A Primer on Metering Pumps

8/17/2019 A Primer on Metering Pumps

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A Primer on

Metering Pumps

Written by Laurel Bloch, Milton Roy

Pumps & Systems, July and August 2007

A review of traditional metering pump designs, along with a look at some of the new

technologies that are being introduced.

The metering pump is a positive displacement chemical dosing device with the ability

to vary capacity manually or automatically as process conditions require. It features a

high level of repetitive accuracy and is capable of pumping a wide range of chemicals,

including acids, bases, corrosives or viscous liquids and slurries.

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Basic Components

he pumping action is de!eloped by a reciprocating piston "hich is either in direct contact

"ith the process #luid, or is shielded #rom the #luid by a diaphragm$ %iaphragms are

actuated by hydraulic #luid bet"een the piston and the diaphragm$ Metering pumps aregenerally used in applications "here one or more o# the #ollo"ing conditions e&ist'

• Lo" #lo" rates in ml(hr or gph are re)uired

• *igh system pressure e&ists

• *igh accuracy #eed rate is demanded

• %osing is controlled by computer, microprocessor, %+, PL+, or #lo" proportioning

•

+orrosi!e, ha-ardous, or high temperature #luids are handled

• .iscous #luids or slurries need to be pumped

he metering pump is usually dri!en by an A+ constant speed motor$ .ariable speed,

pneumatic, and hydraulic dri!ers are also utili-ed$ he li)uid end design and materials o#construction are determined by the ser!ice conditions and the nature o# the #luid to be

handled$ emperature, #lo" rate, #luid !iscosity, corrosi!eness and other #actors are

considered$


3/18

he dri!e mechanism translates the

rotary motion o# the dri!er into reciprocating mo!ement$ /ndustrial duty pumps "ill

submerge this portion o# the pump in an oil bath to assure reliability during continuous

operation$ Pump #lo" rate is adustable by !arying stro1e length, e##ecti!e stro1e length, orstro1ing speed$ Most metering pumps are supplied "ith a micrometer scre" adustment$ he

micrometer can also be replaced by an electronic or pneumatic actuator to adust pump #lo"rate in response to process signal$

etering !ump Characteristics

he pumping action is de!eloped by a reciprocating piston$ his reciprocating motionde!elops a #lo" easily represented by a sine "a!e$ Actual #lo" rate is determined by the

#ollo"ing #ormula'

lo" rate 3 %isplacement & +ycles per unit o# timehe metering pump #lo" !s$ stro1e characteristic cur!e is linear$ /t is not, ho"e!er,

necessarily proportional, in that 40 percent stro1e setting may not e)ual 40 percent #lo"$his is due to the #act that the calibration line may not pass through 0 on both a&essimultaneously$

By measuring #lo" at t"o stro1e settings,

plotting both points and dra"ing a straight line through them, other #lo" rates !s$ stro1e can be accurately predicted$ he steady state accuracy o# a correctly installed industrial grade


4/18

metering pump is generally 5(6 $0 percent or better$ Although a metering pump can

generally be adusted to pump at any #lo" rate bet"een 0 and its ma&imum capacity, its

accuracy is measured o!er a range determined by the pump8s turndo"n ratio$Most metering pumps ha!e a turndo"n ratio o# 0', "hich means that the pump is "ithin

its accuracy rating any"here bet"een 0 percent and 00 percent o# capacity$ 9e"generation metering pumps ha!e been introduced that #eature higher accuracy, and a greater

turndo"n ratio o# 00'$ ome o# these ne" designs "ill accurately dose any"here bet"een percent and 00 percent o# capacity$

he ma&imum capacity o# a

metering pump is determined by gear ratio, piston diameter, and motor RPM$

"iquid #nd $esigns

he li)uid end, "hich is re#erred to as the "etted part o# the pump, is selected to meet thespeci#ic ser!ice conditions o# the application$ Re)uired #lo" and pressure ratings are

considered, as "ell as the physical and chemical properties o# the li)uid$ he li)uid end8s

ability to protect the en!ironment is also a maor consideration "hen dealing "ith to&ic or

ha-ardous chemicals$

All li)uid ends ha!e se!eral #eatures in common$ irst,

the li)uid is dra"n into the "etted end by the rear"ard motion o# a piston, and e&pelled bythe #or"ard motion$ o achie!e this, the metering pump is supplied "ith chec1 !al!es at the

suction and discharge connection points$ he chec1 !al!es contain and release the chemical

based on system conditions and gra!ity$


5/18

%uring the suction portion o# the stro1e, the motion o# the piston li#ts the suction ball chec1

