BOILER FEEDWATER REQUIREMENTS

BOILER FEEDWATER REQUIREMENTS

Boiler Make-up Water Sources:

Salinity Classes

Typical Total Dissolved Solids (TDS), mg/L

Fresh Water

less than 1500

Brackish (Well or Aquifer) Water

1500 to 10000

Brines

greater than 10,000

Seawater

35000-60000

Feedwater Impurities:

1. Dissolved gases such as oxygen & carbon dioxide, which lead to corrosion.

2. Dissolved solids which are usually inorganic salts of calcium & magnesium

3. Dissolved organics (oil, organic chemicals) that can foul Heat Transfer areas.

4. Microbiological & macrobiological organisms

5. Particulate matter such as suspended solids (mud).

6. Caustic which can cause caustic embrittlement of steel.

Scale & Deposit forming impurities are:

- Calcium Ions

- Magnesium ions

- Silica

- Phosphates (likely to be generated by chemical treatment)

- Iron & Copper

Water analyses are conventionally expressed for both cations (+ve charged ions) and anions (-ve charged ions), in parts per million by weight (ppmw) except for hardness & alkalinity, which are usually expressed in ppw of calcium carbonate (CaCO3). These ppmw values can be converted to a common basis such as milli-equivalents/liter. this permits the summation of oppositely charged ions such that total cations will equal total anions. Cation & anion meq/liter can be converted to ppmw CaCO3.

Total hardness is defined as the sum of calcium and magnesium ions in ppw of CaCO3. Total alkalinity is the sum of CO3-2, HCO3-1, and OH-1 ions in ppw of CaCO3.

CONSTITUENT

CHEMICAL FORMULA

DIFFICULTIES CAUSED

MEANS OF TREATMENT

Turbidity

None. Usually expressed in Jackson Turbidity Units

Imparts unsightly appearance to water , deposits in water lines, process equipment, boilers & so on; interferes with most process uses

Coagulation, Settling & filtration

Color

None

Decaying organic material and metallic ions causing color may cause foaming in boilers; hinders precipitation methods such as iron removal, hot phosphate softening; can stain product in process use

Coagulation, filtration, chlori-nation, adsorption by activated carbon

Hardness

Calcium, magnesium, barium, and strontium salts expressed as CaCO3

Chief source of scale in heat exchange equipment, boilers, pipe lines, and so on; forms curds with soap; interferes with dyeing

Softening, distillation, internal boiler water treatment, surface active agents, reverse osmosis, electrolytes

Alkalinity

Bicarbonate (CHO3-1), carbonate (CO3-2) and hydroxyl (OH-1) expressed as CaCO3)

Foaming & carryover of solids with steam; embrittlement of boiler steel; bicarbonate and carbonate produce CO3 in steam, a source of corrosion.

Lime & lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying.

Free Mineral Acid

H2SO4, HCl etc., expressed as CaCO3 titrated to methyl-orange end-point

Corrosion

Neutralization with alkalies

Carbon Dioxide

CO2

Corrosion in water lines & particularly steam & condensate lines

Aeration, deaeration, neutralization with alkalines, liming & neutralizing amines.

pH

Hydrogen ion concentration defined as pH = log (1/H+1)

pH varies according to acidic or alkaline solids in water; most natural waters have a pH of 6.0-8.0

pH can be increase by alkalies & decreased by acids.

Sulfate

SO4-2

Adds to solids contents & increase corrosive character of water.

Demineralization, distillation, reverse osmosis, electrodialysis

Chloride

Cl-1

Adds to solids contents & increase corrosive character of water.

Demineralization, distillation, reverse osmosis, electrodialysis

Nitrate

NO3-1

Adds to solids contents, but is not usually significant industrially; useful for control of boiler metal embrittlement.

Demineralization, distillation, reverse osmosis, electrodialysis

Fluoride

F-1

Not usually significant industrially.

Adsorption with magnesium hydroxide, calcium phosphate or bone black; Alum coagulation, reverse osmosis, electrolytes.

Silica

SiO2

Scale in boilers & cooling water systems; insoluble turbine blade deposits due to silica vaporization.

Hot process removal with magnesium salts; adsorption by highly basic anion exchange resins, in conjunction with demineralization, distillation.

Iron

Fe+2 (ferrous); Fe+2 (ferric)

Discolors water on precipitation; source of deposits in water lines, boilers etc.; interferes with dyeing, tanning, paper manu-facture.

Aeration, coagulation & filtration, lime softening, cation exchange, contact filtration, surface active agents for iron retention.

Manganese

Mn+2

same as iron

same as iron

Oil

Expressed as oil or chloroform extractable material, ppmw

Scale, sludge & foaming in boilers; impedes heat exchange; undesirable in most processes.

Baffle Separators, strainers, coagulation & filtration, diatomaceous earth filtration.

Oxygen

O2

Corrosion of water lines, heat exchange equipment, boilers, return lines, etc.

Deaeration, sodium sulfite, corrosion inhibitors, hydrazine or suitable substitutes.

Hydrogen Sulfide

H2S

Cause of "rotten egg" odor; corrosion.

Aeration, chlorination, highly basic anion exchange.

Ammonia

NH3

Corrosion of copper & zinc alloys by formation of complex soluble ion.

Cation exchange with hydrogen zeolite, chlorination, deaeration, mixed-bed demineralization.

Conductivity

Expressed as micromhos, specific conductance

Conductivity is the result of ionizable solids in solution; high conductivity can increase the corrosive characteristics of a water.

Any process which decreases dissolved solids content will decrease conductivity; examples are demineralization, lime softening.

Dissolved solids

None

"Dissolved Solids" is a measure of total amount of dissolved matter, determined by evaporation; high concentrations of dissolved solids are objectionable because of process interference & as a cause of foaming in boilers.

