variable speed fluid drives for pumps

8/19/2019 Variable Speed Fluid Drives for Pumps

1/20

Variable Speed Fluid Drives for Pumps

February 4, 2010

5 Votes

The contents of this post were originally published in the Variable Speed Drive chapter of the

International Pump Handbook .

In almost all pumping applications, some means is required to provide variable flow andorvariable pressure of a liquid as it leaves a pump and enters a process application! "wo alternativeapproac#es e$ist, eit#er %a& a Variable 'peed (rive for a pump, or %b& a )onstant 'peed (rive for a pump wit# a disc#arge control valve t#at can provide a variable disc#arge flow to t#e processapplication!

"#e primary emp#asis of t#is post is to focus on Variable 'peed Fluid (rives for pumps suc# as boiler feed pumps, condensate pumps, and pumps for ot#er fluids and applications! *evert#eless, for completeness, ot#er types of Variable 'peed (rives for load equipment are briefly discussed!

Variable 'peed (rives t#at are available today include Fluid (rives, +ec#anical (rive 'team"urbines, lectronic Variable Frequency (rives, and +agnetic (rives!

In almost all situations, a pump and t#e associated variable speed drive will be substantially moreefficient and more reliable t#an a constant speed pump wit# a disc#arge control valve! "#ec#oice of w#ic# variable speed drive to use usually depends upon t#e application, including suc#c#aracteristics as t#e environment %temperature, dustiness, remoteness&, t#e e$pected reliability,t#e importance of #aving critical parts not becoming obsolete, maintainability, capital cost, andoverall efficiency!

+agnetic (rives are discussed elsew#ere in t#is International -ump .andboo/ ! "#ey #aveapplications up to 00 or 400 #p or per#aps slig#tly #ig#er! "#ey are limited in power due to t#edifficulty of dissipating #eat losses!

lectronic Variable Frequency (rives can be used in a variety of applications! "#ey are verycommon for low power applications of less t#an 1000 #p, and in ot#er cases t#ey can be used inapplications over 10,000 #p! "#ey can also be used for applications up to several t#ousand rpm!it# gear bo$es, t#ey can go to very #ig# speeds or very low speeds! .owever, t#ese drives

http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446http://www.amazon.com/Pump-Handbook-Igor-Karassik/dp/0071460446


2/20

consist of very sop#isticated solid state electronics to convert incoming p#ase 3) power atconstant frequency %50 or 0 #ert& to () power and t#en to invert t#e () power bac/ tovariable frequency p#ase 3) power! It is quite typical for isolation transformers to be used ont#e incoming power, for substantial air conditioning pac/ages to be used to cool t#e electronics,and for step up transformers to be used to convert t#e voltage to suit t#e motor! For #ig# power

applications, t#e motors are usually made specifically for variable frequency drives in order to beable to #andle t#e very #ig# frequency components t#at arise in t#e solid state electronics! 3c#aracteristic of t#ese applications t#at s#ould be addressed is t#e very #ig# frequency pulsationst#at are in#erent in t#e inverter process %() to 3)&, as t#ese pulsations affect bot# t#e electricalsystem and t#e mec#anical system, and t#ese pulsations are emp#asied during rapid c#anges oft#e rotational speed of t#e motor and pump!

+ec#anical (rive 'team "urbines are used to provide variable speed power to pumps of manydifferent applications, small to very large! In main power plant applications, steam from ane$traction port of t#e main turbine can be used to drive variable speed steam turbines t#at drive boiler feed pumps!

Variable 'peed Fluid (rives of t#e type /nown as 6.ydro/inetic7 are t#e most important onesfor driving larger pumps, and for t#is reason, t#ey are t#e focus in t#is )#apter! "#is type offluid drive can be made from a few #orsepower to over 40,000 #p! .owever, today, most .ydro/inetic fluid drives are made for powers over 00 to 400 #p! For powers under t#is level,electronic variable frequency drives and t#e ot#er types of fluid drives are used more often!

3s an aside, t#e terms Fluid )oupling and .ydraulic )oupling are sometimes usedinterc#angeably wit# Fluid (rive!

3 .ydro/inetic Variable 'peed Fluid (rive usually ta/es input power from a constant speed

power source suc# as a motor of eit#er induction or sync#ronous type and delivers variable speed power to a process pump! "#e fluid drive output s#aft speed %pump speed& is controllable instepless speed c#anges in a range, typically, from 208 to 9:!58 of t#e input s#aft speed! "#eoutput s#aft speed is e$tremely smoot#, t#at is, t#ere are no torsional e$citation pulsations! 3ninteresting and valuable application for certain main power plants occurs w#en input power ista/en from t#e main turbinegenerator s#aft into a fluid drive and t#e fluid drive delivers variablespeed power to a large boiler feed pump! "#is arrangement is called a 6s#aft driven fluid driveand boiler feed pump7!

It s#ould be noted t#at every type of variable speed drive can be used wit# a constant ratiogearbo$ to ac#ieve t#e actual speed range desired by t#e pump application! .owever, t#e onlytype of variable speed drive t#at includes a gearbo$ wit#in t#e drive is t#e type called 6Variable'peed ;eared Fluid (rive7!


