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Wall Blowers or Water Cannon for Slagging Furnaces - A

Comparison

written by: Dr V T Sathyanathan edited by: Michele McDonough updated: 5/9/2010

Slagging furnaces in boilers need water cannons and not steam wall blowers. This needsunderstanding by boiler owners, operating groups, and designers. Water first cools the slag,

makes it friable, and dislodges it from the furnace walls.

Furnace wall cleaning is a requirement in any coal fired boiler. Depending upon the property ofthe ash (inorganics), designers adopt different methods for cleaning the deposits on the furnace

walls. There are many types of deposit-cleaning devices used for boiler furnaces firing coal.

Steam wall blowers, water lancers, and water cannons, also called HydroJet, are used to remove

furnace deposit or slag.

Steam wall blowers

Large numbers of steam wall blowers are provided in the furnace walls to make sure the deposits

are cleaned effectively. If the furnace wall ash deposits are expected to be loosely adhered,

designers go for a short travel retractable-type soot blower called a wall blower which directssteam on the furnace wall to remove the deposits. This consists of a poppet valve, feed tube,

lance tube, and a drive mechanism for inserting the soot blower tube and rotating it in the fully

inserted position. The lance tube has a nozzle near the tip, which during non-operating conditionis located outside of the boiler and within a sleeve to protect it from the furnace radiated heat.

When it is activated, the lance tube is advanced to where the nozzle is about 40 mm from the

face of the boiler tubes. The operation is synchronized in such a way that steam flows through

the nozzle when it comes to the blowing position. The cleaning radius is normally around 3meters and is a function of steam pressure, the type of nozzle, the nature of the deposit, and the

boiler surface

Steam acts as a good cleaning medium when the deposit is loosely adhered as it needs only

penetration and expansion to give the required impact energy to dislodge the deposit from the

wall. Some designers use air instead of steam, but this has been largely discontinued.

Water lance

When the ash deposit is mainly a dense slag layer, mechanical impact energy alone may not be

sufficient to clean effectively. A water lance removes slag by water jet and precisely controlling

the pattern and progression velocity of the water. The water first absorbs the heat from the slagmaking it friable in nature and then the wall is cleaned by the steam produced during heat

absorption from slag. The operation is more like the steam wall blower except that the water

flow is delayed until the nozzle is about 50 cm from the rest position.

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Water cannon

The water cannon directs a high pressure jet of water through a side furnace wall opening toclean surfaces on the opposite wall of the furnace. Here the nozzle is at the leading tip of the

lance tube and the lance tube is articulated by a two-axis drive system. The water cannon offers

several advantages that make it an attractive cleaning option. The 45 deg in both the horizontaland vertical directions, single water cannon can provide the cleaning coverage equivalent to

many water lances, depending on the size and layout of the furnace.