#rom its seat, allo"ing li)uid into the pump$ At the same time, the piston8s motion and

system bac1 pressure hold the upper chec1 !al!e :discharge; closed$ his is then re!ersedduring the discharge stro1e$

+hec1 !al!es are a!ailable in se!eral di##erent designs and con#igurations$ here are singleor double ball con#igurations and poppet style chec1 !al!es$ electing "hich type to use can

be determined by the manu#acturer based on capacity re)uired o# the speci#ic pump$

or e&ample, slurries or li)uids "ithlarge #ibers or particles can cause a single ball to lea1 i# particles are trapped bet"een the

ball and seat$ here#ore, a double ball chec1 o##ers more stability and accuracy$


6/18

Additionally, the #riction bet"een the piston and the pac1ing results in "ear that increases

lea1age$ Periodic pac1ing adustment is necessary to maintain !olumetric e##iciency$ o

a!oid problems associated "ith lea1age, consider a diaphragm style li)uid end$ he pac1ed plunger can handle pressures up to 4,0006psi, and temperatures to >006deg :"ith special

modi#ications;$

Disc Diaphragm

+ertain disc diaphragm li)uid

ends use a te#lon diaphragm to act as a barrier bet"een the piston and the process #luid$ he piston8s pumping motion is applied to hydraulic #luid "hich causes the diaphragm to #le&

bac1 and #orth as the piston reciprocates$

he hydraulically actuated diaphragm operates "ith e)ual pressure bet"een the hydraulicand process #luids$ his eliminates diaphragm stress, since the pressure is essentially e)ual

on both sides at all times$ "o contour plates encase the diaphragm to contain its tra!el$

he hydraulic and process #luids pass through care#ully engineered holes in the contour plates in order to come into contact "ith the diaphragm$ Relie# and re#ill !al!es control the

!olume o# hydraulic #luid$ An automatic air bleed !al!e continuously purges air #rom the

hydraulic #luid$

he diaphragm style pump is sealed, ma1ing it an e&cellent choice #or ha-ardous, to&ic, orcorrosi!e chemicals$ or e&tra protection, double diaphragm and lea1 detection

modi#ications are a!ailable, although they are considered redundant since this design ise&tremely durable$Because the process #luid must pass through relati!ely small holes in the contour plate, the

disc diaphragm li)uid end is not the best choice #or slurries$ With a #e" e&ceptions, disc

diaphragms are usually not the best choice "hen pumping !iscous #luids$ he discdiaphragm is capable o# handling #luids "here the re)uired inection pressure is ?4006psi or

greater and the #luid temperature e&ceeds 2406deg $


7/18

Mechanically Actuated Diaphragm Design

A mechanically6actuated diaphragm

pump represents the best balance bet"een lo" pump cost and high )uality per#ormance$Because it has -ero diaphragm lea1age, it ma1es a great pump #or critical and other"ise

e&pensi!e chemicals or "here en!ironmental issues are in!ol!ed$

he mechanically6actuated series is an e&cellent choice "here slurries and abrasi!echemicals are re)uired up to the pump8s ma&imum #lo" and pressure ranges$ hey are also

"ell tolerant o# high !iscosity li)uids, pro!iding an economical solution #or a !ariety o#

di##icult applications$Mechanically6actuated pumps operate "ith a plunger directly attached to the diaphragm$his attachment generally ta1es place #rom a bolt and clamp being placed through the

plunger and through the diaphragm$ he direct attachment o# the piston to the diaphragm

connects the pump8s dri!e and motor to the li)uid end$ he motion o# the pump dri!e mo!esthe plunger bac1 and #orth, thereby causing suction #rom the supply tan1 and pumping the

