Specific speed (Ns) is a dimensionless number, or index, that identifies the

hydraulic similarity of pumps. Pumps with practically the same Ns, even of

different make or size, are considered hydraulically similar - one pump being a

hydraulic size-factor or model of the other.

[An old approach defined Ns as the speed in RPM at which a pump, if sufficiently reduced in

size, would deliver a flow rate of one gallon per minute, at one foot of differential head. The

definition is useless, has no practical application, and simply a poor attempt at restating its

equation. In fact, Ns is not a unit of speed and its equation has inconsistent units, and thus is

considered dimensionless.]

The symbol Ns comes from the old practice of using**N** to designate speed in

number of revolutions and the subscript**s** stands for "specific".

Pump specific speed (Ns) is calculated from the equation:

**Ns = [N x Q^0.50] / [H^0.75]**

where:

N = pump speed, in RPM

Q = capacity at best efficiency point (BEP) at maximum impeller diameter, in GPM

H = head at BEP at maximum impeller diameter, in FT; in multistage pump, the

head is the head per stage

For simplicity the U.S. customary units are used in this article.

**Example:**

What is the specific speed of a two-stage pump whose capacity and total head

at BEP is 400 GPM, and 200 FT, respectively at 1780 RPM?

**Solution:**

Ns = [1780 x (400)^0.50 / (200/2)^0.75] = 1126

**Importance of pump specific speed**

The concept of pump specific speed has several significant and practical

applications:

Specific speed identifies the type of pump according to its design and flow

pattern. According to this criterion a pump can be classified as radial flow, mixed

flow, or axial flow type. A radial flow pump is one where the impeller discharges

the liquid in the radial direction from the pump shaft centerline, an axial flow

pump discharges the liquid in the axial direction and a mixed flow pump is one

that is a cross between a radial and an axial flow pump design.

Specific speed identifies the approximate acceptable ratio of the impeller eye

diameter (D1) to the impeller maximum diameter (D2) in designing a good

impeller.

Ns: 500 to 4000; D1/D2 < 0.5 - radial flow pump

Ns: 4000 to 8000; D1/D2 > 0.5 - mixed flow pump

Ns: 8000 to 12000; D1/D2 = 1 - axial flow pump

Francis vane impellers are of the radial flow type.

The figures for Ns and D1/D2 ratio are not restrictive and there can be a big

amount of overlap in the numbers as pump designers push the operating range

envelope of the different types of pumps.

The types of pumps are also indicative of the manner energy is imparted by the

impeller into the liquid.

In radial flow type, the pressure is developed by the centrifugal action of the

impeller. In mixed flow type, the pressure is developed mainly by the centrifugal

force in combination with the lifting action of the impeller. And in axial flow type,

the pressure is developed solely by the lifting action of the impeller.

Specific speed is used in designing a new size pump by modeling, or

size-factoring, a smaller pump with the same specific speed, or within the range

of + or - 10% of the specific speed. The performance and construction of the

smaller pump are used to predict the performance, and to model the

construction, of the new pump.

The specific speed is also a good indicator of pump efficiency. Over the years

charts have been developed showing plots of average pump efficiency versus

pump specific speed. These charts are valuable tool in comparing pump

efficiencies - whether a competitive pump is inferior, efficiency-wise, with another

pump, or whether a particular pump shows an usually high efficiency whose

accuracy might be doubtful.

**Rule-of-Thumb:** For similar pumps with about the same capacity at BEP, the

pump with the higher specific speed will typically have a higher efficiency also.

**Discharge specific speed**

Persons familiar with the term suction specific speed (Nss) know that Nss is

affected by parameters on the suction side of a pump, such as the impeller eye

area and eye diameter, the suction nozzle size, the suction area development of

the casing, etc., hence the word suction in suction specific speed.

Similarly, the specific speed (Ns) of a pump is mainly affected by such factors as

the impeller width (or impeller BA), the volute throat area, and by the discharge

nozzle size.

