Engineering guide
The NKRE module determines the specific speed of centrifugal pumps and the NPSH available of the system. The specific speed nq is the key characteristic number for classifying the impeller type: it links rotational speed, best-efficiency flow rate and stage head, and thereby assigns the pump to the radial, mixed-flow or axial impeller range. The calculation is based on the Energietechnische Arbeitsmappe (14th edition), a standard German energy engineering reference.
Anyone who wants to size a centrifugal pump or calculate the specific speed needs this characteristic number when selecting pumps, comparing quotations, and judging whether a given flow–head combination can sensibly be achieved with a single-stage pump or requires a multistage design. In parallel, the module determines the NPSH available from vessel pressure, suction pressure, vapor pressure of the fluid, inlet velocity, density and elevation loss — the decisive quantity for avoiding cavitation at the impeller inlet.
In process engineering, the NPSH assessment is particularly critical for boiling or hot media, for example condensate, column bottoms or boiler feed pumps, where the margin between suction pressure and vapor pressure becomes small.
Typical workflow
Define the operating point
The inputs are the pump's best-efficiency flow rate, the head or stage head, and the rotational speed. For multistage pumps the head of a single stage is decisive, not the total head.
Calculate the specific speed
From rotational speed n, flow rate Q and stage head H, the specific speed nq = n·√Q/H^(3/4) is formed. It corresponds to the speed of a geometrically similar pump delivering 1 m³/s against a head of 1 m.
Classify the impeller type
The nq value indicates the impeller shape: low values characterize radial impellers for small flows and large heads, high values mixed-flow and axial impellers for large flows at low head.
Determine the NPSH available of the system
From vessel pressure, suction pressure, vapor pressure of the fluid, density, inlet velocity and elevation loss of the suction line, the NPSH available of the system is calculated — the energy head remaining above the vapor pressure at the impeller inlet.
Assess cavitation safety
The NPSH available of the system must exceed the NPSH required of the pump (manufacturer's data) by a sufficient margin so that no cavitation occurs in operation.
Frequently asked questions
Which units enter into the specific speed nq?
In the definition customary in Germany, n is entered in 1/min, Q in m³/s and H in m; nq then formally carries the unit 1/min. Entering Q in m³/h yields values distorted by a factor of 60 — one of the most common sources of error. In addition, there is the dimensionless type number as well as the US specific speed Ns using gpm and ft, which produces numerically completely different values.
Why is the stage head used for multistage pumps?
The specific speed characterizes the hydraulics of a single impeller. In a multistage pump the total head is split among the stages; the head per stage is therefore what determines the impeller shape. Calculating with the total head would incorrectly classify the pump as an extreme radial design.
What is the difference between the NPSH of the system and the NPSH of the pump?
The NPSH of the system (NPSHA, available) follows from the pressure and elevation conditions on the suction side: absolute pressure at the inlet minus vapor pressure, converted to meters of liquid column, plus velocity head and minus losses. The NPSH of the pump (NPSHR, required) is a property of the pump measured by the manufacturer. Cavitation is avoided when NPSHA exceeds NPSHR by a safety margin (typically at least 0.5 m).
Why is the vapor pressure of the fluid so decisive?
If the static pressure at the impeller inlet locally drops below the vapor pressure, vapor bubbles form and implode as the pressure rises through the impeller — cavitation. With cold water the vapor pressure is small and uncritical; with hot or boiling media (condensate, column bottoms) the suction pressure is close to the vapor pressure, and even small elevation losses in the suction line lead to cavitation.
Worked example
A single-stage centrifugal pump delivers Q = 180 m³/h at its best-efficiency point against a head of H = 48 m. The rotational speed is n = 2,900 1/min. Calculate the specific speed nq in this worked example.
Given
| Flow rate Q | 180 m³/h = 0.05 m³/s |
|---|---|
| Head H (one stage) | 48 m |
| Rotational speed n | 2,900 1/min |
Solution
Convert the flow rate
Q = 180 m³/h = 180 / 3,600 m³/s = 0.05 m³/s
√Q = √0.05 = 0.2236 (m³/s)1/2
Head term
H3/4 = 480.75 = 18.24 m3/4
Specific speed
nq = n · √Q / H3/4 = 2,900 · 0.2236 / 18.24
nq ≈ 35.6 1/min
The value lies in the range of radial impellers of medium specific speed — a typical single-stage standard pump hydraulic design.
Result
| Specific speed nq | 35.6 1/min |
|---|---|
| Impeller type | Radial impeller |
Technical content source: LV-Solved-Problems-Content 628d313247e7