There is an important distinction between fluids and liquids that is vital to understand. A liquid is anything in a state between its solid and gas phases. Whether or not a substance is in its liquid state depends on the temperature and pressure it experiences, as well as the intrinsic properties of the substance itself.
The term fluid refers to any substance that can flow continuously and is able to take the shape of whatever container houses it. While this perfectly describes a liquid, it could also be used to describe a gas. In other words, all liquids are fluids, but not all fluids are in their liquid state. So generally, when the term fluid is used in pumping, it is referring to a liquid since pumps are not designed to transfer gas.
Liquids have primary physical properties that are important to consider in pumping applications, namely viscosity, density and vapor pressure. These properties are vital for understanding how a liquid will behave and therefore which pump would be best suited for it.
Viscosity refers to how resistant a liquid is to flow, or how “thick” or “sticky” it is. This will affect the pump flow rate, total head, efficiency and power.
Density refers to the mass contained within a given volume of a substance. In pumping it is also common to refer to a relative density (specific gravity), which is the ratio of a substance’s density to that of water at a specific temperature. Density and specific gravity are needed for determining the power required to move one fluid as compared to another.
Vapor pressure is the pressure at which a liquid begins to vaporize, and it is crucial to monitor this within a pump system. If the pressure in the pump falls below the liquid’s vapor pressure, cavitation may occur.
Understanding the differences between fluids and liquids and the behaviors of liquids is vital to pump function. These concepts often serve as early steps in the process of a pump engineer’s training. These concepts are discussed more extensively in the Hydraulic Institute’s Introduction to Pump Fundamentals training at training.pumps.org.