Calculate boiler water blowdown flow

To control the level of total dissolved solids (TDS) within the boiler

As water is boiled within the boiler and steam is produced, then the solids remain in the water and concentrate. Thus, over time the level of total dissolved solids (TDS) increases. Further evaporation causes these dissolved solids to come out of solution, and to produce suspended solids (sludge). As the dissolved solids increase there is a risk of ‘carry over’ of boiler water into the steam. It is therefore extremely important to control the level of Total Dissolved Solids. This is achieved by either continuous or intermittent blow down. Manual bottom blow down through the main bottom blow down valve should still be carried out at regular intervals to remove sludge.

Manual Control of TDS and Blowdown

Boiler Water TDS Maximum Allowable Average

Automatic Control of TDS and Blowdown

Boiler Water TDS Maximum Allowable Average

Boiler Water TDS Boiler Water TDS

How to calculate the required blowdown rate of a boiler

Blowdown rate =

𝐹𝐹 × 𝑆𝑆 Where: 𝐵𝐵−𝐹𝐹F


For example:

To keep a steam boiler producing 4000 kg/hr of steam below 3500 ppm TDS when fed from a feed tank having a TDS of 80ppm, it will need to blowdown at least;

F 80 ppm 80 × 4000 S 4000 kg/hr 3500 80
B 3500 ppm

Typical permissible levels of boiler water TDS

= 93.6 kg/hr

Feed tank TDS in ppm

Actual boiler steam production in kg/hr

Maximum TDS allowed in the boiler in ppm

Type of Boiler

TDS level in parts per million (ppm)

Water tube – High Pressure


Vertical shell


Modern packaged 3 pass


Older economic 2 pass


Water tube – Low Pressure





Aldo Zaffalon
Aldo Zaffalon


Water treatment automation expert, with over 25 years of experience in industrial water treatment automation, marketing, sales and engineering.

Leave a comment

Comments will be approved before showing up.