Selecting a Centrifugal Pump for Viscous Flow

8/12/2019 Selecting a Centrifugal Pump for Viscous Flow

1/12

PDHonline Course M102 (2 PDH)

Selecting a Centrifugal Pumpto Handle a Viscous Liquid

2012

Instructor: Randall W. Whitesides, PE

PDH Online | PDH Center

5272 Meadow Estates DriveFairfax, VA 22030-6658

Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.com

An Approved Continuing Education Provider
http://www.pdhonline.org/http://www.pdhcenter.com/http://www.pdhcenter.com/http://www.pdhonline.org/


2/12

www.PDHcenter.com PDH Course M102 www.PDHonline.org

Selecting a Centrifugal Pump to Handle a Viscous LiquidCopyright 2002-2011

Randall . hitesides! P.".

Introduction

#his course pro$ides the student with an understanding o% %luid $iscosity and its e%%ects on the per%orm-ance o% centri%ugal pump operation. #he di%%erent types and interrelation o% $iscosity $alues are dis-cussed. Most importantly! it presents the esta&lished means to con$ert water standard pump per%ormancecur$es so that they can &e used to select or si'e pumps in $iscous li(uid ser$ice. "(uations are pro$idedto ena&le the student to estimate the %low! pressure! e%%iciency! and power re(uired %or a gi$en pumpingapplication.

Content

What exactly is viscosity anyway?)iscosity is a measure o% a %luid*s resistance to %low and molecular shear. +tudents interested in gaining amore in-depth! scienti%ic e,planation o% $iscosity are in$ited to $isit

www.u$i.edu Physics +C/ ,, e& Plum&ing luidDynamics.html .

#here are a num&er o% su& ects co$ered at the a&o$e we&site. +croll down the page until the sectionentitled Viscosity is displayed.

)iscosity is an elusi$e %luid parameter partly &ecause o% the layman*s o$ersimpli%ication o% the physical phenomenon. Consider the %ollowing two common e,amples

+electing a Centri%ugal Pump to Handle a )iscous 3i(uid 2002-2011 Randall . hitesides! CP"! P" 1

In reality, all that is practically important asit relates to this course is that fluid viscosityaffects a centrifugal pumps performance.
http://www.pdhcenter.com/http://www.uvi.edu/Physics/SCI3xxWeb/Plumbing/FluidDynamics.htmlhttp://www.uvi.edu/Physics/SCI3xxWeb/Plumbing/FluidDynamics.htmlhttp://www.pdhcenter.com/


3/12


)iscosity characteristics must not &e con%used with %luiddensity4 they are totally independent o% each other. 3i(uidsthat are dense! i.e., those that ha$e a high speci%ic gra$ity! arenot necessarily highly $iscous. Mercury has a $ery highspeci%ic gra$ity while e,hi&iting an e,tremely low $iscosity.Many lu&ricants that are less dense than water are e,tremely$iscous. /s it no wonder that $iscosity is a misunderstood%luid property that deser$es a $ery important consideration incentri%ugal pumping applications5

6ecause $iscosity e,ists in two %orms and in a multitude o% dimensional units that are more or lesscon$erti&le! can also &e con%using. )iscosity units can consist o% $arious impractica&le! incomprehensi&le!or otherwise un-relata&le (uantities some o% which are l& % -sec %t7! dyn-sec cm7! +ay&olt +econds8ni$ersal! and Reyns. Conse(uently! &e%ore we can really &egin! we must em&ar9 on a $ery &rie%

technical treatment o% $iscosity.

Whats the difference and importance of absolute and kinematic viscosity?)iscosity is e,pressed in absolute and kinematic terms. :;&solute $iscos-ity is also o%ten re%erred to as dynamic $iscosity+electing a Centri%ugal Pump to Handle a )iscous 3i(uid 2002-2011 Randall . hitesides! CP"! P" 2

1. #he adage blood is thicker than water imparts an incorrect three dimen-sional aspect to $iscosity implying that it has something to do with $olume.

2. >ne car owner as9s another! ? hat weight oil are you using5@ #his e,- pression incorrectly associates the oil*s $iscosity with its speci%ic weight! ormore accurately! its %luid density.
http://www.pdhcenter.com/http://www.pdhcenter.com/


4/12


>n the other hand! 9inematic $iscosity is a measure o% a li(uid*s resistance to %low and shear under the%orces o% gra$ity. /t is mathematically de%ined as the ratio o% the a&solute $iscosity to the %luid speci%icgra$ity. #he &ase unit o% 9inematic $iscosity is the stoke 4 one sto9e is 1 cm 2 sec. #he commonly used en-

gineering unit is the centisto9e :1 100 sto9e

6ecause pu&lished technical re%erences and physical property data %or $iscous li(uids may pro$ideeither a&solute or 9inematic units! it is important %or the student to 9now that a&solute and 9inematic $is-cosities are related! as pre$iously stated! &y

centisto9ecentipoise

speci%ic gra$ity=

Put another way! the a&solute $iscosity o% a li(uid is the product o% the 9inematic $iscosity and the speci%-ic gra$ity.