3/20

Figure 1< 6.ydro/inetic7 variable speed fluiddrive

Figure 2< 'iing )#art for Fluid (rives.ydraulic diameters are s#own on diagnol

$ample< 6:07 signifies a :!0 inc# diameter

Design of Hydro-kinetic Variable Speed Fluid Drives:

"#e design of a 6.ydro/inetic7 Variable 'peed Fluid (rive wit# an oil conditioning system iss#own in Figure 1! "#e primary mec#anical components of a variable speed fluid drive assemblyare %a& an outer #ousing or tan/ t#at supports t#e bearings, #olds t#e oil, and provides a

protective cover, %b& an input element comprising an input s#aft and coupling #ub, an impeller,an intermediate casing, and an outer casing %or scoop tube casing& all attac#ed to eac# ot#er androtating toget#er, %c& an output element comprising an output s#aft, a coupling #ub, and a runnerall attac#ed to eac# ot#er and rotating toget#er, %d& =ournal and t#rust bearings t#at support eac#s#aft, %e& oil used as a power transmitting medium between t#e impeller and runner and as alubricant for t#e bearings, %f& a moveable scoop tube t#at slides in and out to control t#e amountof oil t#at remains resident in t#e rotating casings> and %g& an oil conditioning system!


4/20

"#e impeller and runner are structures wit# vanes and poc/ets between t#e vanes s#aped muc#li/e a #alf of a grapefruit wit# t#e linings t#at separate t#e fruit intact and t#e fruit removed! "#eimpeller and a runner face eac# ot#er wit# a small gap between t#em, on t#e order of 18 of t#ediameter of t#e impeller %or runner&, but t#ey do not touc# eac# ot#er mec#anically!

"#e impeller and t#e attac#ed rotating casing structure retain t#e wor/ing fluid, usually oil, ass#own in Figure 1! "#e runner is located wit#in t#e envelope of t#e rotating input element! "#eactual mec#anism causing t#e runner to rotate is t#e /inetic action of t#e oil< t#e impellertransmits energy to t#e oil and t#e oil transmits energy to t#e runner, as described below!

Fluid (rives of t#e Variable 'peed Fluid (rive type are sied according to t#e equation<

#ere<

Factor ? appro$imately 2!1, but depends on many design details suc# as e$act poc/et s#ape, vaneangles, and t#e circuit oil flow rate!

* out ? 0!9:5 $ * in %rated speed point& %rpm&

(ia ? .ydraulic diameter of t#e impeller and runner %inc#es&!

Figure 2 is a c#art s#owing power %#p and /w& as a function of speed for various #ydraulicdiameters! "#e actual sectors t#at are applicable for different #ydraulic diameters will varyslig#tly depending upon t#e many design details of a drive! )onsequently, t#is c#art s#ould be

considered to represent trends for different fluid drives, but s#ould not be considered to representa specific fluid drive!

3 fluid drive of t#e .ydro/inetic Variable 'peed design operates as follows< "#e tan/ contains alevel of oil, often described as I'@ V; 2, as lig#t turbine oil! 3n oil pump supplies cooled andfiltered 6lube oil7 to t#e bearings and 6circuit oil7 or 6wor/ing oil7 to t#e 6circuit7 meaning t#ecavity comprising t#e poc/ets between t#e vanes of t#e impeller and runner, as s#own in Figure1!

Aecause t#e impeller and runner are rotating, t#e circuit oil is t#rown outward under centrifugalforce, so t#at t#e oil forms a torus in t#e rotating casing assembly! Aecause t#e impeller is being

driven by a driver, generally at a fi$ed speed, t#e impeller rotates faster t#an t#e runner!)onsequently, oil t#at is in t#e impeller is sub=ect to greater centrifugal acceleration t#an is oilt#at is located in t#e runner poc/ets! )onsequently, a parcel of oil t#at starts in an impeller poc/et moves in a generally circular pat# %relative to t#e rotating poc/et& to t#e radially outer portion of t#at impeller poc/et, gaining momentum along t#e pat#, and leaves t#e impeller poc/et traveling across t#e gap wit# bot# circumferential and a$ial velocity components, t#en t#e pac/et of oil enters a runner poc/et w#ere because t#e runner is rotating slower t#an t#e impeller,t#e oil pac/et #its t#e wall of t#at runner poc/et, and departs momentum to t#e runner vane, after


5/20

w#ic# t#e pac/et of oil moves radially inward along a generally circular pat# %relative to t#erunner poc/et& until it leaves t#e runner poc/et in a pat# wit# a$ial and circumferential velocitycomponents, until t#e pac/et of oil enters an impeller poc/et w#ere because t#e impeller isrotating faster t#an t#e runner, t#e wall of t#at impeller poc/et #its t#e pac/et of oil and impartsmomentum to t#e pac/et of oil, and t#en t#e pac/et of oil again follows a circular pat# %relative

to t#e impeller poc/et& radially outward wit#in t#e impeller poc/et until it again crosses t#e gapto enter t#e runner!

"#e combination of bot# circular pat#s wit#in t#e poc/ets and a circumferential pat# of t#e poc/ets around t#e s#aft centerline provides a general tra=ectory of eac# pac/et of oil t#at is aspiral t#at travels in circles, as s#own in Figure !

*ote t#at in a .ydro/inetic Fluid (rive, t#e energy is transmitted from impeller to runner usingt#e principal of 6momentum e$c#ange7< "#e vane of t#e impeller #its a pac/et %mass& of oil andincreases its velocity, and t#en t#e pac/et %mass& of oil #its t#e vane of a runner and t#e velocityof t#e oil is reduced, transmitting momentum, and t#e process continues!

Fluid drives are 6constant torque7 mac#ines, t#at is, t#e torque t#at is absorbed by t#e impellerfrom t#e input s#aft to drive t#e circuit oil equals t#e torque t#at t#e circuit oil imparts to t#erunner t#at drives t#e output s#aft and t#e load, w#ic#, in t#e present case, is a pump!

Figure ! "#e "ra=ectory of ac# -ac/et of @il is a 'piral "#roug#t#e Impeller and Bunner as "#ey Botate about t#e '#aft )enterline!

