Duty & suction
0 = skip Nss
Awaiting Valid Duty Parameters
Adjust the parameters on the left to compute live hydraulic values, ranges, and sizing verdicts.
Impeller selection or exam prep?
Specific speed classifies impellers in one number. Our engineers use it daily for impeller selection and re-rating — ask us anything.
Specific speed explained
What specific speed tells you
Specific speed Ns = 3.65·N·√Q/H^0.75 classifies impellers by shape: low values (below ~140) mean radial impellers for high head and low flow; mid values (140–450) mean mixed-flow; high values mean axial impellers for high flow and low head. It is the number behind "what shape of impeller does my duty need?"
The calculator returns the type with the value, so the theory maps straight to a selection.
Suction specific speed and recirculation
Suction specific speed Nss = N·√Q(gpm)/NPSHr(ft)^0.75 measures how hard the impeller inlet is working. Above about 11,000 (US units) the impeller starts recirculating — vapour recirculation that looks like cavitation but is a design limit, not a suction-system problem. Above 14,000 it is severe.
The calculator uses the US convention so the thresholds match the published practice, and flags both levels. For double-suction pumps, use half the flow for Nss.
Using the numbers
With Ns you can sanity-check a vendor’s impeller choice, and with Nss you can separate recirculation problems from true NPSH problems on plant. Teflow’s engineers use both when re-rating impellers and selecting pumps — call +91 98251 62709 for a selection check.
Technical & Operational FAQs
Q1.What is pump specific speed?
A dimensionless number classifying impeller shape: Ns = 3.65·N·√Q/H^0.75 (metric). Below ~140 it indicates radial impellers (high head, low flow); 140–450 mixed flow; above 450 axial (high flow, low head).
Q2.What is a good suction specific speed?
Below about 11,000 (US units) is normal; above 11,000 flags suction recirculation risk and above 14,000 severe risk. High Nss pumps need extra suction margin and stable conditions.
Q3.How do I calculate Ns for my pump?
Ns = 3.65 × speed (rpm) × √flow (m³/s) ÷ head (m)^0.75. For 2900 rpm, 25 m³/h, 40 m: 3.65 × 2900 × √0.00694 ÷ 40^0.75 = 55 — a radial impeller. The calculator does it instantly.
Q4.Radial or axial impeller?
Radial impellers suit high head and low flow (chemical transfer, boiler feed); mixed flow suits mid duties (ETP, sewage); axial impellers suit very high flow and low head (circulation, irrigation). Ns decides the class.
Q5.What is Nss in the calculator?
Suction specific speed, Nss = N·√Q(gpm)/NPSHr(ft)^0.75 — it uses US units by convention so the 11,000/14,000 thresholds match published practice. Enter your NPSHr to get it; leave 0 to skip.
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Need a custom hydraulic audit or specific duty curve verification? Call +91 98251 62709 or Request a Detailed Engineering Quote.