What are some examples of everyday fluids and their viscosity values?#o pro$ide the student with a relational %eel %or $iscosity range! here are some common %luids! and theirrespecti$e $iscosity! arranged in ascending order. /t should &e intuiti$ely o&$ious that $iscosity is e,-tremely dependent on temperature.

+electing a Centri%ugal Pump to Handle a )iscous 3i(uid 2002-2011 Randall . hitesides! CP"! P"

Now that we have these technical definitionsbehind us, let it be said that we only need toconcern ourselves with the relational aspectsof these two viscosity forms along with acomparative understanding of each.
http://www.pdhcenter.com/http://www.pdhcenter.com/


5/12


The case for the Centrifugal ump>% all o% the generic types o% pumps! the centri%ugal pump is the most commonly used. /t has %ound%a$or &ecause o% its many ad$antages simple construction! low relati$e cost! low maintenance!

(uiet operation! and relia&ility.

8n%ortunately! the single &iggest disad$antage o% thecentri%ugal pump is its lowered e%%iciency in handling $is-cous li(uids. /t should &e noted that the hydraulic lossesand there&y hydraulic ine%%iciencies that occur in all

pumps are due in large part to the $iscosity o% the %luid &eing handled.

8se o% centri%ugal pumps %or a $iscosity o$er 00 centis-to9es generally is not recommended &ecause the pump e%-%iciency is so poor. /n some cases howe$er! the high relia&ility o% a centri%ugal pump may ma9ethis type o% pump the &est o$erall economic choice e$en when the $iscosity e,ceeds 00 centis-to9es.

+electing a Centri%ugal Pump to Handle a )iscous 3i(uid 2002-200A Randall . hitesides! CP"! P"

B

8nleaded gasoline at am&ient temperature has a $iscosity o% 0. cs ater at EF.BG has a $iscosity o% 1 cs! also 1 cP the standardI 1.JK hydrochloric acid at 10JG has a $iscosity o% 1.E cP +;" 10 lu&e oil at E0G has a $iscosity o% AE cP B0 motor oil at room temperature has a $iscosity o% 00 cs Castor oil at 100G has a $iscosity o% 00 cs )arnish at EFG has a $iscosity o% 00 cs Lo. E %uel oil at E0G has a $iscosity o% FJ0 cP Peanut &utter at am&ient temperature has a mean $iscosity o% 11!000 cs

Pumped liquids are generally considered viscouswhen their viscosity exceeds 40 centipoise.


6/12


=enerally spea9ing! centri%ugal pumps are e,tremely ine%%icient con$eyors o% li(uids when %luid$iscosity e,ceeds 1!B00 centisto9es. ith that said! the pu&lished correction charts that will &ediscussed momentarily indicate centri%ugal pump material handling applications where the %luid$iscosity may range upward to ! 00 centisto9es. /t must &e noted that e,tremely $iscous %luidhandling applications are &est le%t to positi$e displacement! rather than centri%ugal pumps.

The selection of a Centrifugal umpCentri%ugal pumps are normally selected &y the "ngineer %or a hydraulic application %rom pumpmanu%acturer*s pu&lished pump cur$es. ; pump cur$e in its simplest %orm is a plot o% the pump*simpeller per%ormance indicating change in discharge head :pressure< $ersus pump capacity! i.e. !%low rate. #hese graphs are also re%erred to as

pump per%ormance cur$es or characteristic cur$es.8n%ortunately %or the "ngineer! pu&lished pump

per%ormance cur$es are deri$ed %rom la&oratorytest data utili'ing clean water. #his o&$iously

presents no pro&lem when the pump application athand in$ol$es water or a %luid whose physical

properties appro,imate those o% water. Howe$er!when the %luid properties de$iate signi%icantly%rom those o% water! serious error can &e intro-

duced &y using the standard manu%acturer*s pump per%ormance cur$e without correction.

#his course will instruct you on how to normali'e or correct the pumping re(uirements o% a particu-lar hydraulic application to compensate %or a $iscous %luid. #hese corrected $alues can then &e usedin con unction with the manu%acturer*s standard pump water per%ormance cur$es.

