Specific speed is one of the first parameters that a centrifugal pump designer looks at when evaluating a pump application. Specific speed may be used to rapidly determine the most feasible designs for the service conditions. Combined with some knowledge about the trade-offs associated with various pump selections, it becomes a great tool for quickly making some basic design choices.
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Dale B. Andrews - Editor |
Printable Version |
Specific Speed (Ns), or sometimes (NsQ), is often treated as a dimensionless number that represents the physical design of an impeller regardless of pump size. The equation for specific speed is:
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Where:
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n = rotative speed in revolutions per minute
Q = Pump flow at the best efficiency point
H = pump differential head at the best efficiency point
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US customary units |
SI Units |
| Flow (Q) |
Gallons per minute |
cubic meters per second |
| Head (H) |
feet of head |
meters of head |
| Note: Dividing the US units by 51.64 will yield the SI equivalent specific speed.
In a constant speed pump, flow rate is largely determined by the area at the pump inlet, and head is determined by impeller diameter.
High Ns pump impellers have inlet diameters (D1) that approach or equal the outlet diameter (D2), and relatively large open flow passages. Low Ns pump impellers have outlet diameters (D2) that are much larger than the inlet diameters (D1) and relatively narrow flow passages.
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High Ns |
Low Ns |
| The chart shown below is commonly used as a reference and shows the relationship between Ns and impeller style.
Impeller design is not so much a choice of the designer as it is a result of the application parameter considerations and selection of operating speed.
For Example:
An application calls for 1000 GPM (0.062 M3/sec) at 200 ft. (62 M) TDH.
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Where:
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| Substituting values for "n" yields very different impeller design options. |
n = 1200
RPM
Ns = 714
(SI = 14) |
n = 1800
RPM
Ns = 1070
(SI = 21) |
n = 3600
RPM
Ns = 2140
(SI = 41) |
| Considerations such as the presence of abrasive solids, solids passing capability, efficiency, suction conditions (NPSHA), equipment cost, and desired reliability all come into play when selecting the desired rpm.
This table provides a simple set of guidelines relating to pump RPM and Ns. Which pump is the right choice depends on how all of these factors weigh into your specific pump application. |
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RPM & NsConsiderations |
| Abrasives |
Lower RPM is better. Wear life for sliding abrasion varies roughly as (rpm1/rpm2)2.5 factor. |
| Solids Passing |
Higher Ns pump impellers have more openness to the blade passages than lower Ns Pumps. It is not uncommon to be able to "see through" from the OD to the eye on axial and mixed flow impellers. |
| Efficiency |
Efficiency tends to increase with Ns up to around 2500-3000 and then decrease with further increasing Ns. (More on this in our next newsletter.) |
| Cavitation |
Lower RPM pumps generally have lower NPSH requirements and a broader allowable operating range than higher RPM pumps. Low NPSH pumps are also often low Ns, as both flow and rpm are key factors in NPSHR determination. |
| Capital costs |
Higher RPM pumps are physically smaller than lower RPM pumps for the same duty point. Both motors and pumps are often less expensive and occupy a smaller footprint than their lower speed counter-parts. |
| Reliability |
Higher RPM pumps are often less tolerant to difficult service conditions than lower speed counterparts. Factors such as solids, cavitation, and off design-point operation may severely impact reliability. Any capital cost benefit may be eliminated by pre-mature equipment failure. For difficult applications, especially where high horsepower is involved, it is good practice to be conservative with equipment speed. |
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