pump binder articles

7/30/2019 Pump Binder articles

1/31

2005 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Webhttp://www.lawrencepumps.com Contact:[email protected]

Page 1 of 3

October 2005 vol.2_iss 10

Often, some of the most costly problems are the result of the mostfundamental errors. Recently, while reviewing some of our past issues, Irealized that we have not discussed some of the fundamentals of pumpoperation. Like many things, a thorough understanding of basic principles isoften a key to successfully solving complicated problems. Therefore, in thenext few issues, well present some of the basics of centrifugal pumpoperation. As always, we welcome your feedback and comments.

Dale B. Andrews Editor

A centrifugal pump is a kinetic device. Liquid entering the pump receiveskinetic energy from the rotating impeller. The centrifugal action of the impelleraccelerates the liquid to a high velocity, transferring mechanical (rotational)energy to the liquid. That kinetic energy is available to the fluid to accomplishwork. In most cases, the work consists of the liquid moving at some velocitythrough a system by overcoming resistance to flow due to friction from pipes,and physical restrictions from valves, heat exchangers and other in-line devices,as well as elevation changes between the liquids starting location and finaldestination. When velocity is reduced due to resistance encountered in the

system, pressure (P) increases. As resistance is encountered, the liquid expendssome its energy in the form of heat, noise, and vibration in overcoming thatresistance. The result is that the available energy in the liquid decreases as thedistance from the pump increases. The actual energy available for work at anypoint in a system is a combination of the available velocity and pressure energyat that point.

HeadHead (H) is the term that is used to define the energysupplied to the liquid by the pump. It is independent ofthe type of liquid being pumped. Head is expressed inFeet or Meters. In the absence of any velocity, it is equal tothe height of a static column of liquid that could besupported by the pressure (P) at a given point in thesystem. In practice, pressure is usually measured by apressure sensing device such as a gage or pressuretransducer.
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


2/31

2005 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Web http://www.lawrencepumps.com Contact:[email protected]

Page 2 of 3

Head (H) is the ratio of pressure to the Density (Specific weight) of a liquid. Forwater at 60oF, head (H) may be calculated, from a pressure reading, using thefollowing equation:

ft

ftin

inlbs

lbsessure

H

3

2

2

2

4.62

144)(Pr

=

This may be simplified to H = P * 2. 31. The units cancel out so that only feetremain. For a liquid with a Density other than water, divide by the specificgravity of the liquid.1

..

31.2

GrSp

PH

=

Because specific gravity is an index number (dimensionless) the units remain asfeet of head.2

Flow Rate

Flow rate is determined by the impeller geometry and itsrotational speed. Pump designers manipulate the impellervane design to achieve an optimum throughput velocity3 for

an impeller. The throughput velocity (ft/sec) multiplied bythe usable area of the impeller inlet (ft2) yields the flow rate(ft3/sec). Every impeller has one optimum design flow ratefor a given speed and diameter. This is the best efficiency

point of the pump. At other flow rates there will be a mismatch between thevane angle at the pump inlet and the flow rate, resulting in increased turbulenceand loss of efficiency within the pump.

Total Dynamic HeadTotal Dynamic Head (TDH) is the difference in head between the pump outlet

and inlet. In actual practice, readings must be corrected for piping losses, gauge

1 Specific gravity is the ratio of the Density of a liquid to the Density of water @ 600F.2

In the SI system pressure Head is expressed in Meters (M) and Pressure is expressed inKilopascals (KPA). (PSI = KPA * 0.145). H = M

3/hr * Specific gravity/366

3 Also called the meridional velocity. It is the velocity of the liquid traveling within an impeller vane

passage from inlet to outlet.
http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


3/31

2005 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Webhttp://www.lawrencepumps.comContact:[email protected]

Page 3 of 3

location and differences in pipeline velocity between the measurement point sonthe pump inlet and outlet; all unique to the specific pump/system setup.

Pump EfficiencyPump efficiency is the ratio of hydraulic horsepower to the brake horsepower

required to drive the pump. Hydraulic horsepower is the kinetic poweravailable at the pump discharge. It is calculated by the following equation4.

3960

.)()( GRSPftTDHUSGPMFlowHHP

=

Brake horsepower is measured at the pump input shaft by a torque-metercoupling or similar device. The difference between brake horsepower andhydraulic horsepower is the amount of power consumed by mechanical losses,noise, heat, viscous drag, and internal recirculation. OurSept 2004issue

discusses this topic in greater detail.

Characteristic CurvesCurves are available from pumpmanufacturers that depict the as newperformance characteristics for any givenpump model. These may be either genericcatalog curves that represent typical values,or they may be test curves that show theactual performance of a customers

particular pump unit. Performance curvesshow plots of TDH, Efficiency, BHP and,when specified, NPSHR5 as functions offlow rate6.

The shape of a pump curve is primarily determined by the geometry of theimpeller. High flow - low head pumps typically have steeper curves than lowflow - high head units.

The performance of a pump when placed in a system is a function of theinteraction between the pump and system as defined by their relativecharacteristics. Next month, well discuss system basics, including start-upconditions, operation, and troubleshooting.

4 To obtain Kilowatts multiply HP * 0.7465

Net Positive Suction Head Required. The minimum absolute suction pressure required by the pump. Seethe Oct 2004 issue for a discussion of NPSHR.6 Characteristic curves are based on the pump performance while pumping clean water at 20oC (68oF).
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Sep.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Sep.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Sep.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i5_oct.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i5_oct.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Sep.htmlmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


4/31


Page 1 of 3

November 2005 vol.2_iss 11

In this months issue, we discuss system curves and how understanding

the relationship between pump and system is a useful tool in system

operation and troubleshooting.


In last months issue we discussed howevery pump has a unique set ofperformance characteristics. Pump curvesprovide a graphical representation of thesecharacteristics that typically display Flow,TDH, Brake power and Net PositiveSuction Head Required (NPSHR) . Singleline curves, such as the one shown here, arebased on a fixed impeller diameter at asingle speed. Most pump manufacturersalso provide multi-line curves that displaypump characteristics over either a range of

speeds or diameters.

The pump curve in Fig 1 is marked with a rated operating point flow rate of 50M3/hr flow and 50M Total Dynamic Head (TDH). This rated operating point,specified by the purchaser, is based on an analysis of the system where the pumpis to be used.

