AUTOMATIC BLOWDOWN IN STEAM BOILERS

Blowdown

Boiler blowdown is necessary in order to control the quality of boiler water in terms of dissolved and suspended solids.
The concentration of impurities, which enter the boiler via the feed water, rises as pure steam is evaporated from the
boiler. High levels of dissolved solids (TDS) can cause a number of problems:

Scale

Poor control of blowdown coupled with poor water treatment, can encourage the formation of scale within the boiler.
Scale insulates the heat transfer surfaces, reducing their heat transfer efficiency. This can cause fuel consumption
to rise.
There are also safety implications, as the scale may cause the heat transfer surfaces to overheat and become damaged.
Scale and sludge may also block valves, pipework and for example, level control chambers, causing malfunction and
danger.
Corrosion may attack the metal beneath the scale.

Boiler blowdown cannot eliminate the problem of scale, but can keep water conditions fairly consistent, allowing
the chemical dosing of the feed water to do its job, which helps to discourage scale from forming.

Carryover

High concentrations of dissolved and suspended solids may cause the water in the boiler to foam, or to boil in an
unstable manner. This may result in carryover of dirt and water into the steam system, which is highly undesirable.

Clean, dry steam is necessary to;

Avoid contamination of the product,

Avoid blockages in the steam and condensate system which may be caused by dirt,

Maximise the heat transfer efficiency of the system, which may be reduced if the steam is wet, or the heat transfer
surfaces are contaminated,

Avoid damaging and dangerous water hammer,

To avoid sudden drops in the level of boiler water. If carryover (or priming) is severe, the boiler may even shut down
on heavy loads, as foaming of the water can make the level difficult to meausre. This is a major operational
consideration.

Automatic TDS Control

Automatic TDS Control systems work by measuring the conductivity of the boiler water (which is related to the
TDS level) and comparing it with the set point on the controller. This is the most effective method.

If the TDS (in ppm or μS/cm) is below the maximum permissible level, the blowdown valve remains closed. If the TDS
exceeds the set point, the boiler blows down until the TDS falls below the set point.

Automatic Bottom Blowdown Control

Bottom blowdown removes sludge from the bottom of a boiler. A build-up of sludge will lead to inefficient operation,
and even a risk of boiler explosion. A bottom blowdown valve is opened, allowing blowdown to clear the boiler.