DOSINGPUMPS

INSTALLATIONNPSH

PULSATION DAMPERS

1CONTENTSI. INTRODUCTION, features of a dosing pump

II. PRINCIPLES GOVERNING FLOW AND PRESSURE FOR A DOSING PUMP2.1. Flow2.2. Velocity and acceleration of fluid2.3. Pressure

III. RESULTANT INSTALLATION CONTRAINTS3.1. Contraints from flow3.2. Contraints from pressure3.3. Some simple precautions

IV. NPSH4.1. Introduction4.2. NPSH Calculations4.3. The general case of non-viscous fluids4.4. Improving the NPSH conditions4.5. NPSH tests

V. ADDITIONAL CHECKS AND CALCULATIONS5.1. In general5.2. Maximum pressure in the liquid end5.3. Criteria for overflow/siphoning effect5.4. Usual solutions

VI. PULSATION DAMPERS6.1. Balancing pot6.2. Constant level tank6.3. Pulsation dampers6.4. Determining the damper

VII. TYPICAL INSTALLATIONS7.1. Schematics of typical installations7.2. Particular case of slurries7.3. Particular case of aerated liquids

VIII. ACCURACY OF A DOSING PUMP8.1. In general - API Standards8.2. Linearity8.3. Repeatability8.4. Steady state accuracy8.5. Deviation from rated flow

2This handbook is designed for use byengineers and technicians who maybe dealing with new projects or main-tenance, involving the selection andinstallation of a Dosapro dosing pump(MILTON ROY EUROPE).Included is all necessary informationto determine standard installation, andto avoid the all too frequent classicalfaults responsible for so many pumpmalfunctions.For non standard special/sophisticated applications, MILTONROY EUROPE can provide engineersand designers with further assistance:

Technical Assistance Department Applications and ProjectsSpecialists

A dosing pump is a positivedisplacement, reciprocating pumpwith adjustable flow whilst running orstopped. Thus it is essential todifferentiate between a dosing pumpand a centrifugal pump; a dosingpump has a pulsating flow wheresuction and discharge phases can bedistinguished.This feature is always consideredwhen choosing a pump and designingan installation. Various parametersdictate the final selection of the pump:The first stages of selection come fromconsiderations of flow rate and pres-sure. Additional consideration of thepower required from the pump thendetermines which range to choose.Viscosity of the process fiuid is thesecond factor to be taken intoaccount; this determines the type ofvalve (standard ball or springassisted) and the maximum strokerate. Figure 1 shows this graphically;zone I shows that a pump in standardconfiguration will operatesatisfactorily; Zone Il, pumpingviscous liquids, will need springloaded valves; Zone III shows a regionwhere special designs would benecessary for satisfactory operation.

Unlike a centrifugal pump, thereciprocating action of a dosing pumpinteract greatIy with the suction anddischarge pipework in an installation.For this reason NPSH must be also aconsideration in the selection process(see 3.2. for discharge pipework inte-raction and 4.2. for NPSHcalculations).

I - INTRODUCTIONFeatures of a dosing pump

vis

cosi

ty c

p

stroke/mm

Fig. 1

32.1. FlowThe internal mechanism of the dosingpump generates a reciprocating ac-tion resulting in a particular flow pat-tern at suction and discharge. Mostdosing pumps piston motion is createdby a connecting-rod/crank assembly,which is almost sinusoidal. A typicalrelationship for a simplex single ac-ting pump, is shown in figure 2; themaximum instantaneous flow rate is3.14 () times the mean flow!

Fig. 2

2.2. Velocity and acceleration of fluidSuction phase:

Start: Velocity is zero: as the suctionvalve opens, the suction liquid columnis stationary. Acceleration is a maximum: thepump causes a depression due toinertia of the liquid column to be moved.

Middle: Velocity is a maximum: greater thanthree times the mean velocity. Viscousfriction loss is a maximumand causesa depression. Acceleration is zero: fluid is in mo-tion.

End: Velocity is zero: as suction valve clo-ses, fluid speed reduces. Acceleration is a maximum: liquidcolumn is being stopped - fluid inertiacreates an overpressure at the suctionvalve.