"#e degree of coupling between t#e impeller and runner depends upon t#e level of filling of t#eimpeller and runner c#amber, and t#e level of filling is controlled by t#e position of t#e scooptube tip! "#e more mass of oil t#at can traverse t#e spiral pat#, t#e more coupling t#at e$ists!"#e degree of coupling is controlled by t#e radial position of t#e scoop tube tip!


6/20

3 scoop tube is a #ollow tube wit# a tip t#at is s#aped to scoop oil from t#e inside surface of t#erotating torus of oil and to drive t#e oil t#roug# t#e tube so t#at it e$its t#e rotating fluid driveelement!

Figures 4!a and 4!b s#ow two different levels of filling! In Figure 4!a, t#e scoop tube tip is

moved radially outward %away from t#e s#aft centerline& providing a small torus of oil wit# asmall mass of circulating oil between t#e impeller and runner, wit# t#e result of low coupling!

In Figure 4!b, t#e scoop tube is moved radially inward %toward t#e s#aft centerline&, providing alarge torus of oil wit# a large mass of circulating oil between t#e impeller and runner wit# t#eresult of #ig# coupling!

Figure 5 presents a generalied c#art t#at s#ows t#e torque transmission of a fluid drive as afunction of scoop tube position and output s#aft %pump& speed! It is important to realie t#at t#elocations of t#e constant scoop tube position lines vary according to many design detailsincluding t#e poc/et s#apes in t#e impeller and runner, as well as vane t#ic/nesses, vane angles,

and t#e circuit oil flow! )onsequently, t#is c#art s#ould be considered to represent t#egeneralied trends of variable speed fluid drives and not any particular fluid drive!

Fundamental Equations for Fluid Drives:

'everal fundamental equations for fluid drives are given below<

"orque in ? "orque out ? "orque %lbfin&

-ower in ? "orque in %lbfin& $ Input BotCl freq %radsec&

-ower out ? "orque out %lbfin&$@utput BotCl freq %radsec&

-ower loss ? -ower in D -ower out ?

"orque %lbfin&$%Input BotCl freq D @utput BotCl freq& %radsec&

.eat power %btu #r& ? -ower Eoss %#p $ 2544 btu #p#r&

It is t#e #eat t#at is generated by t#e lost power t#at requires t#e circuit oil in a fluid drive to bee$c#anged continuously!


7/20

Figure 4!a! 3 'mall "orus of )ircuit @il -rovides 'mall )oupling!

Figure 4!b! 3 Earge "orus of )ircuit @il -rovidesEarge )oupling

)ertain losses are not included in t#e above equations! "#ese include bearing power losses andcircuit oil #andling losses! Aearing losses for t#e input s#aft are constant and for t#e output s#aftincrease wit# increasing speed! )ircuit oil #andling losses are a function of scoop tube positionand circuit oil flow rate! +ost fluid drives #ave a constant, or almost constant, circuit oil flowrate, so t#e circuit oil #andling losses are greatest w#en t#e scoop tube tip is at t#e largestdistance from t#e s#aft centerline, w#ic# corresponds to minimum output s#aft %or pump& speed,and t#ese losses are t#e least w#en t#e scoop tube tip is closest to t#e s#aft centerline, w#ic#corresponds to ma$imum output s#aft %or pump& speed! "#e net result is t#at t#e sum of t#e bearing losses and circuit oil #andling losses can be considered to be a constant, typicallyranging from 8 to 4 8 of t#e rated power of t#e fluid drive!


8/20

"#e efficiency of a variable speed fluid drive of t#e #ydro/inetic type can typically be e$pressedas

It is very common to use a slip speed %Input speed D @utput speed& of 2!58 at t#e design point,t#oug# in some applications, t#e slip speed at t#e rated operating point is between 2!0 8 to over4 8 ! sing a slip speed of 2!58 at t#e design point, t#e design point efficiency is 1008 D 2!58%slip speed loss& D !58 %mec#anical losses& ? 948 !

If a set of gears is included wit#in t#e fluid drive tan/ and it is located on t#e output side of t#efluid drive circuit, t#e runner output s#aft and a gear s#aft can be combined, eliminating two =ournal bearings and a pair of t#rust bearings! In t#is case, t#e power losses will increase byappro$imately 1!5 8, rat#er t#an t#e normal 2!0 to 2!5 8 additional loss w#en an independentgear bo$ is used between t#e fluid drive and t#e pump!

omparison of Efficiencies of Variable Speed Drives:

#en comparing efficiencies for alternative types of variable speed drives, it is essential todetermine t#e following factors<

First, before t#e efficiency is calculated it is essential to determine if t#e equipment beingevaluated will provide variable speed power under all of t#e conditions t#at are required G

'econd, determine if t#e equipment will respond as fast to a c#ange of load as is required ordesired G

"#ird, determine if t#e efficiency of alternate types of equipment will be evaluated at severaloperating points from low load, say 208, to full load being included, or will t#e evaluation be based on only one operating point, suc# as full power G +any types of equipment can bedesigned to operate well at or near a single operating point suc# as full load, but t#ey do not perform well w#en operating away from t#e design point of full load!


9/20

Figure 5! )#art of @utput "orque Batio vs! @utput 'peed Batio for (ifferent 'coop "ube-ositions!

