Centrifugal Pumps Head and Pressure

Pump Head and Pressure in Centrifugal Pumps:

In centrifugal pumps, head and pressure are two critical concepts used to describe the pump’s ability to move fluid and overcome system resistance. Although they are closely related, they are not the same thing. Understanding the difference between head and pressure is key to selecting, sizing, and operating pumps effectively.

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1. Pump Head:

Definition:

The head is a measure of the height a pump can raise a fluid, usually expressed in meters (m) or feet (ft). It represents the energy imparted to the fluid by the pump, measured as the height of the fluid column it can lift.

Head does not depend on the type or density of the fluid but rather the amount of energy per unit weight that the pump can supply to the fluid.

Types of Pump Head:

There are several components that make up the total head in a pump system:

- Static Head (Hₛ): The vertical distance between the fluid level at the pump's suction (inlet) and the discharge point. It's the energy required to lift the fluid.

- Suction Head: The height of the fluid above the pump's inlet. When the fluid source is below the pump, it’s called suction lift.

- Discharge Head: The height from the pump’s discharge to the point of fluid delivery.

- Friction Head (Hᶠ): The head loss due to friction as the fluid moves through pipes, fittings, and valves. Longer pipes, bends, and smaller diameters increase friction head.

- Velocity Head (Hᵥ): The energy required to impart velocity to the fluid. It is typically small compared to static and friction head, especially in low-velocity systems.

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- Total Dynamic Head (TDH): The sum of all the head components a pump must overcome to move fluid through a system. It is calculated as:

2. Pump Pressure:

Definition:

Pressure refers to the force exerted by the fluid on a surface per unit area, measured in units such as Pascals (Pa), bar, or pounds per square inch (psi). In a pump system, pressure indicates the energy imparted to the fluid to overcome system resistance, compress the fluid, or move it to a higher elevation.

Pressure in a pump system is a direct function of the fluid density and the pump head. A pump that provides a certain head will generate a specific pressure depending on the density of the fluid being pumped. For example, pumping water and oil to the same height will produce different pressures because water is denser than oil.

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Relationship Between Head and Pressure:

The relationship between head (H) and pressure (P) is given by the equation:

For practical purposes, the relationship between head and pressure for liquids like water can be simplified as:

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If you are using common units like psi and feet, the relationship between pressure and head can be simplified using a conversion factor:

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- For water:

Where 2.31 is the conversion factor for water at standard conditions (at 60°F).

3. Key Differences Between Head and Pressure:

- Head is a measure of energy per unit weight of the fluid, whereas pressure is the force per unit area exerted by the fluid.

- Head is expressed in terms of height (meters or feet), while pressure is expressed in force per area (Pascal, bar, psi).

- Head remains constant for a given pump, regardless of the type of liquid being pumped, but pressure varies depending on the fluid's density.

4. Head-Pressure Conversion Example:

For a centrifugal pump delivering a total head of 50 feet of water:

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- For water (density = 1000 kg/m³):

- Using the simplified conversion factor:

- The pump creates less pressure when pumping oil compared to water for the same head.

5. Head and Pump Selection:

When selecting a pump, understanding the required total head is essential. The pump’s performance curve, provided by the manufacturer, shows the relationship between the pump’s flow rate and head. Operating outside of the pump’s intended head range can result in:

- Cavitation: If the pump is under too much suction pressure.

- Overheating: If the flow is too low.

- Reduced efficiency: If the pump operates too far from its Best Efficiency Point (BEP).

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6. Cavitation and NPSH:

Cavitation: A condition where bubbles form due to a drop in pressure at the pump inlet. These bubbles collapse as pressure increases, causing shockwaves that damage the pump.

Net Positive Suction Head (NPSH): To prevent cavitation, the available NPSH (NPSHa) must exceed the required NPSH (NPSHr). NPSHa is related to the pressure at the pump inlet, while NPSHr is a property of the pump design.

Summary:

- Head refers to the height a pump can lift fluid, measured in meters or feet.

- Pressure refers to the force exerted by the fluid, measured in psi, Pa, or bar.

- Pressure is dependent on the fluid’s density, while head is not.

- Conversion between head and pressure helps in pump sizing and selecting the right pump for different fluids.

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Understanding the relationship between head and pressure is vital for proper pump selection, system design, and operation.