Systems take on an almost infinitenumber of forms, ranging from thesimple to the complex. However,regardless of the complexity, all

systems have some combination ofstatic pressure and frictionalresistance that will vary with fluidcharacteristics and velocity.

Fi 1

Fig 2
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i10_oct05.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i10_oct05.htmlmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


5/31


Page 2 of 3

Fig 2 is a simple schematic of a typicalsump pump system where a submergedpump is moving liquid through asystem of pipes and valves to anotherlocation1. In this example, we have a

submersible pump located 3 M belowthe liquid surface in an open pit. Thepump transfers water a distance of 380M through 3in pipe to another sump.The elevation change from the pumpimpeller to the discharge into the 2ndsump is 8M.

The amount of TDH required for any given flow may be plotted on a systemcurve (Fig 3).The net static head, at zero flow, is the difference between the 3 M

of submergence and 8 M of discharge head, or 5 M. The engineer can then usesoftware or published hydraulic tables to calculate the system head for anyparticular flow. For this application the total system resistance for our 50M3 flowrate is 50 M TDH made up of 5M static head, 4M of head losses in various valvesand fittings, 40M of head loss due to pipe friction, and about 0.5M of velocityhead associated with the kinetic energy of the moving water. The pumpproduces 50M TDH at a flow rate of 50M3/hr; therefore, the pump and thesystem will be matched. This is graphically depicted in fig 3 above.

At the moment when a pump starts, there

is little frictional resistance to flow (Fig 4).Friction builds as the piping system fills.The pump operates at a very high flowrate during the period that the system isfilling. In the example of our sump, thepump, at the moment of start-up, wouldhave almost zero resistance and wouldoperate at its right-hand most point ofnearly max flow and zero TDH.Manufacturers refer to this as the run-outpoint. As the system fills, resistance

would build until the design system curveis established. This is a characteristic ofall centrifugal pump start-ups. Centrifugal pumps that have power curves thatincrease with flow should be started with a throttled discharge to prevent a highcurrent condition associated with the combination of starting inrush current and

1 Fig 2 courtesy of ePUMP-FLOhttp://www.pumpflo.com/solutions

Fig 4

Fig 3
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://www.pumpflo.com/solutionshttp://www.pumpflo.com/solutionshttp://www.pumpflo.com/solutionshttp://www.pumpflo.com/solutionsmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


6/31

2005 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Webhttp://www.lawrencepumps.com Contact: [email protected]

Page 3 of 3

maximum pump load. The notable exception to this are axial flow pumps whichhave a power characteristic that decreases with an increase with flow. For thisreason, axial pumps should be started in a valve open condition .

It is important to remember that a pumping

system will always operate at the intersectionof the pump curve and the system curve. Thepump and system curve always intersect. Thisshould be kept in mind when troubleshooting.For example: Low flow in a system suggests thateither the system curve has shifted so as to crossthe pump curve at a lower flow, or the pumpperformance has degraded so as to intersect thesystem curve at a lower flow (Fig 5).

An obstruction to flow downstream of the pump would result in the systemcurve shifting to cross the pump curve at a lower flow and higher pressure thanthe rated point. If the pump dischargepressure corresponds to a low flow on theoriginal pump curve, an obstruction is agood possibility.

If the pump is momentarily run near shut-off2 , a TDH that is below the originalpump curve is indicative of pump wear

(Fi6 5). A TDH at or near the originalTDH, that falls off rapidly as thedischarge valve is opened, is indicative ofa blockage in the pump inlet (Fig 6).

Using the as built system and pumpcharacteristics, in conjunction with collection and careful analysis of operatingdata, can be a time and money saving tool when troubleshooting productionproblems.

Next month well look at systems that involve parallel pump operation and therequirements for continuously rising head curves.

2 Consult with the pump manufacturer before running this test to verify minimum flow rates. This test is

generally only viable for pumps below 150 kW.

Fi 5

Fig 6
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/


7/31

2004 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Web http://www.lawrencepumps.com Contact: [email protected]

Page 1 of 4

Oct 2004 vol.1_iss5

Cavitation is a recurring cause of premature pump failures that result

from related seal, bearing, and impeller damage. In this issue we

discuss the fundamentals of cavitation and the selection of pumps to

avoid it.


In our July newsletter on Inducers, we described the occurrence of cavitation in acentrifugal pump as follows:

Vapor bubbles form in a pump inlet whenever thelocal absolute pressure of the liquid falls below itsvapor pressure. These bubbles collapse rapidly andviolently when the local absolute pressure increasesdue to kinetic forces being imparted by the impeller.Cavitation is the rapid formation and collapse of thesevapor bubbles. Collapsing cavitation bubbles cause

noise, vibration, and erosion of material from theimpeller. Pump service life is shortenedsignificantly when cavitation occurs. The severityof the effects of cavitation varies as a function of amachine's horsepower. Fig. 1 shows a photographof a cavitation bubble implosion. Fig 2 shows animpeller that has been severely damaged bycavitation. Fig 3 is a diagrammatic view of thecavitation bubble implosion sequence.

Fig 3: Cavitation bubble implosion onto a solid surface, arrows indicate fluid pressure.

Fig 1


8/31


Page 2 of 4

For any given flow rate, every pump has an absolute suction head at whichcavitation will occur. This suction head is referred to as the Net Positive SuctionHead Required (NPSHR). Head is always expressed in feet or meters to make itindependent of any specific fluid. The absolute suction head available at thepump inlet is termed the Net Positive Suction Head available (NPSHA). To avoid

cavitation, the available NPSH must be greater than the required NPSH.

NPSHA is determined by subtracting the absolute vapor pressure of the fluidpumped from the total suction head available. Total suction head is the statichead (suction gage pressure) corrected to the impeller centerline (or impellerinlet if vertical), plus the velocity head (found in most pipe friction tables), plusatmospheric pressure. All values should be expressed in feet of liquid.

NPSHR is determined by hydraulictesting and is available from the pumpmanufacturer. Pump manufacturersperform a series of breakdown tests todetermine the NPSHR. The pump isoperated at a constant flow rate whilethe NPSHA is steadily decreased. Asudden drop in the total output head isevidence of cavitation. Industrystandards establish that a 3% drop intotal head as point where the NPSHRreading is taken.

It is important to note that an actual test curve showing NPSHR testresults reflects a pump that is cavitating.

To operate cavitation free, pumps need a margin of additional NPSH above thetest values. The amount of margin depends on the suction energy of the pump.Suction energy reflects energy available for cavitation damage, and it is a functionof the suction specific speed (S) of the pump.

