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Category Archives: Centrifugal Pump Fundamentals

Getting the Install RightThe most important factor in ensuring a pump system offers a long service life is to get the installation right. o amount of

good engineering! proper pump selection! or correct application of pumping technology can overcome the challenges a poor

installation can thro" at a pump system. #o it$s critical that "e all %no" the bare minimum re&uirements to ensuring a good

installation.

Step 1: Read the Manufacturers Installation Instructions

'irtually all pump manufacturer$s offer (peration and )aintenance )anuals *(+)$s,. In addition to information on proper

pump operation and maintenance these manuals usually also provide information ho" to install the pumps. It is critical that the

nstaller revie" and understand these inst ructions prior to attempting to install the pumps. ot only "ith this information prove

invaluable! failing to comply "ith the instructions could void the manufacturer$s "arranty. #o al"ays al"ays al"ays start by

as%ing for! and thoroughly revie"ing the manufacturer$s "ritten instructions.

Step 2: Visual Inspection Looing for !"vious Signs of #ro"le$s

(nce you$re certain the Installer has ta%e care of #tep - it$s time for a &uic% visual inspection of the e&uipment. First! as% the

manufacturer if they have an Inspection hec%list. If one is available use it to chec% any items they indicate are important. If one

isn$t available call the manufacturer up and as% "hat items you should be loo%ing for.

/epending on the type of e&uipment! typical installation inspection re&uirements may include0

All e&uipment appears to be properly installed0 guards are in place! anchor bo lts are installed! seals are not lea%ing

e1cessively! electrical connections are neatly terminated in appropriate 2unction bo1es! e&uipment is installed level as

verified by a machinist level *not a carpenter$s level,! etc.

A simple verification of alignment using a straightedge to compare the alignment of the coupling hubs at several

increments around the perimeter of the coupling.

'erification that the e&uipment is properly anchored and that baseplates are grouted *depending on e&uipment design

and manufacturer re&uirements,.

Add lubricant and3or chec% lubricant levels.

'erify that the seal flush arrangement is correctly assembled and appears to be functioning.

onduct a bump test to ma%e sure the motor is rotating in the right direction *only done "ith the motor decoupled from

the pump! or "ith the pump filled "ith li&uid,.

4emove the bolts from the pump flanges to confirm that the piping is properly anchored and not transmitting any thrust

to the pump.

These are simple inspections that can be conducted by any %no"ledgeable pump professional. The %ey is to move slo"ly! be

thorough! and al"ays refer bac% to the manufacturer$s (+) and Installation Inspection hec%list. In the case of small pumps

this may "ell be the final step prior to startup. Ho"ever! for larger pumps additional inspections are often re&uired.

Step %: &actory'Authori(ed Service

n the case of large e&uipment it "ill probably be necessary to bring either a service technician employed by the e&uipment

manufacturer! or a service technician formally approved by the e&uipment manufacturer to perform additional inspections and

commissioning.

This is the step "here the pump sales and application professional "ill bo" out and give "ay to personnel "ho speciali5e in

inspecting and starting large rotating e&uipment.

Typical services at this step may include0

Alignment inspection0 (nce the simple straight edge method has been applied the ne1t step in terms of accuracy "ould

be to use dial indicators to actually measure the degree of misalignment. The highest level of accuracy "ould be attained

by utili5ing laser alignment e&uipment to measure misalignment "ith a great degree of accuracy.

'ibration measurement0 There are many vibration standards that apply to pumps depending on the application and the

customer$s preference. T"o of the more commonly applied standard "hich conta in vibration limitations are the Hydraulic

Institute #tandards and the American Petroleum Institute. It is fairly common for vibration testing to be conducted on

large e&uipment to verify that the vibration levels e1hibited in the field are "ithin the allo"able levels according to therelevant standard.

Flatness measurement0 #ome installations may even re&uire that sophisticated e&uipment be used to verify the flatness

of the installed e&uipment beyond "hat can be verified "ith a machinist level. This is particularly important and more

Most Recent Articles

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ettin! the "nstall Ri!ht

Most #iewed Articles

$PSHr% What "s "t and Why &oes "t

Matter'

&efinin! the (asics% #olutes )

"mpellers

What are the Main Parts of *+ery

Centrifu!al Pump'

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I)*R! *! #+M#S An Introduction to entr i fugal Pump Fundamentals

Page 1 of 12Centrifugal Pump Fundamentals - Intro To Pumps

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commonly re&uired "hen dealing "ith large vertical e&uipment due to this e&uipment$s sensitivity to seemingly minor

out6of6plumb installation conditions.

f services at the #tep 7 level are re&uired it$s important that these be handled by &ualified professional se rvice technicians

approved by both the )anufacturer and the ("ner of the e&uipment.

