Fluids versus Liquids
What Pumps Really Move
When we talk about pumps, we often say they move “fluids.” But what does that really mean? A fluid is any substance that can flow, including liquids and gases. Understanding the distinction between fluids and liquids is essential for selecting the right pump, designing efficient systems, and avoiding issues like cavitation, loss of prime, or equipment damage.
What is a Fluid?
A fluid is any substance that flows continuously. This includes liquids and gases and can conform to the shape of its container. Examples of fluids include water, oil, and chemicals, as well as gases like air and steam.
What is a Liquid?
A liquid is considered a state of matter. It is a type of fluid that has a definite volume but has no set shape at a certain temperature or pressure, adapting to the shape of its container. It is anything in a state between its solid and gas phases. Unlike gases, liquids are nearly incompressible, where the volume changes very little when pressure is applied. This makes them predictable and easy to move with standard pumps. Common examples: water, oils, and chemicals.
Why it Matters for Pumps
Pumps are not designed to handle all fluids the same way. Because pumps generate head (energy), and not pressure (force), they rely on the high, stable density of liquids to produce usable energy and predictable performance, something low-density gases cannot provide. While a fluid can be a liquid or a gas, most pumps are specifically engineered to move incompressible fluids that retain their volume such as liquids. Liquids and gases behave very differently inside a pump, and those differences directly affect performance, safety, and equipment selection.
In the pump industry, the term “fluid” commonly refers to liquids, as standard liquid pumps are not designed to handle gases effectively.
Key reasons it matters:
Liquids such as water, oil, and chemicals, are nearly incompressible, while gases like steam and air are highly compressible.
- Pumps rely on the incompressibility of liquids to create predictable flow and head.
- Gases compress, expand, and change volumes, which makes them unstable in standard pumps.
- Standard liquid pumps cannot generate stable or predictable head or pressure when handling gases.
- Head and pressure calculations are meaningful and reliable primarily for incompressible liquids; they lose validity for compressible gases. Head is based on liquid density.
- If gas enters a pump designed for liquids, flow collapses and the pump may lose prime.
- Cavitation and damage risks increase when gases are present.
- Vapor pockets or entrained air can cause cavitation, vibration, overheating, and premature wear.
- Equipment selection depends on fluid type.
- Liquids: Centrifugal or positive displacement pumps
- Gases: Compressors, vacuum pumps, blowers
*Standard liquid pumps cannot move gases safely or effectively.
- System performance becomes unpredictable with gases.
- Gases change density and volume with temperature and pressure, making flow rates and pump performance inconsistent.
Differences Between Liquids and Fluids
| Property | Liquid | Fluid | Why it Matters for Pumps |
|---|---|---|---|
| Definition |
Definite volume, takes shape of container |
Anything that flows (includes liquids & gases) | Determines what type of pump can be used |
| Shape | Adapts to container | Conforms to container; gases expand to fill space | Affects flow predictability and system design |
| Volume | Constant | Variable (especially gases) | Influences pump type and control requirements |
| Compressibility | Nearly incompressible |
Compressible (especially gases) |
Impacts pressure, head, and pump selection |
| Density | Higher than gases | Low (gases) | Determines required head, power, and efficiency |
| Common Examples | Water, oils, chemicals | Air, steam, water, oil | Guides appropriate pump selection |
Wilo is Your Solutions Provider
Understanding the difference between fluids and liquids is essential for selecting the right pump, designing efficient systems, and ensuring safe, reliable operation. While all liquids are fluids, not all fluids are liquids, and pumps are specifically engineered to move incompressible, dense liquids rather than gases. At Wilo, we support you at every stage of your pump system, from helping you select the proper pump for your fluid, to providing installation guidance, service, and lifecycle management, ensuring optimal performance, efficiency, and reliability for your application.
January 2026 | tlk