Oil fired boiler explosion

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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barts
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Re: Oil fired boiler explosion

Post by barts » Mon Sep 21, 2015 5:03 pm

dhic001 wrote:Well we've tried hard to make vegetable oil explode, but it just doesn't happen. The same steam atomising burner will do it with smelly long-chain hydrocarbon on the same setting very easily. The resulting whoomph made all the floorboards jump out of the bilge. The vegetable oil just ends up coating everything and when it does light, just burns like any other fire. Of course it does take time to burn off, so you try and avoid putting the flame out for long periods, but with five different boilers in four different boats, we haven't had a vegetable oil explosion yet.
I know what fuel I'll use when i'm oil firing.

Daniel
Interesting. This may have something to do with the size oil droplets you're generating in that burner. Vegetable oils tend to be significantly more viscous than "diiesel" fuel, and typically require more preheat to burn properly (no soot, no deposits on walls of combustion chamber). Steam atomizing burners are very flexible, and will handle a wide variety of fuels, esp. with appropriate preheating. If you're burning oil on top of wood fire, of course, the burner can emit pretty large droplets and everything still works, since the oil will vaporize as it hits the hot coals in the wood fire.

Otter's burner and firebox will look like the inside of a metal melting furnace - just a blue haze against orange walls with bits of orange flame - once its up to temperature, which can take 20 minutes or so. This burner has no target, just tangential strike against the combustion chamber walls. I'll try and snap a pic of the burner in operation next weekend at the B & W meet.

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Re: Oil fired boiler explosion

Post by fredrosse » Thu Sep 24, 2015 4:13 pm

Any oil can cause a puff or explosion, just depends on circumstances. Hot enough oils can be atomized to explosive conditions, just more difficult with heavier oils. In power plant practice we must purge a furnace for 5 minutes (fans on, no fuel) then light ignightors (usually oil or gas fired), then prove ignition flame (scanners usually), then insert main fuel.

Without electric safety interlocks the same safety precautions can be followed on a steamboat, and the operator has to remember to do all of this, every time.

I once had a 20 gallon gasoline tank replaced on my international truck at the farm. With plenty of space between myself and the tank, I tried to get a gasoline tank explosion going, like we see in every action movie.

I tried half full, 10% full, preheated with a wood fire external to the tank, cap on, cap off, M-80 at the tank mouth, electric spark inside the tank, etc. After about 30 attempts the most I got was a moderate "woof" when flames got to the mouth of the open gas tank. No damage.

But I am well aware that these experiments could have exploded the tank with an impressive fireball, I just could not find the right conditions.

We have also had boiler casing destruction when firing pulverized coal, the air purge before ignition is very necessary.
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Re: Oil fired boiler explosion

Post by Oilking » Thu Sep 24, 2015 8:47 pm

Hi All,
#2 Deisel has a flash point of ~150F Biodeisel is ~250F.
The main issue in "involutary reignition" is not so much the flash point or the droplet size as it is the fuel vaporizing as it hits the hot refractory leaving a fire box full of fuel vapor on reignition.
On the destroyers if fires were lost the fuel to the burners was quickly secured, the forced draft blowers maxed out to purge the fire box of fuel vapors, the torch lit, air registers closed, torch inserted, and fires restored starting with #1 burner, or you could relight off the brick work, a highly frowned upon and unapproved practice since it would ignite any accumulated vapors. With the strength of the double wall air casing construction, the issue was damaging the brick work from shock of an uncontrolled ignition.

I can not quote chapter and verse, but I remember reading of an event where an engineer told of relighting fires after what he thought was a proper purge to shortly after hear a muffled thud from above, finding that the stack had split open!

Dave
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Re: Oil fired boiler explosion

Post by barts » Thu Sep 24, 2015 8:59 pm

Oilking wrote:Hi All,
#2 Deisel has a flash point of ~150F Biodeisel is ~250F.
The main issue in "involutary reignition" is not so much the flash point or the droplet size as it is the fuel vaporizing as it hits the hot refractory leaving a fire box full of fuel vapor on reignition.
On the destroyers if fires were lost the fuel to the burners was quickly secured, the forced draft blowers maxed out to purge the fire box of fuel vapors, the torch lit, air registers closed, torch inserted, and fires restored starting with #1 burner, or you could relight off the brick work, a highly frowned upon and unapproved practice since it would ignite any accumulated vapors. With the strength of the double wall air casing construction, the issue was damaging the brick work from shock of an uncontrolled ignition.

I can not quote chapter and verse, but I remember reading of an event where an engineer told of relighting fires after what he thought was a proper purge to shortly after hear a muffled thud from above, finding that the stack had split open!

Dave
Otter's burner sometimes goes out, usually when she's cold.... the wait time for relight is about 4 seconds w/ an open firebox door; if I wait that long, I've never had a woof.

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Re: Oil fired boiler explosion

Post by Oilking » Fri Sep 25, 2015 1:38 am

Yes, there is a matter of scale when dealing with a system that burns 1 to 2 gallons per hour compared to one that is guzzling 15 to 25 gallons per minute.

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Re: Oil fired boiler explosion

Post by Maltelec » Mon Oct 05, 2015 8:43 pm

The whole point of the fan starting well before ignition and oil feed is to blow out any combustible gasses to prevent such an explosion.

Should the burner fail, a good control system will keep the fan running and attempt ignition again only after 10 seconds or so, to allow all the unburnt fuel vapour to be pushed out.

I have known this happen on a steam car for the same reason.
I've got the vehicle, just need the boat.
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Re: Oil fired boiler explosion

Post by dhic001 » Wed Oct 07, 2015 7:01 am

That requires a fan, which a steam atomising burner doesn't have....
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Re: Oil fired boiler explosion

Post by barts » Wed Oct 07, 2015 9:53 pm

Note that shutting off the fuel and letting the steam continue to enter the firebox through the burner should very quickly remove any oil vapors from the firebox - it does on my boiler, anyway. This may take longer if you have a really large firebox because you're also setup to burn wood - but in that case, keep a small wood fire going and the burner will relight right away.

Of course, always keep the firebox door shut whenever possible.

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