It's not only centrifugal pumps that can suffer from cavitation (although it's more commonly experienced in that type of pump). A badly designed suction line can cause cavitation in any type of pump. And understanding of fluid mechanics was rudimentary at the time that Trevithick was working (Osborne Reynolds hadn't even been born when Trevithick died).
No, but they will have had one-way valves to control liquid flow into and out of the pumping cylinder (so that the liquid is actually pumped, rather than just being moved backwards and forwards by the motion of the piston). Such valves typically involve tortuous liquid flow paths that will involve acceleration of the fluid flow...
There was also a case (Nene Valley Railway?) where the wrong taper thread was cut for a fusible plug, which, held in by a minimal amount of thread, came out at pressure. I think that killed someone.
I think that's the example I was trying to recall. Certainly dropping a fusible plug can lead to serious injury in the wrong circumstances, so I'd hope it would come within the gambit of the RAIB (pace D7666), but it's not there !
I remember reading of another incident with dropped plugs on a line in the USA, run by a small team of volunteers on a shoestring like many UK preserved lines. The sight gauges were furred up due to inadequate procedures at overhaul IIRC, but as usual there was a catalogue of missed opportunities to catch it. A good investigative report was printed (don't remember if it was by the NTSC though ;-))
Most North American steam engine did NOT have fusible plugs, instead they relied, if that's the right word, on low water alarms and crew vigilance. I think the story you are referring to was the collapse of the crown sheet on the Gettysburg Railroad engine number 1278, an ex Canadian Pacific 4-6-2 on June 16th, 1995 while working a six-car train at about 15 mph near Gardners Pennsylvania. The locomotive was not fitted with fusible plugs, a not uncommon practice in North America.
For wrought iron boilers, I assume (in castings they'd be part of the main lump out of the mould)?
You'd need flanges at the boiler ends, to attach the end plates. These could be formed over a mandrel - within the expertise of a colliery blacksmith? The makers of early wrought boilers don't seem to have been bothered about having rivets through from outside to inside (nor, until the 1840s, about having rivets from fire-side to water-side in the flue or firebox), so the boiler plates could be either lap-jointed (overlapping and rivetted through) or joined with a butt strap, with the rivets going through the strap. Once again, within the abilities of a good colliery blacksmith? The result would be crude - but these locomotives were!
Firstly, not much in 1891 was superheated. Superheating in steam locos appeared slowly, from 1900. It appeared according to the preference of CMEs, some embracing it, others avoiding it. They mostly recognised the efficiency advantages, but the problem was cylinder lubrication. The high temperatures of superheating tended to break down the lubricants of the period, leading to varnish buildup and sticking pistong rings and valves. This was particularly a problem with slide valves - why the piston valve also started to become popular around this time.
Secondly, superheating still doesn't work well in trams or shunters, even today. Superheating requires a hot superheater element, which requires gasflow past it. Fine on a long-haul run, but hard to achieve with stop-start work, or long periods standing idle. Some superheater designs also suffer if cycled between hot & cold and may start to leak. A more common arrangement for donkey engines (and this might have applied to trams too) was the "steam drier". This was a very mild superheater whose purpose wasn't to change efficiency by shifting the enthalpy significantly, but merely to heat the steam enough to ensure that thoroughly dry steam was delivered to the cylinders, and that it would avoid condensation during expansion - even when these were distant, or went cold between operations. For intermittent use, condensation and wet carry-over (even though this wasn't as bad as priming) was a problem. Steam driers were particularly common in small vertical boilers, which otherwise tended to deliver wet steam.
Tried out much earlier, of course - possibly in the middle 1830s[1], certainly in the 1840s - in locomotives. By that time it was moderately common in marine and stationary plant (IIRC first application of superheating in a stationary engine was about 1801). It provided a much greater boost to efficiency in a low-pressure engine, of course.
Again, less of an issue at low pressures, where steam temperatures were lower. Most of the early (pre-1880s) attempts on locomotives were smokebox superheaters, probably providing a fairly low order of superheat. Reasons for non-adoption varied, but generally seem to come down to greater maintainance costs and poorer reliability - much the same story as with piston valves in locomotives in the same period (first adopted in a locomotive in 1826, but in use in stationary plant slightly earlier). All these early superheaters seen to have been mainly intended to avoid condensation in the cylinder (Ahrons 1825-1925 is a good starting source on them). The smoke-tube superheater did, as you say, place much more severe demands on lubricants.