#luid o# choice through the attached con!eyance in#rastructure$

his type o# pump generally #inds pressure pea1s at 746psi, but is only limited to #lo" as amatter o# "etted end !olume$ Ma&imum li#e o# the pump can be achie!ed by replacing the

diaphragm at the recommended ser!ice inter!al$ Lea1 detection can be easily #ound #rom

the air6#illed chamber residing generally at atmospheric pressure on the dri!e side o# the

li)uid end$As "ith any chemical "here gas binding can be a problem, it is recommended that a

degassing !al!e be used to release o##6gases #rom the agitation or pressure changes

e&perienced by a li)uid ha!ing o##6gas characteristics$ ome o# these li)uids that cangenerate o##6gases as a result o# pressure losses are 9a


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turndo"n as high as 00'$ Mechanically6actuated diaphragm pumps are easily maintained

and pro!ide years o# ser!ice #or little e##ort$

Metallic Diaphragm Liquid End and Critical Head Service

Metallic diaphragm metering pumps are ideal #or use in critical, high pressure applications such as oil and gas plat#orms

and specialty industrial applications$ hey are especially use#ul "here temperatures and

pressures o# both the en!ironment and the process chemical can be !ariable or other"isedi##icult$ hese pumps are 1no"n #or their longe!ity and durability in many di##icult

applications$

Metallic diaphragm metering pumps are hydraulically6actuated in the same manner andstyle as a standard hydraulically6actuated dri!e li)uid end$ *o"e!er, the te#lon or other

usual diaphragm material is replaced "ith a special metal alloy particular to the application

to produce higher pressures than more traditional materials$ he metal design o# the

diaphragm also manages di##icult chemicals such as abrasi!es, slurries and other specialre)uirement compounds easier and more e##iciently than its more standard !ersion$

Many oil and gas o##shore drilling plat#orms re)uire metallic diaphragms because o# their

high reliability and longe!ity$

Advanced Liquid End Technology High Per!ormance Diaphragm


9/18

A high per#ormance diaphragm :*P%; li)uid end

operation is similar to a disc diaphragm in that it is hydraulically actuated and utili-es the

same shape and diaphragm$ /t is similar to a tubular diaphragm in the respect that the process #luid has a @straight through@ path through the li)uid end$ /ts lo" 9P*

re)uirements are similar to that o# a pac1ed plunger li)uid end$

he primary ad!antages o# a *P% are the uni)ue design #eatures that separate it #romtraditional design$

A hydraulically actuated diaphragm li)uid end design re)uires a re#ill system to compensate

#or hydraulic #luid that bleeds past the piston or through an air bleed !al!e during normaloperation$ *ydraulic #luid is also e&pelled #rom the chamber through the internal relie#

!al!e "hen the system e&periences e&cess pressure, and there#ore must also be replenished$

A *P% #eatures a mechanically actuated re#ill system :MAR; that o##ers a number o#ad!antages o!er traditional re#ill systems$ o understand the ad!antages o# a MAR,

traditional re#ill systems must #irst be e&plored$

Traditional Designs

raditional designs use a system that re#ills the chamber "hen a !acuum is created by the

inability o# the diaphragm to mo!e beyond the hydraulic contour plate$ /t also re#ills "hen

the suction is momentarily or permanently star!ed by accidental !al!e closure, insu##icient 9P*, or other similar occurrences$ When this happens, the hydraulic #luid chamber is

o!er#illed because a !acuum has been created e!en though the diaphragm has not been able

to tra!el rear"ard$


10/18

o a!oid diaphragm rupture due to o!er#illed hydraulic

oil, a process side contour plate stops the diaphragm8s #or"ard tra!el, and #orces thehydraulic relie# !al!e to open, thus e&pelling the e&cess #luid$ he contour plate is a conca!e

:actually, conca!o6con!e&; disc that supports the diaphragm and limits its tra!el$ he plate

has a series o# holes bored through it to permit the #luid to come into contact "ith thediaphragm$ he pattern and si-e o# these holes re)uires care#ul engineering to maintain thecontour plate strength re)uired to "ithstand the #orce o# the diaphragm e&perienced at

operating pressure$

he hydraulic contour plate does not cause any problems in pump operation since thehydraulic #luid passes easily through the contour plate holes$ *o"e!er, a process contour

plate, re)uired by traditional disc diaphragm li)uid ends, places limitations on the types o#

process #luids the pump can handle :such as slurries; since the process #luid must also passthrough contour plate holes$ he process contour plate also creates a pressure loss "hich

raises the 9P* re)uirement o# the li)uid end$

MA"S Design

A MAR eliminates the need

#or a process contour plate by assuring that the hydraulic #luid can only be re#illed "hen thediaphragm has tra!eled all the "ay bac1 to the hydraulic contour plate$ he diaphragm

presses against the MAR !al!e, "hich only then permits a poppet !al!e to open #rom the

