What is the difference between head, pressure, and flow in a centrifugal pump?

In a centrifugal pump, head, pressure, and flow are all interrelated concepts, but they represent different aspects of the fluid’s behaviour:

• Flow (volumetric flow rate): This is the volume of liquid passing through the pump per unit of time. It’s measured in units like cubic meters per second (m³/s) or gallons per minute (GPM). Flow represents the quantity of liquid being moved.
• Pressure: This is the force exerted by the liquid per unit area. It’s measured in units like Pascals (Pa), pounds per square inch (psi), or bar. Pressure indicates how much “push” the liquid has.
• Head (pump head): This is a concept specific to pumps, and it relates pressure to the height of a liquid column. It’s essentially the pressure converted into the equivalent height a column of the liquid would need to reach to exert that same pressure due to gravity. Head is typically measured in meters of liquid column (MLC) or feet of liquid column (FLC). The head represents the potential energy the liquid gains due to the pump. The head takes into account the specific density of the liquid because it relates pressure to height. However, for liquids with similar densities (like water and many common liquids), the head will be relatively constant even if the pressure readings (in psi or bar) differ slightly. This simplifies the selection and comparison of centrifugal pumps for applications using fluids with similar properties.
• Relationship between Head vs Flow of a centrifugal pump: The relationship between head (H) and flow (Q) in a centrifugal pump is inversely proportional. This means as the flow rate increases, the head generated by the pump decreases. This relationship is typically depicted by the pump’s performance curve or H-Q curve.

• Low Flow Rate: At lower flow rates (left side of the H-Q curve), there’s less liquid volume passing through the pump at any given moment. This allows for a more complete and efficient conversion of kinetic energy into pressure within the volute casing. Less energy is wasted due to turbulence or friction because there’s less liquid experiencing these effects. As a result, the pump achieves a higher head (pressure converted to height) at lower flow rates.
• High Flow Rate: As the flow rate increases (right side of the H-Q curve), there’s more liquid volume to process. This creates a situation where the conversion of kinetic energy to pressure becomes less efficient. Two main factors contribute to this:
  • Increased Turbulence: With more liquid flowing through the pump at higher velocity, there’s greater internal friction and turbulence. This energy dissipation reduces the amount of kinetic energy available for pressure conversion.
  • Less Time for Conversion: Due to the higher flow rate, individual liquid particles spend less time within the volute casing where the conversion from kinetic energy to pressure happens. This reduces the efficiency of the conversion process, leading to a lower head at higher flow rates.