NPSHR

QrpmS

43

=

Fig 4


9/31


Page 3 of 4

Chart 1 provides some basic guidelines on

determining whether a pump falls under high or low

suction energy.

Table 1 reflects the recommended margin that should

be maintained between the NPSHA and the NPSHR.

TABLE 1

NPSH MARGIN RATIO GUIDELINES (NPSHA/NPSHR)

SUCTION ENERGY LEVEL

Market Low High Very High

Petroleum 1.1a

1.3c

Chemical 1.1a

1.3c

Electric Power 1.1a

1.5c

2.0c

Nuclear Power 1.5b

2.0c

2.5c

Water/Waste Water 1.1

a

1.3

c

2.0

c

General Industry 1.1a

1.2c

Pulp and Paper 1.1a

1.3c

Building Trades 1.1a

1.3c

Cooling Towers 1.3b

1.5c

2.0c

Slurry 1.1a

Pipeline 1.3b

1.7c

2.0c

Water Flood 1.2b

1.5c

2.0c

a. or 2 feet whichever is greaterb. or 3 feet whichever is greaterc. or 5 feet whichever is greaterNote: Vertical turbine pumps often use a NPSH margin of 1.0 without damage, butwith slightly reduced discharge head.

Chart 1


10/31


Page 4 of 4

When selecting a pump, sufficient NPSH margin should be applied to cover the entire

operating region for the application. Very high suction energy level pumps are those withspeeds 1.5 to 2 times the levels shown in chart 1. These are approximate guidelines. The

pump manufacturer should be consulted where high suction energy levels are suspected.

Descriptions of each of the pump categories and the reasons for the selected margins can

be found in Hydraulic Institute Standard HI 9.6.1 1998.

Fig 1.Courtesy of Solid State Fusion technologies

Fig 2 & 3. Lohrberg, H., Stoffel B., Intelligent Maintenance Management of Pumps , Pump Users International Forum, 2000

Chart 1 & Table 1 Hydraulic Institute Standard HI 9.6.1 1998 Pump NPSH Margin


11/31

2006 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291

Web http://www.lawrencepumps.com Contact: [email protected]

Page 1 of 4

Aug 2006 vol.3_iss 8Net positive suction head available (NPSHA) is the total suction headavailable at the eye of the first stage impeller expressed in terms ofabsolute pressure. Pump manufacturers use NPSHA to select a pump

with a lower net positive suction head required (NPSHR) so as toavoid cavitation1. This issue covers how to determine NPSHA for a

new or existing pump installation.


(1) NPSHA = ha

hvp + hs = where:

ha = atmospheric headhvp = vapor pressure expressed as headhs = total suction head

Atmospheric Head (ha)Atmospheric pressure approximately equals 101.3 kPa.(14.7 psia) at sea level. The headassociated with this pressure is dependent on the specific gravity of the fluid being pumped. Theequation for determining head from any pressure reading is:

(2) h =..

102.0

GrSp

PkPaor h =

..

31.2

GrSp

psi

Vapor Pressure (hvp)Vapor pressure is the absolute pressure atwhich a liquid will boil for any giventemperature. For example, at sea levelthe boiling point of water is 100

oC (212

oF).

101.3 kPa is the vapor pressure of waterat its sea level boiling point. Vaporpressure vs. temperature properties offluids may be found in fluid propertytables.

Notice that the equation for NPSHA has

the term ha

hvp. For any boiling liquidthe resultant of this term is always zero.For boiling liquids the equation for NPSHAmay be simplified to:

NPSHA = hs

1 For more information on cavitation see our Oct 2004 Issue.


12/31



Page 2 of 4

Total Suction Head (hs)Total suction head is the total fluid head available to the eye of the first stage impeller. It is thecombination of static and dynamic heads corrected for losses and gauge location. The equationfor hs is:

(3) hs = +/-hgs + hvs +/-z - hfWhere:h

gsis the suction gage reading converted to

meters or feet of head. It is positive if it is aboveatmospheric pressure and negative if it is avacuum gage reading

2.

hvs is velocity head. Velocity head is the headassociated with the kinetic energy of the fluidmoving at some velocity(V) in the suction pipe,toward the impeller eye. Velocity head iscalculated from the equation:

(4) hvs = V2/2g

Where g is the acceleration due to gravity, 9.81 m/sec2

or 32.2 ft/sec2

The velocity through the standard piping for a givenflowrate may be found in most hydraulic tables orcalculated by dividing the flowrate by the crosssectional area of the piping.

3

z is a correction for the vertical distance between thecenter of the suction gage and the centerline of the1

ststage impeller. The value of z is expressed in

either meters or feet. It is positive if the gage islocated above the impeller centerline and negative iflocated below the impeller centerline.

Note: For a vertical pump with no suction gage, z is

the distance from the centerline of the impeller to thefluid free surface. It is negative if the fluid surface isbelow the impeller centerline and positive if the fluidsurface is above the impeller centerline.

hf represents friction and other losses associatedwith the piping and fittings located between thesuction gage pressure tap and the pump inlet or, inthe case of vertical pumps with tailpipes, lossesbetween the suction inlet and the impeller. hfforstandard pipe, valves, and fittings can be found inmost fluid tables. However, for most pumps, hf is arelatively small number and can be ignored if the

pressure taps are within a few feet of the pump andthe piping is relatively straight.

2 For conversion of vacuum readings to head: 1 mm hg = 0.0133 m (H 2O) , 1 in. hg = 1.13 ft (H2O)3 See ourJuly 2006 newsletter for more information on velocity head.


