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Boiler Blowdown

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Dissolved solids and particles in the make-up water will remain in the boiler when steam is generated. During operation the total dissolved solids (TDS) builds up finally reaching a concentration level where the operation of the boiler becomes impossible.

  • Total Dissolved Solids - TDS - is normally measured in ppm

If solids are not purged from the boiler they can lead to

  • scale formation
  • carryover
  • corrosion
  • embrittlement

Chemical treatments and continuous surface blowdowns are commonly used to eliminate solids at the same rate as they are added from the feed water.

Recommended maximum TDS levels depends on the type of boiler - normally ranging 2000 - 10000 ppm. TDS can be monitored by measuring the electrical conductivity of the boiler water. A rise in conductivity indicates contaminated water.

In general

  • short frequent blowdowns are more effective in removing boiler water sludge than
  • a one long infrequent blowdown

Note that blowdown tanks are required by the Uniform Plumbing Code (UPC)1) and Uniform Mechanical Code (UMC)1) Codes on any hi-pressure installations exceeding 15 psig.

1) The International Association of Plumbing and Mechanical Officials (IAPMO) develops the Uniform Plumbing Code (UPC) which is widely adopted in western United States.

Due to high pressure and temperature in the boilers - large amounts of flash steam are generated during blowdown. BE AWARE that the hot water and flash steam are DANGEROUS. Care must be taken during design, construction and operation.

Manual Blowdown

Manual blowdown can be accomplished through taping at the bottom of the boiler where settled solids are removed. With manual blowdown water level control devices and cutoff devices are kept clean of any solids interfering with their operation.

All steam boilers requires an arrangement for manual blowdown even if they are supplied with automatic continuous blowdown systems or not.

Automatic Continuous Blowdown

A continuous blowdown system use a calibrated valve and a blowdown tap near the boiler water surface. Water is continuously taken from the top of the boiler at a predetermined rate.

ASME Guidelines for Water Quality in Watertube Boilers

Maximum allowable water impurities:

For full table with Alkalinity CaCO3, Total Dissolved Solids and Specific Conductivity - rotate the screen!

Boilers - Maximum Allowable Water Impurities
Boiler Pressure
Iron Concentration
Copper Concentration
Hardness CaCO3
Silica Concentration
Alkalinity CaCO3
Total Dissolved Solids
- TDS -
Specific Conductivity
0 - 300 0.1 0.05 0.3 150 700 3500 7000
300 - 450 0.05 0.025 0.3 90 600 3000 6000
450 - 600 0.03 0.02 0.2 40 500 2500 5000
600 - 750 0.025 0.02 0.2 30 400 1000 4000
750 - 900 0.02 0.015 0.1 20 300 750 3000
900 - 1000 0.02 0.015 0.05 8 200 625 2000

*) electrical conductivity is measured in 1/ohms m, 1/Ω m, siemens/m, S/m or mho/m.

  • 1 S/m = 104 μS/cm
  • 0.042 µS/cm - Ultrapure water (20°C)
  • 0.5  - 5 µS/cm - Deionized water
  • 100 - 300 µS/cm - Soft ground water
  • 45000 - 55000 µS/cm - Sea water
  • > 100000 µS/cm - Concentrated acids or lyes (highly concentrated aqueous solution of potassium hydroxide or sodium hydroxide (caustic soda))
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Related Topics

Steam and Condensate

Design of steam & condensate systems with properties, capacities, sizing of pipe lines, system configuration and more.

Related Documents

ASME - International Boiler and Pressure Vessel Code

The International Boiler and Pressure Vessel Code safety rules governing design, fabrication, and inspection of boilers and pressure vessels, and nuclear power plant components during construction.

Boiler - Efficiency

Combustion gross and net calorific value.

Boiler Blowdown Rate

Calculate boiler blowdown rate.

Boiler Capacities

Steam boilers output can be expressed in Boiler Horsepower, MBTU or in Pounds of Steam delivered per hour.

Boiler Feed Water - Chemistry and Impurity Limits

ABMA recommended feed water chemistry limits for steam boilers.

Boiler Water - Total Dissolved Solids (TDS)

Impurities from the feedwater will concentrate in the boiling water.

Boilers - Classification

Classification of boilers according the ASME Boiler and Pressure Vessel Code.

Feed Water Treatment to Avoid Corrosion

Make-up water to steam boilers should be treated with oxygen scavengers to avoid serious corrosion problems.

Steam Boiler Rating vs. Feed Water Temperature

Boiler output vs. feed water temperature.

Steam Boiler Shells - Stress vs. Boiler Pressure

Calculate the stress in steam boiler shells caused by steam pressure.

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