Discharge phase:

Start: Velocity is zero: as the dischargevalve opens, the liquid column isstationary. Acceleration is a maximum: interiaof liquid in discharge line results in

2.3. PressurePressure fluctuations in the suctionand discharge lines results from thepulsating flow and associated

an overpressure, as pump tries todisplace it.

Middle: Velocity is a maximum: greater thanthree times the mean velocity. Viscousfriction loss is a maximum and cau-ses an overpressure. Acceleration is zero: fluid is in mo-tion.

End: Velocity is zero: as discharge valvecloses, fluid speed reduces. Acceleration is a maximum: liquidcolumn is being stopped - fluid inertiacreates a depression at the pumpdischarge valve.

acceleration phenomena. Figure 3illustrates the effect of a simplex sin-gle acting pump.

DISCHARGE

SUCTION

Fig. 3

II - PRINCIPLES GOVERNING FLOWAND PRESSURE FOR DOSING PUMPS

Simplex single effect

43.1. Constraints from flow

A pulsing flow may be unacceptabledue to process or instrumentationrequirements (most flow meters aredesigned to respond to a steady flow).In these circumstances, pulsationsmust be avoided. (See section VI).

3.2. Constraints from pressureIn figure 3, if points A1 & A2 reach thefluid vapour pressure or hydraulic oilvapour pressure (in the case ofhydraulically actuated diaphragmpump), cavitation will occur. NPSHcalculations determine this.If pressure at points B, C1 and C2risen too much, the system or pumpmaximum allowable pressure may beexceeded = damage or malfunctionof relief valves, drive motor, flanges,etc.

Pressure at points B or D must not besuch that a siphon effect is set upbetween suction and discharge:dynamic suction pressure at B mustnot be above the static dischargepressure; similarly the dynamicdischarge pressure at D must not belower than the static head at thesuction.See corresponding calculations in sec-tions IV and V.

3.3. Some simple precautionsKeeping pipe lengths to a minimum,increasing pipe diameter and the ins-tallation of a damper at discharge,greatIy reduce inertia effects - pressureat C1, and C2 reduce whilst the pres-sure at D improves (increases).Similar rules apply to the suction -reducing pipe lengths and increasingthe diameter plus the inclusion

of a damping system (balancing pot,constant level vessel or accu-mulator). Inertia effects are reducedwhich diminishes pressure at B andincreases pressure at C1 and C2.Introduction of a back-pressure (orloading) valve increases the diffe-rential pressure between suction anddischarge thus reducing the siphoningeffect between B and D.Low stroke speed and triplex arran-gement are sometimes solutions thatstand out.

IV - NPSH

4.1. IntroductionNPSH (Net Positive Suction Head)relates to the available hydraulicenergy at a given point in a systemwhen centrifugal pump (constant flow)is installed. The system will have anAvailable NPSH and the pump willrequire a certain NPSH to function.The required NPSH will thus be acharacteristic of the chosen pump.Similarly, a dosing pump requires anNPSH value less than the availableNPSH for its correct performance.

However, the calculations made for acentrifugal (based on constant flow)will no longer apply.In fact, for a dosing pump, frictionlosses due to viscosity should becalculated for maximum fiuid velo-city: 3.14 ( ) times the mean value.Also acceleration effects must beincluded and they depend on pumpand characteristics of the suction line(Iength and diameter, etc).In addition, unlike most centrifugalpumps, a dosing pumpss

performance can be affected by thedischarge system. (see section 3.2).Thus a knowledge of the installation(or assumptions) is necessary tocalculate the NPSH available andtherefore to determine the perfor-mance of the pump in the system.

III - RESULTANT INSTALLATIONCONSTRAINTS

54.2. NPSH CalculationsDuring the suction cycle, cavitation isavoided when the absolute pressureat any point remains above the vapourpressure expressed as follows:

NPSH available > NPSH required

NPSH available and NPSH required abbreviated in future to NPSHa andNPSHr .

Fig. 4

In the schematic installation shown infigure 4, the Bernouilli formuladescribes the conservation of energy.Applied to an incompressible,

viscous fluid whose motion is notconstant, results in the formula beingwritten:

+H+ dVdt

Lg

+P-TVg 1+(1+2)

Ld

V22g = C

ste

with:

P : static pressure : fluid densityg : acceleration due to gravityH : physical heightL : lenght of pipeV : fluid velocity1 : specified load loss coefficient2 : pipe loss coefficientd : pipe diameterTV : vapour pressure at pumping temperature.