To a lesser degree, Ns is also effected by the number of impeller vanes and

their discharge angle. In short, Ns is affected by parameters on the discharge

side of the pump, hence CENTRIFUGALPUMP.COM and The EMA Project refers

to it as**discharge specific speed** to highlight its difference from **suction **

specific speed.

Application of specific speed in pump selection

There are many ways the concept of specific speed can be applied in pump

selection.

**Here is one example:**

A plant has process requirement for 3,000 gallons per minute (GPM), and 900

feet head. It was estimated that a pump of this size, operating at 3560 RPM, will

require a 1,000 HP motor driver based on assumed pump efficiency of 80%. The

plant has two spare 500 HP motors and control panels that they want to use so

they would like to buy two smaller pumps. (Besides, the lead time for buying a

new 1,000 HP motor is well beyond their start-up schedule.)How should one go

about selecting the two pumps on the basis of these facts alone, assuming all

other factors have equal weight and are to be ignored?

**Solution:**

The two pumps can be selected to operate either is series, or in parallel

connection. For pumps to operate in series, each pump will be rated for 3,000

GPM and 450 feet head. The pump specific speed is:Ns = [ 3,560 x

(3,000)^0.50] / [ (450)^0.75] = 1996For pumps to operate in parallel, each pump

will be rated for 1,500 GPM and 900 feet head. The pump specific speed is:Ns =

[ 3,560 x (1,500)^0.50] / [ (900)^0.75] = 839

For simplicity, CENTRIFUGALPUMP.COM uses the U.S. system of units in the

calculations through-out this web site .Many charts have been published for

estimating the efficiency of pumps based on their flow rates and specific speed.

It was observed that no matter how well-designed the pumps are their peak

efficiency is still correlated to their hydraulic size. (Some would call it the

hydrodynamic size.)Based on using one of those efficiency charts (*) , it is

estimated that a pump with a flow rate of 3000 GPM and NS of 1996 would have

an efficiency of 86%. In comparison, a pump with a flow rate of 1500 GPM and

Ns of 839 would have an efficiency of 77% - a significant difference of 9 points.

Expectedly, different efficiency charts will show different values of estimated

efficiencies but the point to keep in mind is the significant relative difference in

the efficiencies.(*) a copy of a typical efficiency chart can be requested from

www.centrifugalpump.com.

**Application of specific speed in hydraulic re-rates**

There are many ways the concept of specific speed can be applied in hydraulic

re-rates. Here is one example:

An engineering firm was doing a feasibility study on using a radial flow ,

20x20x22 (*) pump, in a flood control system. The pump was originally rated for

20,000 gallons per minute (GPM), and 400 feet head, at 1780 RPM. The

proposed new rated conditions are 40,000 GPM, 200 feet head, using some

existing motors.(*) 20" suction nozzle x 20" discharge nozzle x

22" maximum impeller diameter pump size. The firm contacted the original

vendor (A) to study the feasibility of the hydraulic re-rate. Engineers from

vendor A reviewed the operating conditions, pulled out some drawings, reviewed

some test curves, and after two days came back with the conclusion that the

re-rate is not feasible. Not satisfied with that answer, the firm contacted pump

vendor B and made the same inquiry. Within 15 minutes of receiving the inquiry

its engineer come back with the same conclusion that the re-rate is not

doable.Questions:1. Why was the hydraulic re-rate not feasible?2.