!acuum created by insu##icient hydraulic #luid$ *ydraulic o!er#ill is there#ore impossible$With the process contour plate gone, the straight through path o# the process li)uid ma1es a

*P% a per#ect choice #or slurries and !iscous materials$ /t also lo"ers the 9P*re)uirements o# the pump, since pressure loss through a process contour plate is eliminated$A MAR also simpli#ies *P% start6up$ nli1e other hydraulic li)uid ends, the re#ill !al!e

does not need adustment$ Additionally, since a *P% hydraulic #luid cannot be o!er#illed,

there is no need to per#orm delicate procedures to synchroni-e hydraulic #luid balances :adi##icult tas1 re)uired #or tubular and other double diaphragm li)uid ends;$ With a *P%, ust

#ill the reser!oirs and turn it on$


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HPD Preshaped Composite Diaphragm

A *P% #eatures a preshapedP(elastomer composite disc diaphragm$


12/18

ome designs #eature a

hydraulic bypass mechanism that uses a piston "ith a constant stro1e length that pumpshydraulic #luid, thus trans#erring the pumping motion to a diaphragm$ here#ore, this type o#

dri!e can only mate "ith a hydraulically actuated diaphragm li)uid end$

+apacity is !aried by changing the location o# a hydraulic bypass port o!er the piston8s patho# tra!el$ /# the port is positioned at 40 percent o# the piston8s stro1e length, hydraulic #luid

"ill be relie!ed through the port during the #irst hal# o# the piston8s stro1e, and pumped

against the diaphragm during the remaining hal#$his type o# dri!e is o#ten called hydraulic lost motion, because a portion o# the piston8s

tra!el does not transmit pumping energy "hen the capacity adustment is less than 00

percent$ *ydraulic bypass style pumps can de!elop reciprocating piston motion by "ay o# a

"orm gear set and eccentric$/n these type o# pumps, a piston pumps hydraulic #luid, "hich either #orces the diaphragm to

#le& or is relie!ed through the bypass port$ A control !al!e positions the port based on a

desired capacity setting$ A disc diaphragm li)uid end can #eature simple& or duple& li)uid

ends, "ith ma&imum capacities ranging bet"een 0$F?6gph and G46gph :706gph %uple&;,and ma&imum pressures up to G006psi$

/n some hydraulic bypass style pumps, capacity can be !aried by positioning a stro1e adust slee!e o!er bypass ports bored

through the hollo" piston$ When operating at 00 percent the ports are co!ered, "hich traps

hydraulic #luid in the hydraulic pumping chamber$


13/18

A cup !al!e attached to the diaphragm closes all hydraulic paths to the diaphragm "hen it

reaches the #ull #or"ard position$ his eliminates a process contour plate, as "ell as

e&cessi!e hydraulic pressure on the diaphragm, since any e&cess hydraulic #luid in thehydraulic pumping chamber cannot reach the diaphragm and is #orced through the internal

relie# !al!e to the #luid reser!oir$A hydraulic bypass style pump can be an e&cellent choice #or a mid6range metering pump

capacity at lo" pressure$ /ts design is more economical than high pressure pumps in thesame capacity range, "ithout sacri#icing ruggedness and accuracy$ he @straight through@

process #luid path allo"s this metering pump to be applied to many o# the same ser!ices as

the *P% :high per#ormance diaphragm; li)uid end$

Polar Crank

ome metering pumps use a polar

cran1, an ad!anced and reliable !ariable stro1e length dri!e a!ailable in high pressure(high#lo" industrial duty metering pumps$