4.2.1 Calculation ofavailable static NPSH - NPSHa

Applying Bernouillis formula at pointA where the fluid is considered to bestatic, velocity and acceleration termsbecome zero.

NPSHa =10.2 (Pa - Tv) + Ha

NPSHa in metres of liquid column(mlc) with: : specific gravity of fluidHa : physical height in metresPa, Tv : pressures in bar abs.

Some diaphragm pumps withhydraulic lost motion require a mini-mum static NPSHa. This is so forDosapro mROY and MAXROY pumps(MILTON ROY EUROPE).

4.2.2. Determination of head lossalong a pipe H

Caused by acceleration Z

Z = 0.016 LQNd

Z is expressed in mlc with:

L : total lenght of pipe inmeters (m)

Q : mean flow in liters per hour(l/h)

d : diameter of the pipe inmillimeters (mm)

N : stroke speed of the pumpin strokes per minute (spm)

6Friction loss YFriction loss in the suction can usuallybe obtained from standard table/graphs, but be careful: maximum flowrate is 3.14 times the mean flow.Calculation is also possible by usingthe formula:

Y = 3.63

NoteIn general, for non viscous fluids, fric-tion losses Y are small compared toacceleration losses, since they are outof phase, have no effect since k = 1.Refer to paragraph 4.3.

4.2.3. Determination of internalNPSHr

This value depends on the pumpselected and the viscosity of the fluidto be pumped, and is provided by thepump manufacturer. In general, fornon viscous fluids, the internal NPSHof a dosing pump is not significantcompared to acceleration loss. (Seeparagraph 4.3).

Le Q d44444

Fig. 5

4.2.4. Conditions for pumping:Usually a 2 mlc safety margin isapplied to give:

NPSHa - H > 2 + NPSHr

or

NPSHa > 2 + H + NPSHr

If this condition is not met the pumpwill cavitate and may lose its primeresulting in:- Loss of flow and accuracy- Noise and vibration- Increased wear.

Y is expressed in mlc with:

Le : equivalent pipe lenght in meters

: viscosity in centipoiseQ, , d : l/h, s.g, mm.

Determination of total head loss HAs shown above, losses due to fric-tion are out of phase compared toacceleration losses. The combinationof Z and Y depends of their relativelevels and H is determined asfollows and according to graph ofFig. 5: Calculate Y/Z Find coefficient k using graph in figure 5 Calculate total head Ioss (mlc):

H = kZ

74.3. The General Case of non viscous fluids

Most application can be treated asnon-viscous where friction lossescan be ignored and internal NPSH

NOTE Ha is + ve for a flooded suction Ha is - ve for a suction lift

is negligible compared toacceleration losses. In this situationthe condition for good performanceis:

10.2

(Pa - Tv) + Ha > 2 + 0.016 L Q Nd

4.4. Improving the NPSH conditions

When it is found that the NPSH con-ditions are inadequate, a number ofmodifications can be suggested whichusually include:- Increasing NPSHa by increasing Ha.- Siting the pump close to the suctiontank reducing L and Le and thusdecreasing H.- Increasing the diameter, d, of thesuction line reduces Y and Z resultingin a decreasing H- Introducting a suction damper closeto the inlet of the pump, reduces theinertia effects and so reduces H.

4.5. NPSH TestsThe internal NPSHr depends on theprocess fluids viscosity and acce-leration losses are determined by theinstallation arrangement. There istherefore no standard NPSH testavailable which is meaningful. Testscan be performed using water with aflexible suction line sized to simulate:

The available NPSH at the suction ofthe liquid end, ie:

NPSHa - H

The internal NPSHr when a viscousfluid is to be used, ie:

NPSHr

The energy balance, ie:

NPSHa - H - NPSHr

8V - ADDITIONAL CHECKS AND CALCULATIONS5.1. In GeneralCalculations of viscous friction loss,acceleration loss together with thecombined effect is analysed for the

discharge in the same manner as thesuction was (using the same style ofnotation d represents discharge).See figure 3.

