Specific Speed (Ns) & Pump Type Selection

The Geometry of Flow

If you need to move 1,000 $m^3/h$ of water, what kind of pump do you buy? A standard centrifugal pump? A massive propeller in a pipe?

Pump impellers come in wildly different physical shapes. The shape dictates how the fluid moves through the casing:

• Radial Flow: Fluid enters the eye and is thrown 90 degrees outward by centrifugal force. (Looks like a flat disc with vanes).
• Axial Flow: Fluid enters and exits in a straight line, parallel to the shaft. (Looks like a boat propeller).
• Mixed Flow: A hybrid, throwing fluid out at a 45-degree angle.

To determine exactly which impeller geometry is thermodynamically optimal for a given duty, engineers calculate a dimensionless index called Specific Speed ($N_s$).

The Specific Speed Formula

Specific Speed is defined as the speed (RPM) a geometrically similar pump would run at to deliver exactly 1 unit of flow at 1 unit of head.

The universal equation (evaluated at the Best Efficiency Point) is:

Where (in Metric Units):

• $N$ = Pump rotational speed (RPM)
• $Q$ = Volumetric flow rate ($m^3/h$ or $m^3/s$ or USGPM)
• $H$ = Total Dynamic Head per stage (meters or feet)

Warning: Because engineers refuse to standardize units, $N_s$ values are completely different depending on the country. A metric $N_s$ (using $m^3/s$ and meters) multiplied by 51.6 equals the US Customary $N_s$ (using USGPM and feet).

Interpreting the Specific Speed

By calculating $N_s$, you instantly know what the impeller must look like:

1. Low $N_s$ (Radial Flow)

Values: $N_s \approx 10$ to $30$ (Metric $m^3/s$) | $500$ to $1500$ (USGPM)

Duty: High Head, Low Flow.

The impeller is large in diameter but very narrow. The fluid is aggressively thrown outward by pure centrifugal force to generate massive pressure (head). Standard ANSI or API 610 chemical process pumps are almost all radial flow.

2. Medium $N_s$ (Mixed Flow)

Values: $N_s \approx 40$ to $100$ (Metric) | $2000$ to $5000$ (USGPM)

Duty: Medium Head, Medium Flow.

The impeller vanes are twisted. The fluid gains energy from both centrifugal force and the aerodynamic "lift" of the vanes. Often used in massive cooling water pumps or large municipal water intakes.

3. High $N_s$ (Axial Flow)

Values: $N_s \approx 150+$ (Metric) | $7500+$ (USGPM)

Duty: Low Head, Massive Flow.

There is no centrifugal force. The impeller is literally a propeller pushing the fluid straight ahead like a fan. These generate very little pressure (head < 10 meters) but move oceans of water. Used in flood control, massive irrigation, and evaporator recirculation.