Basic Understanding of Total Dynamic Head

Total Dynamic Head (TDH) is the total energy a pump must generate to move a fluid from its starting point to its desired destination. This head is usually measured in units of length (e.g., meters) rather than pressure units, so that pipe system analysis remains independent of the type of fluid.

Total Dynamic Head Components

Based on Bernoulli's Equation, the total head that a pump must overcome consists of 3 main components:

1. Static Head ($h_s$)

This is the difference in vertical elevation between the suction liquid level and the discharge liquid level (Static Discharge Head - Static Suction Head).

• Characteristics: It is a constant value for a given system, independent of the flow rate (unless the liquid level in the supply tank changes).
• Impact of changes: If you need to pump water to a higher location (the destination tank height increases), the static head will increase, making the pump work harder to lift the water to that level.

2. Friction Head ($h_f$)

This represents the energy loss due to friction when fluid moves through straight pipes (Major Loss) and through pipe fittings like elbows, valves, or joints (Minor Loss).

• Relationship: It varies approximately with the square of the flow rate (velocity) and is inversely proportional to the pipe diameter.
• Impact of changes: Increasing the flow rate, using smaller pipes, or running complex piping with many fittings will rapidly increase the friction head.

3. Velocity Head ($h_v$)

This is the kinetic energy in the form of fluid velocity, calculated by $\frac{v^2}{2g}$, where $v$ is the fluid velocity and $g$ is the acceleration due to gravity.

• Characteristics: In typical pumping systems, fluid velocity in pipes is not very high, making the velocity head very small compared to the static and friction heads. In preliminary calculations, it can sometimes be ignored.
• Impact of changes: It increases when the fluid has a higher velocity or when the pipe size is reduced at the destination.

Relationship Equation

The total head the pump must generate (TDH) is the sum of these three components:

$TDH = h_s + h_f + h_v$

Understanding these components is crucial for proper system design and appropriate pump sizing. If a pump is selected with a head lower than the system's TDH, it will deliver less flow than designed or no flow at all. Conversely, selecting a pump with an excessively high head will waste energy and may cause various problems within the system.