R,MI)-,R./0 and Conclusion

It bears repeating that at every step reference should always be made to the manufacturers instructions. They are the ones who

designed the equipment, are intimately aware of the capabilities and sensitivities of the equipment, and will be responsible to

warranty the equipment against defects. eeping that warranty intact is important. !o following the manufacturers instructions at

every step along the way is a non"negotiable.

(nce these steps have been completed the installation is ready for startup! and you can rest easy %no"ing that you$ve done

everything you could to ensure that the e&uipment installation "as done right.

What are the Main Parts of Every Centrifugal Pump?There are a fe" components that virtually every centrifugal pump has in common. These components include the impeller!

casing! shaft! shaft sleeves! bearings! and seals. These parts can be subdivided into the 8"et end9 and the 8mechanical end9. The"et end of the pump includes those parts that dictate the hydraulic performance of pump. The mechanical end includes those

parts that support the impeller "ithin the casing! seal the casing "here the shaft passes through it! and enable rotation0 the

means by "hich the "et end creates flo" and pressure.

et ,nd

:e$ve already tal%ed about the t"o primary parts of a pump0 the casing and impeller. #o "e "on$t spend much time on those

here. The short e1planation is that the impeller rotates at a high speed creating energy. The impeller is positioned "ithin the

casing "hich converts the energy created by the impeller into flo" and pressure. For a more detailed loo% at this process ta%e a

loo% at the article on this topic.

Mechanical ,nd

The impeller is mounted on a shaft. The shaft is usually made of steel or stainless steel and is si5ed to support the impeller.

mpellers have to be si5ed carefully. An undersi5ed shaft can result in increased pump vibration! shorter bearing life! the

potential for shaft brea%age! and an overall reduced pump life.

;enerally spea%ing the pump shaft is covered "ith a shaft sleeve. The shaft sleeve is a sleeve of metal! usually bron5e or

stainless steel! that is designed to either slide or thread onto the shaft. The shaft sleeve is used to position the impeller correctly

on the shaft! and it also protects the shaft.

n order for the shaft to hold the impeller "ithin the casing it must pass through the casing. The point "here the shaft enters

the casing is called the stuffing bo1 and must be sealed. The most typical sealing mechanism is the mechanical seal.

)echanical seals vary tremendously in design! performance! and cost. The simplest seal consists of 2ust a fe" primary parts0 a

stationary face! a rotating face! a gland! and a spring. The rotating face is a ring of smooth hard material that is fastened to shaft

sleeve! and the stationary face is a second ring of smooth hard material fastened to the casing. The gland bolts to the outside of

the casing and the spring is placed under tension bet"een the gland and the stationary seal face causing it to press against the

rotating face. As the pump shaft rotates the rotating face "ill rotate against the stationary face. A small amount of the pumped

li&uid "ill ma%e it$s "ay bet"een the faces %eeping them cool and lubricated. As long as the seal faces stay clean! smooth! and

lubricated they "ill virtually eliminate lea%age around the shaft.

The final part of the mechanical end is the bearing arrangement. ;enerally spea%ing centrifugal pumps are e&uipped "ith

standard ball6type anti6friction bearings. These are the same bearings used in everything from electric motors! to roller s%ates!

to automobiles! and they are lubricated by grease or oil. The pump shaft is supported and held in place by the bearings "hich

have to be designed to handle all of the loads created by the rotation of the impeller! and si5ed to provide a reasonable service

life. Bearing failures are one of the most common causes of pump do"ntime so designing


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Why Should You Consider Using Standard Pumps?The general consensus "ithin the municipal and industrial pumping mar%ets is that standard6material pumps aren$t &uite good

enough for today$s demanding applications. There may be some truth to the idea that special materials and construction

methods can provide a longer and more reliable service life. Ho"ever! there are also good reasons "hy you should thin% long

and hard before as%ing for an e1pensive material or construction upgrade.

A #roven *rac Record

(ne reason "hy a pump manufacturer$s standard product should at least be given cursory consideration is that the materials

and design that ma%e up the standard product are proven. There$s nothing theoretical or ris%y about using the standard

materials and design. (n the other hand "ith special materials and designs the specifier has ta%en on all the ris%! and if

something goes "rong the specifier "ill be in hot "ater right along "ith the manufacturer.

There are certainly cases "here standard materials and designs "on$t cut it! but before ma%ing the leap to a special pump

design it$s al"ays best to ma%e sure this is one of those cases.

Replace$ent #arts

A second reason to thin% about using standard product has to do "ith getting replacement parts on do"n the road. )ost

manufacturer$s can offer standard material replacement parts off the shelf! and "ith ne1t day shipping those replacement parts

can generally be on site "ithin a matter of days. (n the other hand! if that pump has a sophisticated bearing lubrication system

and you need a replacement bearing you may be loo%ing at some e1tended do"ntime "hile the replacement parts are

manufactured.

*he otto$ Line

(ne final reason to consider using standard product is cost. In virtually all cases the manufacturer$s standard product "ill be the

cheapest configuration available. This isn$t necessarily because it$s inferior as much as because the manufacturer has been able

to accomplish economies of scale in building their standard product.