13/31



Page 3 of 4

Example 1 Horizontal Pump (300 mm Suction)Flow (Q) = 1000 m

3/hr

Suction Pressure hgs = 200 kPaFluid vapor pressure hvp = 50 kPaSp.gr. = .89Suction gage elevation z = 1 m above the pumpcenterlineSuction gage tap location 1 m from the pump inlet

From equation (1): NPSHA = ha hvp + hs

ha hvp =

89.0

102.0503.101 kPakPa= (+)5.9m

From Equation (3)hs = +/-hgs + hvs +/-z - hf

hgs =

89.0

102.0200 kPa= (+)22.9 m

Velocity = 3.9 m/sec

hv from equation (4) =

)secsec/81.9)(2(

sec)/9.3(2

m

m = (+)0.8 m

z = (+)1 m

hf may be disregarded due to the close proximity to the pump inlet.

hs = (+)22.9m + (+)0.8m + (+)1 m = (+)24.7mAdding in the result from ha hvp yields

NPSHA = (+)5.9 m + (+)24.7 m = 30.6 m

Example 2 Vertical Pump (300 mm Suction) - open sumpFluid level = 1 m below impellerFlow (Q) = 1000 m

3/hr

Suction Pressure hgs = (-)225 mm hgSuction velocity = 3.9 m/secFluid vapor pressure hvp = 50 kPaSp.gr. = .89Suction gage elevation z = 2 m above the impeller centerline,located at the coverplate

From equation (1): NPSHA = ha hvp + hs

ha hvp =

89.0

102.0503.101 kPakPa= (+)5.9m

From Equation (3)hs = +/-hgs + hvs +/-z - hf

hgs = (-)225 mm hg * 0.0133 m (H2O)/mm hg = (-)3 m


14/31



Page 4 of 4

hvs from equation (4) =

)81.9)(2( secsec/

sec)/9.3(2

m

m = (+)0.8 m

z = (+)2 m

hf may be disregarded due to the close proximity to the pump inlet.

hs = (-)3m + (+)0.8m + (+) 2m = -0.2 m

Adding in the result from ha hvp yields

NPSHA = (+)5.9 m + (-)0.2 m = 5.7 m

For existing installations, investigating NPSHA is one of the first steps in investigating a cavitationproblem. In a new installation, designing a system with sufficient NPSHA so as to accommodatea pump with a reasonable suction specific speed (S)

4will significantly lower the life cycle cost of

the installed equipment.

4 Refer to ourOct 2004 newsletter for a discussion of NPSH margin, cavitation and suction specific speed.


15/31



Page 1 of 2

Sep 2006 vol.3_iss 9What happens to the net positive suction head required (NPSHR)when an impeller is trimmed? Does it change? In this months issuewe will discuss impeller diameter changes, and the impact on a

pumps NPSHR.


NPSHR is the amount of absolute suction pressure that the pump manufacturer specifies to be

made available to the pump inlet so as to avoid damaging levels of cavitation1

. The interaction ofthe physical geometry of the pump inlet, inclusive of the casing, impeller, and all associatedwetted parts within the inlet field of flow determines the NPSHR characteristic of a pump. Thevalue of NPSHR for any centrifugal pump is determined through performance testing. FromNPSH test data, Suction Specific Speed (S) is calculated using the following equation, where Qrepresents flow at the best efficiency point of the pump.

In can be seen in the above equation, that NPSHR should not change, with changes in impeller

diameter, as long as flow and RPM remain constant. There is no factor in the S equation thatrelates to impeller diameter. Suction specific speed (S) remains constant, for any defined inletgeometry, as long as the field of flow into the impeller eye is not disrupted by events taking placedownstream of the impeller inlet.

The accepted standard for minimum NPSHRmeasurement is the absolute suction pressureat which a 3% drop in the total dynamic head(TDH) occurs, under conditions of constantflow and speed. This head drop occursbecause cavitation restricts the inlet flowpassages of the impeller. In order to maintaina constant flow-rate around the restricted vanepassage, relative fluid velocity in the impellerinlet increases, while relative fluid velocity atthe impeller exit remains unchanged. Thisresults in a decrease in the total headdeveloped by the impeller.

1 For a discussion of cavitation and NPSHR see ourOctober 2004 issue.


16/31



Page 2 of 2

When trimming impellers on pumps that are of a low specific speed (Ns < 30 SI, 1500 US), testshave shown that there is little effect on NPSHR within the allowable impeller cut range. Beyondthe allowable impeller cut range, recirculation between impeller discharge and the impeller inletstart to disrupt the inlet field of flow, increasing the NPSHR

2.

Higher Ns pumps are characterized by an increase in the impeller vane passage area inrelationship to impeller vane length. Higher Ns impeller are more susceptible to internalrecirculation. Test have shown that trimming a higher Ns impeller often can have a direct impacton a pumps NPSHR.

For low Ns applications, full diameter NPSH values may be used for estimating NPSHR for cutimpeller performance. For applications with Ns values above 30 (1500 US), a NPSH test isrecommended to determine the NPSHR for any impeller trim.

References:

D. Konno, Y. Yamada, Does Impeller affect NPSHR?, Proc. International Pump Symposium - 1984

2 To be technically correct, the disruption to the inlet field of flow decreases the net positive suction head

available (NPSHA) to the impeller. However, the industry standard is that NPSHA is determined at the

pump inlet flange. Any changes to NPSH downstream of the inlet flange are associated with NPSHR.


17/31

2007 Lawrence Pumps Inc. 371 Market St., Lawrence. MA. 01843. Tel :(978) 682-5248 Fax:(978) 975-4291Webhttp://www.lawrencepumps.comContact: [email protected]

Page 1 of 2

March 2007 vol.4_iss 3

While at the Houston pump symposium earlier this month, I attendeda technical session where there was some confusion about how to

treat NPSHR when changing pump speed. The effect of speed

changes and, for that matter, impeller cuts on NPSHR is one whereeven the experts do not always agree. This month well discuss theways in which pump speed and impeller trim affect NPSHR, and the

most common methods for estimating the impact of those changes

on NPSHR values.

The net positive suction head required (NPSHR) of a pump is the minimum amount of suctionhead required at the inlet of the impeller to avoid cavitation levels that would seriously impairpump hydraulic performance

1. A 3% drop in total dynamic head (TDH) at constant flow is the

accepted standard for establishing NPSHR at test.

For any specific set of inlet conditions, the NPSH characteristics of a pump are driven by thegeometry of the impeller inlet. Suction specific speed (Nss) is used to describe the relativeNPSHR capabilities of pumps [1]. First presented by Karrasik, et al., in 1939, Nss is relateddirectly to the geometry of the impeller inlet.

[1]

RPM Changes

Suction specific speed for any impeller is a constant. Holding Nss at a constant value, NPSHRwill arithmetically vary as the square of the ratio of change to RPM [2]. There is some empiricalevidence to suggest that NPSHR varies as the speed ratio to approximately the 1.5 power [3].Different manufacturers use different exponents based on various products and individual testexperience. There is no proven absolute predictive value for the exponent. The majority ofauthors agree that the NPSHR should behave according to equation 2, but that changes in thebehavior of fluid within the inlet flow field, as the fluid nears its critical pressure, is responsible fordifferences between theoretical values and actual test results.

