Boiler Failure

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Licensed Steam Engineering Company posted the following on eBay:

Most of the older Cagney Boilers built after 1913 appeared to be built from 3/8" plate as is customary for the size and pressure range. You need to pay close attention to wasting away in the water legs as a result of poor storage or improper wet layup procedures. If water is left standing in the water legs oxygen pitting and acid grooving will be heavy in these areas. Most live steam clubs do not do a full internal inspection as they usually go with a quick hydro yearly and would not catch this type of damage.
A quick example is Chet Peterson's 1 1/2" Scale Daylight with an all steel boiler. Chet had a current non code boiler certificate at a local live steam club for years and hauled children around the club track at Griffith Park for free. Then one day the engine was simmering in the station the crown sheet let go unloading the boiler water contents onto the ground. If an internal inspection had been performed yearly the clogging of hardness scale and corrosion would have been caught. The local clubs response to this dilemma was not to change their practices to include an internal inspection it was to install a reverse osmosis unit.

Fred V. writes:

I have to take some issue with this statement "little EXPLOSIVE devices". We are not covered by the ASME codes because, if below the 5 cu. ft. of volume, our boilers don't explode. They may split open a seam and scald you to death but they don't explode. There has never been an explosion of a miniature boiler that I have ever heard about. I have heard of seams splitting open. I haven't heard of any death related to a miniature boiler accident.

Cold Water Stress

Dean Willoughby wrote on Allen Locos Yahoo group:

I don’t recall if it was on the Chaksi board but there have been a number of boilers that after many years are showing wear on the tubes where cold water is admitted. If you happen to have Joe Nelson’s book, look at page 98 showing boiler water circulation. I have heard that some boiler builders are now installing a diversion shield inside the boiler to eliminate the shot of cold water directly on the tubes.

Russ Noe wrote on Allen Locos Yahoo group:

On full-sized locomotive boilers I've seen significant scale buildup on the tubes at those forward water injection points. Common. It makes sense that would also happen with miniature boilers, but to a lesser extent. Use of water treatment and judicious blow-downs plus cleaning are the best defenses against such scaling, especially around the mud-ring areas.
During fire-up, water circulation can be enhanced and encouraged by opening the blow downs. This is common practice with full sized boilers, and actually shortens the fire-up time. Once water flow is established out the blow down, a fast closure forces circulation around the firebox walls.
I've witnessed a fast fire-up without doing this. The result: 150 psi on the gauge, and the lower firebox outer sheets were barely warm to the touch! Expected result = broken stay bolts.
Model injectors do pre-heat the water, which helps general steaming. My axle pump (Allen Ten Wheeler) provides a very constant water input, so I never see any real temperature differentials developing during operation. A smokebox pre-heating coil makes sense, using otherwise wasted heat.

Dale wrote on Allen Locos Yahoo group:

It is true that dumping cold water into the boiler at temperature and pressure is hard on the boiler. There are probably several reasons for it, but primarily, I think, is this:
When cold water enters, it lowers the overal water temperature in the boiler, and the boiler pressure usually drops. The more cold water introduced, the more pressure drop. This reduces the amount of expansive force on the interior surfaces of the boiler. In essence, the boiler "relaxes" a little. As the pressure picks back up and returns to full, the boiler re-expands. This constant expanding and relaxing of the boiler can work on the metal, and certainly on any joints that are weaker than the rest of the structure. So the trick is to introduce enough water to drop the pressure a few pounds, but hopefully less than ten -- unless you're in a real urgent situation and you're in danger of letting the crown sheet become uncovered.
There may be additional factors I'm not familiar with, but when you think about how a boiler operates, the above becomes pretty evident. It is one of the reasons that many designs include some kind of water pre-heater, usually inside the smokebox, to take the chill off the water before it's forced into the boiler.

Proper Maintenance

The Live Steamer, May-June 1951

by George Murray

The question is sometimes asked about our small gauge locomotives:

"Is there any danger of a boiler explosion with these engines?"

This is, I suppose, a common thought in the mind of the average person who is not too familiar with these small gauge locos, as soon as they are told there is 100 pounds of steam in the boiler. Some builders themselves are apt to be a bit uneasy about it if they are not long in the hobby and building their first engine.

Under average good type of workmanship there should be no fear of a boiler ever "exploding". The thickness of the boiler materials used is always figured out to be many times stronger than needed for the working pressure. All joints are made steam tight by brazing, with a few rivets to hold it while doing the brazing, such a joint will be stronger than other parts of the boiler when properly done. A hydrostatic test of anywhere from 50% to twice above the working steam pressure is made before the boiler is actually "steamed" and periodically tested this way is a safety precaution to check on the condition of the boiler. A copper boiler that has been brazed at all joints will give many years of safe service without fear of structural failure or weakness.

Steel boilers when properly taken care of will also last safely for a long time. However, in the steel boiler, rust and pitting inside the boiler have to be watched for by periodic inspection and hydro testing. Any moister left in a steel boiler can do plenty of damage in a very short time, so it is important to thoroughly "blow-down" a steel boiler after each steam run.

The "insurance" against any chance of a boiler explosion is well taken care of by one or two reliably designed safety valves that are kept in good working order. All excess steam above the needs of the boiler is released by this "pop-valve" as we call it. Should it stick to its seat for any reason without blowing off and a light pull up on its stem with small pliers fails to unseat it, then the operator should immediately shut off all draft on the fire and in extreme case, dump the fire if necessary and take steps to lower the excess pressure generated by the stuck valve. Opening the blowdown valve or the whistle or other means to get rid of the excess and pumping water into the boiler will help to lower the pressure to a safe level.

Few are the men in this hobby that care to "live dangerously" by neglecting to regularly check on the condition and working of these small loco boilers and their safety devices, to guard against any chance of a stuck pop-valve, etc. Nore would anyone with an ounce of sense neglect to watch the backhead gauges at all times when running an engine. The steam pressure gauge will show indication of any excess pressure, to allow corrective measures before it might reach a dangerous stage.

The only other chance of boiler failure would be due to doubtful workmanship, such as joints held by a few rivets and sweat soldered over. This kind of structure is unsafe. Builders new in the hobby are to be warned that soft solder has little or no place in a live-steam boiler, except to seat over stayheads or to seal some minor crevice or weep. A brazed joint is to be preferred in every way.

Summing things up, it is a safe bet that a brazed boiler that has passed the hydro test with no sign of weakness or leaks, will never give any fear of "explosion" if the pop-valve is kept in good working order at all times.

Air Dry

Harlock posted on Chaski.org:

Another trick that several friends use is they pressurize the boiler on air while it is still hot, after blowdown. They then open up every orifice and appliance and blow every last drop of water out, and roll the engine back and forth to blow it out of the cylinders with the drains open. Then the boiler is dry and still warm, and any remaining moisture evaporates before it cools down, so it is bone dry when put away.

References