Su$$ary

There are certainly times "here process or pumpage characteristics dictate that special materials or special designs are re&uired.

Ho"ever! if these conditions do not e1ist it$s al"ays a good idea to give standard product serious consideration due to the

significant benefits standard product can afford.

aria!le Speed "perationt is increasingly common for pumps to be operated by motors that are controlled by variable fre&uency drives *'F/s,. 'F/s

control motor speed by varying the fre&uency of the po"er being sent to the motor. #o a >? H5 motor operating at -@?? 4P)

"ill reduce to -?? 4P) if the fre&uency is varied do"n to ? H5. There are t"o primary reasons "hy someone might consider

operating pumps on variable fre&uency drives0

-. To simplify the pumping system by minimi5ing the different si5es of pumps needed.

C. To ensure the pumps operate as efficiently as possible.

Lets e1plore each of these reasons independently.

Si$plify the #u$ping Syste$

#o lets say a pumping system needs to produce a total flo" of -?!??? ;P). Ho"ever! it is also e1pected to operate regularly at

flo"s of C!?? ;P) and !??? ;P).


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-. The pumps could be different si5es. #i5e one pump for -?!??? ;P) at C?? Ft! a second pump for !??? ;P) at -?? Ft! and a

third pump for C!?? ;P) at ? Ft. The problem here though is that the initial purchase cost 2ust "ent up dramatically! and

any redundancy bet"een pumps of e&ual si5e has been lost.

C. The pumps could be si5ed for C!?? ;P) at C?? Ft but selected carefully so that they can also operate efficiently at C!??

;P) at ? Ft "hen the operating speed is reduced.

n this scenario option 7 is the best option. It allo"s us to retain the redundancy and simplicity of having four identical pumps

and ensures "e are operating the pumps at an acceptable point on the curve. #o in this case 'F/$s are an obvious choice.

,nsure ,fficient !peration

Another reason someone might employ 'F/$s is to ma1imi5e system efficiency. It$s no secret that pumps operate most

efficiently "ithin a small portion of their total potential operating range. This range has been dubbed the 8Preferred (perating

4ange9 and formally defined by the Hydraulic Institute #tandards and the American Petroleum Institute. In most cases the P(4

e1tends from ?J to -C?J of the flo" at the best efficiency point on the pump performance curve. 'F/s are often employed to

ad2ust the number of pumps in operation and their operating speed to %eep pumps operating as close to the P(4 as possible

as system demands change.

Here$s an e1ample of ho" this might "or%. Imagine that in the scenario discussed above "e decided to ma1imi5e efficiency at

the !??? ;P) condition. In doing so "e might decide that it ma%es the most sense to have t"o at reduced! three at a further

reduced speed! or four pumps at a very slo" speed in operation in order to achieve the best possible efficiency. 'F/$s provide

the fle1ibility to analy5e a pumping system and tailor the operating scenario to ma1imi5e pump efficiency at all operating points

thereby minimi5ing po"er costs! e1tending pump life! and increasing the mean time bet"een failure of the pumping units.

Su$$ary

'F/$s are an increasingly common technology that is applied to simplify pumping systems and ma1imi5e system efficiency.

=nderstanding the most common reasons "hy 'F/$s are used "ill enable you to provide intelligent assistance to pump system

designers in selecting the right pumps for the 2ob.

Pi#$ the RIG%& Motor RatingPic%ing the right motor rating "ill result in a pumping unit that is more efficient and

provides a longer service life. Failing to pic% the motor correctly "ill result in a pumping

unit that demands more po"er than it should! and that may not last as long as it "ould

other"ise.

&irst a 3ord a"out $otors

The first thing you need to

%no" is that motors are

generally si5ed at

predetermined intervals. :hat this means is that in most cases you "ill need

to round up to the ne1t available motor si5e "hen determining ho" large a

motor to couple up to a pump. #o if the pump po"er re&uirements indicate

that the motor should be rated for at least . HP you$ll have to round up to

the ne1t normal motor rating of HP.

The second thing to %eep in mind is that the vast ma2ority of motors are

designed "ith a -.- service factor. :hat this means is that a motor rated for

-?? HP is actually capable of continuous operation at up to -- HP *-?? 1 -.-,. Ho"ever! if the motor is operated on a variable

fre&uency drive the service factor is lost and does not apply.

