BIZOMAT 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). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Pump at rated speed
What this means: Shut-off head
- What it is
- The head the pump develops at zero flow (shut-off) when running at rated speed, read from the maker’s pump curve.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 5000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read it at rated speed and for the impeller diameter actually fitted.
What this means: Best-efficiency flow
- What it is
- The flow at the pump’s best-efficiency point at rated speed, from the maker’s pump curve.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 10000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter litres per minute; convert from m³/h by multiplying by 1000 and dividing by 60.
What this means: Head at best efficiency
- What it is
- The head the pump develops at its best-efficiency flow at rated speed, from the pump curve.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 5000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be below the shut-off head.
What this means: Best efficiency
- What it is
- The pump’s efficiency at its best-efficiency point, from the maker’s curve.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 5% to 95%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both powers, and so the power saving, vary in inverse proportion to it.
- Get it right
- Type the percentage itself (70 for 70 %). Use pump efficiency, not motor or drive efficiency.
What this means: Rated speed
- What it is
- The rated speed of the pump at which the pump curve was drawn.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The speed for the target flow is a fixed share of this rated speed.
- Get it right
- Use the speed on the curve, which for a motor-driven pump is the motor’s rated speed under load.
System
What this means: Static head
- What it is
- The head the system needs at zero flow: the static lift from suction to discharge level plus any pressure difference between them, as head.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 5000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Include any back-pressure at the delivery point converted to metres of liquid.
What this means: Known system flow
- What it is
- A flow at which the system head is known, from a measurement or system calculation, used to fit the system curve.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 10000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Pick a reliable measured or calculated point, ideally near the target flow.
What this means: System head at that flow
- What it is
- The total system head at that known flow, static head plus friction losses.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 5000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be above the static head.
What this means: Liquid density
- What it is
- The density of the liquid being pumped at its pumping temperature.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 20000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both powers and the saving rise in direct proportion to density.
Duty
What this means: Target flow
- What it is
- The reduced flow you need from the pump, which speed control or throttling must deliver.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 10000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher target flow needs a higher pump speed.
- Get it right
- Use the same flow unit as the other flows, litres per minute.
What this means: Hours per year at this flow
- What it is
- The number of hours per year the pump runs at the target flow.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 8784 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The yearly saving rises in direct proportion to running hours.
- Get it right
- Count only hours at this flow, from run-time records or the duty plan.
What this means: Electricity price
- What it is
- The price you pay for each kilowatt-hour of electricity, from your electricity bill or contract.
- Units and format
- Enter a number in $/kWh: money per kilowatt-hour. The form accepts $0/kWh to $100/kWh. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $0.25/kWh as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The yearly saving rises in direct proportion to the price.
- Get it right
- Use the average energy rate for the hours the pump runs; demand charges are not included.
What this means: Least speed
- What it is
- The lowest speed you allow, as a share of rated speed, to protect motor cooling, seals and stable pumping.
- Units and format
- Enter a number in % of rated: % of rated. The form accepts 10 % of rated to 100 % of rated. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 % of rated as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the pump and motor makers; type the percentage itself.
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