/n the polar cran1 dri!e, a high speed "orm gear reduces the RPM supplied by the motor,and pro!ides the lo"er RPM to a rotating cran1$ A connecting rod "ith spherical bearings

on each end lin1s the cran1 to the crosshead and piston assembly$ he "orm gear and cran1assembly pi!ots in an arc about the "orm sha#t center to change stro1e length$ he piston

stro1e length is determined by the angle o# the assembly$

or e&ample, "hen the pump is at -ero stro1e, the "orm(cran1 assembly is in a !ertical position$ he cran1 then rotates in a !ertical plane, and one end o# the connecting rod

re!ol!es "ith it$ he crosshead and the piston remain stationary because no reciprocating

action is produced$ When the pump is adusted #ull stro1e :or ma&imum capacity;, therotating cran1 is mo!ed to its ma&imum angle #rom the !ertical a&is$

At the top o# the rotation cycle, the connecting rod is pushed #or"ard, mo!ing the crosshead

and piston to the #ull #or"ard position at the end o# the discharge stro1e$ As the cran1continues to rotate, the angle o# the cran1 causes the connecting rod to pull the crosshead

and piston until it reaches the #ull rear"ard position, at "hich point the connecting rod has

reached the bottom o# the rotation cycle$


14/18

Regardless o# the stro1e length setting,the top o# the rotation cycle al"ays #orces the crosshead and the piston to the #ull #or"ard

position at the end o# each discharge stro1e$ his assures complete sca!enging o# the li)uid

end during each stro1e cycle$ he angle o# the polar cran1 can be adusted in in#initeincrements bet"een -ero and ma&imum stro1e #or e&tremely accurate controlled !olume

pump settings$

A polar cran1 dri!e can #eature ma&imum capacity ranges bet"een 0$0??6gph :246mL(hr;and 2406gph depending on #rame si-e, stro1ing speed, and plunger diameter$ %ischarge

pressures are rated up to 74006psi and up to eight pumps can be multiple&ed and dri!en by

one motor$ Polar cran1s can include *P%, pac1ed plunger, disc diaphragm, or tubular

diaphragm li)uid ends$o achie!e a high thrust capacity and e&tend component li#e, some polar cran1 dri!es

#eature a pressuri-ed lubrication system$ his positi!e oil pressure lubrication ensures long

bearing li#e and permits the pump to operate at !ery high suction and discharge pressures$As the crosshead mo!es #or"ard during the discharge stro1e, oil #rom the reser!oir is dra"n

up through a ball chec1 into a ca!ity in the crosshead$ %uring the suction :rear"ard; stro1e

the lubricant is trapped$ /t is then #orced through the crosshead, into the crossheadconnecting rod bearing, through the hollo" connecting rod, and #inally to the cran1

connecting rod bearing$By #orcing the oil through this path, e!ery mo!ing part is lubricated during e!ery complete

cycle o# the pump$ o reduce the "ear o# mo!ing parts and e&tend oil li#e, a magneticstrainer cleans the oil be#ore it enters the pressuri-ed system$

Advanced Drive Technology

A ne" inno!ation in metering pumps combines ad!anced

gear reduction design "ith electronic !ariable speed dri!es to achie!e t"ice the accuracy

o!er a turndo"n ratio :00'; ten times greater than traditional designs, "ith E 0$4 percent

steady state accuracy$


15/18

he most signi#icant di##erence #rom traditional systems is the close relationship bet"een

the dri!er and dri!e mechanism and ho" they enhance o!erall pump per#ormance$ he

operating characteristics o# this technology are the result o# a uni)ue constant stro1e lengthdri!e mechanism that depends on a special electronic !ariable speed dri!e to !ary pump

#lo" rate$raditional designs utili-e "orm gear sets to con!ert motor rotation to reciprocating motion

through an eccentric or similar mechanism$ Worm gears are the best choice "hen the dri!emechanism is re)uired to incorporate a !ariable stro1e length adustment$ hey operate "ell

at high rotary speeds, due to an oil shield that de!elops bet"een the gear sur#aces$

n#ortunately, they lose that shield at lo"er speeds, "hich causes "ear and raises motortor)ue re)uirements$ his limits "orm gears to a 0' turndo"n ratio$

his ne" technologyutili-es a special helical gear set$ ince there is no need #or stro1e length adustment, the

gear set is !ery simple$ *elical gears are 1no"n #or operating "ith lo" noise and lo"