Ma4or considerations for si(ing a $otor

There are three ma2or considerations to %eep in mind "hen si5ing a mo tor to drive a centrifugal pump0

-. :hat are the po"er demands of the pumpD

C. :hat "ill typical operation loo% li%e for this pumpD

7. :ill the pump be operated on a variable fre&uency drive *'F/,D

hat are the po3er de$ands of the pu$p5

This first &uestion has t"o parts0


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:hat is the po"er demand at the design conditionD

:hat is the ma1imum po"er re&uired to operate the pump at any point on the pump performance curveD

The bra%e horsepo"er at the design condition should al"ays be less than the nameplate rating of the motor. #o if the pump "ill

re&uire . HP at the design condition! at a minimum the motor should be si5e for HP *the ne1t nominal si5e above . HP,.

n many cases it "ill ma%e sense to select a motor large enough to handle the ma1imum po"er demanded by the pump at any point

along the curve. This ma1imum po"er value is referred to as the non6overloading bra%e horsepo"er of the pump! or (L BHP. #i5ing

a motor to handle this po"er demand "ill ensure that the motor "ill be able to operate the pump even "hen the pump operatingcondition changes dramatically! and acts as an insurance policy against damage in the event that the pump operates at une1pected

points on the curve due to unforeseen circumstances.

hat 3ill typical operation loo lie for this pu$p5

#ome pumps are installed into systems "here it is %no"n that the pump "ill operate at the design condition -??J of the time.

Ho"ever! most systems "ill re&uire that the pump be capable of operating across a large range of conditions. For those cases "here

the pump "ill operate continuously at a single condition si5ing the motor for the po"er demands at that condition is generally

acceptable. In other cases "here a range of operating heads "ill be encountered it is generally best to si5e the motor to handle the

(L BHP of the pump.

ill the pu$p "e operated on a varia"le fre6uency drive5

f a pump "ill be operated on a 'F/ then the motor should be si5ed for the pump (L BHP! and the motor nameplate HP should bemore than the pump (L BHP "ithout any consideration given to the motor service factor. 4emember! the motor service factor

disappears "hen the motor is operated by a 'F/. The only e1ception to this is a case "here the pump "ill operate at a single

condition -??J of the time.

f the pump "ill not be operated on a 'F/ then si5ing the motor becomes a matter of choice. Best practice dictates that in all cases

the motor nameplate HP rating should e1ceed the po"er demanded by the pump at the design condition! and many customers also

"ant the (L BHP to be less than the motor nameplate HP. In other cases the motor nameplate HP rating "ill be selected to e1ceed

the po"er demanded by the pump at the design condition! but "ill be less than the (L BHP. This is o% as long as the customer

understands ho" the motor "as selected! and as long as (L BHP is less than the nameplate HP rating of the motor plus the service

factor.

A 4o" 3ell done/

#electing the right motor rating is an important step in ensuring that the entire pumping unit provides reliable and efficient

performance for years to come. =nderstanding and follo"ing the basic guidelines listed above "ill help the pump professional ensure

the right motor rating is selected every time.

%o' (o I Ma$e the RIG%& Pump Sele#tion?8Eust pic% the pump "ith the highest efficiencyG 4ightD9 That$s the line of reasoning most novice pump professionals "ill follo".


6/12

The first thing to do when provided a single design condition and no additional information is to demand more information. Insist on

knowing the full range of heads and flows that the pump will be expected to produce. In cases where you get pushback to this

request, insist on getting as much information as possible. Youre protecting yourself and your customer by requiring this information.

A selection should be based on the most frequent operating condition.

ext you need to know the average or most!typical operating condition. "here will the pump operate the vast ma#ority of the time$

This is the point that needs to fall at the highest efficiency, and as close to the pump best efficiency point as possible.

%inally you need to know the range of operating heads that the pump is expected to operate against continuously. "hat is the

complete range of heads that the pump will be required to pump against$ Take the maximum and minimum values and make sure

they fall within the allowable operating range as defined by the pump manufacturer.

&nce you locate a selection that puts the average condition near pumps best efficiency point and keeps the maximum and minimum

head values within the allowable operating range youve #ust made the 'I()T pump selection.

Disaster averted!

&nce youve made the right pump selection you will still need to verify that you have adequate suction pressure *+)a-, that youve

selected the right pump design for the application, and that the pump is a good fit for other application specifics. &nce youve

covered all of these bases you are well on your way to avoiding the mistake of picking the wrong pump.

NPSHr: What Is It and Why Does It Matter?&ne common mistake pump novices sometimes make is to overlook a little detail

called et +ositive uction )ead *+)- when making a pump selection. "hen

considering pump selections the first thing most people are taught is to look for is

a selection with high!efficiency, and one where the design condition falls close to the best efficiency point. "hen operating

under an overwhelming mountain of new information the pump novice will sometimes forget to make sure the et +ositive

uction )ead 'equired *+)r- characteristics of the pump are suitable for the application. "hile understandable, this mistake

can lead to large problems further down the road. "hile a pump may potentially provide acceptable performance at lower

efficiencies and at a point that is not very close to /+, a pump that is tasked with operating with inadequate et +ositive

uction )ead 0vailable *+)a- will see a considerable decrease in the head and flow produced and further deterioration in

performance due to internal damage can be expected in relatively short order. 0s a result, ensuring the +)r characteristics of

the pump are compatible with the +)a characteristics of the system is a critical step in the pump selection process.

