Static Head, Dynamic Head and the Importance of the Best Efficiency Point

By Simon Anthes, Senior Application Manager at DESMI

Modern aquaculture facilities increasingly operate pumps using frenquency converters (VFDs)

This allows flexible flow control, but it also introduces a new challenge:

The pump may operate far away from its Best Efficiency Point (BEP)

What is the BEP?

The Best Efficiency Point is the operating condition where a pump converts energy most efficiently.

At this point:

• hydraulic losses are minimal
• internal forces are balanced
• vibration is lowest

Static vs Dynamic Head

To understand where the duty point occurs, we must distinguish between two components.

1. Static Head

Static head is the height difference between suction and discharge levels.

It does not change with flow.

Example:

• pumping water to the top of a tank 5 meters high
• water level in tank before pump = 3.5 m

Static head = 1.5 m

2. Dynamic Head

Dynamic head represents friction losses in the piping system.

These increase with flow rate.

Factors include:

• pipe length
• fittings
• valves
• filters

Example of Static & Dynamic Head, plus a duty point intersecting the system curve and the pump curve (AI-generated image)

Why VFD Operation Changes Everything

When pump speed changes, the pump curve shifts.

This means the operating point moves along the system curve.

If not properly designed, the pump can operate far away from BEP.

Recommended Operating Range

Most pump manufacturers recommend operating between:

70% – 120% of BEP flow

Outside this range, problems may occur:

• vibration • recirculation • reduced efficiency • mechanical stress

Example with DESMI NSLH400-390 pump (AI-generated image)

Why This Matters in Aquaculture

In RAS systems, flow requirements change depending on:

• feeding cycles
• biomass
• treatment processes

VFDs allow flexible operation, but the system must be designed so that all expected operating points stay near BEP.

Takeaway

Pump efficiency is not determined only by the pump itself.

It is determined by how the system curve interacts with the pump curve across the full operating range.

Proper hydraulic design ensures pumps operate near their BEP even when speed changes.