Boiler pressure rise?

[Lopez Mike]

I'm been reviewing all the information I can find online about the 2001 steam traction engine failure in Medina Ohio. It seems fairly sure that the crown sheet of the locomotive type boiler had eroded to a ridiculous extent and would not have survived the most modest hydro test. Pretty much a 'no brainer'. Fools will blow themselves up from time to time if not supervised.

What has caught my attention are repeated claims in multiple forums that when a crown sheet fails or there is any sudden and large escape of steam from a boiler, then the pressure rises in the boiler thus making the failure more violent. Claims that the pressure doubles or more are common!

Now I understand that if some part of the boiler gets hotter than the water or steam and is hit with liquid that there can be a rapid conversion of water to steam. Dry crown sheet issues for instance. But this doesn't seem to be what these contributors are talking about.

They are saying such things as "Don't use too big of a safety valve or blow down valve as you can blow up your boiler that way!"

Now I have a 1.5 inch quick action blow down valve on my VFT (33 square feet) and I have opened it all the way many times and never seen the slightest bump on the main gauge. On my little test boiler on the bench with propane firing, I have often turned off the fire and then blown it all the way to zero. Leaves it nice and dry for storage. Makes the cat run around of the shop!

Have I missed something? Is there something lurking in the steam tables waiting to kill me when I blow down my boiler? I think not but I am very interested in our members' thoughts on this. I've been wrong before.

[fredrosse]

I spent the first 20+ years of my career performing analysis of nuclear plant steam-water accidents. These conditions generally considered pressurized reservoirs of steam/water, and a pipe rupture or breakage that lets out the hot water/steam, destroying equipment and structures downstream. In all that time there were not incidents as you describe where internal pressure increases. As the steam and/or water is blown out, the pressure in the vessel reduces.

One could envision cases where the pressure would increase, in particular in a flash boiler with attached continuous superheater. In this case, the superheater tubes may be very much hotter than the boiling section tubes, and when sudden release of steam occurs, the saturated water (in the boiling coils) is driven into the superheater section, where there may be enough temperature energy stored in the hot tubes to overheat this residual water and cause a spike upward in pressure.

On the blowdown side of the boiler, opening this connection will only reduce the inventory of liquid water, and some flashing of saturated water will occur, but pressure will stay close to the original pressure until all of the saturated water is blown out. After that, steam pressure will decay more rapidly, but it will not go up.

[Lopez Mike]

Thanks, Fred. I thought so. But I was ready for something new.

I spent much of my life trying to get more and more power out of small internal combustion engines and the more I learned, the more of my most cherished theories turned out to be based on received dogma.

As a typical example:

After modifying quite a few stock motorcycle engines including a bunch of careful port changes, I finally acquired a flow testing bench. I dug out some of my race winning cylinder heads with great anticipation. Every one of them flowed less than a stock head. Instant humility.

The state of the art these days is to get 90 hp out of a 15 cu. in. engine on ordinary gasoline and no supercharging at 15,000 rpm.

My boat engine has a swept volume of about 28 cu. in. times two for being double acting = 56 cu. in. and is rated at 5 hp at 500 rpm.

And my friends wonder why I'm not obsessed with boat speed!

[marinesteam]

I don't know if this helps, but it seems to support that a sudden release of pressure could cause water to flash to steam faster than the pressure can be released therefore increasing the amount of energy available during a failure. From my reading it looks like you would need a large release like a rupture to cause this occurance and the effect of the water flashing to steam would make the rupture worse.

http://books.google.com/books?id=POJQAA ... re&f=false

Like many things, this information might of been taken to a level beyond of what can actually physically happen by the well meaning but uniformed. But then again, maybe I'm missing something as I'm no expert.

Ken

[Lopez Mike]

Nice reference. Thanks, Ken.

Lots of solid information in that article but also some howlers.

The business about being able to heat the water beyond the boiling point until there is a disturbance is real but has no relevance to practical boiler operation. To make that happen, the water has to be very pure and free of any contaminants and incredibly low mechanical vibration. Not in any boiler I have dealt with.

The author mentions having water contact an overheated part of the boiler and thus 'flash' into steam but repeats the notion that a sudden demand (read, pressure drop) will cause the pressure to rise. Old Wives Tale, I think. (Sorry for the sexist language)

A lot of good engineering has come down the pike since 1910. Steam didn't roll over and die with Liberty Ships and the Lima locomotive works. The fact that all of the steam cleaners around aren't flying to bits would seem to be pertinent. Talk about sudden demands and then waiting for the next surge.

Also, every propane barbecue tank is a boiler in operation. You can knock the top off one and all that happens is a big long whoosh and a change of trousers. No smoking please!

[gondolier88]

This thread is a sad reflection on conjecture from those that need to find a book with steam tables in and an explanation of what steam is.

It is scary to think that you would compare a gas cylinder and a boiler, let alone even think about it as an analogy. Both are pressure vessels, yes, but their hazards lie in entirely different territories- one is calorific potential (ie. a spark finding released LPG), and actual calorific content (ie. a boiler at max.SWP)

Boiler explosions are just that- they have killed, do kill, and will kill people around the world. Predominantly through extremely bad mismanagement, neglect or ignorance rather than bad design.

