WHAT IS SOOT BLOWING IN BOILER OPERATION & THEIR OPERATION PROCEDURE?
One of the important by product of the Combustion of solid fuels is ash. During Combustion some, some of the mineral constituents and compounds can be in a molten or plastic state. The ash constituents can be deposited on the heat absorbing surface causing Slagging and fouling. If ash reaches the heat absorbing surface at a temperature near its softening temperature, the resulting deposits are likely to be porous and can be removed by soot blowing. Slagging is the deposition of molten, partially fused deposit on the furnace walls and the radiant super heater exposed to radiant heat. Fouling is the deposition of more loosely bonded deposit on the heat absorbing surface in the convective path soot blowers using steam are generally used for removing ash deposits on fouled tube surfaces , while water lances or water cannons using thermal shocks by room – temperature water may be needed to clean the slagged tube surfaces.
WHY SOOT BLOWING NECESSARY IN BOILER?
A soot blower is a system for removing the soot that is deposited on the furnace tube of a boiler during Combustion. Various types of soot blowers such a wall blowers, long retractable blowers and air heater blowers are used for cleaning. Steam is normally used as a medium for blowing away the soot. The soot which sticks to the heating surface insulates them from the heat, efficiency soot must be blow off to maximize heat transfer in the boiler. Elevated stack temperature may indicate excessive soot build up. High exit gas temperature at normal excess air indicates poor heat transfer performance.
Stack temperature should be checked and recorded regularly as an indicator of soot deposits. When the flue gas temperature raises by about 20 deg. C above the normal temperature for newly cleaned boiler soot deposits are the main concern. It is estimated that 3 mm of soot can causes an increase in fuel consumption by 2.5 % due to increased flue gas temperature.
The frequency of soot blowing is depending upon type of fuel used. Coal is used as powder condition in pulverized coal fired boiler. So lot of soot is deposited in this type of boiler. Frequency of soot blowing at an interval of 8 to 10 hours is required in this cause.
In case of oil and gas fired boilers ash percentage in fuel is very less so soot deposits is very negligible. Soot blowing is required once in 10 to 12 days.
Normally dry steam at 12 to 14 kg/cm2 is used as blowing medium. Soot blowers are placed in the flue gas path to clean some portion of the tube. When blowing is starting this high pressure steam comes out at high velocity from the nozzle of blower. This high velocity steam clean the soot deposits on the tube. The blower rotates by 360 degree by means of electrical motor. So the high velocity steam can clean tube surfaces around it. Dislodged soot is taken out by flue gas.
TYPE OF SOOT BLOWERS IN BOILER
There are two types of soot blowers. These are rotary soot blowers ( RSB ) and long retractable soot blowers (LRSB ) . Rotary soot blowers rotate at a fixed position. In long retractable soot blowers the lancer enters into flue gas path and also rotates like rotary blowers. Movement of lancer and rotation of blower is done through a motor.
Retractable blowers is installed at high temperature zone and rotary blower or fixed blowers is installed at lower temperature zone like air preheater and exit end of economizer.
All the soot blowers are connected to common soot blowing line. Soot blowing is started one by one in the direction of flue gas flow. Normally blowing is done sequentially once it is started. Otherwise this can be done manually one after another in the flue gas direction. Steam used should be dry. Steam is supplied to that particular blower when it is in service. Steam is cut off automatically through a poppet valve, when blowing of that blower is finished. Movement of the blower is finished. Movement of the blower is controlled by limit switch. Soot blowing is preferably done at a load 50% of MCR (maximum continuous rating) .
BOILER SOOT BLOWING PROCEDURE:-
Following important steps are followed during soot blowing:-
1.Open the isolation valve of soot blowing steam line.
2.Open drain valve of this line to drain out condensate if any. Allow steam to flow for some time through this valve to increase temperature of this line
3. Put the control valve of the line in auto mode to get 12 to 14 kg/cm2 pressure.
4. Check all retractable blowers are in withdraw position
5. Increased the draft a little higher
6. Start the sequence. Blowing will be started one by one. In case of manual system start blowing one by one in the direction of flue gas flow.
7. After completion of blowing sequence bring back the draft to normal and close the steam isolation valve.
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