Conclusion:It is necessary to keep these pres-sures below the maximum rated forthe pump, or any single component,to prevent mechanical failure, oroverloaded motor, etc.

5.3. Criteria for overflow/ siphoning effect

During Suction:

Pmax = Pa + (Ha + Za)10.2

Ensure that this dynamic pressure atpoint B (reached at the end of thesuction stroke) is less than the staticdischarge line pressure Pd, otherwiseoverflow may take place.

During discharge:

This Pmin is minimum pressure at D,at the end of the discharge phase andmust remain above Pa to ensure thereis no overflow.

Also check Pmin is above the vapourpressure of the process fluid or thehydraulic oil (where applicable).

5.2. Maximum pressure in the liquid end

During Suction:

Pmax = Pa + (Ha + Za)10.2

The maximum pressure is that at the point B at the entry to the liquid end.

During Discharge:

Hd is the geometric height of discharge. This maximum pressure iscorresponding to pressures C1 or C2.

Pmax = Pd + (Hd + Hd + NPSHr)10.2

Fig. 3

LAW OF PRESSURE

Pmin = Pr + (Hd - Zd)10.2

95.4. Usual Solutions

Pressure fluctuations can be dampedwhen a mutiplex assembly is used orby a pulsation damper.

Also note the inclusion of a back pres-sure/loading valve usually illuminatesoverflow/siphoning problems due toacceleration.

When a back pressure valve is used,the NPSH calculation incorporates theadditional counter pressure. In somecases, the calculation result shows animpossibility, the only remedy beingis a pulsation damper.

The combined effect of dampers, backpressure/loading valve and

multiplexing is summarised in thefollowing table:

Triplex unit(1)

SuctionPulsation damper

(2)Discharge

Pulsation damper(2)

Meanflow

Z

Accelerationloss

Y

Friction loss

H

Total loss

3 Q

Q

Q

0.5 Z

0.2 Z

0.05 Z

Y

0.2 Y

0.15 Y

0.5 Z + Y

0.2 Z + 0.2 Y

0.05 Z + 0.15 Y

(1) Q, Z, Y based on a single head(2) Typical values.

10

Fig. 8

VI - PULSATION DAMPERS

A pulsation dampening device atte-nuates both flow and pressure pul-sations caused by a reciprocatingdosing pump. Its effect is to reducethe inertia effects as shown infigure 6.

6.1. Balancing Pot (Fig. 7)A suction damping device fitted whenpipe lengths cause NPSH problems.During the suction phase of the pumpfluid is drawn from the pot, which iskept at the same pressure as the bulkstorage tank. Refill of the pot occursby gravity, during the discharge phaseof the pump.

NPSH calculations can now be madefor the section of pipe between the potand the pump. Often the pot can beused as a calibration device or as ameans of allowing heavily aeratedfluids to gas-off.

To size the pot use, 15 to 20 timesthe pumps swept volume.

6.2. Constant level tank (Fig. 8)

Fig. 7

Fig. 6

11

Principle similar to a balancing pot,plus:- (in left sketch): gravity fed with floatvalve- (in right sketch): pump fed by atransfer pump or pressurised bulkstorage with high and low level control- calibration service can be used aswell as gas-release- sizing is similar to a balancing pot:15 to 20 times pumps swept volume.

6.3. Pulsation Dampers6.3.1. Principle of operation:

Pulsation dampers or dampeners arevessels filled with an inert gas. Com-pression of the gas dampens the pul-sations and reduces the inertiaeffects.Dampers may be fitted to suction ordischarge lines, but have limitedefficiency in suction lines. In practice,most dampers operate only with apositive pressure.

6.3.2. Dampers withoutseparators

Pumped fluid is in contact with thedampening gas. An inert gas is usuallyused, mostIy nitrogen. Frequent main-tenance is required since the gas maybe absorbed or dissolved by thepumped fluid.

Pulsation dampers withoutseparators, without precharge:

The damping gas is not under anypressure.

In time, when the pulsations havebecome too large the following mustbe done (see Fig. 9): Switch off the pump

Isolate the damper by closing valve 1and opening valve 2

Drain the damper

Close valve 2 and open valve 1

Re-start pump

Pulsation damper without separatorand with precharge:

Damping gas is introduced underpressure into the vessel. Thisprecharge must be less than the nor-mal line pressure.