Why did it take two days for vendor A, but only 15 minutes for vendor B, to

arrive at that conclusion? The simple answers: specific speed! And, apparently,

the engineer of vendor B knows how to use that concept to respond quickly to

its customer inquiry. Let us analyze the situation and, for simplicity, let us

assume that the pump should be operating close to its best efficiency point

(BEP) at both original and proposed re-rate conditions.Based on its original

rated conditions, the pump specific speed would have been:Ns = [1,780 x

(20,000)^0.50] / [(400)^0.75] = 2,814^ is used here as an exponential

symbol.This value of specific speed confirms that the pump is of radial flow

design, even if the actual NS deviates slightly from this value depending on the

actual location of its BEP.To meet the re-rate conditions, the pump specific

speed (Ns) should be:

Ns = [1,780 x (40,000)^0.50] / [(200)^0.75] = 6,694

Even if a slight deviation from this Ns value is allowed to account for the actual

location of its BEP, when re-rated, this Ns value indicates that it will require a

pump of mixed flow design to meet the re-rate conditions. There is simply no way

a radial flow pump can be modified to become one of a mixed-flow design. The

engineer from vendor A failed to realize this and wasted valuable time to review

and made some lay-outs on something whose result is quite obvious to the

engineer from vendor B.

It is, of course, very simplistic to turn down a potential business opportunity on

one factor alone so such conclusion should be validated in some other way. In

this situation, one way of validating it is to estimate the impeller diameter

required to meet both the original and the re-rate conditions. The impeller

diameter required to develop a certain head can be estimated roughly from the

equation:

D = [ (3,377,200 x H) / (N)^2 ]^0.50

Where:

D = required impeller diameter, in inches

H = developed head, in feet

N = pump speed, in RPM

The derivation of this equation is available on request from

www.centrifugalpump.com.

For the original rated condition, the impeller diameter required to develop 400

feet head is approximately:

D = [ (3,377,200 x 400) / (1,780)^2 ]^0.50 = 20.6

This is approximately 93.6% of the 22 maximum impeller diameter (or,

20.6/22=0.936)

For the re-rate condition, the impeller diameter required to develop 200 feet

head is approximately:

D = [ (3,377,200 x 200) / (1,780)^2 ]^0.50 = 14.6

This is approximately 66.4% of the 22 maximum impeller diameter (or,

14.6/22=0.664).

The above figures indicate that, even assuming for the sake of discussion, the

pump could be converted from being a radial flow type into a mixed flow type,

still the impeller diameter required to meet the reduced head would fall below the

acceptable minimum diameter for the pump.

**Rule-of-thumb**: Typical acceptable minimum impeller diameter, as a

percentage of maximum diameter, for various pump types are: radial flow = 80%,

mixed flow = 85%, axial flow = 90%

Application of specific speed in new pump design

There are many ways the concept of specific speed can be applied in new pump

design. Here is one example:

A pump manufacturer was requested to submit a proposal for the design,

manufacture, testing, and installation of four identical single stage pumps. Each

pump will be rated for 95,000 gallons per minute (GPM) and 1500 feet of head,

at 1780 RPM. The manufacturer has not yet built a pump of this size. Given the

short time frame with which to submit a proposal, there was no sufficient time to

do an initial design concept, predict and simulate a predicted performance, and

do some CFD analysis to validate the hydraulic design.

So how did it respond to the inquiry? By size-factoring, or modeling, an existing

and proven pump design based on the concept of specific speed. The

application calls for pumps with a specific speed (Ns) of:

Ns = [ 1,780 x (95,000)^0.50] / [ (1,500)^0.75] = 2,276

Next, the manufacturer checked its existing same type product line for the

biggest pump it has ever built whose specific speed is within + or - minus 5% of

the calculated Ns (or, Ns of 2160 to 2390.) It found a smaller but similar pump

type, a 28x30x30 pump, with a specific speed of 2200. Using this pump size as a

model, and applying the principles of size-factoring, it was able to offer a newly

designed pump, a size 40x40x42, complete with predicted performance,

preliminary linear dimensions, and estimated component weights. The

procedures for size-factoring will not be discussed in this article but is available

on request. The intent of this article is mainly to highlight the application of the

specific speed concept in new pump design.

Text below in red color is omitted:

Hydraulic considerations

Some hydraulic considerations apply to the impeller and volute design based on

specific speed.

