I must take issue with a couple of Rays points.
The fact that pumping water into an overheated dry boiler is a bad idea is of course correct. If the firebox crown plates are red hot then hitting them with cold water will cause local contraction and stressing, and although it may cause problems later it is unlikely to cause a huge rise in pressure, even if the boiler is in fact dry at the time. A working dry boiler probably does not have much pressure - no water - no steam, unless it has been left standing under fire but no steam draw. Under these conditions of low water and normal steam pressure it is probably either blowing off or near to it, and adding water will generate some extra steam pressure, but much less than you might expect. Consider the physics of the thing. The specific heat of copper or steel is far lower than that of water (Copper = approximately 0.09) so even if the plates are red hot the contained stored heat in them is not sufficient to evaporate much water and raise it to dangerous pressures - the figure of 1400 x volume water to steam is probably correct but quoted at standard or atmospheric pressure, it will be far less as pressure rises (Gas Laws). The formula is something like Heat Energy in plates = temp x Specific Heat of metal x Mass of metal. To raise water temp this must = Specific Heat of Water x temp rise of water x mass of water. But to boil the water to steam will take a further 540 cals per gramme of water. That means that the amount of water actually converted to steam will be quite small, and the pressure rise will be fairly small, since the mass of metal is much less than the water, and the specific heat of the metal is less than 0.1 of that of the water.
Where the failur hazard comes from is the reduced strength of the plates at high temperature, allowing them to be forced down by steam pressure, often at a pressure below the normal working pressure of the boiler. At least one commercial Gauge 1 boiler will fail this way if operated at low water level with the gas burner full on and the water level too low so the crown is exposed. It usually fails unspectatularly by collapsing so a cross tube leaks and the burner gets extinguished.
Note the crown is forced down by the steam pressure not vice versa.
The issue of fusible plugs is not simple either. They do bring their own problems - At least one fireman has been killed in recent years when a fusible plug failed on a full sized loco, when the plug blew when a loco boiler was operating correctly with adequate water over the crown sheet. I know of another case a few weeks ago in full size, where the plug failed with about 20 psi on the gauge and also with adequate water over the crown sheet while steam was being raised. In 7.25 gauge on a very large narrow gauge ride in loco, a similar failure at full pressure dumped most of the contents of the firebox into the drivers lap. This one was not actually a fusible plug, but a firebox coil which perforated with a hole about the size a fusible plug would have opened.
Jim Gregg.
At 03:04 PM 10/6/03 -0400, you wrote:
I've been following this thread, and I'd like to put my two cents in. I've worked with full sized stationary steam engines, and thought I do not have my operator's license as yet, I'm planning to take the test some time in the not too distant future.
First the fusible plugs primary job is not to extinguish the fire, but to notify the operator that a problem exist, and when it melts you'll know it. When the fusible plug melts, the first responsibility of the operator is to extinguish the fire.
To understand the dangers, one must first understand the why they occur. Two things to know about water and steam. For every on pound of pressure in the system, the temperature required to create steam rises one degree Fahrenheit. At just forty psi the temperature of the water and steam is well over 250 degree, and at 100 psi, its over 312 degree. The second thing to know is that one cubic inch of water will produce over 1400 cubic inches of steam.
With this in mind, when water hits a dry heated crown plate, and instantly turns to steam, the pressure in the boiler raises drastically, the crown plate which is already too hot, is usually the first failed component, forcing the steam downward, as the failure occurs, pressure in the boiler drops drastically, as the pressure drops, the temperature required to turn water to steam decreases, causing the water in the boiler to turn to steam, again 1 cubic inch of water, produces over 1400 cubic inches of steam, and the boiler is history, and no longer setting where it was, this is the primary cause of steam explosions. The biggest danger with a boiler is not the steam in it but the water within. For this reason the fusible plug needs to be sized for the boiler, so as to allow steam to escape but to fast as to greatly effect the pressure within.
To me this is the reason not to use soft solder, once it starts to soften and gives a little the drop of pressure coupled with the resulting increase in the amount of steam can cause an explosion even at a lower pressure.
For this reason I believe water level is a much more import than pressure, assuming a well working relief valve. Don't get me wrong, pressure is important, and I don't think I'd want to run with a pressure gauge, but watching the sight gauge is much more import than watching the pressure gauge.
Ray Baughman > > From: Harry Wade <[EMAIL PROTECTED]> > Date: 2003/10/06 Mon PM 12:20:44 EDT > To: Multiple recipients of sslivesteam <[EMAIL PROTECTED]> > Subject: Re: Boilers, Burners etc. > > At 05:57 PM 10/5/03 -0400, you wrote: > >On the other hand, a well designed silver soldered boiler will build up a > >very high pressure (probably more than 10 times operating pressure). > > In order for a theory to hold water all conditions, components, and > influences on a system must be considered in proper proportion. I agree > the above scenario could happen, in fact possibly as much as 14 x WP, if > there was a constant and unlimited source of heat, but what's being ignored > is that this condition could not occur with a burner the size of a fat > pencil. There are simply not enough BTUs available to the system. If I > were to apply a 1" oxy/acetylene rose tip (150,000+BTU/min) to a Ga1 boiler > boiler of course I should expect trouble, but in actual typical operation > that's not going to occur. The reason for building silver soldered boilers > the way they have been, and should continue to be, is so that even a worst > case scenario, where the boiler is dry and the burner is still on, a rather > large margin of safety remains. I fail to see what some people in the Ga1 > live steam hobby find so difficult and/or objectionable about that. > > >When this eventually leads to failure, the steam/water escapes much > >more violently with parts being torn off and hurling through the air. > > Unsubstantiated rhetoric like this can do a great deal of harm if read > and repeated by or to the wrong person. Rather than to conjur up disasters > based upon incomplete and unrealistic physical scenarios, or constructing > far-fetched justifications for doing it on the quick and cheap or > indictments of doing it properly, I think the hobby would be much better > served if that same energy were used to learn and promote good boiler > building pratices. I continue to be amazed how many people still fail to > undersand that there is a direct connection between certain time-proven > boiler building practices and safety, meaning that if good practices are > used there is no further need for discussions or conjecture about what > might happen in the event of a boil-dry. We would, and do, know what would > happen. As for the actual dangers, historically then overwhelming majority > of steam and boiler-related injuries are from scalds and burns and it's no > different for us. > > >The ultimate solution could be a well designed copper boiler with one or > >two soft soldered melting plugs as "last chance safety valves". > > These are called "fusible plugs" and are being used by some folks in > large scales but there is disagreement as to their advantages, if any, and > whether they should be made mandatory. But Ga1 boilers are a different > animal and it's my opinion that the problems of using a fusible plug in a > typical Ga1 boiler outweigh any benefits. Ga1 boilers are a different > animal because they are much less stable in operation than large scale > boilers and fusible plugs rely to a great extent upon some degree of > stability to do their job when it's called for and NOT do their job if it's > not. By "stable" I mean operating temperatures and pressures are lower > than larger scales but fluctuate much more widely than in larger scales, > water levels fluctuate much more widely, and only a very small percentage > of locomotives have the firebox configuration required for a fusible plug, > most don't have a firebox in the usual sense at all. Fusible plugs rely > upon plate temperature and water level to operate. Also most Ga1 locos > have fires which can be extinguished instantly in the event of low water, > which is one important job done by a fusible plug, but in the event of low > water nothing of much danger happens to a properly built Ga1 boiler anyway. > > Regards, > Harry > >
