Pump Selection: What You Need To Know About Liquids Vs Fluids

Selecting a pump in Singapore for a particular application requires a thorough consideration of various factors, one of the most important being the material to be pumped. This matters greatly since its characteristics will affect its behaviour in the pump as well as the overall network of pipelines and passages through which it will travel.

The two common materials typically related to pumping systems are none other than liquids and fluids, and knowing which one you are working with is vital to optimise pump performance and efficiency. In many instances, these two terms tend to be used interchangeably but they are not quite the same from an engineering perspective. Specifically, they differ in terms of their density, viscosity, and vapour pressure, which are the key aspects that determine how substances behave as they flow through a pump system and affect things like overall system design, energy consumption, and flow rates. 

A Brief Overview of Liquids vs Fluids

A liquid is a specific state of matter within the fluid category, distinguished by its relatively incompressible nature and its fixed volume. Liquids cannot expand or compress easily, which is why they do not exert significant volume changes due to pressure when pumped. However, they do take the shape of their container, filling the lower part and creating a surface. This incompressibility makes liquids ideal for certain pump types, such as centrifugal or piston pumps, which rely on consistent flow without drastic changes in volume.

Meanwhile, a fluid is any substance that can flow, which means it includes both liquids and gases. Fluids lack a fixed shape and easily conform to the shape of their container. They are defined by their ability to deform continuously when any amount of force is applied. This ability to flow is why gases, despite being invisible and compressible, are considered fluids just like liquids.

The Key Factors That Differentiate Liquids and Fluids

1. Density

Density, or the mass per unit volume of a given substance, significantly impacts pump system design and energy needs. Fluids, encompassing both liquids and gases, vary widely in density, with gases generally being much less dense than liquids. A higher-density fluid requires more energy to move through a system, influencing both pump selection and the overall system’s energy consumption.

For liquids, their density remains relatively constant under standard conditions, though it can shift with temperature and pressure changes. For instance, water has a greater density than oil, meaning it typically requires more power to pump under similar conditions. Accurately accounting for a fluid’s density is essential to determine system pressure requirements and the appropriate pump size to achieve the desired flow rate.

2. Viscosity

Viscosity is the measure of a fluid's flow resistance, impacting how easily a substance can be moved through a pump system. While all fluids have viscosity, gases generally have low viscosity and are relatively easy to pump with minimal energy. Liquids, however, can vary greatly in viscosity—from low-viscosity water to high-viscosity substances like syrup or oil.

Higher-viscosity liquids require more energy to pump, often necessitating specific pumps like positive displacement pumps that efficiently handle thicker fluids. Conversely, centrifugal pumps are typically recommended by water pump suppliers for low-viscosity liquids due to their ability to maintain high flow rates with little resistance. As such, understanding the viscosity of the fluid being pumped is crucial for selecting the appropriate pump and ensuring efficient operation.

3. Vapour Pressure

Vapour pressure is the pressure exerted by the gas in equilibrium with a liquid or solid in an enclosed container at a specific temperature. This characteristic is crucial to understand for pump systems since it impacts the risk of cavitation—the formation of vapour bubbles within the pumped liquid. Cavitation can be highly damaging to pump components and significantly reduce system efficiency.

Fluids with high vapour pressures, especially volatile liquids, are more susceptible to cavitation, particularly at elevated temperatures. When the pressure within the pump falls below the liquid’s vapour pressure, vapour bubbles form and collapse, potentially causing severe wear and reducing performance. Centrifugal pumps are particularly vulnerable to this because of rapid pressure changes within the pump. To avoid cavitation, engineers must ensure the pump system maintains pressures above the liquid’s vapour pressure.

Liquids with lower vapour pressures are generally easier to handle, as they pose less cavitation risk. By selecting pumps capable of maintaining appropriate pressure levels, operators can prevent cavitation and ensure long-term reliability and efficiency of the system.

Conclusion

The differences between fluids and liquids go beyond theory as they bring practical implications to the effective design and operation of pump systems. Key factors like viscosity, density, and vapour pressure directly influence pump selection, system efficiency, and reliability. Understanding these characteristics helps in choosing the most suitable pump, optimising energy use, preventing issues like cavitation, and ultimately designing robust, efficient systems that meet operational demands and ensure long-term performance.