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Water Treatment Training

Dakhni Ex ansion Pro ect

Sulfur Recovery Unit

Oil & Gas Development Company Limited

Dakhni Gas Processin Plant

Dakhni, Pakistan

November 2010

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Typical Water Cycle

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Impurities in water

CATIONS:

a c um a

Magnesium Mg

Sodium Na

Bicarbonates HCO3

ar ona esChlorides Cl

Sulfates SO4

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Impurities in water

PROBLEMS IN BOILER SYSTEMS DUE TO WATER

IMPURITIES:

These impurities/salts make hard / non-conductive

scales inside boiler and on heat transfer surfaces.

Scales are very hard to remove.

Heat transfer rates are decreased so energy is lost. O O .

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Soft & Hard Water

SOFT WATER:

o wa er con a ns on y -car ona es.

HARD WATER:

Hard water contains carbonates, chlorides and sulfates.

When soft water is heated, bicarbonates are converted in

carbonates.

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Steam System Requirements

Boiler Operating

PressureTotal Dissolved Solids

Total Alkalinit Total Sus ended Solids

(psig)

(bar)

- -

(ppm)

(ppm)

(ppm)

0 - 50

0 - 3.52500 500

50 - 300

3.5 - 203500 700 15

300 - 45020 - 30 3000 600 10

450 - 600

30 - 402500 500 8

600 - 750

40 - 501000 200 3

750 - 90050 - 60

750 150 2

900 - 1000

60 - 70625 125 1

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Soft & Hard Water

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Soft & Hard Water

-(scale + carbonic acid)

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Total Dissolved Solids (TDS)

When a boiler generates steam, impurities in the feed water

will concentrate. When the concentration of impurities

ecomes g er, oam s genera e an e vo ume o e

boiling water increases, resulting in more and more lowquality wet steam generated.

The concentration of impurities is often termed TDS - Total

conductivity or density method.

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Total Dissolved Solids (TDS)

Measuring TDS with the density method:

-. -

where

TDS = Total Dissolved Solids (ppm)

= relative density of the boiler water at 15.5oC

This is a manual method and requires measuring a cooled down

sample with a very sensitive hydrometer.

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Total Dissolved Solids (TDS)

Measuring TDS with the conductivity method:

T .

where

= conductivity (s/cm)

This method can be used with an automatic resistance meter

controlling the blow down cycles of the boiler. Note that electrolytic

polarization of the probe may be a problem and that an ac (amplitude

current) resistance meter normally is used. Be also aware that

the conductivity increases with temperature.

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Components of Steam System

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Deaerator

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Blow Down Rate

BLOWDOWN RATE :

e ow own ra e epen s on e

Steam consumption (steam used in the process and not

returned as condensate to the boiler)

Concentration of impurities in the feed water

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Blow Down Rate

BLOWDOWN RATE :

e ow own ra e can e ca cu a e as

qBD = qS fc / (bc - fc)

where

qBD= blowdown rate (kg/h)=

fc= Total Dissolved Solids - TDS - in the feed water (ppm)bc= maximum allowable Total Dissolved Solids - TDS - in

e o er wa er ppm

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