boiler blowdown

Boiler Blowdown

Engr. Sahadat HussainSr. Consultant, Environmental & Process

Engineering Resources International (ERI)

Common Impurities in Raw Water

• Dissolved solids - scale forming substances. The principalones are the carbonates and sulphates of calcium andmagnesium. Not all dissolved solids will form scale.

• Suspended solids - sludge formingorganic

substances which areinusually solid mineral or particles, carried

suspension. Not generally a problem with most waters.

• Dissolved gases - corrosive. Oxygen and carbon dioxide.

• Scum forming substances - mineral scum forming or foamingimpurities usually containing soda in the form of a carbonate,chloride or sulphate.

Water quality needs to be such that weoperate under the following criteria:-

• Freedom from scale

• Freedom from corrosion

• Good quality steam

• Safety

ppm = mg/l

Units•

of ConcentrationParts Per Million (ppm)1 kg in 1,000,000 kg (1,000 Tonnes) of water

1 lb. in 100,000 lmp. gallons of water

• Milligramme Per Litre (mg/l)11

1

mg

mg

mg

in

in

in

1,000,000 mg of water1,000 g of water

1 litre of water

100 ppm1,000 ppm

10,000 ppm

= 100 mg/l = 99.99% purity= 1,000 mg/l = 99.9% purity

= 10,000 mg/l = 99% purity

ppm = mg/l

Total Dissolved Solids

NonHardn

ess

TotalDissolv

edTotal

Hardness Salts Solids

Keeping the Steam Clean.

Pure steamto plant

Build up ofFeed waterimpurities inwith

impurities

the boiler

Impurities‘blown down’ to

waste

Deciding on the Required Boiler Water TDS.

Maximum TDS tolerated Maximum TDSby boiler type

Lancashire

(ppm)10,000

4,5002-Pass Economic

3,000-3,500Package and 3-Pass Economic

2000-3000Low Pressure Water Tube

1,500Medium Pressure Water Tube

2,000Coil Boilers and Generators

••

The above figures are offered as a broad guide onlyThe boiler maker should always be consulted for specific recommendations.

Calculating Blowdown Quantity.

Let:

F = Feed water TDS (ppm)

B = Boiler target TDS (ppm)

S = Boiler steaming rate (kg/h)

F x SAmount to be blown down = B - F

This equation is valid for any consistentunits eg lb/h, μS/cm

The effect of too high TDS in the boiler.

High TDS and/or Contamination of Fouling of HeatExchangersControl Valvessuspended solids.

Blockage of Steam Traps

Boiler

Automatic TDS Control

Project Boiler

10,000 kg/hsteam toplant

Feed waterwith 250

Maximumallowable

ppmimpurit

ies

boiler TDS =2,500 ppm

Boiler operating pressure = 10 baroCSaturation temperature = 184

Project Boiler

Given Information

F = Feed water TDS (ppm) = 250 ppm

B = Boiler target TDS (ppm) = 2,500 ppm

S = Boiler steaming rate (kg/h) = 10,000 kg/h

F x SAmount to be blown down =B - F250 x 10,000= 2,500 - 250

= 1,111 kg/h

actual blowdown?How much is the

Ba - F

=

=Amount of excess blowdown = 1282 - 1111

= 171 kg/h

1,282 kg/h

250 x 10,0002,200 - 250

Amount actually blown down = F x S

Given Information

Ba = Actual Boiler TDS (ppm) maintained = 2200 ppm

How much energy is there in theblowdown?

Energy blown down (kW) = 1111kg/h x 782 kJ/kg3600 seconds

= 241 kW

A typical house central heating system is ratedat 13 kW, so this is enough heat for

24113

= nearly 19 houses.

Net saving

Saving in blowdown x Sensible heat lost x Cost of fuelNet Savings =GCV of fuel x Efficiency of boiler x Sp. gravity of fuel (if oil)

=

= Rs. 695 per day

Total Saving = Rs. 20,850 per month

171 x 24 x (184 - 25) x 6 x 1008000 x 80 x 0.88

Boiler Bottom Blowdown Rate

FlowRate

(kg/s)

Pressure (bar g)