mono-tube safety facters

[Lopez Mike]

Pressure cooker 'incidents' are, fortunately, almost always more amusing than catastrophic. I have witnessed two.

The first was at the age of about five watching my grandmother cooking some string beans. The oscillating pressure regulator must have gotten fouled or stuck. The pressure apparently rose far enough for the rubber safety plug to blow out. The beans had swollen to the point where they began to escape through the safety hole one after another like a machine gun.

The second was when a person I knew who ran a small greenhouse decided to try to kill the weed seeds in sheep manure with a pressure cooker. In his kitchen, no less. On cool down he got impatient and was somehow able to force the lid open while there was still some residual pressure. He and the walls of his kitchen were indistinguishable from each other. He said it improved his complexion.

This could go on all night. Goodnight.

Mike

[DetroiTug]

for some reason it was allowed to boil dry resulting in melting the end that was in the fire. Disappointing, but no catastrophy. A pressure cooker that has a constant draw of steam from it will do much the same, run out if water stop making steam, and then then melt if its aluminium. Not unlike a kettle on the stove boiling dry.

The real danger here and what caused almost all steamboat explosions was adding water when the boiler or some interior heated surface of the boiler was dry. It immediately flashes to steam at the same temperature/pressure as the exposed heated surface.

To reiterate: If it is not known how much water is in the boiler i.e.not showing in the sight glass or at the lowest trycock, shut it down. There may be a blow down open, leaking back through the check valves of the feed system etc. and there may not be ANY water in the boiler. Adding water in that situation is a very dangerous thing to do.

Leaking back through the checks- I had that very thing happen. Right back through a United spring loaded check valve at the boiler inlet and an Apollo inline spring checkvalve before the economizer. Both steam rated valves. The United spring valve was mounted horizontally, clean and was simply malfunctioning and has been discarded and replaced with a horizontal swingcheck. The Apollo had a metal shaving in it and was cleaned/repaired. Issue was resolved. I was at a steam meet last summer and a boat there kept losing the boiler water and over flowing the hotwell. Looked over and he had the exact same 1/2" United spring loaded check valve at the boiler.

Re: How much superheating occurs in an exposed vertical firetube. According to the temperatures I've recorded from the funnel at normal boiler water level, which are under 300 degrees, not much. That is a sufficient metric to determine how much of the heat is being transferred to the water.

-Ron

[Lopez Mike]

Ron I've read and heard that about adding water in a low water condition for most of my life and accepted it as 'revealed truth' and not questioned it at all. But I'm starting to have doubts.

Let's say that there is a normal boiler running under normal pressure and we have an electrical heating element in the steam space. We warm that element up to red heat and then spray water on it. Now I grant you that there will be steam generated when that happens.

But unless my understanding of how it all works is way off, the pressure is the same all through the boiler. Any change in one area is communicated all through the boiler at the speed of sound. Added to that is the fact that water has many times the specific heat of steel. When I dunk a pound of red hot steel into ten pounds of water. The steel cools down way more than the water heats up. That is, less heating of the water than the ten to one mass ratio might imply.

So the steam generated in cooling down this electric heater would be instantly condensed to water at the temperature and pressure of the rest of the boiler water. The pressure would rise, of course, but I just don't see this producing the sort of 'spike' that I've been hearing about associated with progressive shell failure.

People have been telling me anecdotal stories for years about how when a crown sheet or shell fails that the pressure went to some high number and produced the damage seen. I don't buy that scenario. If that were the case, I would see some pressure jump, however small, when I open my big 1.5" blow down valve on my little VFT-30. I don't. It just starts dropping. Relatively slowly until the water is gone and then very rapidly as the valve starts passing steam.

I have a sneaking suspicion that these accounts of failure at the moment that water was added are a classic case of "After this therefore because of this". It seems to me that a high percentage of the time the noticing of low water, the adding of water and a structural failure are logically happening at about the same time. A Crownsheet getting ready to collapse is strongly correlated with low water and there is (should be!) a strong correlation between noticing low water and adding feed water. But I'm not convinced (as yet!) that there is a causal connection between adding water and failures.

I could be way wrong on this. It's happened before and will happen again. But I just don't see the physics of it all coming together.

Mike

[DetroiTug]

Cooling a piece of heated steel in a bucket, small-thin piece - little mass it cools quickly. Larger piece-higher mass it cools much more slowly. In the case of a boiler all this heating surface that we rely upon to make steam normally will be potentially over a few thousand degrees. And it's not thin steel, it's thick plating that is not going to cool in an instant. It's going to be flashing the water to steam and initially at a pressure that correlates to the temperature which would be supercritical. If the boiler internal volume is enough to contain it - If the strength of the boiler is adequate to contain it - If all of this mass consisting of the thick waterwall crownsheet and even the shell depending on long it's been fired will cool in time - If the relief valve is adequately sized to release this event. Lots of If's.

On cooling steel with high mass, over at Dearborn Ford Rouge complex, they have a steel mill (Presently owned by Severstal from Germany) - Henry built it to get away from the steel companies in Pittsburgh as he felt they were banding together and overcharging him on prices. Anyways, when they bring these 40k pound coils roughly 40" wide X 60" diameter with a 12" hole in the center, out to cool, it takes three weeks sitting on the ground in the winter time to cool completely.