3 good way to evaluate alternative forms of variable speed drives is to e$amine t#e annual costsfor operating eac# complete drive train at several operating points, giving weig#t for eac# load

point in proportion to t#e number of #ours t#at t#e equipment is e$pected to operate at eac# oft#ese points during a year! "#e reason is t#at w#en operating at or near full load, t#ere is #ig# power, t#ere is #ig# efficiency, and t#ere are usually a large number of #ours, so t#e actual costsat t#is operating point are quite favorable! "#en t#ere are times w#en t#e drive train and pumpwill be operating at low load, at w#ic# time t#e efficiency is low, very little power is transmitted,and t#e number of #ours at t#is point is limited, wit# t#e consequence t#at, w#en averaged wit#t#e #ig# efficiency operating points at #ig# load conditions, t#e average efficiency is quitesatisfactory! #en performing t#ese evaluations, it is very critical t#at an accurate value of t#eactual power consumption for eac# evaluation point be obtained and used! "#is includes suc#t#ings as %a& for steam turbines, t#e actual #eat rate at 6off design7 conditions w#en e$tractionsteam is not available in sufficient quantity and main steam or steam from a 6pony boiler7 may

be needed, and %b& for electronic variable frequency drives, t#e air conditioning load for t#eelectronics as well as t#e transformer, wiring, and motor losses, especially t#e losses due to t#every #ig# frequencies! !

onstruction of Fluid Drives and t!e "il onditioning

Systems:

Variable speed fluid drives are built by a number of manufacturers around t#e world! +ostmanufacturers #ave developed a range of designs of fluid drives for specific applications, andt#ese t#at #ave become 6standardied lines7 for t#at manufacturer! +any applications require

specialied designs t#at are not among t#e standardied designs, due to t#e environment, power,speed, or turn down conditions, and upon request, manufacturers will often provide t#osespecialied designs!

Fluid drives can be made wit# #oriontal s#afting or vertical s#afting!

"#e materials of construction of fluid drives vary widely! @uter #ousings and certain bearing#ousings are usually made of fabricated steel, may be split #oriontally or may be stac/ed fromone end, and may be made from t#ic/ steel or from t#in steel! In certain cases, t#e outer#ousings and certain bearing #ousings may be made of cast iron or cast steel!

"#e impellers and runners may be cast aluminum of a variety of grades and #eat treats, cast steel,fabricated steel, or milled from solid forgings of alloy steel! For a fluid drive wit# a #ydraulicdiameter of 25 to 2: inc#es, t#e impeller and runner poc/ets are eac# about inc#es deep a$iallyand t#e gap between t#e impeller and runner faces is on t#e order of 14 inc# to H inc#! -oc/etsies and gaps for ot#er fluid drive sies are larger or smaller in proportion to t#e sie of t#e fluiddrive #ydraulic diameter!


10/20

'#afts and ot#er rotating casings are usually made of carbon steel or alloy steel! .eat treatmentmay or may not be used! "#e =ournal and t#rust bearings can be rolling element design orAabbitted bearings, and t#e c#oice depends upon t#e speed, sie limitations, operating duty,required life, and price! Bolling element bearings come in a range of duties and E10 %or A10&lives! Aabbitted bearings may be made of simple fi$ed s#apes or t#ey can be made wit#

e$tremely robust tilting pad designs! Fle$ible coupling types range from gear to diap#ragmstyles, and t#ey may be supplied by t#e customer, by t#e manufacturer, or in some cases, t#e#ubs may be made integral wit# t#e s#afts! Instrumentation suc# as t#ermocouples or resistancetemperature devices %B"(s& and vibration pic/ups, pro$imity or seismic, may or may not beincluded!

@il conditioning systems can be e$tremely simple, comprising one pump, electric or s#aft driven,one cooler, one filter, and one sig#t glass to determine t#e oil level in t#e tan/! 3lternately, t#eycan be quite comple$ being built according to t#e #ig#est level of 3-I specifications wit# duple$3) main oil pumps, a () emergency pump, duple$ oil coolers, duple$ filters, all stainless steel piping, various isolation valves, pressure control valves, actuator for t#e scoop tube, additional

lube oil requirements for t#e main motor and main pump, and wit# electronic transmitters for pressure, temperature, and oil levels for interfacing wit# ()' systems! Aecause t#ere are noindustry standards t#at broadly apply to all fluid drives, it is essential for t#e user to detail all oft#e desired features in t#e purc#asing specification! 3s will be seen below, t#e #eat loss in afluid drive depends upon t#e specific operating points in t#e application, and several operating points s#ould be provided to t#e manufacturer, including t#e ma$imum power and speed point,t#e minimum power and speed point, and two or t#ree interim operating points, especially t#elowest pressure wit# t#e ma$imum flow! "#e reason for providing suc# details is t#at t#ere is noreason to use larger pumps andor larger #eat e$c#angers t#an are required to meet t#e userCsoperating conditions and, at t#e same time, to maintain t#e ma$imum oil temperatures below t#eo$idation limits of t#e particular oil to be used %generally in t#e range of 1H5 degrees F&! #en

gears are included in t#e mec#anical design of t#e fluid drive, t#e service factors for gear setslocated on t#e output side of t#e fluid drive do not need to be so #ig# as w#en t#ey are located ont#e input side, particularly w#en sync#ronous motors are involved!

Aecause t#ere is suc# a wide range of design features t#at can be used in fluid drives, and because t#e tec#nology content is t#e principal controller of price, t#e pricing can vary widelyfor different fluid drives for t#e same ma$imum power and ma$imum speed! In ot#er words,specifications addressing only t#e ma$ speed and ma$ power are not sufficient to specify a fluiddrive adequately, especially t#e oil system for a fluid drive, because t#e most difficult operating points for fluid drives are in t#e 6turn down7 operating points w#en t#e output s#aft speed is int#e neig#bor#ood of 2 of t#e input s#aft speed! It is e$tremely #elpful to t#e fluid drivemanufacturerdesigner if t#e user actively participates in developing t#e design details of t#eequipment!

#en a Variable 'peed Fluid (rive and its mating @il )onditioning 'ystem are properlydesigned for a given application, t#ey become relatively simple mec#anical systems wit#e$tremely #ig# reliability and availability! It is quite common for variable speed fluid drives tooperate for over ten years wit#out inspection or any maintenance ot#er t#an c#anging oil filters!#en maintenance of a fluid drive is required, parts can be obtained or can be made as required!