On the impeller side, the specific speed dictates the D1/D2 ratio of the impeller,

as does whether the impeller is going to be of radial, mixed flow, or axial flow

design.

On the volute side, the specific speed dictates the recommended volute B-gap,

as does the area ratio and diffusion angle at the diffusion chamber of the volute.

[This is a raw article that is undergoing revision. Please bookmark this page and come back

later to read the latest update.]

hydraulic similarity of pumps. Pumps with practically the same Ns, even of

different make or size, are considered hydraulically similar - one pump being a

hydraulic size-factor or model of the other.

[An old approach defined Ns as the speed in RPM at which a pump, if sufficiently reduced in

size, would deliver a flow rate of one gallon per minute, at one foot of differential head. The

definition is useless, has no practical application, and simply a poor attempt at restating its

equation. In fact, Ns is not a unit of speed and its equation has inconsistent units, and thus is

considered dimensionless.]

The symbol Ns comes from the old practice of using

number of revolutions and the subscript

Pump specific speed (Ns) is calculated from the equation:

N = pump speed, in RPM

Q = capacity at best efficiency point (BEP) at maximum impeller diameter, in GPM

H = head at BEP at maximum impeller diameter, in FT; in multistage pump, the

head is the head per stage

For simplicity the U.S. customary units are used in this article.

at BEP is 400 GPM, and 200 FT, respectively at 1780 RPM?

applications:

Specific speed identifies the type of pump according to its design and flow

pattern. According to this criterion a pump can be classified as radial flow, mixed

flow, or axial flow type. A radial flow pump is one where the impeller discharges

the liquid in the radial direction from the pump shaft centerline, an axial flow

pump discharges the liquid in the axial direction and a mixed flow pump is one

that is a cross between a radial and an axial flow pump design.

Specific speed identifies the approximate acceptable ratio of the impeller eye

diameter (D1) to the impeller maximum diameter (D2) in designing a good

impeller.

Ns: 500 to 4000; D1/D2 < 0.5 - radial flow pump

Ns: 4000 to 8000; D1/D2 > 0.5 - mixed flow pump

Ns: 8000 to 12000; D1/D2 = 1 - axial flow pump

Francis vane impellers are of the radial flow type.

The figures for Ns and D1/D2 ratio are not restrictive and there can be a big

amount of overlap in the numbers as pump designers push the operating range

envelope of the different types of pumps.

The types of pumps are also indicative of the manner energy is imparted by the

impeller into the liquid.

In radial flow type, the pressure is developed by the centrifugal action of the

impeller. In mixed flow type, the pressure is developed mainly by the centrifugal

force in combination with the lifting action of the impeller. And in axial flow type,

the pressure is developed solely by the lifting action of the impeller.

Specific speed is used in designing a new size pump by modeling, or

size-factoring, a smaller pump with the same specific speed, or within the range

of + or - 10% of the specific speed. The performance and construction of the

smaller pump are used to predict the performance, and to model the

construction, of the new pump.

The specific speed is also a good indicator of pump efficiency. Over the years

charts have been developed showing plots of average pump efficiency versus

pump specific speed. These charts are valuable tool in comparing pump

efficiencies - whether a competitive pump is inferior, efficiency-wise, with another

pump, or whether a particular pump shows an usually high efficiency whose

accuracy might be doubtful.

pump with the higher specific speed will typically have a higher efficiency also.

affected by parameters on the suction side of a pump, such as the impeller eye

area and eye diameter, the suction nozzle size, the suction area development of

the casing, etc., hence the word suction in suction specific speed.

Similarly, the specific speed (Ns) of a pump is mainly affected by such factors as

the impeller width (or impeller BA), the volute throat area, and by the discharge

nozzle size.