[2]

1 For a more detailed discussion of cavitation, see ourOctober 2004 newsletter

NPSHR

Flow

NBEP

ss

RPM

43

=

NPSHRNPSHR

Rpm

Rpm

1

2

2

1

2 =
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i5_oct.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i5_oct.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i5_oct.htmlmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i5_oct.html


18/31


Page 2 of 2

[3]

In practice the most conservative approach would be to conduct NPSHR tests at operating

speed. Also, for estimating NPSHR in the absence of testing, use equation 2 when increasing thepump speed, and equation 3 when reducing the pump speed.

Diameter Changes

For a specific inlet condition and rpm, flow into an impeller iscontrolled by the inlet area of the impeller eye and the inlet bladeangle of the impeller vanes; both of which are fixed by design.For a specific flow rate and RPM, changes at the OD of theimpeller should not impact the inlet of the impeller. When theimpeller diameter changes, the cavitation characteristics of theimpeller should remain unchanged unless discharge recirculationto the impeller eye occurs (Fig 1). Recirculation can besignificant in a poorly designed impeller, an impeller operated off-design, or an impeller that has been trimmed excessively.

It should be noted that any change to impeller diameter will also result in a change in flowrate,and therefore NPSHR, unless there is also a change in the system curve

2.

Some pump manufacturers display their curves holding NPSHR constant with flow changes (fig2). Others show lines of varying NPSHR (Fig 3). The difference between the two curves is thatthe curve in fig. 3 has been drawn by applying the line of constant NPSHR using a 3% drop in

TDH. Because TDH lowers as the impeller is cut, the 3% TDH drop criteria causes the NPSHRpoint to be defined at decreasing levels of TDH drop. Hence, the ISO lines of constant NPSHRare drawn across decreasing flows. The cavitation in the impeller is not changing. It is strictly aresult of how NPSHR is defined. Fig. 3 is more conservative.

NPSHR is derived from test data and is specific for each individual set of pump and applicationconditions. Manufacturers provide data from water tests that provide a valuable baseline of apumps NPSHR characteristics. The methods outlined above provide some estimating tools fordealing with changes to equipment and operating conditions. After any diameter or speed re-rate, a pump should be checked for signs of cavitation. Vibration and noise testing are probablythe best tools for detecting cavitation outside of a test facility.

2 For a descussion of system curves, please see ourNovember 2005 Newsletter.

NPSHRNPSHR

Rpm

Rpm

1

2

5.1

1

2 =
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i11_nov05.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i11_nov05.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i11_nov05.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v02_i11_nov05.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]


19/31


Page 1 of 4

June 2009 vol .6_iss 6

InthethirdinstallmentofourseriesentitledTheDeadlySinsofCentrifugalPumpOwnershipwediscusspumpcavitation.Pumpcavitationisamuchtalkedaboutandoftenmisunderstoodsubject.WhatfollowsisadescriptionofcavitationandtheapplicationofappropriateNetPositiveSuctionHead(NPSH)marginstocentrifugalpumps.

Vaporbubblesforminapumpinlet

wheneverthelocalabsolutepressureof

theliquidfallsbelowitsvaporpressure.

Thesebubblescollapserapidlyand

violentlywhenthelocalabsolutepressure

increasesduetokineticforcesimpartedby

theimpeller.Cavitationistherapid

formationandcollapseofthesevaporbubbles.Collapsingcavitationbubblescausenoise,vibration,and

erosionofmaterialfromtheimpeller.Pumpservicelifeisshortenedsignificantlywhencavitation

occurs.Theseverityoftheeffectsofcavitationvariesasafunctionofamachine'ssuctionenergywhich

isdiscussedfurtherherein.Fig.1showsaphotographofacavitationbubbleimplosion.Fig2showsan

impellerthathasbeenseverelydamagedbycavitation.Fig3isadiagrammaticviewofthecavitation

bubbleimplosionsequence.

Fig3:Cavitationbubbleimplosionontoasolidsurface,arrowsindicatefluidpressure.

Foranygivenflowrate,everypumphasanabsolutesuctionheadatwhichcavitationwilloccur.This

suctionheadisreferredtoastheNetPositiveSuctionHeadRequired(NPSHR).Headisalwaysexpressed

infeetormeterstomakeitindependentofanyspecificfluid.Theabsolutesuctionheadavailableatthe

pumpinletistermedtheNetPositiveSuctionHeadAvailable(NPSHA).Toavoidcavitation,theNPSHA

mustbegreaterthanNPSHRbyanadequatemargin.

NPSHAisdeterminedbysubtractingtheabsolutevaporpressureofthefluidpumpedfromthetotal

suctionheadavailable.Totalsuctionheadisthestatichead(suctiongagepressure)correctedtothe

impellercenterline(orimpellerinletifvertical),plusthevelocityhead(foundinmostpipefriction

tables),plusatmosphericpressure.Allvaluesshouldbeexpressedinfeetormetersofliquid.


20/31


Page 2 of 4

NPSHRisdeterminedbyhydraulictestingandis

availablefromthepumpmanufacturer.Pump

manufacturersperformaseriesofbreakdown

teststodeterminetheNPSHR(Fig4.)Thepumpis

operatedataconstantflowratewhiletheNPSHAis

steadilydecreased.Asuddendropinthetotal

outputheadisevidenceofcavitation.Industry

standardsestablishthata3%dropintotalheadis

pointwheretheNPSHRreadingistobetaken.

ItisimportanttonotethatanactualtestcurveshowingNPSHRtestresultsreflectsapumpthatiscavitating.Tooperatecavitationfree,pumpsneed

amarginofadditionalNPSHabovethetestvalues.

Theamountofmargindependsonthesuctionenergyofthepump.Suctionenergyreflectsenergy

availableforcavitationdamage,anditisafunctionofthesuctionspecificspeed(S)ofthepump.

NPSHR

QrpmS

43

=

Table1reflectsthemarginthatshouldbemaintainedbetweentheNPSHAandtheNPSHRas

recommendedbytheHydraulicInstitute.

TABLE1

NPSHMARGINRATIOGUIDELINES(NPSHA/NPSHR)

SUCTIONENERGYLEVELSuctionEnergyLevel Margin

Low 1.11.3

High 1.32.0

VeryHigh 2.02.5

MarginistheratioofNPSHAtoNPSHR

Suctionenergyisafunctionofthemomentumofthefluidapproachingtheimpellereye.

