and higher viscosities can be handled bye.g. reducing the pump stroking speed, orby installing a pulsation damper. A calcula-tion of the installation however, is requi-red. If desired this can be carried out byLEWA at a nominal fee if all informationrequired is available.

Information concerning the installation of LEWA ecodos

metering pumps.Reciprocating positive displacement pumpsproduce a pulsating output. The meteredfluid in the pipeline is accelerated to themaximum velocity and then decelerated tostandstill again during each stroke. For single cylinder pumps this velocity is3,2 times as high as for a pump with iden-tical, but continuous, output Qm (see fig. 1).

Friction and mass forces of the pulsatingoutput cause pressure fluctuations inthe pipeline. Therefore the pipelines for reciprocating positive displacement pumpsmust be dimensioned following differentcriteria as for centrifugal pumps.

Dimensioning of pipelinesProperly dimensioned pipelines are a basicrequirement for the trouble-free functionof any precision metering pump.The permissible pipeline lengths, LS and LD(see fig. 2) depend on the maximum flow-rate, the internal diameter DN of the pipe,the suction lift and the viscosity of thefluid metered. It is easy to determine thesedimensions from the diagrams. Longer ormore narrow pipelines, higher suction lifts

2 Installation schematic

DNsuction lift

suction headpump

Q

suction vessel

LS

LD

A Unit of the HERBERT OTT - GROUP

Metering Pumps Principles D 10 - 311 en

Diagrams: Dimensioning of the suction pipeline Dimensioning of the discharge pipeline

The diagrams are valid for fluids simi-lar to water (density = 1 g/cm3, visco-sity 10 mPas, vapour pressure 0,1bar). Higher viscosities mandate biggerpipe sizes (DN) resp. a calculation ofthe pipeline.

Example: Suction line for ecodos 180(yellow)At a suction lift of 1 m the maximumlength of the suction line is 2,4 m for DN15, 4,2 m for DN 20 and 6,4 m for DN 25.The same values apply to ecodos 350 with2,3 m suction head.

Example: Discharge line for ecodos 180(green)The total length of the discharge line mustnot exceed 2,7 m for DN 10, 6,3 m for DN15, 10,8 m for DN 20 and 17,1 m for DN 25.

internal diameter ofsuction line DN

internal diameter ofdischarge line DN

2-25

50, 90

180

350

550/750

1100

1500ecodossize

-3 -2 -1 0 1 2 3 1 2 3 4 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20suction lift total length of suction line LS [m]

LEW

A ec

odos

-siz

e =

Q max

[l/h

]

total length of discharge line LD [m]

ecodos 180

ecodos 350

ecodos 550

/750

ecodos 110

0

ecodos 150

0

ecodos 50, 9

0

ecodos 2-25

DN 10

DN 15

DN 20

DN 25

DN 32

DN 40

DN 50

DN 65

DN 80

DN 100

DN 10

DN 15

DN 20

DN 25

DN 32

DN 40

DN 50

DN 65

DN 80

DN 100

floodedsuction [m]

1 Output characteristics for a reciprocating positive displacement pump

Qm

time t

Q

What must be considered?

3 Start-up arrangement

Avoid the following duringinstallation:

Excessive suction lift. Thin walled hoses, which can contract,as suction line. Pipes which are hard to vent, the airenclosed is slowly carried along and leadsto metering errors. Drawing from the lowest point of thetank when the protection against dirt is in-sufficient, e. g. by a suction basket and astrainer. Long, vertical riser pipes directly afterthe discharge flange of the pump. Duringshut-down periods contaminations settle in the discharge valve and which leads toproblems.

The pumphead must be filled with the fluid metered at start-up and vented by this procedure!

Metering pumps with a low stroke volumehave a rather low compression ratio only.The self-priming capabilities with the suc-tion pipe empty are very limited because ofthis. When the pump must be started upagainst pressure a start-up piping arrange-ment similar to figure 3 is recommended.First the fluid metered is circulated untilpump and piping are filled. Then the 3-way valve is switched over for deliveryinto the discharge line. When the pump must draw from the topof a tank a lifter pipe (fig. 4) is recommen-ded. During start-up metered fluid is filledin until suction pipe and pump are vented.

Minimum differential pressureProper valve function is guaranteed at apressure of at least 1 bar above the suc-tion pressure. An external back pressurevalve is recommended if this is not possi-ble at all operating conditions.

Minimum operating pressureWhen a diaphragm rupture sensor is in-stalled a minimum operating pressure of1.5 bar is required, to trigger a diaphragmrupture signal.

Protection against overpressureThe permissible operating pressure of thepump must not be exceeded to protect the pump. Therefore a safeguard must be installed on site by e. g. mounting a safetyvalve.

M~

4 Suction lift via a lifter pipe

M~

LEWA Herbert Ott GmbH + CoUlmer Strasse 10D-71229 Leonberg Telephone +49 (71 52) 14 -0Telefax +49 (71 52) 14 -13 03E-mail lewa@lewa.deInternet www.lewa.de

D 10 - 311 en 11.2000 Subject to change without notice Printed in Germany