of lowest pressure), which consists of static pressure energy and kinetic energy, and the saturated vapor pressure head of the liquid at operating temperature, is known as the pump’s Net Positive Suction Head (NPSH), also referred to as the pump’s net suction head. The net positive suction head of a pump is divided into the required net positive suction head (also known as the necessary net positive suction head), denoted as NPSHr (NPSH Required) or NPSHR, and the available net positive suction head (also known as the effective net positive suction head), denoted as NPSHa (NPSH Available). b) To ensure the proper operation of the pump without cavitation, the net positive suction head must be greater than a specified minimum value; this minimum value is known as the net positive suction head required by the pump (NPSHr). NPSHr is related to the type and structural design of the pump, and it varies with the pump’s speed and flow rate. The lower the NPSHr, the greater the pump’s resistance to cavitation. NPSHr is generally determined and provided by the pump manufacturer. The measurement conditions for NPSHr are based on transporting clean water at 20°C. If the NPSHr provided by the pump manufacturer is not available, or if the fluid being pumped differs from the conditions under which NPSHr was determined, the calculation or correction can be carried out using the formula in section 3.1.2 of these regulations. c) After the equipment and piping layout of the given plant is determined, the net positive suction head provided to the pump by its suction system is referred to as the effective net positive suction head (NPSHa) of the pump system. NPSHa depends only on the plant system and not on the characteristics of the pump itself. d) To ensure the pump operates properly without cavitation, it is necessary that NPSHa > NPSHr; under normal conditions this difference should be at least 0.3 m. For certain pumping conditions (such as transporting liquids at near-boiling points), NPSHa should be ≥ 1.3 NPSHr. The formula is available online

The saturated vapor pressure of the medium needs to be taken into account; the installation height of the pump determines the hydrostatic head of the liquid

The amount of flooding depends on factors such as the structure inside the tank and the flow rate; if there is a anti-rotation design, the liquid level can be kept lower, while in the absence of such a design, it needs to be higher.