Fig 4


21/31


Page 3 of 4

Itisdenotedbytheequation:SuctionEnergy=DeyexNxSxSp.Gr.

Where:

Deye =Impellereyediameter(in)

N =Pumprotativespeed(rpm)

S =Suctionspecificspeed(seeabove)

Sp.Gr. =Specificgravityofthepumpedfluid.

Chart1showssuctionenergylevelsforvariousinletsizesat1800rpm

ThresholdValuesforEndSuctionPumps

Highsuctionenergy160E6

Veryhigh

suction

energy

240

E6

Anyvaluelessthanthehighsuctionenergythresholdislowsuctionenergy Thresholdvaluesforsplitcase/radialinletpumpsare0.75xendsuctionvalues Thresholdvaluesforverticalturbinepumpsare1.3xendsuctionvalues Forotherpumprotativespeedsuctionenergyvariesdirectlywiththerpmratio. Theabovedoesnotapplytoinducers,whichrequiretheirownspecialconsiderations*.

Itisgenerallygoodpracticetoaddanadditional25feetofNPSHAoverthemarginvaluestoaccountfor

disparitiesbetweentestdataandactualsiteconditions.

Itshouldalsobenotedthatcavitationisnotthesameasgasentrainment.Oftentimes,especiallywith

verticalpumps,apumpthatispullingairintothesuctionwillbedescribedascavitating.Althoughthe

symptomsaresimilarthecureisnotthesame.Gasentrainmentwarrantsaseparatediscussionthan

cannotbeaccommodatedhere.Forsomeinformationonthesubjectofgasentrainmentseeour

December2007newsletteronthesubject.
http://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i12_dec07.html


22/31


Page 4 of 4

Finally,itshouldbenotedthatmanycavitationproblemsarecreatedbymodificationandnotbydesign.

Changesinprocessconditionssuchasincreasedflowortemperature,andphysicalchangestothe

suctionsideofthepumpsuchasinstallationofupstreamequipment,modificationofpiperoutes,or

failuretomaintainupstreamequipmentsuchasheatexchangers,maycreatecavitationproblems.In

ordertokeepcapitalcostslowmanysystemdesignsprovideforadequateNPSHmarginsbutnotmuch

ofacushionbeyondthat.Therefore,anyproposedchangestoasystem,oritsoperation,shouldbe

scrutinizedcarefullytobesurethatadequatesuctionconditionsaremaintained.

Fig 1.Courtesy of Solid State Fusion technologies

Fig 2 & 3. Lohrberg, H., Stoffel B., Intelligent Maintenance Management of Pumps; Pump Users International Forum, 2000

References on NPSH Margin Hydraulic Inst itute HI 9.6.1 and Pump Users Handbook, Bloch/Budris, Fairmont Press (2004).

*See - http://www.lawrencepumps.com/newsletter/news_v01_i2_july.htmlfor a brief article on inducers
http://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i2_july.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i2_july.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i2_july.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v01_i2_july.html


23/31


Page 1 of 2

April 2007 vol.4_iss 4

The Suction side of a pump is not the only part of a pump that can bedamaged by cavitation and recirculation. Discharge cavitation

damage is less prevalent, but its effects can be equally damaging.

When most pump users talk about cavitation, they are referring to the classic condition where thetotal available suction head (NPSHA) falls below the vapor pressure of the pumped fluid. Therapid collapse of the resultant vapor pocket causes noise, vibration and results in damage to theadjacent impeller blade.

While its true that cavitation is the result of local pressure that falls below the vapor pressure of

the pumped fluid, the cause of the localized low pressure may be independent of the suctionhead available at the inlet of the pump. Cavitation may also occur as a result of low pressures inregions of turbulent flow.

The image at the left shows turbulence that occurs arounda wing in an airstream. Turbulence is a complex subject,the mechanics of which are still not fully understood.

Turbulence is characterized by chaotic changes in fluidflow that occur as a result of shearing forces betweenadjacent streamlines or boundary layers. Whenever fluidstreams of different velocities merge, shear occurs due tofriction between the fluid streamlines. At the risk of over-simplification, think of a grocery cart as a fluid streamline,

and a broken wheel as drag on one side of thatstreamline. The combination of the carts momentum andthe uneven drag causes the cart to turn. A fluid stream reacts in similar fashion, turning inwardon itself, forming a vortex. Pressures at the center of fluid vortices can be low enough for pumpcavitation even in regions of relatively high pressure, such as in the volute or at the impelleroutlet.

At right is a typical pump curve showing efficiency risingfrom zero to the best efficiency point and falling awayagain. If drawn to the maximum run-out flowrate, theefficiency curve would intersect the baseline at zero TDHand efficiency. The lower efficiency at off BEP conditionsreflects energy consumed by recirculation and itsconsequential heat, vibration, and noise. Thisrecirculation can create zones of high fluid shear, withresultant vortices and cavitation. The amount ofrecirculation is related to the distance the pump isoperating from BEP and the energy available for damage is related to the kinetic energy withinthe pump. High energy pumps have more energy available to do damage off design than lowenergy pumps.


24/31


Page 2 of 2

Discharge recirculation occurs when high pressure flow streamsre-enter the impeller on the low pressure side of the impellervane. This is caused by the pump operating back on its curveor with an inlet restriction. The reverse flow within the impellerpassage shears across the outgoing flow, sets up vortices alongthe pressure wall of the impeller, and causes cavitation along

the pressure wall and shrouds adjacent to the impeller outlet.

The photograph below is of an impeller from a pump recently returned for repair. The cavitationcraters are evident at the pressure side blade tips and the adjacent shroud. Each vane has wornthrough on the pressure side along a section extending between 70% and 85% of the impellerdiameter. The impeller was in a slurry service and the large holes in the vanes are most likelyaccelerated wear due to the internal turbulence. Note the almost pristine condition of the vanetrailing edge at the far left of the photograph. Normal wear for a slurry pump impeller would be theevolution of knife-edge shrouds and blade tips, with the blades eroding back from the blade tipsback toward the impeller eye.

This phenomenon can also manifest itself at the volute cutwater and can occur at both high andlow flow conditions. Off design operation causes a high velocity wake to pass in the gap betweenthe impeller and the cutwater. Cavitation occurs in the turbulent zone around the cutwater and isdistinguishable by visible craters at the cutwater. Pumps designed with the maximum impellerdiameter too close to the cutwater often experience noticeable noise and vibration at bladepassing frequency related to the shock wave generated by the impeller blade impacting the highvelocity wake zone.