11/20

It is rare for a fluid drive design to become obsolete, w#ic# #as been a problem on certain typesof electronic variable frequency drives!

"#e speed of t#e output rotating assembly is controlled by t#e position of t#e scoop tube tipw#ic# is typically positioned by a scoop tube actuator, being driven by a 420 ma signal from t#e

plant control system, suc# as t#e central ()' %(istributed )ontrol 'ystem&, for t#e entire processunit!

It is interesting to note t#at t#e Fluid (rive of t#e .ydro/inetic design was invented by (r!.erman Fottinger in ;ermany in 1905, and fluid drives of t#is design are now used all over t#eworld! 3 te$tboo/ on Fluid (rives, also called Fluid )ouplings, t#at may be useful to t#ose w#o#ave an interest in tec#nical details is entitled 'tromungs/upplungen und 'tromungswandler,Aerec#nung und onstruction, by Ing! +auriio olf, 'pringerVerlag, Aerlin;ottingen .eidelberg, 192! Fluid (rive designs #ave been tuned and ad=usted and adapted to manyapplications, but t#e fundamental features, ot#er t#an instrumentation, #ave not significantlyc#anged!

Po#er $oss and %emperature onditions for Variable Speed

Fluid Drives&

3 very simplified approac! for evaluating t#e #eat loss of a variable speed drive assumes t#att#e power required by a pump is only proportional to t#e cube of t#e pump speed %or output s#aftspeed&! "#is is correct under certain conditions, but because pumps operate at points on t#e#eadcapacity plot t#at are at some distance from t#is simplified curve, a more appropriateevaluation of t#e #eat loss of a variable speed fluid drive for boiler feed pump service is given below via a series of #eadcapacity )#arts!

In t#e simplified approac#, assume t#at t#e power of a pump is proportional to t#e cube of t#e pump speed %or fluid drive output s#aft speed &<

-ower out ? * out J

3nd "orque out ? * outJ2!

*ow "orque in ? "orque out ? "orque,

.ence <

-ower in ? "orque in $ * in

-ower in ? * outJ2 $ * in

-ower Eoss ? -ower in D -ower out ? * outJ2 $ * in D * out J

"#e ma$imum loss occurs at * out ? 2 $ *in


12/20

"#e ma$imum loss at t#is output s#aft %pump& speed is appro$imately 158 of t#e rated input power! Including bearing and circuit oil #andling losses, t#e ma$imum loss is on t#e order of18 to 1: 8 of t#e rated input power!

"#e circuit oil flow pattern wit#in t#e fluid drive c#amber consisting of t#e impeller poc/ets and

runner poc/ets and t#e a$ial gap between t#em can be considered to be a spiral in general, Figure, but it is e$tremely turbulent!

"#e turbulence of t#e circuit oil is affected by t#e speed difference, t#e torque transmitted, andt#e amount of circuit oil in t#e rotating element! 3s indicated above, t#e speed at w#ic# t#ema$imum power loss occurs is w#en t#e output s#aft is at appro$imately 2 of t#e input s#aftspeed, w#ic# is also t#e speed at w#ic# t#e greatest turbulence occurs! "#us, for a fluid driveinput speed of 00 rpm, t#e ma$imum turbulence occurs w#en t#e output s#aft speed %pumpspeed& is in t#e range of 2400 rpm! It is also important to note t#at for a given output s#aft speed bot# t#e turbulence and t#e power loss increases wit# increased torque!

Five #eadcapacity c#arts are provided below t#at are based on t#e #eadcapacity plot for a main boiler feed pump for a :5 + "urbine;enerator! "#e #ead is s#own in units of psig becauset#e intent of t#ese c#arts is to s#ow trends for understanding of t#e issues at #and rat#er t#an to be completely accurate! For specific applications, t#e grids can be calculated wit# all of t#enormal details considered, but it will not c#ange t#e principles at #and! "#e grids on t#e c#artsare made of constant speed lines %generally #oriontal& and constant efficiency lines %generallycurving upward&! "#e grids are for t#e same conditions on all of t#e c#arts! (ata is presented atcertain intersection points of t#e grids!

Figure %)#art 1& presents t#e s#aft power absorbed by t#e pump %out of t#e fluid drive&!

Figure : %)#art 2& presents t#e s#aft power absorbed by t#e fluid drive %out of t#e motor or"urbine;enerator& at t#e same intersection points of t#e same grid!

Figure H %)#art & presents t#e power lost to t#e circuit oil in t#e element %power into t#e fluiddrive minus power out of t#e fluid drive&!

Figure 9 %)#art 4& presents t#e )ircuit @il (isc#arge temperature t#at results from t#e power lostin )#art wit# t#e circuit oil input temperature of 120 deg F and a constant circuit oil flow rateof 400 gpm!

Figure 10 %)#art 5& presents t#e circuit oil flow rate %gpm& required to maintain a constant

temperature rise of t#e circuit oil %t#e difference between t#e circuit oil temperature entering t#efluid drive and t#e circuit oil disc#arge %)@(& temperature from t#e fluid drive rotating element!