Fig. 10

In time, when the pulsations havebecome too large, simple re-chargethe vessel with gas. Unlike theprevious system, the pump may bekept running.

Fig. 9

12

The Particular Case of a liquid in agas-Iiquid equilibrium

It is possible to use a damper withoutseparator which requires no mainte-nance: the damper has a heatedjacket and continuous vaporisation ofthe liquid which maintains the gas-Iiquid equilibrium (Fig. 11).

Fig 11

6.3.3. Pulsations dampers withseparators and pre-charge:

Dampers are pre-charged with gas(Iower than working pressure, usually60% to 80 %). This gas is separatedfrom the working fluid by a flexiblebarrier (diaphragm, bladder), oftenmade of elastomer. See Fig. 12.

Other versions can be used withPTFE diaphragms, bellows or metalpistons, but there are limitations totheir use.

6.3.4. The particular case ofresonators or hydraulic silencers(Fig.13)

Unlike previously described dam-pers, hydraulic silencers do not act asflow dampers. Their use is to filterhydraulic pulsations in order to reducenoise.

Dosing pump installations rarelyrequire such systems since thefrequency of pulsations is so low, eg.a triplex pump running at 150 SPMreaches 15 Hz.

Fig. 13

6.4. Determining the damper6.4.1. In General

Sizing of a damper assumes anisothermic inflation (PV = constant) andits operation follows an adiabatic cycle(PV = constant)Fig. 12

13

6.4.2. Dampers without pre-chargeThe graph in figure 14 gives selectionof damper volume without separatorand pre-charge, for various workingpressures and damping ratios. It isworth noting that 15 Bar is practicalmaximum pressure.

6.4.3. Damper with pre-charge

The graph in figure 15 allows selec-tion of a precharged damper (with orwithout a separator) for variousdamping ratios. A pre-charge of 60%of working pressure has been used.Separator stiffness limits the mini-mum working pressure to 2 Bar.

Standard rule for selection:

Precharge at 60%Max temperature 40C

Volume equal to 15 strokevolumes of the dosing pump

Residual dampeningratio 5%

6.4.4. Important note:

Pre-charge varies with ambienttemperature. When dampers areinstalled outside or in extremes oftemperature, the pre-charge needs tobe adjusted (consult us).

6.4.5. Corrosion

Materials for the vessel body andseparator must be selected to be com-patible with the process fluid.

Fig. 14

Fig. 15

14

VII - TYPICAL INSTALLATIONS

7.1. Schematics of typical installations

7.1.1 Schematics of good installations

Fig. 16: The pump is located above the tank and fitted with a foot-valve (for easier priming). Lines are short on both suction and discharge;suction line is vertical and with a diameter at least equal to the ratedconnection diameter to the dosing pump; the injection nozzle isolatesthe pump and reagent from the main flow.

Fig. 17: The injection nozzle (or a back-pressure valve) creates anartificial resistance allowing accurate dosing.

Fig. 18: Long pipe lengths requiring the use of dampers: balancing pot,pulsation damper...The back-pressure valve creates an artificialcounterpressure of 2 Bar minimum; it is not necessary if existing pres-sure is greater.

Fig. 19: Natural siphoning effect is avoided by using a back-pressurevalve A, adjusted so as to maintain the pressure differential H. Thepump has a flooded suction and it is necessary to fit a shut off valveand a filter on suction line.

Fig. 16

Fig. 17

Fig. 18

Fig. 19

15

Fig. 20: A pump with a high nominal stroke speed (over 140 strokes/min) often requires the installation of a pulsation dampener on thedischarge line (and possibly in addition a back-pressure valve), oncethe pipe length exceeds 10 meters.

7.1.1 Schematics of typical bad installations

Fig. 21: Pumping with high suction pressure (for example liquefied gas)with a back-pressure valve to avoid a natural siphonning effect. TheNPSH calculation is quite important in the case of pumping liquefiedgas in gas-Iiquid equilibrium condition.

Fig. 22: Suction line too long. Suction lift toohigh cavitation.

Remedies: plan for the pipe length on dischargeside increase diameters use a damper install a foot valve and a back pres-sure valve.