So What is NPSHr and What is NPSHa?

+)r1 The minimum suction pressure required by a pump in order for the pump to operate without cavitation and

performance deterioration.

+)a1 The pressure present in the liquid being pumped.

%or a more in!depth and technical look at +)r and +)a check out 2acques 3haurettes article on 3avitation.

n order for a centrifugal pump impeller to generate flow it must have positive pressure available at the point where liquid

enters the impeller. The amount of pressure required will be different for each pump design, and the amount o f pressure

required by a pump will increase as the flow generated by the pump increases. 3onsidering the +)r curve above, the pump

in question would require 4 ft o f +) when producing a flow of approximately 566 (+7. )owever, +)r would double to 86

ft if the pump were producing a flow of about 8946 (+7.

What Happens if NPSHa is nadequate?

imply put1 cavitation. If a pump is operating with inadequate +)a then the combination of velocity head and friction head

will cause small air bubbles to rapidly and repeatedly form and collapse at the lowest pressure point in the pump impeller. If

cavitation is allowed to continue fo r a prolonged period eventually the pump impeller will see considerable damage. &ver time

cavitation will cause wear along the impeller, which will actually increase the pumps +)r therefore worsening the situation

and accelerating future damage due to cavitation. Thankfully, cavitation is generally a noisy phenomenon, and as a result a

pump experiencing cavitation is generally easy to identify allowing remedial steps to be taken before damage to the pump

occurs.

n addition to cavitation a pump operating with inadequate +)a can be expected to see diminished performance in the form

of reduced head and flow production. +umps operating with inadequate +)a are often said to be running :off the curve;

meaning that the point the pump is operating at falls below, and not on, the published performance curve.




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7aking a election the &ld!%ashioned

"ay

Ho Do We Sta" #ut of $rouble?

"hen tasked with making a pump selection be sure to ask about the suction conditions, and a calculated *or at least estimated-

+)a value. Then when making a selection make sure to allow a cushion between +)a and +)r. The general rule of

thumb is to allow at least a 4 ft margin between +)r of the pump and +)a provided by the system at that flow.

What %lse Do Need to &no?

&ne thing to keep in mind is that +)r and +)a both include atmospheric pressure in their calculation. The atmospheric

pressure acting on water at sea level is about 9< ft. o a pump that requires 86 ft of +)r may actually operate by taking

suction from a source that is below the pump *called a suction!lift configuration-. In such a configuration a means would be

needed to prime the pump, but once primed the pump could maintain its prime while taking suction from a source at a lower

elevation.

0nother +) related topic you should be familiar with is uction pecific peed *ss or sometimes simply -. 7uch like

pecific peed was an index of impellers, uction pecific peed is an index of impellers. )owever, ss identifies impellers

according to the relationship between +)r and total dynamic head. The larger the difference between the two, the higher the

calculated ss value will be. )igh ss can be a predictor of a very high!energy pump, or of a pump with an oversi=ed suction

eye. These pumps are ones where care must be taken to maintain operation as close to /+ as possible as they tend to be

prone to vibration and cavitation issues at operation moves away from /+. 7ost centrifugal pump impellers will have ss

values in the range of >666 to 8?666.

Reading the Pump CurveIn years past making a pump selection

meant sitting down with large printed

catalogs and flipping through them

until you reached a pump curve that fit

the pro#ects hydraulic requirements.

Today this process is made much

easier through the use of electronic

pump curve catalogs. &ne of the most

well!known developers of electronic pump catalogs is /ngineered oftware and

their pump selection software pump!flo. 0ll of the curves in th is article were

generated in their web!based pump selection software.

0 pump curve provides a wealth of information regarding the capabilities of a pump. 0t it most basic level a pump curve is a

graphical representation of the performance characteristics of a pump. Information is plotted on an x!y graph where the x!axis

is measured in units of flow and the y!axis is measured in units of head, power, and +)r.

%or the sake of example, today were going to look at a selection made for the following design condition1 8,666 (+7 at 866 ft.

)ere is a possible pump selection that might be a good fit for that operating condition.

3omposite +ump +erformance 3urve




8/12

+ump +erformance 3urve

et +ositive uction )ead 'equired 3urve

Pump Performance 'urve( Head) *lo and %fficienc"

The first piece of information provided by a pump curve is the flow that the pump will develop at any given head. The curve that

provides that information is called the pump performance curve. ome pump curves only provide a single pump performance

curve, but most will provide the maximum performance the pump is capable of achieving with a full!trim impeller, the minimum

performance the pump is capable of achieving with a minimum!trim impeller, and the performance provided by the design!trim

impeller. The design!trim impeller is the impeller trim the pump selection software has selected as the closest fit to the designcondition provided. In this case the design!trim is [email protected]?4;, the max!trim is 8>; and the minimum!trim is 84;.

