Boiler Blowdown

Boiler blowdown will be a really essential aspect of almost any water treatment application. Boiler blowdown has function to control content level of impurities in boiler water. Determination amount of boiler blowdown is crucial: very much will leads to high cost of chemical water treatment and energy loss; very little will leads to too much content level of impurities. There are not specific rules to regulate amount of boiler blowdown due to variation in quality of water may differ from one location to other location. It may range from 1% until 25% according to the flow of feedwater.

Blowdown could be performed intermittently or continuously. Boiler blowdown can be performed either from the bottom of steam drum, header, or mud drum, or it could be performed from the base of steam boiler. The following are several principles to assist determine an efficient boiler blowdown application:

1. For boiler with fire tube type, blowdown may be done either intermittently or continuously. It could be blown down through the base of boiler. Timeframe, rate of recurrence, and type be determined by operating conditions, design boiler, and the kind of water treatment system.
2. For steam boiler which has drum type, the content level of water must be regulated through blowdown via steam drum. Continuous boiler blowdown is definitely recommended.
3. Likewise in steam boiler with drum type, blowdown through bottom header or mud drum will perform removal of suspended solids in steam boiler. Attempting to manage the content level of impurities through blowdown from this place could result in a serious interruption of water circulation and leading to deterioration to steam boiler. If bottom blowdown is performed, it must be in quick timeframe or intermittent. This condition is established by operating conditions, design boiler, and also the deposition rate of suspended solids.

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Coagulation and Flocculation in Boiler Water

Coagulation and flocculation is the process to give coagulant and flocculants materials into boiler water by injection. Coagulation is the neutralization process of charges so the particles can be very close to one another. Flocculation is the unification process between particles that had been already much closed to one another so the particles will attract each other and form flock. Chemicals which are injected to reduce turbidity at inlet clarifier as follow:

1. Alum Sulfate (Al₂(SO₄) . 18 H₂O)

Alum sulfate serves to form clump of suspended particles in water. When alum is contacted with water then hydrolysis will occur which produces aluminum hydroxide and sulfuric acid. The addition of alum depends on water turbidity and flow rate. The following below is the reaction of alum sulfate:

Al₂(SO₄) . 18 H₂O + 6H₂O  → 2Al(OH)₃ + 3H₂SO₄ + 18H₂O

Al(OH)₃ in the form of colloidal will settle with other impurities and shipped with water while H₂SO₄ will result acidic water.

2. Caustic Soda (NaOH)

Caustic soda serves to neutralize the acid caused by reaction in previous process. Concentration of caustic soda is added depends on acidity of solution. pH expected between 6-8. The following below is the reaction of caustic soda:

H₂SO + 2NaOH → Na₂SO₄ + 2H₂O

3. Chlorine (Cl₂)

The addition of chlorine is intended to kill microorganisms in water, in addition it also prevent the growth of moss on clarifier wall which can interfere the next process.

4. Coagulant Aid (Polymer)

Coagulant aid serves to accelerate deposition process because the addition of this material will bind particles that had agglomerated before into bigger clumps (flock) so make it easier and faster to settle. 

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Boiler Water Specification

The sources of water which is used for boiler water are from sea, river, boreholes, rain, and other water sources. The quality of each water source is not same although using same type; it is influenced by environment in each water sources. Water source from river had been contaminated by people’s activity and industrial activity; therefore it needs to be performed water treatment system.

Boiler water specification must meet the requirement to prevent further problem in operation of boiler. The boiler water must be free from mineral and other impurities that are not desired to reduction of efficiency steam boiler.

Boiler water specification must meet certain prerequisites as described in following below:

PH Condensate          = 8.0 – 9.0

Sulfite residual           = 20 – 50 ppm

Phosphate residual    = 20 – 50 ppm

Iron                             = max 2 ppm

Silica                           = max 150 ppm

T. Hardness               = -

O – Alkalinity            = min 2.5 x SiO₂        

M – Alkalinity            = max 800

P – Alkalinity             = -

TDS                            = max 3500 ppm

Conductivity              = max 500 µhos/cm

PH                               = 10.5 – 11.5 

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