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Operating modes

The following methods detail how the serviceliquid is handled outside the pump.

Figures 20 to 22 show some examples ofsingle pumps and separators of standarddesign. Generally, the illustrations also applyto separators of different design and toarrangements of several pumps connected toa common separator. A distinction is madebetween three modes of operation which arebriefly described below:

• combined flow operation• recirculation flow operation• make-up liquid operation

Combined flow operationCombined flow operation is the modecommonly used under normal circumstances.The flow of make-up liquid is limited to thecapacity required to remove the heat.

The arrangement is shown in figure 20:Service liquid B consists of a mixture of make-up liquid F and circulating liquid U. The make-up liquid is tapped from the mains supply,whereas the circulating liquid is obtained fromthe separator. Before entering the service

liquid connection, the two flows U and F aremixed to form flow B. The required make-upliquid flow F is calculated using the followingequation:

where

F = make-up liquid flow rate in m3 /hB = service liquid flow rate in m3 /h

36 Liquid ring vacuum pumps and liquid ring compressors

Small supply of

make-up liquid

required

Make-up liquid

flow rateBF = 2

t3

t–.

2t

4t–

(23).

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Recirculation flow operationThis mode of operation is used for processesinvolving corrosive, waste-water-polluting or

hazardous fluids, as well as in condensaterecovery systems.

Figure 21 illustrates the principle of operation.In this mode of operation, the liquid separ-ated from the product flow is re-used asservice liquid. The liquid contained in thesystem is therefore continuously recirculated(closed circuit).

During continuous operation, the liquidheated up during compression has to be


Fig. 21:

Circulating flow

operation

A Drainage liquid

B Service liquid

K Cooling liquid

F Make-up liquid

M1 Medium

pumped-suction

side

M11 Medium

pumped-

discharge side

P F Liquid pump

PG Liquid ring

vacuum pumpor compressor

XBp Separator

b K Heat exchanger

h K Shut-off valve

i K Regulating valve

i U Regulating valve

l B Service liquid

line

l K Cooling liquid

line

m B Manometer

vacuum guage

mU Liquid level tube

t Thermometer

t M1 Non-return

valve

u A Liquid outlet

u B Service liquid

connection

uc Cavitation

protectionue Drain

u1 Connection for

vent clock

u M1 Suction line

connection

u M11 Discharge line

connection

u ml Connection for

drain valve

u se Dirt drainuU Circulating flow

connection

uA

Liquid level

mU

XBp

MII

MI

uC uMII tMI

uMI

u l

PG

uml

ue

uBuseuelB

mUuU

XBp uA

A

uMIIuB

PG

uMI

uB

tMI

t B

U

lB PF i U

uUbK

K

mB

bK

i K

hK

lK

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cooled by means of a heat exchanger installedin the circulating flow line. The pressure dropof the heat exchanger has to be small if thereis no liquid pump installed in the circulating flow line.

The heat flow rate to be discharged throughthe heat exchanger is calculated according toequation (17).

A liquid pump must be installed in thecirculating flow line to boost the pressurewhenever the viscosity of the service liquid ishigh (>2mm2 /s), or if the liquid ring gas pumpis operated with a small differential betweenthe discharge and suction pressures.

The portions of the flow condensing in theliquid ring gas pump as well as the liquidconstituents of the product being pumped willdrain out of the separator. If the discharge

pressure is higher than atmospheric, eitherthe drainage liquid is removed via the liquidtrap, or the separator is fitted with some sortof control device in order to maintain therequired liquid level.

If the gas outlet from the separator (product from the discharge line MII) has a highervapour mass flow rate than the product

entering the pump (MI), the difference has tobe made up to prevent the liquid level in thepump from dropping below a given minimumlevel. Gas must be prevented from enteringthe recirculation line.

Make-up liquid operationThis mode of operation is used when re-useof the service liquid as such is not required. In

this case, the entire volume of service liquidneeded to operate the system is taken fromthe mains water supply.

Operating modes 39

Booster pump

Monitoring the

liquid level

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The principle of operation is shown in figure22. Drainage flow A is the sum total of the

following: Make-up liquid flow F, the vapourportions condensing in the pump, and anyliquid constituents contained in the product. Ifthe discharge pressure is higher thanatmospheric, either the drainage flow has tobe removed via a liquid trap, or the separatormust be fitted with some sort of controldevice for the purpose of maintaining therequired liquid level. There is no need for a

separator as long as the medium beingpumped and the liquid do not have to bedrained separately.


Fig. 22:

Make-up liquid

operation

A Drainage liquid

B Service liquid

F Make-up liquid

M1 Medium

pumped-suction

side

M11 Medium

pumped-

discharge side

PG Liquid ring

vacuum pump

or compressor

XBp Separator h F Shut-off valve

i F Regulating valve

l B Service liquid

line

l F Make-up liquid

line

m B Manometer

vacuum gauge

mU Liquid level tube

t Thermometer

t M1 Non-return

valve

u A Liquid outlet

u B Service liquid

connection

uc Cavitation

protection

ue Drain

u1 Connection for

vent clock

u M1 Suction lineconnection

u M11 Discharge line

connection

u ml Connection for

drain valve

u se Dirt drain

uA

F

Liquid level

mU

XBp

MII

MI

uC uMII tMI

uMI

u l

PG

uml

ue

uBuseue

lB

mU

XBp

uA

A

uMIIuB

PG

uMI

uB

tMI

tB

F

lF hF i F mB

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