Fig. 23: Risk of gas accumulation and lossof prime.

Remedies: arrange for a connection on the tankbottom (use a filter) or vertical suctionfrom above the tank through a foot-valve.

Fig. 24: Risk of siphoning effect. The pumpnon return, suction and discharge valvescannot stop siphoning.

Remedies: use a back-pressure valve install a valve on the suction line(case of a flooded suction).

Fig. 20 Fig. 21

Fig. 22Fig. 23 Fig. 24

16

Fig. 25: Inefficient damper

Remedy: reverse the relative positions of dampener and back-pressure valve.

7.2. Particular case of slurries

The use of a packed plunger liquid end for this type ofproduct is not recommended. Usually, we recommend adiaphragm pump with general advice for installation asfollows:- vertical or inclined suction; horizontal discharge- plan for a slight flooded suction- avoid an outlet on the tank bottom- an agitator is recommended- install a flushing line

Standard working cycle1. Agitation of the product in the preparation tank.

2. Pumping

3. When stopped - plan a 15 minute flushing cycle:3.1. open the water flushing system3.2. flush while pump is in operation3.3. shut off MILTON ROY EUROPE PIC valve (item 1)3.4. stop the process

7.3. Particular case of aerated liquids

Fig. 27: Installation of a gas-freeing pot on the pump suction line (thusavoiding frequent loss of prime) and an inclined connection line betweenpump and gas-freeing pot to facilitate gas-freeing

Fig. 25

Fig. 26

low level

Preparationtank

flushingwater(2 bar)

to waste

200 m

ini

100 m

m

Fig. 27

17

VIII - ACCURACY OF A DOSING PUMP

8.1. In general API Standards

Standards of construction and testsfor a dosing pump are defined by API675 (American Petroleum Institute).However, only piston pumps andhydraulic diaphragm pumps are bythis standard. API does not considerthe case of mechanically actuateddiaphragm pumps represented byDosapro pumps of LMI, and G series(MILTON ROY EUROPE).

8.2. Linearity

This characterizes the alignment ofthe flow measured for differentsettings:

Fig. 28

API 675 standard stipulates that flowpoints measured are within a 3 %range of rated flow.

This characterizes the capacity of apump to always deliver the same flowfor same stroke setting:

8.3. Repeatability

API 675 standard stipulates thatrepeatability must remain within a 3 % range of the rated flow.

All parameters remaining unchangedand constant (NPSH, pressures,temperatures, ...), steady stateaccuracy expresses the precision ofthe dose at each pump stroke.

API 675 standard stipulates thatrepeatibility must remain within 1%.

8.5. Deviation from rated flow

8.4. Steady state accuracy

8.5.1. Proportionality

Dosing pumps always deliver a flowgreater than the rated flow indicatedin brochures with correction of speedand pressure factors. Moreover, theflow curve, even if perfectly linear, isalways shifted in comparison with theproportional theoretical straight line,as shown by figure 31:

This characteristic should be allowedfor in the case of open loop regulationmanaged by a single set point.

Fig. 29

Fig. 30

Fig. 31

8.5.2. Influence of pressure

Being a reciprocating pump, thedosing pump flow is hardly affectedby the pressure parameter. However,phenomena of compressibility (offluids, seals...) and hydraulicefficiencies (valves, leaks at stuffing-boxes, vents...) result in a slight in-fluence on outputs as pressureincreases. Figure 32 shows flow curvesof the same pump working at 10 and100 Bar discharge pressures.

Usually one notices a 0.4 % (plungerpumps) to 1.5 % (certain diaphragmpumps) flow drop for each 10 Bars.

8.5.3. Influence of set adjustment

Obviously the relative influence of aIlthese deviations and the ones due tofluid and installation varies along thescale of adjustment. Fig. 33 gives anidea of the relative error on a givenpump that corresponds to drive clea-rances, hydraulic efficiencies, fluid va-riations.

The same level of relative error maynot be reached on the full scale ofadjustment. In the range 0-10 % rela-tive errors are much more higher.

Fig. 33

Fig. 32

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NPSH leaflet - Ref. 1NPS 900 401N - 03/06 - Rev. B - No copy allowed.