KEVOS® MAKEOS · Hydraulics
Pump Speed Control Plan (Speed for Target Flow, Power, Saving over Throttling)
What speed must my pump run at to deliver the flow I need, how much power does that take, and how much does speed control save compared with throttling a valve at full speed?
Plan for reducing a pump’s flow by speed instead of throttling: pump and system curves fitted from the shut-off head, best-efficiency point and one known system point, the speed for the target flow (as the pump operating point tool), the head, efficiency and power at that speed, and the power when the same flow is got by throttling at full speed, with the saving per year, checked against the least speed and that the target is reachable.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pump at rated speed | ||
| Shut-off head | m | Head at zero flow at rated speed. |
| Best-efficiency flow | L/min | Flow at best efficiency at rated speed. |
| Head at best efficiency | m | Head at best efficiency at rated speed. |
| Best efficiency | % | Best efficiency of the pump. |
| Rated speed | rpm | Rated speed of the pump. |
| System | ||
| Static head | m | Static lift plus pressure difference the system needs at zero flow. |
| Known system flow | L/min | A flow at which the system head is known. |
| System head at that flow | m | System head at that flow. |
| Liquid density | kg/m³ | Density of the liquid. |
| Duty | ||
| Target flow | L/min | Flow you need. |
| Hours per year at this flow | h | Hours per year the pump runs at the target flow. |
| Electricity price | $/kWh | Electricity price. |
| Least speed | % of rated | Lowest speed you allow (cooling, seal and stability limits). |
Method and formulas
- Pump curve H = s² H₀ − a Q² with a = (H₀ − H_b) ÷ Q_b²; system curve H = H_s + k Q²; speed ratio for the target s = √[(H_s + (a + k) Q_t²) ÷ H₀] (as the pump operating point tool).
- Efficiency from the curve relative to the best-efficiency flow at that speed (Q_b s); power = ρ g Q H ÷ η.
- Throttled at full speed: pump head H₀ − a Q_t² at efficiency for Q_t ÷ Q_b; saving = throttled − speed-controlled power.
- Example: run at 1,223.9 rpm (84.41%) for 300 L/min using 1.986 kW instead of 3.0945 kW throttled, saving 1,108.5 $ a year.
Limitations
- Quadratic pump and system curves and a simple efficiency curve; use the maker’s curves for important decisions.
- Drive and motor losses at part speed are not included.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Justify a variable speed drive on a pump
- Find the speed for a reduced flow
- Compare throttling and speed control
Who it is for
- Energy managers
- Plant engineers
- Maintenance engineers
What you get
- Speed for the target flow
- Power with speed control and with throttling
- Yearly saving
Interpreting the result
- Speed control saves most where friction head dominates; with high static head the saving is smaller.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3021–3027. Topic reference; no source tables or recommended settings are reproduced.
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Open the Pump Speed Control Plan (Speed for Target Flow, Power, Saving over Throttling) calculator
