Need A Break?

[Overmod]

"Foaming" and "priming" are technically two somewhat different things, but they have the same general effect.

Remember that you can't, and don't, fill a conventional boiler 'all the way up' with water.  This is true even if you're going to be running the engine up and down severe grades and both the longitudinal 'slosh' and the danger of uncovering part of the crown need to be addressed ... often via carrying more water than is sensible 'on the level'.  If you wonder why the top of the firebox is so far below the shell in a radial-shell firebox ... or why the crown and top in a Belpaire box slope backward toward the cab in a good design ... this is part of the 'reason why'

MEANWHILE most people have no real idea of what water in a boiler actually looks and behaves like, just as they don't realize quite how many drop plugs would be required in a Super-Power firebox to have the 'desired effect' in the advertising.  Porta famously commented that it is like 'boiling milk' and if you've ever had to snatch a saucepan off the stove when scalding and the foam rises up you will appreciate just what he means.  In that foam are both solids (see a discussion of 'TDS' about them) and a considerable amount of water, and if any of that gets through the mouth of the dry pipe you have carryover.

Further meanwhile, on many locomotives the dry pipe mouth is 'safely' up in a dome, but on modern locomotives this can be abbreviated in actual height, or in modern designs actually 'missing' and replaced with some fancy arrangement of steam-separator paraphernalia as in Niagaras.  The fun comes in when you open the throttle a bit too fast, or the engine requires substantial mass flow of steam: this can cause a relative loss of pressure in the region around the mouth of the dry pipe, and this can extend in effect down toward the foaming region, and even to the substantially-liquid part of the 'interface'.  At which point the same effect that drives boiler explosions starts to be facilitated in the water 'under' the low pressure, and the developed distributed nucleate boiling makes the liquid, and of course all the foaming above it, rise up in that area.

In the 'why can't I catch a break' department: modern boiler-water treatment greatly predisposes foaming, to the point that any good treatment must include a 'sacrificial' antifoam, which has to be continually replenished.  This is one of the secret weapons in the Porta-McMahon treatment (now commercialized as "Porta Treatment" by McMahon, who I continue to think deserves his own mention in the name) that makes it possible to run a 'mobilized sludge' in the boiler rather than blow down early and often, sliming everything including railheads in the vicinity as you go. 

Some very clever minds developed various patent thingies to cope with this foaming and priming, most notably the Elesco Steam Dryer that has featured in some threads here.  The principle is generally to impart some spin to the steam flow going into the dry pipe, the initial idea being to provide a little drip channel "outside" the cyclone action where the 'water' would be flung and then obediently run down into the boiler again.  Some savvy operators, Canadians prominent among them, soon figured out that the separated liquid-phase carryover was not obediently following the little arrows in the drawings ... and that it was best to pipe them directly out of the boiler and 'over the side' rather than trying to reclaim them.

You have to have a LOT of carryover to pose a great danger in a working locomotive; much of the actual problem is that much of the carryover will have its solids 'plate out' going through the superheater, which over time can induce a number of not-very-happy problems in the element metallurgy and construction.  The greatest part of the fun, though, comes in slipping relatively soon after a stop, where the elements and combustion gas are relatively cool and the steam circuit cold enough to cause prompt condensation of the inlet steam.  Here much of the carryover goes, often TDS and all, right down to the valves and cylinders, where even if you have inadequate little cylinder cocks open and Okadees present and accounted for, you might easily get enough instantaneous hydraulic lock to stretch cylinder-head studs... or worse.  (And see the famous picture of Niagara spaghetti rods to see what some of the 'worse' effects of suddenly-arrested reciprocating motion can involve!)

General practice of 'wise heads' is to keep the cocks open any time you detect any sort of mushing or carryover -- in the old days you could 'hear this in the exhaust' and I suspect you still can in proper modern low-back-pressure front ends, with enough experience or 'ear training'.  Of course your overall water rate, net of all that expensive chemistry and 'enthalpic enhancement', goes toward the moon when you do this.  Many current operators, aware of the value of intact equipment to their franchise and knowing that fuel cost is only about 5% of overall venture cost, 'err on the side of conservatism' and run to minimize carryover effects wherever they can...

[Deggesty]

Even with low pressure systems such as steam heating systems, you have to be careful not to put too much water into the boiler. My first two years in college, I had the (paid) responsibility to go around every night, fill stokers, and check water levels. About midnight, one Saurday night in my last January in college, the house president, apparently thinking I still had some responsibilty, came pounding up to my room about midnight, and exclaimed, "A STEAM PIPE HAD BURST IN THE BASEMENT!" I went down, and found hot water dripping from an overhead radiator (you do not notice a steam leak overhead, but when hot water starts dripping on you....) I immediately knew WHY the water was dripping--the boy who had the responsibility had looked at the water glass, thought more water was needed, opened the inlet valve, filled the stoker, and went on his way. 

I went back to my room, put some clothes on (it was cold outside), went down, out and around to the boiler room. went in, closed the inlet, opened an 1 1/2" valve--an hour later, I closed that valve and went back to bed. I do not think that the boy would have known any more than to close the inlet, for I doubt that he had been exposed to the existence of the valve I opened. 

Monday morning, I told the Superintendent of Buildings and Grounds of the event, and told him he owed me an hour--and he laughed (the night attendant was given credit of one hour each night; it seldom took me even half an hour)