Forced Draft to Clean Fire Tube in Boiler

The main purpose of running air before igniting a boiler is to ensure that there is not an overall explosive atmosphere when it is hit with flame. (blow out all potential gas before igniting to allow for proper flame and not explosion)

normally blower switch on for a while before combustion begins - this is known as pre-purging (not cleaning tubes) to remove any left over combustible materials before ignition takes place. There is also a post purge where after the boiler is switched off the blower purges and then stops. Your burner programmer has a set purging time and this is a flame safeguard safety that the blower first starts and pre-purges as per the set timing of the programer and then starts your burner. If this safety is not there then boiler/burner back-fire (explosion) takes place.

It sounds as though you are describing steam-activated soot blowers. Here is a description: http://facworld.tripod.com/fwezine13.htm

thx for the link...however according to my boilerman...they are not using soot blower to clean to fire tube. annually they will clean the tube manually using brush attached to a steel rod....im not sure what it is called.....

anyway the blower im referring to is actually for air suction to create a forced draft in the fire tube.

ducon's reply is quite what im looking for. but regarding the combustible material and explosive gas im assuming it is the unburned fuel and fuel impurities?

the boiler is using Medium Fuel Oil.

im assuming it is the unburned fuel and fuel impurities? yes + left over surplus if any.

Let me explain you in a layman term. To burn 1 cfm gas you need 10 cfm air. Your kitchen cooking range has a big burner 12 cfm and 3 small burners each 8 cfm. Now to burn, the big burner needs 120 cfm combustible mixture and small 80 cfm. If you were lighting with a match, say on the 3rd try you will get a "backfire". Now imagine if the burners were all on and your kitchen closed all night, in the morning with all the accumulation of gas just ONE match strick lightning = BOOM.

Your Boiler say 5000 lbs/hr requires 1.25 cfm/lb = 6250 cfm x 10 = 62,500 cfm of combustible mixture , a 10,000 lbs/hr = 125,000 cfm (comparing to your cooker burner 120 cfm imagine the size).

Any delayed ignition is boom. Now since your burner is say designed for 5000/10000 lbs/hr boiler and if there were any left over gas due to tripping/ gas leakage via the main soloniod gas valve(dirt on the diaphgram) there would/can be excess gas + the 62,500 cfm. This is why the blower does pre-purge and post purge for safety of BIG BOOM = NO BOILER, NO BOILER HOUSE , NO OPERATORS/PERSONNEL CLOSE BY.

Note : The BOOM will take place even with purging due overnight gas valve leakage. since blower not designed for that volume collection of gas. This is why there is a system known as " Double-block-bleed" . This is a 1" Solenoid valve with NO position for venting in between 2 nos main valves NC.

For boiler tube claening go to google any type "Goodway" boiler tube claening equipments.

Ducon explained it.

Clarifications for you: A boiler burner using oil (your case), has a valve (1 or more) that opens when firing to release the oil to an atomiser device. this produces a fine mist of oil mixed with the combustion air from the fan/blower.

While the flame is on, there will be no problem since the fuel is burning continuously.

At the end, when the burner is turned off, if the valves are leaking, you can have the flame OFF but still produce a small amount of Oil/Air mixture in a fine mist. This will fill all the boiler tubes and combustion chamber. The reason it is not ignited at this stage is somewhat dependent on the volume and others...

but it can happen. Also, when the blower stops, the leaks continues and since there is heat in the chamber, oil will evaporate without burnig and still fill the space inside.

When starting a new cycle, the purging of all this mess is required to avoid an explosion as described by DUCON.

The securities on the boiler (Photocells, flame detectors, Pre-purging and post purging etc are to prevent this explosion from happening and at worst, reduce its power if the leak was substantial. The small explosions that might be present at ignition could indicate (amongst others) a leaky valve even if very small.

On manually-fired steam locomotives, the tube-cleaning technique is simple. When the engine is working hard, the fireman throws a shovelful of dry sand into the firebox!

PWSlack you're jokeing Really

http://www.youtube.com/watch?v=bwuFEZBsxSA

watch above video on boiler tube cleaning with Goodway equipments.

thx for the video...my boiler configuration is as below...it doesnt has that many tubes...

Ok happy you now understand the difference of purgeing and tube cleaning.

Everybody's 3-pass fire-tube is like this. The tubes' quantity & dia depends on the steam generation capacity rated for each boiler.