Pumps operating off-design for extended periods are subject to increased operating costs and

mechanical failure. The conditions that a pump is designed for are not necessarily the sameoperating conditions it will be exposed to 10-15 years later. A re-rate of an impeller can beworthwhile on large equipment and should be considered as a part of equipment life cycle costmanagement.


25/31


Page 1 of 2

May 2007 vol.4_iss 5

The ideal centrifugal pump would operate reliably over its entirecurve, be very efficient, be low cost and require little NPSH. These

can sometimes be conflicting objectives that involve trade-offs

between operating performance, cost and reliability.

Operating Range

A centrifugal pump is designed to operate at a single operating point, the best efficiency point(BEP). At the best efficiency point the angle of the impeller blades in the impeller eye closelymatches the angle of the approaching flow, and recirculation within the pump is at its minimumlevel. Operation at any point, other than best efficiency point flow, results in an increase in

internal recirculation. Internal recirculation causes efficiency to diminish on either side of the bestefficiency flow rate. The most common form of recirculation, often referred to as leakage bydesigners, travels from the volute/diffuser along the casing walls, past the wear rings, back to theimpeller eye. As pump operation moves further away from the BEP, the amount and severity ofrecirculation will increase and, at some point, the pump will also experience the onset of suctionor discharge recirculation. Suction and discharge recirculation is recirculation between the inlet,or respectively the volute, and the confines of the impeller blades. Every pump has an operatingpoint at which suction recirculation will occur, and a point where discharge recirculation will occur.

The extent of damage that may occur from operating in these recirculation zones is dependent onthe fluid pumped, the speed of the pump and the power density level present.

Damage to the impeller may result from cavitation that is independent of the net positive suctionhead available (NPSHA). A recirculation vortex will often have a significantly low, localized,

pressure at its eye so as to enable the onset of cavitation, even at the impeller discharge.

1

Ifsolids are present, off-BEP operation will decrease the wear life of both the impeller and wearrings due to increased recirculation velocities. The charts below are adaptations of chartsdeveloped by Fraser

2for safe operating ranges established for pumps operated away from BEP.

1Last months issue, April 2007, presents a more detailed discussion on the topic of discharge recirculation.

2Flow Recirculation in Centrifugal Pumps, Warren Fraser, Worthington Pump Company, ASME 2002
http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Apr07.htmlmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/http://localhost/var/www/apps/conversion/tmp/scratch_14/Newsletter/news_v04_i4_Apr07.html


26/31


Page 2 of 2

Efficiency and NPSHR

As mentioned at the beginning of this article, optimum flow through a centrifugal pump isdetermined by the design of the pump inlet. Every centrifugal pump has a best efficiency point.

The level of efficiency attainable is partly determined by the impeller eye design. The impellerinlet design is also a major determining factor of NPSHR. However, the inlet design that yieldsthe best NPSHR is not the inlet that yields the best efficiency. Pumps that are designed for lowerNPSHR generally have a larger eye than an impeller designed for optimum efficiency.

Increasing the eye diameter of an impeller for lower NPSHR will result in a correspondingly largersuction specific speed. High suction specific speed pumps are prone to reliability problems. Inrecognition of the inherent problems associated with high suction specific speed pumps, manypurchasers routinely specify that centrifugal pumps should not exceed a suction specific speed of194 (10,000).

Low cost

Cost is among the decisions that a purchaser makes. For a specific set of hydraulic conditionsand pump stages, the required impeller diameter will vary directly with the operating speed. Acentrifugal pump operating at higher speeds will, in most cases, cost less than its lower speedcounterpart. The pump will be smaller in diameter, have thinner casing walls, smaller diametermechanical seals, and take less time to manufacture. In an environment of lowest cost thatmeets the specification procurement, the higher speed, lower cost pump is often appealing.However, there is a trade-off in reliability that, in some circumstances, will have a dramaticnegative impact on the reliability related ownership costs over the lifespan of the equipment.

Higher speed pumps are also higher wear pumps when solids are present. Wear will increase bya exponential factor of about 2.5 applied to the relative velocity difference between two pumps ofsimilar materials in the same application. A pump with twice the relative velocity will have a wearrate of about 22.5 or over 5x the wear rate of a lower velocity unit.

Higher speed pumps with identical NPSH requirements are also higher suction specific speedpumps. A significant portion of mechanical seal failures, bearing failures, and wear problems arerelated to cavitation and off-design operation. Taken as a group, high suction specific speedpumps will account for more pump problems than their lower suction specific speed counterparts.

In closing, every pump application should be evaluated; giving priority to the most critical desiredattributes. Standard operating conditions and upset conditions should be clearly differentiated,identified as such, and communicated to the pump manufacturer. The pump should be a part of anew process design and not the result of one. Once a mistake is installed it is very difficult, if notimpossible, to change, and is always more expensive.


27/31

2004 Lawrence Pum ps Inc. 371 M arket St., Lawrence. M A. 01843. Tel :(978)682-5248 Fax:(978)975-4291W ebhttp://www.lawrencepum ps.com Contact:dandrews@ lawrencepumps.com

Page1 of 3

August 2004 vol.1_iss3

Specific speed is one of the first param eters that a centrifugal pum p designer looksat when evaluating a pum p application.Specific speed m ay be used to rapidlydeterm ine the m ost feasible designs for the service conditions. Com bined with som eknowledge about the trade-offs associated with various pum p selections, it becom esa great tool for quickly m aking som e basic design choices.


Specific Speed (Ns),or som etim es (NsQ),istreated as a dim ensionless num ber thatrepresents the physical design of an im peller regardless of pum p size. The equationfor specific speed is:

75.0H

QnN s

=

where:

n = rotative speed in revolutions per m inuteQ = Pum p flow at the best efficiency pointH = pum p differential head at the best efficiency point.

US custom ary units SI unitsQ gallons per m inute cubic m eters per second

H feet of head m eters of headNote:Dividing the US units by 51.64 will yield the SI equivalent value.

In a constant speed pum p,flow rateis largely determ ined by the area at the pum pinlet, and head is determ ined by im peller diam eter.

High Ns pum p im pellers have inlet diam eters (D1) that approach or equal the outletdiam eter (D2), and relatively large open flow passages. Low Ns pum p im pellers haveoutlet diam eters (D2) that are m uch larger than the inlet diam eters (D1) and

relatively narrow flow passages.