Various softening processes such as lime softening & cation exchange by hydrogen zeolites will reduce dissolved solids; demineralization; distillation; reverse osmosis; electrolytes.

Suspended Solids

None

"Suspended Solids" is the measure of undissolved matter, determined gravimetrically; suspended solids, plug lines & cause deposits in heat exchange equipment, boilers etc.

Subsidence, filtration, usually preceded by coagulation & settling.

Total Solids

None

"Total Solids" is the sum of dissolved & suspended solids, determined gravimetrically.

See "Dissolved Solids" & "Suspended Solids".

Blowdown:

All dissolved & suspended solids entering a boiler with the feedwater remain in the drums and tubes as steam is generated. The continual addition of feedwater produces higher & higher concentration of solids in the boiler water. A point can be reached beyond which operation is completely unsatisfactory. this situation may be caused by dissolved solids, silica content or alkalinity. Every boiler has a limit above which scaling, foaming & carryover occur. In order to keep boiler water concentrations below this limit, some of the concentrated boiler water must be removed from the unit as blowdown.

The intermittent or manual blowdown is taken from the bottom of the mud drum. This blowdown is mainly intended to remove any sludge formed in the boiler water. Both suspended & dissolved are present in the water. These must be removed to prevent solids from settling & caking on the heat transfer surfaces. the manual blowdown should be used approximately once per day for a few seconds to remove suspended solids which may have settled in the mud drum.

A continuous blowdown system helps to keep the boiler water within the concentration limits on a

relatively constant basis. Removing a small stream of water continuously saves water, chemicals,

and heat. The heat in the continuous blowdown water can be recovered in a heat exchange

system installed in the blowdown system. The continuous blowdown connection is usually

located below the low water level in the steam drum.

Proper regulation of boiler blowdown is very important in boiler operation. Too little blowdown

allows the concentration of suspended and dissolved solids to become too great, resulting in scale

formation and carryover of impurities in the steam. Too much blowdown wastes fuel and

feedwater. Globe valves with position indicators allow for accurate control of the blowdown rate.

Boiler concentration limits to control corrosion and fouling in the boiler vary as a function of the

operating pressure. In some cases, the blowdown from a high pressure boiler may be suitable for

makeup to a lower pressure boiler.

Steam that is dirty and wet can cause deposits in superheaters, turbines, and control valves. A

good separation of water and steam must occur inside the boiler steam drum to produce clean and

dry steam. Most boilers have effective mechanical separators in the boiler steam drum when the

water boils smoothly. When boiler water primes (high level) or foams, however, impurities are

carried over in the steam. While water priming and foaming are partly controlled through careful

operation of the boiler drum level and chemical injections, respectively, they are highly dependent

on maintaining proper boiler blowdown.

Some blowdown systems have automated blowdown control based on a continuous conductivity

measurement of boiler water. Conductivity is a measure of the dissolved solids in the boiler

water.

The blowdown system must safely dispose of the flashing steam and hot liquid as previously

discussed.

Cycles of Concentration:

A dissolved salt entering the boiler system will not leave the boiler in the steam. The dissolved

salt will concentrate in the boiler water as steam is formed. The dissolved salt concentration can

be controlled to a given level by adjusting the blowdown rate. A simple mass balance on the salt

will demonstrate this principle.

The mass of salt entering the boiler is the mass of boiler feed water (BFW) times the

concentration of the salt. Since there is no salt in the steam, the mass of salt leaving the boiler is

the mass of blowdown (BD) times the concentration of salt in the BD as shown in the figure below.

The concentration of salt in the blowdown will be the same as the concentration of salt in the steam

drum. The cycles of concentration (CC) is defined by the concentration in the blowdown divided

by the concentration in the boiler feedwater which is equal to the BFW rate divided by the

blowdown rate.

The percent blowdown is defined as the blowdown rate divided by the BFW rate times 100. The

cycles of concentration is the inverse of the percent blowdown multiplied by 100. The steam

drum (blowdown) concentration can readily be controlled by blowdown rate since they are

directly related.

X = (FBD / FBFW) *100

FBFW*CBFW = FBD*CBD

CBFW / CBD = FBD / FBFW = X / 100

CC = CBD / CBFW = 100 / X

where:

FBFW = Feedwater flow, kg/h (lb/h)

CC = Cycles of concentration, dimensionless

FBD = Blowdown flow, kg/h (lb/h)

X = % blowdown, percent of boiler feedwater

FSTM = Steam flow, kg/h (lb/h)

CBFW = Solids concentration in boiler feedwater, ppm

CBD = Solids concentration of blowdown (circulating boiler water), ppm

Deposits are most likely to occur in the riser tubes. The concentration of dissolved solids will be

greater in the riser tubes than in the steam drum because some water has been vaporized. The

steam drum concentration is controlled at a level that minimizes deposits in the riser tubes.

Care should be exercised in use of % blowdown because some use % blowdown to mean % of

steam and not BFW as defined here.

Example:

Water is fed into a boiler from a DM water plant. The chloride specification of this water also controls the solids specification. The chloride content of the steam drum water must be ≤10 ppm by weight. The boiler feedwater has a chloride content of 0.2 ppm by weight. The boiler produces 200,000 kg/h of steam. Calculate the BFW & Blowdown rates.

Calculations:

CBFW =

0.2

CBD =

10

FSTM =

200000

kg/h

FBD / FBFW = CBFW / CBD = X /100

FBD / FBFW =

0.02

FBD =

0.02*FBFW

1

FBFW = FSTM + FBD

2

From 1 & 2

0.98*FBFW = FSTM =

200000

FBFW =

204081.6

kg/h

Boiler Feedwater quantity

FBD =

4081.6

kg/h

Blow down quantity