To a lesser degree, Ns is also effected by the number of impeller vanes and

their discharge angle. In short, Ns is affected by parameters on the discharge

side of the pump, hence CENTRIFUGALPUMP.COM and The EMA Project refers

to it as

specific speed.

Application of specific speed in pump selection

selection.

feet head. It was estimated that a pump of this size, operating at 3560 RPM, will

require a 1,000 HP motor driver based on assumed pump efficiency of 80%. The

plant has two spare 500 HP motors and control panels that they want to use so

they would like to buy two smaller pumps. (Besides, the lead time for buying a

new 1,000 HP motor is well beyond their start-up schedule.)How should one go

about selecting the two pumps on the basis of these facts alone, assuming all

other factors have equal weight and are to be ignored?

connection. For pumps to operate in series, each pump will be rated for 3,000

GPM and 450 feet head. The pump specific speed is:Ns = [ 3,560 x

(3,000)^0.50] / [ (450)^0.75] = 1996For pumps to operate in parallel, each pump

will be rated for 1,500 GPM and 900 feet head. The pump specific speed is:Ns =

[ 3,560 x (1,500)^0.50] / [ (900)^0.75] = 839

For simplicity, CENTRIFUGALPUMP.COM uses the U.S. system of units in the

calculations through-out this web site .Many charts have been published for

estimating the efficiency of pumps based on their flow rates and specific speed.

It was observed that no matter how well-designed the pumps are their peak

efficiency is still correlated to their hydraulic size. (Some would call it the

hydrodynamic size.)Based on using one of those efficiency charts (*) , it is

estimated that a pump with a flow rate of 3000 GPM and NS of 1996 would have

an efficiency of 86%. In comparison, a pump with a flow rate of 1500 GPM and

Ns of 839 would have an efficiency of 77% - a significant difference of 9 points.

Expectedly, different efficiency charts will show different values of estimated

efficiencies but the point to keep in mind is the significant relative difference in

the efficiencies.(*) a copy of a typical efficiency chart can be requested from

www.centrifugalpump.com.

re-rates. Here is one example:

An engineering firm was doing a feasibility study on using a radial flow ,

20x20x22 (*) pump, in a flood control system. The pump was originally rated for

20,000 gallons per minute (GPM), and 400 feet head, at 1780 RPM. The

proposed new rated conditions are 40,000 GPM, 200 feet head, using some

existing motors.(*) 20" suction nozzle x 20" discharge nozzle x

22" maximum impeller diameter pump size. The firm contacted the original

vendor (A) to study the feasibility of the hydraulic re-rate. Engineers from

vendor A reviewed the operating conditions, pulled out some drawings, reviewed

some test curves, and after two days came back with the conclusion that the

re-rate is not feasible. Not satisfied with that answer, the firm contacted pump

vendor B and made the same inquiry. Within 15 minutes of receiving the inquiry

its engineer come back with the same conclusion that the re-rate is not

doable.Questions:1. Why was the hydraulic re-rate not feasible?2.

Why did it take two days for vendor A, but only 15 minutes for vendor B, to

arrive at that conclusion? The simple answers: specific speed! And, apparently,

the engineer of vendor B knows how to use that concept to respond quickly to

its customer inquiry. Let us analyze the situation and, for simplicity, let us

assume that the pump should be operating close to its best efficiency point

(BEP) at both original and proposed re-rate conditions.Based on its original

rated conditions, the pump specific speed would have been:Ns = [1,780 x

(20,000)^0.50] / [(400)^0.75] = 2,814^ is used here as an exponential

symbol.This value of specific speed confirms that the pump is of radial flow

design, even if the actual NS deviates slightly from this value depending on the

actual location of its BEP.To meet the re-rate conditions, the pump specific

speed (Ns) should be:

Ns = [1,780 x (40,000)^0.50] / [(200)^0.75] = 6,694

Even if a slight deviation from this Ns value is allowed to account for the actual

location of its BEP, when re-rated, this Ns value indicates that it will require a

pump of mixed flow design to meet the re-rate conditions. There is simply no way

a radial flow pump can be modified to become one of a mixed-flow design. The

engineer from vendor A failed to realize this and wasted valuable time to review

and made some lay-outs on something whose result is quite obvious to the

engineer from vendor B.