A few simple facts;

- Water heated at pressure has it's boiling point raised in direct correlation to the pressure it is under.

- Water at any pressure at a temperature corresponding to it's boiling point at that pressure, when released into atmospheric pressure will expand as it flashes into steam at a ratio of 1600:1- ie. on pint of water at 100psi at boiling point will flash into 1600 pints of steam at atmospheric pressure.

- It is the expansion of released water in a boiler rupture that causes an explosion- imagine a boiler with perhaps 200gal of water in expanding at a ratio of 1600:1.........!

- If a boiler runs dry without rupturing the VERY worst thing you can do is put water in it to cool it down- you will flash the water into superheated steam in a moment, this will overpressure the boiler quicker than the safety valve can handle and will rupture the boiler, at which point an explosion will ensue. However this explosion will be of much higher temperature than under normal steam operating pressure/temperature and would be a very nasty accident indeed.

Please do not play down boiler explosions, or the hazards and conditions under which they occur- one misguided word on a forum like this can give people the wrong impression and lead to accidents which have comeback onto us all.

Greg

[marinesteam]

Here is a much more recent article talking about sudden pressure release as related to boiler explosions

http://www.hsengco.com/nbexplosion/nbx.htm

The takeaway as I see it (and not well explained in the first post) is that this is a real phenonemon but not one that could be caused by the opening of a fairly normal sized orifice on a boiler. There is reference to this happening with the fast opening of "grossly oversized" valves but all references I can find refer to plate failure as the root cause of this type of explosion. I'm taking "grossly oversized" to mean well beyond ths size one would normally find on a boiler. So as far as the original post is concerned, a ridiculously sized safety might be of some concern but I can find no reference to normal sized valves being opened as the cause of this type of failure.

P.S> Here is one related to liquified gas showing that the mechanism is roughly the same.

http://www.theenergylibrary.com/node/11368

Ken

[Lopez Mike]

Very good stuff. The article about the mechanism in a BLEVE is about the best explanation I have read. And brings to the front something that I was unclear about. That the phenomenon is not restricted to flammable liquids.

It would seem that if we keep our boilers at safe water levels, hydro test at reasonable intervals and test our safeties at least as often, that we are probably relatively safe.

That said, leaving aside the flammability of propane or butane (I don't!), I think that the analogy with a steam boiler is valid. We have a liquid under pressure and in equilibrium at a temperature. When we draw off either liquid or gas from the pressure vessel, then the source of the heat, the surrounding air usually, provides energy to evaporate more gas and thus restore equilibrium. If the thermal demands of our energy withdrawal exceed the energy from the source (surrounding air again), then the pressure drops. A common problem for me when I'm doing foundry work in cold weather! So I stoke up the 'fire' with some water from the tap. Pressure rises and I go on drawing off 'steam' from my 'boiler'.

I love stirring up trouble. You guys are so good and I learn more than any school.

[S. Weaver]

Greg:

Very good summary. I was about to weigh in, but your succinct response put that to rest. The only point that is debatable is the last argument.

Standard railway operating practice in N. America upon encountering a "white bottle" - no ascertainable way to determine water level - is to put on both injectors, and then see about reducing the intensity of the fire by shut off (oil) or dumping the grates (coal/wood). In the business, I personally have inspected "quilted" crownsheets, post-incident, that were plastic and deforming on the stays and then stabilized by the re-introduction of feedwater and dowsing the fire. Were the enginemen lucky? Yes, in spades. But some of us have taken this as evidence that introduction of feedwater per se is not the primary culprit, but the large voiding to atmospheric pressure caused by sheet failure that you ably describe. Could the introduction of feedwater "flashing" on the deformed sheet blow the sheet off the stays? Perhaps, but how would we know? We would need to be Schrodinger's cat to measure that. The number of saved "quilts" over the last century, has led us to take the risk with the feedwater.

Back to the original question, if we don't remember anything else from this discussion, we should watch our water as if our lives depended on it, remember the wisdom of the commonly available steam tables and drill the ratio of expansion - 1600:1 - into our heads and those of our trainees. I'll step down now ...

[Aheadslow]

As to the accident in Ohio,, That was a case of someone operating a boiler that was glaringly unsafe. The boiler on the case 110 in question should never have seen any kind of pressure . according to the report by John D. Payton, Director of the Certified Boiler Engineers for the Commonwealth of Pennsylvania
there were fusible plugs that had been seal welded (very bad idea) the crown sheet had thinned in places as much as 77%. . But even a cursory inspection of the boiler by a knowledgeable person would have determined it to be unsafe to operate. For those interested here some pics of the boiler after the explosion. http://www.dli.mn.gov/CCLD/BoilerIncidentsHobby1.asp Ive run my grandfathers old peerless portable at many many shows over the years with no incident. The crown sheet in the old beast is still nearly 90% of its original thickness, but I still pressure test and inspect every year. and the fusible plugs get replaced every 2 years whether they need it or not. I get the pressure relief valve reset and tested every year. you can not be too safe when operating a boiler. especially a loco style firetube boiler. So remember be safe and happy steaming.