3onsidering the design trim curve we see that at =ero

flow, also known as shutoff, the pump will develop about

896 ft of head. This is the head the pump would develop

if it were operating against a closed valve. Aeep in mind

that the actual pressure experienced between the pump

and the closed valve might exceed this value because a

pump 0BB head to the liquid being pumped. In other

words, if this pump were operating at shutoff with

suction pressure of ?6 feet the total head experienced at

the pump discharge flange would be 846 ft *?6 ft C 896

ft-. o considering the design trim impeller we see that

shutoff occurs at about 896 ft, the design condition fallsvery close to the pumps best!efficiency!point */+-, and the pump will operate down to approximately @6 ft of head and

produce a flow of approximately 8956 (+7 at @6 ft. The maximum and minimum trim curves also tell us the possible conditions

that the pump could be modified to meet in the fu ture by installing an impeller of a different trim.

n addition to head and flow most pump performance curves will also provide efficiency information. 0 pumps efficiency is the

relationship between the power required to drive the pump at a given operating condition and the water horsepower being

created by the pump. If a pump were 866D efficient then the input power required would be equal to the water horsepower

being generated by the pump. )owever, since no pump is 866D efficient every pump will require more input power than it will

generate in water horsepower. In the case of this pump the best!efficiency!point falls at approximately 86>4 (+7 at E4 %t, and

efficiency at /+ is 56D to 8?6D of flow

at /+ for most centrifugal pumps. This would mean that the +&' for this pump would be from approximately >46 (+7 to 8?E6(+7 *>6D to 8?6D of 86>4 (+7-. %or some pumps with high specific speed impellers the +&' is a more!restrictive 54D to

886D of /+. It is best to select a pump that will operate most of the time in the +&' since this will have implications for pump

life and power consumption.

There is another region of operation that is defined by the pump manufacturer. This is the 0llowable &perating 'egion *0&'-

and is made up of the portion of the curve shaded in light yellow. This is the region that the pump manufacturer has

determined comprises all of the points that this pump can operate at continuously. "hile it is preferable to select pumps to

operate within the +&', pumps should always be selected to operate within the 0&' without exception. Fery short!term

operation outside of the 0&' might be acceptable, but the pump manufacturer should be consulted before selecting a pump

that will see even intermittent operation outside the confines of the 0&'.

The pump performance curve above has two more items which should be mentioned. %irst, the red line on the left hand side of

the pump curve is the 7inimum 3ontinuous table %low *73%- line. This is the point beyond which the pump manufacturer

has determined the pump should not be allowed to operate for any extended period of time. econd, the blue curve beginningat 6 (+7 and 6 %t and extending through the design condition is the ystem 3urve and represents the operation of the system

in which the pump is being applied. This curve can be manipulated by manually entering data points and is particularly useful

when evaluating the variable!speed performance of a pump.

NPSHr 'urve

The next part of the pump curve is the et +ositive

uction )ead 'equired *+)r- curve. The +)r curve

provides information about the suction characteristics of

the pump at different flows. The x!axis is still measured in

flow units, but the y!axis is now measured in feet of

+)r. /ach point along the curve identifies the +)rrequired by the pump at that flow to avoid cavitation

issues that would be damaging to the pump and would have a negative impact on overall pump performance.




9/12

+ower 3urve

/nd!uction +ump from "ikimedia

3ommons

Gooking back at our example design flow of 8,666 (+7 we can see that this pump will require approximately > ft of +)r at

that condition. 0 typical safety margin between et +ositive uction )ead 0vailable *+)a- and required *+)r- is 4 ft. o in

this case it would generally be recommended that this pump not be applied in applications where +)a at the design flow of

8,666 (+7 is less than approximately 8? ft.

(enerally speaking +)r does not vary dramatically between variations in impeller trim which is why we do not see separate

curves for the minimum and maximum impeller trims. Those curves are actually present, but they are overlaid by the design!

trim +)r curve.

Poer 'urve

The final portion of the pump curve is the power curve.

&nce again the x!axis is measured in units of flow, but

the y!axis is now measured in power units. In this case

the unit of measurement is horsepower. This curve tells

us how much power the pump will demand at any

particular flow point. This information is useful in

ensuring the selected motor is suitably si=ed, and is also useful when calculating power consumption costs.

0t our design flow, 8,666 (+7 we can see that power demand is approximately 96 )+ and that power demand is greatest at

approximately 8,966 (+7. ased on this information, if the pump were to be driven by an electric motor, most pumpmanufacturers would recommend that the next largest motor rating be used. In this case that would be a motor rated for


10/12

ubmersible +ump from "ikimedia

3ommons

Fertical Inline +ump from tock %ree

Images

plit!3ase +ump from tock %ree Images

Fertical

Turbine

from

"ikimedia

3ommons

/nd!suction pumps, such as the submersible, which have been designed to handle

solids are often fitted with mixed!flow impellers or specialty impellers such as the vortex

or screw impeller. These specialty impellers are designed to handle solids without

clogging.