For instance, in Figures t#roug# 10 % )#arts 1 t#roug# 5&, following t#e constant speed line for2400 rpm from left to rig#t, it can be seen in t#ese c#arts t#at power transmission increases, t#etorque increases, t#e power %#eat& losses increase, and t#e circuit oil disc#arge %)@(&temperature rises! 3ll of t#is is associated wit# increased circuit oil mass flow and associated


13/20

increased turbulence of t#e circuit oil w#en traversing t#e constant speed line going from low boiler feed pump flow to #ig#er pump flow!

onstant %!rottle Pressure "peration Vs& '(educed

Pressure) or 'Sliding Pressure) "peration:In t#e 190 era, almost all turbinegenerators were originally designed for constant pressure att#e t#rottle, e$amples being 1H00 psig, 2000 psig, 2400 psig, and at super critical pressuresabove 00 psig! "#e corresponding system resistance curve for t#e boiler feed pump is a line beginning at t#e rated operating point of 11,0 #p on t#e 510 rpm curve on t#e #ead capacityc#art of Figure %)#art 1& and traverses t#e c#art wit# a downward slope until it reac#ed t#eminimum flow point for t#e pump at 2,929 #p on t#e 2H00 rpm curve! )onsequently, t#e fluiddrive of t#is e$ample would never #ave to operate below 2:00 rpm! "#e typical operating procedure, at least as anticipated in t#e design stage, was t#at t#e startup pump would be used toget t#e unit up to minimum load and full t#rottle pressure! "#is process wor/s, and is used for

supercritical units w#ic# #ave #ig#er operating pressure t#an t#is e$ample, but it is almost never used by plant operators for subcritical "; units! 3s can be seen by inspecting t#ese AF- #eadcapacity c#arts, /eeping t#e operating pressure #ig# so t#at t#e output s#aft speed is alwaysabove 2:00 rpm #as t#e result t#at t#e #eat dissipation in t#e circuit oil is limited to less t#anappro$imately 1500 #p, as s#own in Figure H %)#art &! "wo important consequences are %a& t#att#e #eat e$c#angers do not #ave to be sied for t#e #ig#er #eat dissipation on t#e order of 2200#p t#at occurs if t#e fluid drive were to be operated at 2400 rpm and at ma$imum flowconditions of appro$imately 2,000,000 lbs#r at t#is speed, and %b& t#e circuit oil pump sie can be limited in sie for similar reasons, bot# of w#ic# save capital cost, and p#ysical space on t#eoil conditioning s/id!

Figure ! %)#art 1& -ower 3bsorbed by t#e -ump '#aftFrom t#e Fluid (rive @utput '#aft!


14/20

Figure :! %)#art 2& -ower 3bsorbed by t#e Fluid (rive Input '#aftfrom t#e +otor or "urbine;enerator @utput '#aft

at t#e 'ame @perating -oints of t#e ;rid!

Figure H! %)#art & -ower Eost to t#e )ircuit @il in t#e Fluid (rive lement%quals -ower into t#e Fluid (rive +inus -ower @ut of t#e Fluid (rive&

at t#e 'ame @perating -oints of t#e ;rid!

+ost operators of subcritical units operate t#ese units in a different manner, as follows< "#etypical startup ob=ective is to get t#e "; unit onto t#e main pump as soon as is practical! "#e pressure is limited to t#e 6silica7 limit, say 1200 psi at first, and t#e flow would be increased tot#e ma$imum possible by operating t#e t#rottle valves to t#e Valves ide @pen condition! "wo benefits are ac#ieved by operating in t#is manner< 3 greater feed water flow will clear t#e boilerwater faster and, #ence, t#e boiler pressure can be increased more rapidly! "#e second benefit ist#at t#e electrical power generated is ma$imied during t#e water cleanup process! "#e #eatdissipated by t#e fluid drive at 1200 psi disc#arge and 500,000 lbs#r is around 1500 #p, similarto t#e amount of #eat dissipated at points along t#e system resistance curve for t#e constantt#rottle pressure application! .owever, as t#e flow is increased, t#e #eat dissipated increases,

and if t#e #eat e$c#angers and circuit oil pumps are not adequately sied for 6reduced pressure7or 6sliding pressure7 operation, t#en t#e fluid drive circuit oil disc#arge pressure will get #ot, upto t#e 200 degree F range, as s#own on Figure 9 %)#art 4&!

3 reason for limiting t#e oil temperature to 1H5 degrees F is t#at above t#is temperature, oil int#e presence of air will degrade! 3not#er reason for limiting t#e temperature is t#at if waterenters t#e oil, t#en t#e #ig#er t#e temperature, t#e faster sludge will develop, and sludge can


15/20

centrifuge out in t#e casings causing unbalanced conditions wit# associated increased vibrationlevels!

#en t#e fluid drive and t#e oil system are in t#e specification andor design stage, t#e solutionto avoid over#eating conditions involves accounting for t#e #eat to be dissipated and t#e circuit

oil flow rates t#at s#ould be used to limit oil temperatures to 1H0 or 1H5 degrees F at t#e #otoperating conditions t#at correspond to low speed, #ig# AF- flow, and ad=usting t#e plantcooling water #eat balance accordingly!

If t#e fluid drive is already in AF- service for a "urbine;enerator, and t#ere is a desire tooperate t#e unit wit# 6reduced pressure7 or 6sliding pressure7, t#en it is necessary to identify t#eadditional #eat to be dissipated, and t#en install additional #eat e$c#anger and circuit oil pumping capacity! "#e most effective #eat e$c#angers are usually large single pass counterflowdesigns! .eat e$c#angers of t#is design may not be t#e lowest cost by t#emselves, but t#ey willli/ely obtain t#e lowest oil temperature leaving t#e coolers and t#ey will use t#e least amount of plant cooling water, providing significant economic benefits!

Figure 9! %)#art 4& "#e )ircuit @il (isc#arge "emperature "#at BesultsFrom t#e -ower Eost per )#art it# t#e )ircuit @il Input "emperature of

120 deg F and a )onstant )ircuit @il Flow Bate of 400 gpm!

Figure 10! %)#art 5& )ircuit @il Flow Bate %gpm& Bequired to +aintain a)onstant "emperature Bise of t#e )ircuit @il % "#e (ifference Aetween t#e

)ircuit @il "emperature ntering t#e Fluid (rive and t#e )ircuit @il (isc#arge %)@(&"emperature From t#e Fluid (rive Botating lement!