It is, of course, very simplistic to turn down a potential business opportunity on

one factor alone so such conclusion should be validated in some other way. In

this situation, one way of validating it is to estimate the impeller diameter

required to meet both the original and the re-rate conditions. The impeller

diameter required to develop a certain head can be estimated roughly from the

equation:

D = [ (3,377,200 x H) / (N)^2 ]^0.50

Where:

D = required impeller diameter, in inches

H = developed head, in feet

N = pump speed, in RPM

The derivation of this equation is available on request from

www.centrifugalpump.com.

For the original rated condition, the impeller diameter required to develop 400

feet head is approximately:

D = [ (3,377,200 x 400) / (1,780)^2 ]^0.50 = 20.6

This is approximately 93.6% of the 22 maximum impeller diameter (or,

20.6/22=0.936)

For the re-rate condition, the impeller diameter required to develop 200 feet

head is approximately:

D = [ (3,377,200 x 200) / (1,780)^2 ]^0.50 = 14.6

This is approximately 66.4% of the 22 maximum impeller diameter (or,

14.6/22=0.664).

The above figures indicate that, even assuming for the sake of discussion, the

pump could be converted from being a radial flow type into a mixed flow type,

still the impeller diameter required to meet the reduced head would fall below the

acceptable minimum diameter for the pump.

percentage of maximum diameter, for various pump types are: radial flow = 80%,

mixed flow = 85%, axial flow = 90%

Application of specific speed in new pump design

There are many ways the concept of specific speed can be applied in new pump

design. Here is one example:

A pump manufacturer was requested to submit a proposal for the design,

manufacture, testing, and installation of four identical single stage pumps. Each

pump will be rated for 95,000 gallons per minute (GPM) and 1500 feet of head,

at 1780 RPM. The manufacturer has not yet built a pump of this size. Given the

short time frame with which to submit a proposal, there was no sufficient time to

do an initial design concept, predict and simulate a predicted performance, and

do some CFD analysis to validate the hydraulic design.

So how did it respond to the inquiry? By size-factoring, or modeling, an existing

and proven pump design based on the concept of specific speed. The

application calls for pumps with a specific speed (Ns) of:

Ns = [ 1,780 x (95,000)^0.50] / [ (1,500)^0.75] = 2,276

Next, the manufacturer checked its existing same type product line for the

biggest pump it has ever built whose specific speed is within + or - minus 5% of

the calculated Ns (or, Ns of 2160 to 2390.) It found a smaller but similar pump

type, a 28x30x30 pump, with a specific speed of 2200. Using this pump size as a

model, and applying the principles of size-factoring, it was able to offer a newly

designed pump, a size 40x40x42, complete with predicted performance,

preliminary linear dimensions, and estimated component weights. The

procedures for size-factoring will not be discussed in this article but is available

on request. The intent of this article is mainly to highlight the application of the

specific speed concept in new pump design.

Text below in red color is omitted:

Hydraulic considerations

Some hydraulic considerations apply to the impeller and volute design based on

specific speed.

On the impeller side, the specific speed dictates the D1/D2 ratio of the impeller,

as does whether the impeller is going to be of radial, mixed flow, or axial flow

design.

On the volute side, the specific speed dictates the recommended volute B-gap,

as does the area ratio and diffusion angle at the diffusion chamber of the volute.

[This is a raw article that is undergoing revision. Please bookmark this page and come back

later to read the latest update.]

O P Q R S T U V W X Y Z

article/s shall not be reprinted or republished, in whole or in part, in any manner

or form, without the written permission of the author. To obtain permission

please contact: admin@centrifugalpump.com .