!electing a ump for a "iscous #i$uid

#he student will %ollow a &asic %our-step procedure to select the proper pump when handling a$iscous %luid


J


7/12


#he most popular per%ormance correction charts are those de$eloped &y #he Hydraulic /nstitute!Parsippany! Lew ersey. #hey are contained in the pu&lication %&!'()' ump !tandards set, *++*

elease. #his organi'ation*s per%ormance correction charts %or $iscous li(uids are widely pu&lishedand reproduced. /% a copy o% %&!'()' ump !tandards is not accessi&le to the student! reproduc-tions o% the correction charts can &e $iewed in any o% the %ollowing or similar pu&lications

#et us take an illustrative example-;ssume %or our e,ample that pro ect speci%ications dictate that a 0!000-gallon capacity storagetan9 o% a hypothetical li(uid at E0G must &e routinely emptied in a time not to e,ceed two hours.3et us also assume that a%ter consulting a technical re%erence! it is determined that this hypotheticalli(uid has a $iscosity o% FJ0 centipoise at the stated temperature. /n a li9e %ashion! it is determined


E

1. ;ccurately determine or otherwise ascertain the re(uired process :$iscousM


8/12


that the li(uid has a speci%ic gra$ity o% 0.AA at E0G . rom the gi$en in%ormation it can &e (uic9lydetermined that the pump*s capacity or rate o% %low must &e!

This is the pumps required viscous capacity . 3et us %urther assume that a%ter conducting acare%ul hydraulic analysis o% the piping system to which this pump will &e connected! it has &eendetermined that the pump will operate with a total dynamic head o% 1J0 %eet when pumping at thisrate o% 2J0 gpm. This is the pumps required viscous head ( H V ). +imply relying on a pumpmanu%acturer*s standard water per%ormance cur$e at this point! with these $iscous %low and head(uantities! will result in the erroneous selection o% a pump that will not satis%y the pro ect speci%ica-

tions. This is because the pumped fluids viscosity has not been taken into consideration.Increased fluid viscosity decreases pump capacity, head, and efficiency. /n essence! a pumpmust &e selected with water pumping capacity and head greater than that determined %or the pumpedli(uid in order to compensate %or the reduced per%ormance. #he logical (uestion that %ollows ishow much should these actual $iscous %luid per%ormance $alues &e increased to insure the relia&ilityo% the data plotted on the manu%acturer*s water per%ormance cur$e5

#he answer lies in the correction %actors deri$ed %rom #he Hydraulic /nstitute*s per%ormance correc-tion charts %or $iscous li(uids. #wo $iscosity correction %actor charts are di$ided &y %low rate one

chart %or %lows o% 100 gpm and &elow! and one chart %or %lows e,ceeding 100 gpm. /n the now su- perseded 1B th "dition o% The )ydraulic 'nstitute !tandards, these charts appear on opposing pagesstarting at page 112. :Highly signi%icant re$ision to $iscosity correction data rarely ta9es place4ne$ertheless! the student would &e wise to periodically consult the Hydraulic /nstitute*s we& site atwww.pumps.org to determine the rele$ance o% the latest release o% the %&!'()' ump !tandards4.

/t must &e recogni'ed that #he Hydraulic /nstitute per%ormance correction charts are at &est appro,-imations o% estimated $iscous pump per%ormance. #hey were created &ased on empirical rather thanrigorous analytical techni(ues. #heir use assumes uni%orm :Lewtonian< li(uids and use o% pumps o%

con$entional design in a normal operating range. #he student must also &e aware o% the %act thatcertain groups o% %luids do not &eha$e as one would logically e,pect. Lewtonian %luids :water! wa-ter-li9e! most oils! etc. < react in a logical %low &eha$ior4 they are una%%ected &y the magnitude and9ind o% motion to which they are su& ected. Lon-newtonian %luids! two groups o% which are 9nownas dilatants and thi,otrophics! react counter intuiti$ely to e,ternal agitation or motion. luids in


. " = =0 000

11

21

E0 2J0! gal

hr hr min gpm


9/12


these categories are &eyond the scope o% this course. ",amples o% non-newtonian %luids are sandslurries! gels! paper stoc9! and the li9e.

Re%er to igure 1 on page A4 it is a schematic or skeleton representation o% the much more richlydata populated pu&lished correction chart : #he Hydraulic /nstitute

C 0. 0 :the correction %actor %or capacity! dimensionless

Selecting a Centrifugal Pump for Viscous Flow

Documents

Transcript of Selecting a Centrifugal Pump for Viscous Flow