,ertical nline( The vertical inline pump is very comparable to the end!suction

pump in terms of general design. The difference is that the casing has been

redesigned so that the suction and discharge flanges are inline with each other

rather than perpendicular as is the case with end!suction pumps. Fertical inlinepumps are commonly used in water and chemical pumping applications, but are

not typically used in solids!handling applications. They are a very common design

in )F03 and plumbing applications, and are growing in popularity in other

applications. They offer many of the same positive attributes as end!suction pumps

with the added benefit that their vertical design saves on floor space requirements,

and their inline flange design simplifies piping layout work.

Hori-ontal Split+'ase( The designs mentioned have so!far all utili=ed a single!

suction impeller. The most typical pump design that incorporates a double!suction

impeller is the hori=ontal split!case pump. )ori=ontal split!case pumps are often

called between!the!bearing pumps because the double!suction design of the

impeller requires that there be bearings on both sides of the impeller. This is in

contract to the typical end!suction or vertical inline design which generally only

includes bearings above the impeller. 0s a result of the double!suction impeller and

between!the!bearings design, hori=ontal split!case pumps are a more robust and

durable design than the end!suction or vertical inline design. This is why split!case

pumps are often selected for applications where pumping duty is expected to be

very frequent, and where down time for pump repair and maintenance must be

kept to a minimum. 0pplications that fit this description are municipal water applications and industrial process applications.

"hile spilt!case pumps are certainly a lso used in other applications, applications that require extremely dependable

performance, and are able to absorb the additional cost associated with this robust pump design, are typically those that specify

and procure split!case pumps.

,ertical $urbines( Biffuser casings are rarely used in standard centrifugal pumps such as those listed above.

The primary use of diffusers in todays pumping world is in vertical turbines. True vertical turbines incorporate

radial, francis!vane, or mixed!flow impellers which discharge into diffuser!type casing. In high!pressure

applications more than one :stage; may be used. /ach stage is the combination of an impeller and diffuser. o atwo!stage vertical turbine would have a suction inlet followed by the first!stage impeller which would discharge

into the first!stage diffuser. The water would then flow into the second!stage impeller which would discharge

into the second!stage diffuser. The water would then exit the second stage!diffuser and flow up the pump

column to the discharge point. Fertical turbines with up to 86 stages are common, and designs incorporating

?6, 96 or even more stages are not unheard of. The effect of adding stages is that the pump generates

additional pressure. Fertical turbines are a very common pump design used in many different applications

including raw water intake, municipal water pumping, cooling tower applications, and irrigation applications

among many others. ome vertical turbines may be very short and others may be hundreds of feet long

depending on the application requirements.

There are specialty vertical turbine type pumps for unique applications. &ne common specialty des ign

incorporates a fully!open mixed!flow or axial!flow impeller design. These types of pumps are often called

propeller pumps, vertical mixed!flow pumps, or vertical axial!flow pumps. These types of pumps are high!flow, low!head

designs ideally suited to move a large quantity of water at a low pressure. These pumps are rarely staged, and due to their fully!open design are ideally suited for handling large non!stringy solids. Bue to these characteristics vertical mixedJaxial flow pumps

are routinely used in storm water, raw water, and flood control pumping applications. 0nother specialty design incorporates a

non!clog type impeller into a vertical turbine design for pumping raw sewage or o ther solids!bearing liquids.

Summar"( There are many different pump designs. This short list includes the most common designs used in municipal water

and wastewater applications, industrial water pumping applications, )F03 and plumbing applications. This list is far from

exhaustive as many unique pump designs exist and significant variations in the designs listed exist between manufacturers.

)owever, with a general knowledge of these basic pump types the pump professional will have a good handle on the pump

designs required by the vast ma#ority of typical pumping applications. 0dditional detailed knowledge regarding each of these

designs can be gained by studying catalog data freely available from many different pump manufacturers.

Defining the asi!s: "olutes # Impellers"hen you boil them down to their most fundamental components, pumps are

made up of two components1 an impeller and a volute. There are different types of




11/12

image from A Brief Introduction to

Centrifugal Pumps by Joe Evans, Ph.D

image from Pump andboo! by Igor

"arassi!

impellers and volutes and the combination of these different types of impellers and

volutes result in different types of pumps.

#ypes of $olutes

Volutes: #he %ord volute actually describes a specific type of pump casing that

converts energy created by the impeller into pressure. #he impeller pushes %ater into

the volute %hich converts that energy into pressure and directs the flo% to%ard thedischarge point. In the picture on the right notice that the impeller is not located in

the center of the volute. #his is intentional. #he portion of the volute that e&tends

closest to the impeller is called the 'cut%ater(. It is the point %here the flo% is forced

to e&it through the discharge point rather than continuing to s%irl around the

impeller. #he gradually increasing distance bet%een the volute and casing and the

direction of rotation of the impeller )noted by the arro% above the volute* combine to

force the %ater around the volute in a counter+cloc!%ise direction in the pump

section sho%n, and once the flo% reaches the cut%ater it is forced to e&it the volute.

