flame impingment

7/30/2019 Flame Impingment

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FLAME IMPINGMENT

How docs the furnace provide for proper combustion?A For fuel to be burned efficiently, the furnace must have adequate

combustion space to ensure a thorough mixing of air and fuel. The furnace

must also maintain a high enough temperature for complete combustion.

The furnace must be tight so that air cannot leak into, or out

of, the casing and thus affect the air/fuel ratio. Too high a furnace temperature

must also be avoided, as this can lead to rapid deterioration of

linings (if installed), or possible overheating of vital pressure parts such

as tubes or combustion furnaces in scotch marine boilers. Also, the

reaction of combustion should be completed before the flue gases leave

the combustion chamber so as to avoid flame impingement on tubes andpossible overheating

List the precautions needed with refractories on oil-fired boiicrs.

A Refractories (brickwork) are subject to damage from many causes,

such as improperly adjusted fires and vanadium-contaminated oil. Impingement

and the resulting carbon buildup arc common. The flame

should travel down the furnace on sm boiicrs without touching cither the

furnace or the refractory. Long periods of operation on low firing often

cause refractor)' damage. The very small flame reduces the combustion

chamber temperature and causes poor combustion.

Carbon builds up and intense heat is directed at the surface of the

refractory, thus causing spalling (facing breaks off). Unless the oil has

been specifically treated, there is no relief from the damages caused by

vanadium in the oil. Moisture trapped in the refractory may develop

steam, which ruptures the surface.

Q What checks are made on the fireside of boilers?

A Carefully inspect the plate and tube surfaces that are exposed to the

fire. Look for places that might become deformed by bulging or blistering

during operation. Solids in the waterside of lower generating tubes cause

blisters when sludge settles in tubes and water cannot carry away heat.Urheberrechllich geschutzlcs Material

Maintenance, Impaction, and Repair 313

Tiie boiler must be taken out of service until the defective part or parts

have been properly repaired. Blistered tubes usually must be cut out and

replaced with new.

Lap-joint boilers are apt to crack where plates lap in a longitudinal

or straight seam. If there is evidence of leakage or trouble at this point,

remove the rivets and examine the plate'carefully if cracks exist in the

seam. Cracks in shell plates are usually dangerous, except fire cracks that

tun from the edge of the plate into the rivet holes of girth seams. Usually,



a limited number of such fire cracks are not very serious.

Test stay bolts by tapping one end of each bolt with a hammer. For

best results, hold a hammer or heavy tool at the opposite end while

tapping. A broken bolt is indicated by a hollow sound.

Tubes in hrt boilers deteriorate faster at the ends toward the fire.

Tapping the outer surface with a light hammer shows if there is seriousthinness. Tubes of vt boilers usually thin at the upper ends when exposed

to the products of combustion. Lack of water cooling is the cause.

Tubes subject to strong draft often thin from erosion caused by impingement

of fuel and ash particles. Soot blowers, improperly used, will also

thin the tubes. A leaky tube spraying hot water on nearby sooty tubes

will corrode them seriously from an acid condition. Short tubes or nipple;

joining drums or headers lodge fuel and ash, then cause corrosion if moisture

is present. First clean, then thoroughly examine all such places.

Baffles in wt boilers often move out of place. Then combustion gas,

short -circuiting through baffles, raises the temperature on portions of the

boiler, causing trouble. Heat localization from improper or detective

burners, or operation causing a blowpipe effect, must be corrected to

prevent overheating.

What causes one or more bulges in tubes in a wt boiler?

A Usually a piece of scale breaks away from inside

the boiler and settles in a tube, causing localized

overheating of tube metal (Fig 16-1). Flame or heat

impingement and sludge deposits are also causes.

If no evidence of scale is found after a tube bulges

and leaks, it may have been blown free with the tubefailure.

Q Is flame impingement on boiler parts harmful?

. local biiio- * Direct flame impingement causes local overin

B (biiitar) on watoi heating of metal because water circulation may not

, u b*- be fast enough to keep the metal within safe limits.

On ci units, flame impingement can create uneven

expansion stresses that may crack a section. On wt boilers, steam pockets

can form in a tube, leading to tube bulging or rupturing

flame impingment

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