High N s Low N


28/31


Page2 of 3

The chart shown below is com m only used as a reference and shows the relationshipbetween N sand im peller style.

Impeller design is not so m uch a choice of the designer as it is a result of theapplication param eter considerations and selection of operating speed.

For exam ple:

An application calls for 1000 GPM (0.062 M 3/sec) at 200 ft. (62 M ) TDH.

75.0200

1000=n

N s

Substituting values for n yields very different im peller design options.

Considerations such as the presence of abrasive solids, solids passing capability,efficiency, suction conditions (NPSHA), equipm ent cost, and desired reliability allcom e into play when selecting the desired rpm .

n = 1200RPM

Ns= 714(SI=14)

n = 1800RPM

N s= 1070(SI=21)

n = 3600RPM

Ns= 2140(SI=41)


29/31


Page3 of 3

This table provides a very sim ple set of guidelines relating to pum p RPM and Ns.W hich pum p is the right choice depends on how all of these factors weigh into yourspecific pum p application.

Condition RPM & NsConsiderationsAbrasives Lower RPM is better. W ear life for sliding abrasion variesroughly as (rpm 1/rpm 2)2.5factor.

Solids Passing H igher Nspum p im pellers have m ore openness to the bladepassages than lower NsPum ps. It is not uncom m on to beable to see through from the OD to the eye on axial andm ixed flow im pellers.

Efficiency Efficiency tends to increase with Nsup to around 2500-3000and then decrease with further increasing N s.(M ore on thisin our next newsletter.)

Cavitation Lower RPM pum ps generally have lower NPSH

requirem entsand a broader allowable operating rangethan higherRPM pum ps.Low NPSH pum ps are also oftenlow Ns,as both flow and rpm are key factors in NPSH Rdeterm ination.

Capital costs Higher RPM pum ps are physically sm aller than lowerRPM pum ps for the sam e duty point. Both m otors andpum ps are often less expensive and occupy a sm allerfootprint than their lower speed counter-parts.

Reliability HigherRPM pum ps are often less tolerant to difficultservice conditions than lower speed counterparts. Factorssuch as solids, cavitation, and off design-point operation

m ay severely im pact reliability. Any capital cost benefitm ay be elim inated by pre-m ature equipm ent failure. Fordifficult applications, especially where high horsepower isinvolved,it is good practice to be conservative withequipm ent speed.


30/31


Page 1 of 2

November 2007 vol.4_iss 11

Specific speed (Ns) and suction specific speed(S) are terms that areno longer limited to the interest of pump designers. Most peopleassociated with centrifugal pump evaluation have come to see these

terms as valuable tools in pump selection. This months issueaddresses one readers recent inquiry as to whether there is arelationship between Ns and S.

An historical relationship and an indirect hydraulic relationship exist between pump

specific speed and suction specific speed. Both specific speed and suction specific speedrelate to flow through the impeller. Specific speed can be said to describe the impeller

whereas suction specific speed describes the impeller inlet. The inlet and outlet of an

impeller can be considered independent within certain limits.

Specific speed is a function of a relationship between flow and total dynamic head at any

rotative speed (Eq 1). Specific speed will remain constant for a pump design regardlessof its rotative speed. To a designer, specific speed is an indicator of impeller geometry.

A high specific speed value indicates a high rate of flow in relation to the amount of head

developed. For instance, an axial flow (propeller) pump, characterized by high flow and

low head, is a high specific speed pump. Conversely, a radial impeller pump,characterized by low flow high head, is a low specific speed pump. Fig.1 shows the

relationship between impeller types and specific speed.

Eq. 1

Fig 1

H

QrpmNs

43

=


31/31

Suction specific speed is a function of a relationship between flow and net positive

suction head required (NPSHR) at any rotative speed (Eq. 2). Similar to specific speed,suction specific speed will remain constant for a pump design regardless of its rotative

speed. To a designer, suction specific speed is an indicator of impeller inlet geometry. A

difference in suction specific speed, for two pumps with the same rotative speed and

capacity, will be physically evidenced by a difference in the inlet diameters.1

Eq. 2

At the beginning of the 20th

century, as pumps grew rapidly in size and power, severeproblems related to suction conditions began to occur with increasing frequency.

Because cavitation was generally associated with head loss, pump engineers felt that

there was a relationship between NPSHR and total dynamic head. In 1922, as a result of

study conducted in the United States and Germany, L. Moody and D. Thomaindependently described a parameter that became known as the Thoma-Moody cavitation

factor (sigma) (Eq. 3).

Eq. 3

In 1934, M. Tenot2

postulated that sigma was related to specific speed. Tests showed that

NPSHR varied with the affinity rules for speed changes, but the same did not hold true

for impeller trims.3

Engineers knew therefore, that the correlation between specific speed

and sigma was false. In 1937, Igor Karassik, an engineer with Worthington Pump Co,was credited with developing the suction specific speed equation (Eq. 2).

4Suction

specific speed accounts for changes in NPSHR characteristics that are created without a

change in specific speed. A designer can develop an impeller with a specific speed

identical to that of another impeller, but with a lower NPSHR, simply by increasing theeye diameter and lowering the impeller inlet blade angles.

A change to the outside diameter of the impeller changes the geometry and thereby

changes the specific speed. The inlet geometry and NPSHR characteristic will remainunchanged. The suction specific speed value will change because there is a shift in the

best efficiency point capacity that resulted from the impeller trim, but discounting any

recirculation induced effects, the NPSHR will remain the same for any given flow rate.Recirculation induced effects place a restriction on the extent of diameter reduction that

can be practically applied without adversely impacting performance. The range of

allowable diameter change diminishes as either specific speed or suction specific speed

increases. Subsequently, the allowable impeller trim is always design dependant.

1 This assumes that there is no inducer. An inducer should be evaluated as a separate impeller.2 Tenot was also a pioneer in photography, taking some of the earliest known photos of cavitation.3 The Interaction between Geometry and Performance if a Centrifugal Pump, B. Neuman, Mechanical

Engineering Publications, Ltd. 19914 A Map of the ForestUnderstanding Pump Suction Behavior: Where Do We Go From Here?, Igor

Korassik. Proceedings of the 1st

International Pump Symposium, 1984

HNPSHR=

NPSHRQrpmNs

43

=

pump binder articles

Documents