Why LWCO's & closed fill valves are a good idea


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Old 12-23-09, 07:29 AM
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Exclamation Why LWCO's & closed fill valves are a good idea

Recently there was some comment about leaving the water supply auto feed open or closed. B&G states flat out to close the auto-feed on a hot water boiler. Once the system is filled, bled, and up to pressure the valve should be closed.

Otherwise there is the possibility of a steam explosion occurring.

It happens:

Fire in the Hole Photography

:yikes:

Al.
 
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Old 12-23-09, 08:06 AM
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Bang

Boiler explosions are rare but spectacular, and deadly. In the 60's, A Telephone co. boiler in NYC exploded and traveled many feet horizontaly and verticaly much like a rocket, and a hot water heater in a school in the midwest exploded, both events caused severe damage, injuries and as I remember some deaths. Both explosens were safety valve related as this one appears to be.
Sid
BE CAREFUL you cant put a smilie on this one
 
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Old 12-23-09, 10:05 AM
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The only way this explosion can be blamed on the safety valve is that it could not vent the massive quantity of steam that flashed from water being added to a red hot boiler.

Water under pressure contains no energy. Water (as other liquids) is incompressible, so can not cause the explosion that occurred in the link I posted. Even if the boiler pressure reached 100 psi, there is no energy stored in the compressed water.

An explosion of this nature is caused by water instantly flashing to steam. Which happens when a boiler runs dry, the pressure drops, and the auto-fill then adds some water.

Al.
 
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Old 12-23-09, 10:53 AM
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Originally Posted by OldBoiler
The only way this explosion can be blamed on the safety valve is that it could not vent the massive quantity of steam that flashed from water being added to a red hot boiler.

Water under pressure contains no energy. Water (as other liquids) is incompressible, so can not cause the explosion that occurred in the link I posted. Even if the boiler pressure reached 100 psi, there is no energy stored in the compressed water.

An explosion of this nature is caused by water instantly flashing to steam. Which happens when a boiler runs dry, the pressure drops, and the auto-fill then adds some water.

Al.
Not so. If the water contained in the boiler is heated the temp will increase along with the pressure. If there is no means for the pressure to escape, the temp and pressure will continue to climb. At 100PSI, the water would be at approx 330F. If the boiler then split open releasing all that superheated water to atmospheric pressure then BOOM So, it could indeed be completely full of water and still explode.

If the temperature of the water is below it's boiling point at atmospheric pressure, then there isn't much of a problem. This is why high pressure gas cylinders are hydro-tested.
 
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Old 12-23-09, 11:21 AM
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Originally Posted by formula74
Not so. If the water contained in the boiler is heated the temp will increase along with the pressure. If there is no means for the pressure to escape, the temp and pressure will continue to climb. At 100PSI, the water would be at approx 330F. If the boiler then split open releasing all that superheated water to atmospheric pressure then BOOM So, it could indeed be completely full of water and still explode.

If the temperature of the water is below it's boiling point at atmospheric pressure, then there isn't much of a problem. This is why high pressure gas cylinders are hydro-tested.
The water doesn't need to be at 330F for the pressure to be at 100 psi. This can happen on a cold boiler with a failed auto-fill valve. Or even by a mistakenly opened manual fill valve.

For the boiler temperature to be at 330F, the aquastat would also have had to fail. Along with a long call for heat that is now overheating the house. So the thermostat would also have had to fail (unless the primary is set up with the TT terminals jumped).

So to get 100 psi at 330F there has to be the failures of multiple devices: safety valve, aquastat, and the thermostat. I don't see the 100 psi scenario with 330F water being likely. It is more likely that the system was run low on water.

Al.
 
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Old 12-23-09, 11:33 AM
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Unlikely, perhaps. However definitely possible.

But to say "Water under pressure contains no energy" is inaccurate as there are other variables in this equation.
 
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Old 12-23-09, 01:43 PM
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I am stated a comment that may or may not be true, I assume this is a single story home. The boiler off the kitchen means there is no basement as many homes in Delaware. The heating pipes probably run down into a slab or crawl space. With that said I am going to agree the boiler was out of water and may have fed water into a very hot boiler. It probably is more a lack of maintenance than anything. Annual maintenance can avoid these problems by finding problems and repairing them.
This is one reason LWCO are starting to be required everywhere is just for this reason. The second reason is a runaway boiler due to welded control contacts or what ever is keeping the boiler running. As the temperature rises so does the pressure. The water is not going to steam due to raising the boiling point as the pressure goes up. When the pressure gets to 30 psi the relief valve opens and the sudden pressure drop which changes the boiling temperature and allows the water to rapidly expand to steam which is 1500 times it's size and KABOOM.
 
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Old 12-23-09, 04:12 PM
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An explosion of this nature is caused by water instantly flashing to steam. Which happens when a boiler runs dry, the pressure drops, and the auto-fill then adds some water.
I'm going to expand this for the benefit of those who didn't see the previous discussion:

A boiler low on water, and firing somewhat continuously in order to meet a calling thermostat demand could, and would, open the pressure relief to vent the pressure built up when the remaining water started to boil. This pressure would remain at the setting of the relief valve (30 PSI) while that relief valve spewed the steam that formed. Meanwhile, whatever leak happened to exist continued to leak, even faster, because of the increased pressure. SO FAR, there is no explosion...

Once the boiler is DRY from boiling out the remaining water, and the thermostat continuing to call for heat, that boiler just gets hotter and hotter, and as Al said, now the pressure will start DROPPING because there is nothing left in the boiler to maintain the pressure... and there is a leak...

Once that pressure drops below the setting of the reducing valve, that valve will feed some water. Let's say that it feeds ONE CUP of water... once that cup of water is dumped onto that cast iron, and it flashes to steam, it immediately expands in volume to the rough equivalent of ONE HUNDRED GALLONS of water. And THAT's when there is an Earth Shattering Kaboom that Marvin was waiting for.

I don't think Sears is going to honor the warranty on that washer/dryer...
 
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Old 12-23-09, 04:30 PM
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At least they won't have to buy a new door. They should be able to put that one right back with no problems. Maybe use some more screws this time around.
 
 

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