16/20

#ile more #eat is lost in a fluid drive on sliding pressure operation t#an on constant t#rottle pressure, t#e amount of power saved in t#e pumping process and in t#e reduction of t#rottlinglosses in t#e steam turbine is greater, providing a substantial net improvement in t#e overallcycle efficiency! For instance, t#e input power to t#e fluid drive in Figure at full pressure and50,000 lbs#r is 425 #p, w#ereas on sliding pressure conditions for t#e same +, w#ic# uses

about t#e same feed water flow, t#e input power is about 2500 #p, a savings of 1:5 #p, or 1!+, w#ic# at K:0 + represents around K100!#our! "#is savings is only t#e reduced power todrive t#e pump, and does not include reduced t#rottling losses, and reduced fan airflow and ot#er savings!

'liding pressure operation, including unconditional operation anyw#ere on t#e #eadcapacitymap, usually #as tremendous economic benefits! In most cases w#ere t#e process can use t#esecapabilities, t#e improved operating conditions can pay for t#e slig#t increase in capital cost for a#eavy duty fluid drive and oil system in a matter of a few mont#s! 3 reference boo/ foranalying sliding pressure operation in steam turbines is entitled valuating and Improving'team "urbine -erformance, by en )! )otton, 'econd dition, publis#ed by )otton Fact in

199H!

se of a )ontrol Valve to +aintain niform )ircuit @il (isc#arge %)@(& "emperatures<

3t very low speed D low AF- flow operation, t#e minimum speed at low or ero 8 scoop tube issubstantially controlled by t#e circuit oil flow rate! )ontrol of t#e pump speed, and #ence, t#eAF- flow is improved by minimiing t#e circuit oil flow to t#at required to /eep t#e circuit oildisc#arge %)@(& temperature below appro$imately 1H5 degrees F! "#is suggests t#at circuit oilflows s#ould be reduced, w#ic# is t#e opposite of t#e solution for t#e #ig# #eat conditiondiscussed above!

"#e compromising solution to t#is circuit oil flow issue is to install a circuit oil disc#arge %)@(&temperature control valve t#at limits t#e circuit oil flow to t#e fluid drive in order to maintain aconstant circuit oil disc#arge %)@(& temperature! (epending upon t#e application, t#is )@(temperature set point is set in t#e neig#bor#ood of 1H0 degrees F! "#e results of t#e use of t#is)@( valve are demonstrated in )#art 5, w#ic# is based on t#e #eat dissipated in )#art andupon a constant differential temperature %circuit oil disc#arge temperature D circuit oil supplytemperature& of :0 deg F! In t#is c#art, t#e )@( temperature remains constant and t#e flowvaries, w#ereas in )#art 4, t#e circuit oil flow is constant and t#e )@( temperature varies!3dditional benefits arise<

%1& "#e circuit oil flow is limited in t#e low output s#aft speed range, and t#is increases t#eeffectiveness of t#e scoop tube to control t#e output s#aftpump speed, reducing t#e minimumoutput s#aftpump speed w#en t#e scoop tube is at t#e minimum power position, and yieldinggreatly improved feed water control t#roug#out t#e low speed range!

%2& "#e circuit oil pump sie can be increased to suit t#e specific application so t#at w#en t#efluid drive is operated in t#e neig#bor#ood of 2400 rpm and it is generating a large amount of#eat, t#e circuit oil flow is adequate to limit t#e ma$imum temperature of t#e circuit oildisc#arging from t#e fluid drive element to appro$imately 1H0 to 1H5 deg F!


17/20

%& "#e power required to #andle t#e circuit oil is reduced at operating conditions away from t#e#ig# #eat one, increasing t#e operating efficiency of t#e unit!

%4& 3 constant temperature fluid drive maintains uniform elevation #eig#t of t#e input and outputs#afts and maintains uniform t#ermal e$pansion of t#e #ousing and all internal parts! "#is

e$tends t#e life of t#e fluid drive by minimies low cycle stresses, relative movement, or fatigueof t#e various parts!

It s#ould be noted t#at "BI #as been awarded ' -atent 5,15,H25 regarding t#e use of acircuit oil flow control valve t#at is controlled via a feedbac/ from t#e )@( "emperatureInstrument, as described above!

%ypical Vibrations of Hig! Po#er Variable Speed Fluid

Drive (otors:

In almost every fluid drive, t#e first critical speed of t#e input s#aft and t#e first critical speed oft#e output s#aft is eac# measurably above 00 rpm %0 #&! "#e mode of vibration of eac# s#aftin response to unbalance is typically a rigid body conical mode! "#at is, t#ere is almost no bending of t#e s#aft and t#e critical speed of eac# s#aft is controlled primarily by t#e stiffnessesof t#e oil films of t#e bearings and t#e structures t#at support t#e bearings for t#at s#aft!

(epending upon t#e condition of t#e =ournal bearings, it is possible to e$perience some subsync#ronous vibration suc# as 6oil w#ip7 w#ic# occurs at a fractional frequency, generally below#alf running speed! "#is type of vibration s#ould not be confused wit# a very unique type ofvibration t#at often arises in fluid drives, discussed below<

"#e =ournal at t#e inputinboard bearing of a #ig# powered variable speed fluid drive typically#as t#e #ig#est amplitude vibrations because it is t#is =ournal t#at is e$periencing t#e largestvibratory forces, bot# from t#e #eavy casings of t#e input rotor and from t#e unbalance of t#eturbulent oil contained in t#ese casings! 3 typical vibration frequency analysis for t#is inputinboard =ournal appears is s#own in Figure 11! "#e unique features of t#is figure are t#at t#reevibration components usually appear in t#is analysis< %a& t#e sync#ronous component of t#e inputs#aft, w#ic# in t#is case is 0 #, %b& t#e output s#aft rotational frequency, 9 #, and %c& afrequency %41 to 44 #& t#at is slig#tly above t#e output s#aft rotational frequency correspondingto t#e average effective rotational frequency of t#e circuit oil in t#e rotating element!


