Pressure vs. Head
Pressure vs. Head: What's the Difference?
When selecting a pump, there are a few key factors that drive the decision, such as:
- Head (H)
- Flow rate (Q)
- Fluid characteristics (density, viscosity, temperature, etc.)
One area that often causes confusion is the relationship between head and pressure. While they are related, they are not the same thing. Understanding the distinction is crucial for choosing the right pump for your system, especially when dealing with regenerative turbine pumps like those produced by MTH Pumps.
Definition of Head and Pressure
Head refers to the height that a pump can raise a liquid, and it is measured in feet of liquid column (or sometimes meters). The important point is that head is independent of the fluid's specific gravity. This means a pump will lift different fluids to the same height, whether it's water, oil, or another liquid.
Pressure, on the other hand, is fluid dependent. It varies based on the density of the fluid. In simple terms, the same height of fluid will exert different pressures depending on the liquid's density. For example, water and a denser fluid like sulfuric acid will exert different pressures, even if raised to the same height.
Measuring Pressure vs. Head
Head is not measured directly. Instead, pressure is measured at the pump's suction and discharge using gauges, typically in PSI (pounds per square inch). To convert this pressure into head, you need to factor in the specific gravity of the liquid being pumped.
Conversion Example
Consider a centrifugal pump working at 33 gallons per minute (GPM) with a head of 33 feet, pumping both water and sulfuric acid (H₂SO₄). The pump provides the same head for both fluids, but the pressure will be different due to the difference in density:
- For water (specific gravity = 1.0), the pressure is approximately 14.7 PSI.
- For sulfuric acid (specific gravity = 1.8), the pressure is about 26.5 PSI.
The relationship between head and pressure can be expressed with the formula:
\[P[\text{PSI}] = 0.433 \times H[\text{feet}] \times \gamma[\text{specific gravity}]\]
Where:
- 0.433 is the conversion factor for water column height in feet to PSI.
His the head in feet.\gammais the specific gravity of the fluid.
Practical Impact on Power Consumption
Power consumption also varies with pressure, and the formula to calculate it in horsepower (HP) is:
\[N[\text{HP}] = \frac{\gamma[\text{specific gravity}] \times Q[\text{GPM}] \times H[\text{feet}]}{3960 \times \eta}\]
Where:
Qis the flow rate in gallons per minute (GPM).His the head in feet.\etais the pump efficiency.
Notes for Pump Technicians
- The above relationships apply primarily to low-viscosity fluids like water. For higher-viscosity liquids, pump performance must be adjusted.
- A pump’s head at a fixed speed depends on the flow rate and follows its characteristic performance curve.
- Calculating the required head for your pump involves more than just the vertical distance. It includes:
- Geodetic head: The difference in height between suction and discharge.
- Pressure difference between the suction and discharge points.
- Friction losses from the piping system.