Head, Pressure, and the SI System: A Simple Guide for Engineers

Why Head and Pressure Matter?

In fluid dynamics and pump systems, head and pressure are fundamental concepts. Understanding these terms—and how to use them in the SI system—is essential for designing efficient and safe fluid-handling systems.

Here we explain:

• What pressure and head are
• How they’re measured
• Their practical applications in engineering

Understanding Pressure

Pressure is the force applied per unit area. It tells us how much force a fluid exerts on surfaces in a system.

The basic equation is:

Where:

• P = Pressure (Pa)
• F = Force (N)
• A = Area (m²)

SI Unit of Pressure

• Pascal (Pa): 1 Pa = 1 N/m²
• Other common units:
  • Kilopascal (kPa): 1 kPa = 1,000 Pa
  • Bar: 1 bar = 100 kPa
  • Atmosphere (atm): 1 atm = 101.325 kPa
  • Millimeters of Mercury (mmHg): 1 atm ≈ 760 mmHg

Types of Pressure

1. Absolute Pressure (Pₐ): Measured relative to a perfect vacuum
2. Gauge Pressure (P₉): Measured relative to atmospheric pressure (P₉ = Pₐ – P_atm)
3. Differential Pressure: Difference between two points in a system

Understanding Head

Head measures the energy of a fluid per unit weight. Typically expressed in meters (m), it indicates how high a pump can raise a fluid. Unlike pressure, head is independent of fluid density.

The formula:

Where:

• h = Head (m)
• P = Pressure (Pa)
• ρ = Fluid density (kg/m³)
• g = Gravity (9.81 m/s²)

Types of Head

1. Static Head: Height difference between source and destination
2. Friction Head: Losses due to pipe resistance
3. Velocity Head: Energy from fluid velocity
4. Total Dynamic Head (TDH): Sum of static, friction, and velocity heads

Converting Between Head and Pressure

For incompressible fluids like water:

Example for water (ρ = 1000 kg/m³): 

For fluids other than water, multiply by specific gravity (SG):

Applications in Engineering

Understanding head and pressure is essential in:

• Pumping Systems: Selecting pumps based on TDH and pressure requirements
• Hydraulic Systems: Designing fluid power systems safely
• HVAC & Water Distribution: Ensuring sufficient water head in systems
• Petrochemical & Process Engineering: Managing pressure and head in pipelines

Key Takeaways

• Head and pressure are interrelated but distinct concepts
• Use SI units for consistency and accuracy
• Always consider fluid properties, friction losses, and real-world conditions

By mastering these basics, engineers can design efficient, safe, and reliable fluid-handling systems.