I'm going to side with the dangers of adding water to an empty fired boiler are not any sort of urban legend and are very real.

-Ron

[Lopez Mike]

You may be right. But I would like to hear of any sort of engineering study or report that explains or documents this happening. All I've found so far has been stories how this and that happened. Mostly written by journalists and we know how well informed they are! The plural of 'anecdote' is not 'data'.

I'm not an engineer so I'm not in a position to calculate any test cases. I would think that by the time a crown sheet got to anywhere near such high temperatures it would have long since collapsed before the fireman got near the injector. The tensile strength of steel drops pretty rapidly far before red heat. And compared to the mass of the overheated boiler plate, the mass of the remaining water is at least an order of magnitude higher. The crown sheet is exposed to steam with only a few percent drop in water volume.

Not convinced. But I love the discussion.

Mike

[fredrosse]

Time to weigh in here, and the statements to never add water to a fired boiler that has lost its water level is good advice. An overheated crown sheet can have enough energy available to cause a serious spike in pressure when water is introduced and suddenly covers the sheet with liquid water.

The issue here is all an energy balance, so lets look at what might be a typical condition. For the purposes of this discussion, I will use the dimensions of my Crown Manufacturing Co. boiler, with 12 inch diameter furnace crown, 22 inches long, 3/8 inch thick. The steam space in this boiler is only about 1 cubic feet at low water level. The boiler is running at 100 PSIG, 338F saturation temperature.

In a clean boiler, the crown sheet should be at about 340F. Water level gets low, exposing the crown sheet, which begins overheating. Steel retains virtually all of its strength at 650F, and considering safety factors in boiler design, the sheet could probably get to 900F (+/-) before collapse. Lets say the crown sheet is 840F when someone turns the feedwater on.

The 338F water then rises to cover the crown sheet, and begins quenching the crown sheet, generating steam. The energy available is the exposed steel plate, about 1 square foot, 3/8 inch thick generates new steam, until the overheated crown plate is cooled to a new saturation temperature. Without going into calculation details, this scenario results in an equilibrium pressure of about 420 PSIG.

This analysis assumes the boiler shell does not absorb heat, and that the boiler water free surface does not absorb heat. In fact both of these surfaces will begin picking up heat as the flash steam condenses on these surfaces, all at about 340F. If we assume that only one third of the flash energy is used to cause a spike in boiler pressure, then the pressure spike would be to about 225 PSIG, still significant.

[fredrosse]

"Pressure cooker 'incidents' are, fortunately, almost always more amusing than catastrophic. I have witnessed two."

Similar story, when I was about 5 years old I asked my mother how she could make a can of soup? She innocently told me "Just put the can of soup on the stove and heat it up". Evidently she left off the part about opening the can, and putting it into a cooking pan.

Later that day I decided to make some Campbells Tomato Soup, so I got the can out of the cubbard and put it on the coal stove. Several minutes later the can exploded, blowing "flash soup" all over the kitchen. Fortunately nobody was in the kitchen when this happened, a mess to remember.

[Lopez Mike]

Thanks Fred. Good to see some rough numbers that show the possibilities.

I think that the water level would, however, rise relatively gradually. Thus between dumping the fire and not spraying the feed water directly on a hot surface, I think that it is just a possibility rather than a certainty. Also, I can see where a larger boiler that is engineered much more closely for economic and performance reasons might actually be less safe under careless operating than one of our hobby boilers. We tend to over design.

When I was designing my small locomotive I was stunned to find that the shell would handle nearly scuba tank pressures. I have a sneaking suspicion that most of our launch boilers would stand several times our normal operating pressure without damage. I'm much more concerned about piping failures that would get me scalded.

My most recent domestic explosion was due to my late father absentmindedly leaving a can of spray paint on a shop heater. No one was in the room when it finally cooked off. There is still a lot of Krylon Bright Red here and there about the shop. One window cracked. Lots of interesting shadows where familiar objects blocked the paint spray before it reached the walls.

We all have heard and seen the results of home water heater failures. Spectacular! A wonder it isn't worse in most cases.

Mike

[Oilking]

I'll title this one the "Aerospace Plumbing".

Several years back this event took place in Longview, WA. A young couple in a second floor apartment started to have problems with getting hot water when they first opened the cold water faucets, quite hot!* The management called a plumber. The plumber examined the situation, and determined that installing a check valve in the cold water line would solve the problem which it did, for a while . That night with the couple all snug in their bed, the tank let go. The couple some how escaped serious injury. The bedroom wall was pushed up against the bed and they were covered with sheet rock dust. The roof of the building was lifted of the upper plate, and the tank itself was found the next day several blocks away.

The findings:
The tank was an older tank without a direct tank mounted relief valve.
The plumber had mistakenly installed the check valve between the tank and a line mounted relief valve
A stuck thermostat or shorted element was causing the water to over heat and back out into the cold water supply.*
The plumber was not licenced, but did work under another plumbers licence.

*anytime hot water starts to show up in the cold water, and it hasn't been so before, check the thermostats and elements.


Houston We had lift off!

Dave