18/20

Figure 11! 3 "ypical Vibration Frequency 3nalysis of t#e Lournal at t#eInputInboard Aearing of a .ig# -ower Variable Frequency Fluid (rive

Figure 12! Variable 'peed ;eared Fluid (rive wit# -arallel 'tartup )apabilityt#at is sed to 'tart a Very Earge +otor #ic# can "#en(rive a Very Earge -ump, )ourtesy, "urbo Besearc#, Inc!

*lternate *rrangements of Drivers+ Fluid Drives+ and ,oiler

Feed Pumps for %urbine-enerator Service:

"#ese days, t#ere is great emp#asis of improving t#e overall efficiency of fossilfired generating plants!

It is very common to design every new plant as if it is going to operate at only one point< fullload! .owever, once built, many #ave to cycle to a reduced load at nig#t, and t#is includes t#oset#at use fossil fuels! In t#is case, #aving t#e unit designed so t#at it is relatively efficient at fullload as well as over a range of operating points down to low load is more appropriate!

http://www.turboresearch.com/http://www.turboresearch.com/


19/20

"o t#is end, an arrangement t#at s#ould be considered is one w#erein t#e fluid drive and 100 8capacity boiler feed pump are attac#ed directly to a steam turbinegenerator e$tension s#aft, andto #ave t#e entire plant designed for sliding pressure at t#e turbine t#rottle valve!

In t#e 190s, many suc# steam turbinegenerators were built t#is way, t#at is, wit# a single 100

8 6s#aft driven7 fluid drive boiler feed pump attac#ed to eit#er t#e turbine or generatore$tension s#aft! 3 number of t#ese units #ad #eat rates at t#e design point on t#e order of 900 btu /w#r! 'everal were built at t#e 25 + range! Bavenswood , t#en owned by)onsolidated dison of *ew Mor/, Inc!, was t#e first 1000 + unit in t#e world and wascommissioned in 195! It was built for sliding pressure and, incidentally, it #ad coal fired boilers! "#is is a crosscompound unit, and t#ere are two fluid driveboiler feed pumps, eac#rated at 1H,000 #p, one set on eac# end of t#e .- I- "urbine ;enerator 00 rpm s#aft line!it# todayCs tec#nology, a single fluid drive boiler feed pump set on t#e order of ,000 #pcould be used instead of t#e two t#at were used t#en!

)onsequently, today, wit# t#e availability of t#reedimensional computational flow dynamic

analyses for steam pat# design, it s#ould be possible to build large electrical generating plantsusing similar arrangements and obtain net #eat rates measurably below 9000 btu /w#r!

"#e primary advantage of t#e 6s#aft driven7 design is purely to ma$imie efficiency of t#e steam power! In t#e s#aft driven arrangement, t#e steam power is converted to mec#anical power atappro$imately 92 8 efficiency! *ear t#e ma$imum power point, t#e efficiency of a fluid drive isaround 9 8 at full load, and it will e$ceed :08 for t#e bul/ of t#e #p#ours of operation,t#oug# it will drop below t#at for very low load operation! In comparison, very few mec#anicaldrive steam turbines e$ceed :0 8 efficiency at t#e best operating points!

For a motor driven fluid drive and pump arrangement, t#e power supply c#ain is longer and less

efficient< "#e mec#anical power at t#e turbine coupling to t#e generator is 92 8 of t#e steam power %t#e same as above&, wit# additional losses of appro$imately 1!8 for t#e generator, 18for t#e transformer, and per#aps 8 for t#e motor! "#e result is t#at for every + needed todrive a fluid drive by a motor requires at least 98 more of t#e power from t#e turbine s#aft! Fora 1000 + "; t#at requires ,000 #p from t#e turbine to drive a boiler feed pump via t#e6s#aft driven7 feature would require 9,24 #p, or an additional ,24 #p to drive a motor driven boiler feed pump! "#is is significant in #eat rate calculations for a "urbine;enerator of t#issie!

Variable Speed Fluid Drive for $arge Sync!ronous .otors

#it! a Starting apability:

3s mentioned above, wit# todayCs tec#nologies, it is possible to build very large 1008 boilerfeed pumps and comparably large fluid drives for "urbine;enerators on t#e order of 900 to100 +!

Figure 12 presents an e$ample of a fluid drive design t#at can be used for t#e unique applicationof a very large sync#ronous motor driving a geared fluid drive large boiler feed pump! "#is


20/20

variable speed geared fluid drive #as features %a& to start a very large sync#ronous motor so t#att#is very large motor can be connected to t#e grid smoot#ly wit#out a large inrus# current, %b& toseparate t#e starting equipment from t#e gear train, and t#en %c& to use t#e large motor to drive acomparably large variable speed pump! "#is design permits t#e use of one large motor fluiddrive boiler feed pump to be used from startup to full load operation! *o startup boiler is

required as is required if a steam turbine driver is used, and no small startup boiler feed pump isrequired as is required for a s#aft driven fluid drive boiler feed pump!

"#e consequence is t#at a wide range of options for driving large boiler feed pumps, and ot#er pumps as well, is now available to system designers! Aeyond t#ose fluid drive arrangementsdiscussed #ere are alternative configurations t#at can be adapted or developed to meet t#e needsof t#ese and ot#er applications!

For more information about fluid drives and #ig# power applications, call (r! +el at 10H5:0 or visit our web site at #ttp

variable speed fluid drives for pumps

Documents