#riction$ +ombined "ith a basic scotch yo1e mechanism to de!elop reciprocating motion,

this gear set runs )uietly and e##iciently "ith #e" mo!ing parts$ All mo!ing parts aresubmerged in oil to ensure long li#e$

igure indicates the greatest ad!antage o# the special gear design$ he #lat tor)ue cur!e

:!s$ the "orm gear set; allo"s the technology to operate at or belo" percent o# speed"ithout placing e&tra demand on the motor, thus permitting a 00' turndo"n ratio$ he

simple dri!e mechanism also allo"s the product to be easily duple&ed "ithin the same

housing by adding a second piston and li)uid end opposite the primary li)uid end$ hecapacity can there#ore be doubled economically and e##iciently$

+on!entional A+ and %+ !ariable speed dri!es are limited to turndo"n ratios bet"een 4'

and ?0'$ his is insu##icient to ta1e ad!antage o# the turndo"n capabilities o# this geardesign$ 9e" technology in brushless %+ motors and controllers has created ad!anced dri!es

capable o# strong operation at lo" speeds$ he !ariable speed dri!e can deli!er rated tor)ue

at less than percent :00' turndo"n; o# ma&imum rated RPM "hile maintaining steady

state speed control at better than E 0$ percent$ /t "or1s "ell "ith the dri!e mechanism$


16/18

he #le&ibility o# a 00' turndo"n ratio permits these metering pumps to be applied "here a "ide range o# dosage rates are re)uired$

/t can also pro!ide built6in gro"th potential by pumping e##iciently in systems re)uiring a

#raction o# the pump8s capacity #or the short term during startup or early phases o# an

e&panding proect$ hese pumps can pro!ide all this "ithout compromising accuracy ordri!e po"er$

his ne" technology can respond instantly to changes in dosage rate$ /ts constant stro1e

length does not upset the balance "ithin the li)uid end hydraulic system= that is, hydraulic#luid !olume remains constant$ When the hydraulic balance is disturbed, as in !ariable

stro1e length designs, the #ull result o# dosage changes can ta1e minutes or hours$ he

instant response can pro!ide smoother operation in closed loop or automated systems, andalso assure proper dosage at all time in systems re)uiring !ery close dosage tolerance$

he E 0$4 percent steady state accuracy o!er the #ull turndo"n ratio is a result o# the

constant stro1e length and the precise speed control o# the dri!e$ his le!el o# accuracy

pro!ides ma&imum chemical economy "hile assuring stable automatic operation and

optimum process )uality$

etering !ump %ystem Components

Proper metering pump

system operation depends upon the selection o# appropriate system components suited #orthe application re)uirements$

Sa!ety "elie! $alves


17/18

Most piping systems re)uire the use o# an e&ternal sa#ety !al!e to protect the piping #rom

o!er6pressure$ %iaphragm pumps #eature internal sa#ety !al!es to protect the pump, but

e&ternal sa#ety !al!es are still recommended$ a#ety relie# !al!es should match theoperating pressures o# the pumps being used$ ypical !al!e materials include specialty steel,

?>, alloy 20, and P.+$

#ack Pressure $alves

o pre!ent unmetered li)uid #rom #ree6#lo"ing through the pump, metering pump systems

re)uire a greater pressure in the discharge line than the suction or inlet line$ When the

process does not supply a minimum o# 246psi abo!e the suction pressure, a bac1 pressure!al!e is re)uired$ ypical !al!e materials include specialty steel, ?>, alloy 20 and P.+$

Pulsation Dampeners

he metering pump8s reciprocating motion pro!ides a pulsating discharge #lo"$ Applications

re)uiring a steady #lo" can eliminate o!er H0 percent o# the pulsations "ith a pulsation

dampener$ ypical dampeners are a!ailable #or pressures to 0006psi$ i-ing is based oncubic inch(stro1e displacement o# the speci#ic pump$

Cali%ration Columns

Metering pumps should be #actory tested$


18/18

Water )uality meters can be used as standalone products or in conunction "ith a chemical

dosing system$ Water )uality, including p*,

A Primer on Metering Pumps

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Transcript of A Primer on Metering Pumps