Relationship between inlet pressure and head

That’s the general understanding, but you need to consider density

Head refers to the energy gained per unit weight of liquid as it passes through a pump; this is what is known as head. The head of the pump, including the suction head, is approximately equal to the pressure difference between the pump outlet and inlet. Head is denoted by H, with the unit being meters (m). The actual situation refers to the height that the water pump can lift water to. The head indicated on the nameplate refers to the head that the water pump itself is capable of generating; it does not include the head loss caused by frictional resistance in the pipeline flow. This is something that cannot be ignored when selecting a water pump. Otherwise, water cannot be pumped in. For positive displacement pumps, the viscosity of the material and pipe resistance also need to be taken into account

This post was last edited by ylb913 on 2018-8-31 at 18:11. The head of a pump is a fixed property of the pump itself; it is not the same as the pressure at the pump’s outlet. Moreover, the conversion of head into pressure depends on the density of the medium. When pumping water with a head of 50 meters, the outlet pressure of the pump is the inlet pressure plus 5 kilograms. When pumping liquefied gas with a specific gravity of 0.5, the outlet pressure is the inlet pressure plus 3 kilograms; when pumping liquid sulfur with a specific gravity of 1.8, the outlet pressure is the inlet pressure plus 9 kilograms.

It can be understood as you said; the key lies in the operating conditions. If it’s a multi-stage pump station for tap water or a pressure boosting station, then this understanding is correct. However, in cases of natural suction or pumping from tanks, the situation is different. The head of a pump refers to the height that the fluid can reach at the pump’s outlet when clean water is used as the medium. The inlet pressure generally affects the size of the pipes at the pump’s inlet. The head of a pump has nothing to do with the density of the fluid inside it; it’s not the case that if the pump achieves a head of 60 meters when pumping clean water, it will only achieve 40 meters when pumping a fluid with a density of 1.5. Different densities require different motor powers for the pump, and the selection tables for pumps provide information on the relationship between density and motor power, not between density and pump head. The head and flow rate of a pump are fixed performance parameters that don’t change significantly due to differences in density. The viscosity of the fluid affects the pump’s performance parameters, as viscosity influences the flow velocity of the fluid, which in turn affects the pump’s flow rate. Flow rate and head are related to each other – a pump can only achieve its rated head when the flow rate meets the required conditions. For example, reducing the valve at the pump’s inlet often results in the pump not being able to achieve its desired head.

Ps + H*γ ≈ Po Ps——Inlet pressure Po——Outlet pressure H——Head γ——Specific gravity of the medium