Suspended solids in the feed water will remain in the boiler when steam is generated
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
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.
- 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 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:
|Total Dissolved Solids|
- TDS -
|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))