Diffusers: #here is a second %ay that is often used in centrifugal pumps to convert

energy into pressure. #his is through the use of a diffuser. A diffuser functions similarly

to a standard volute in that it contains vanes that begin close to the impeller and then

gradually e&tend a%ay from the impeller periphery. o%ever, unli!e the volute casing a

diffuser casing may contain many vanes. In the case of the assembly dra%ing sho%n

the diffuser contains - vanes as compared the volute casing %hich only has one.

#ypes of Impellers

mpellers be classified according to the relationship bet%een the amount of flo% they create as compared to ho% much head

they generate )specific speed* and they can be classified according to the physical design of the impeller. Entire chapters in

pump handboo!s have been devoted to the topic of specific speed, but %e %ont be spending the time to learn ho% to

calculate specific speed or to consider the implications of e&cessively high or lo% specific speeds. o%ever, a basic

understanding of %hat the term specific speed means is important for anyone %ho %ants to be ta!en seriously in a pump+

design conversation.

Specific Speed: As %as mentioned previously specific speed describes the relationship bet%een ho% much flo% a pump

generates and ho% much head an impeller generates. /or e&ample, one impeller might generate a very large volume of %ater

but at very lo% pressures. A second impeller might generate a very great deal of head but at a very small flo%. #he high flo% 0

lo% head impeller %ould be a high spec ific speed design, and the lo% flo% 0 high head design %ould be a lo% specific speed

design.

1pecific speed is the speed at %hich a geometrically similar impeller if it %ere of such as si2e as to deliver on gallon+per+minute

of flo% at foot of head. )1ource3 4i!ipedia* #his is a commonly+available graph that sho%s this relationship and ho% it affects

impeller design.

As you can see pumps %ith lo%er specific speeds )lo%+flo% 0 high+head* have very tight clearances. 5n the other end of the

spectrum you see pumps %ith high specific speeds )high+flo% 0 lo%+head*. #hese impellers )actually more correctly called

propellers in the a&ial+flo% field* have increasingly large internal clearances until you reach the a&ial+flo% field in %hich case the

impeller are completely open %ith no impeller shroud.




12/12

image from Pump

andboo! by Igor "arassi!

6 5lder posts

Physical Design: Another %ay to classify impellers is according to the design. #his method of classification is not unrelated to

specific speed, and the specific speed of the impeller plays a ve ry large role in determining the physical design of the impeller.

Open vs. Enclosed: 5ne differentiation in impeller design is the shroud design. Impellers %ith a top and bottom shroud are said

to be enclosed impellers. Impeller %ithout any shroud are said to be open impellers. #here are also single+shroud impellers in

some specialty pumps. #hese designs only have a top shroud that leaves the vanes completely open to the product being

pumped. Impellers of the single+shroud variety are ideally suited for applications %here a large number of solids are present.

Single vs. Double Suction: All of the impellers in the specific speed table above are of the s ingle+

suction design. #his means that there is a single portion of the impeller that is designed to ta!e

in %ater. #here are also impellers designed to ta!e suction from t%o locations. A dra%ing of this

type of impeller can be seen to the right. A double+suction impeller is a more balanced design

than a single+suction impeller because the t%o sides of the impeller balance the a&ial thrust

loads being placed on the impeller, shaft, and bearings.

Vane Design: 1ome impellers have many vanes and tight internal clearances. #hese are typically

intended for %ater service and generally fall bet%een the radial+vane and francis+vane specific

speed fields. 5ther impellers have 7ust one or t%o vanes and large internal clearances. #hese

types are often called solids+handling or non+clog impellers and generally fall bet%een the

/rancis+vane and mi&ed+flo% fields. 1till others are designed %ith a single vane and no lo%er shroud, or %ith vanes that do not

e&tend very far do%n into product being pumped. #hese are called scre% and vorte& impellers respectively, and are intended for

applications %ith a high concentration of solids. /inally, there are impellers %ill no shroud at all, top or bottom, such as %hat

you see in the a&ial+flo% field.

1ummary

#here are t%o primary pump volute designs %hich direct the energy created by an impeller into a pressuri2ed flo%. #hese t%o

designs are the volute and the diffuser. #here are many impeller designs, and their intended purpose %ill dictate to a large

degree their design. #he combination of the different impeller and volute designs results in a large range of centrifugal pumps.

n the ne&t article %ell consider a fe% of the most typical pump designs that result from the different combinations of the

available options.

ome Pump /undamentals Pump Business "eep 8earning About I#P

Intro #o Pumps 9 Po%ered by :antra ; 4ordPress.


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