There'd been a brief flurry of piston valves on (UK) locomotives in the 1870s
- the younger Beattie on the L&SW and Bouch on the S&D notably[2], but none were free of problems (piston valves on locomotives go back much further, to Wilson's engine for the S&D in 1826). Interestingly, builders of lower- pressure engines in marine and stationary applications seem to have made piston valves work well much earlier than locomotive builders really did, even though I'd have expected condensation in the valve to be more of an issue with low pressures..
[1] It's been suggested that the re-entrant smokebox fitted to the Dundee and Newtyle locomotive "Trotter" in 1834 may have been a low-order superheater, similar to some used in Germany later (can't recall ref. for this..)
[2] Bouch's machines for the S&D[3] in the early 1870s seem to anticipate (or exceed!) best practice of 50-60 years later, with 13" diameter long (6.5") travel, long lap piston valves serving 17" cylinders. Sadly, metallurgical and lubrication problems with the valves made them near-useless, and Fletcher rebuilt them with slide valves and inside cylinders, after which they did well (obviously no problems with the boilers..).
[3] OK, strictly for the NER (Darlington Committee) by then.
Too complicated to explain in a throwaway usenet post, sorry.
It's also wrong to use these simplistic enthalpy calculations to explain engine efficiency, especially the importance of condensers. Improvements to the low end of the cycle that appear to be unimportant from a simple linear calculation actually turn out to be very important when you integrate over the cycle. This is why stationary engines, and marine engines, and especially turbines, all make the effort to run condensers.
This is overlooked for locomotive practice - probably because the size & weight of condensers would be so impractical anyway. I know of no English language descriptions of steam locomotive performance that explain this properly, or give it the due importance. The only real treatments of it are by Chapelon and Porta. If you want such an explanation (like I said, I don't have time to write it) you'll probably find it best explained by a good book on beam engines and especially something heavily theoretical on the Cornish engine (which isn't just an engine in Cornwall).
Surely the biggest factor would be the reliance of the Trevithick/Hackworth /Stephenson line of locomotive on exhaust steam blast to stimulate the fire and draw it through the flue/tubes? In a stationary engine the only limitation on chimney height is cost and structual limits of materials, and in marine applications uptakes can be carried high - and there's likely to be significant air movement over the top of the uptake anyway. If neither of these suffice, then there's more space available to provide forced draught (either in a closed-stokehold or open-stokehold arrangement) than there is in the limited loading gauge of a locomotive. Another factor would be that a stationary engine can have a very large cooling pond to keep the condenser water cool (a sea-going steamship, of course, has an effectively limitless cool sink available!), whereas a locomotive with a limited on-board water supply will gradually heat that up, reducing the effectiveness (in thermal terms) of the condenser. From what I've read, (virtually?) all usage of condensers on locomotives was aimed at either reducing steam emission (underground locomotives, tram engines) or reducing water consumption, rather than enhancing thermal efficiency.
An issue, but you can do it as Seguiin did, with a mechanical fan. The few condenser locos (mostly turbines) tended to do just this, with a separate little steam engine and a smokebox fan. Several of the turbine locos, mostly the Swedes, did indeed use condensers for their cycle efficiency.
I'm not sure what any of that is supposed to mean. The enthalpy figures I gave were certainly not in any sense linear, and by looking at the difference from maximum to minimum enthalpy I was indeed "integrating over the cycle".
The extra efficiency is certainly worth having, though of course it gets less and less as the boiler pressure and superheat increase. There are other factors too, though. In general marine engines have to recycle their water and the same goes for stationary engines with specially treated water. Doing that means a condenser and if you're going to have a condenser you might as well use it to suck a bit more power out of the system. While locomotives could also benefit from the water recovery, the power gained in the cylinders would be dwarfed by the loss of draught from losing the blast pipe.
The exhaust pressure of a steam loco is atmospheric pressure. The engine actually has to work to push that atmosphere out of the way when getting rid of the used steam. Add a condensor, and the exhaust pressure is near-as-dammit zero. The engine doesn't have to push air out of the way to get rid of the used steam.
Alternatively - the condensor drops the exhaust pressure to nearly zero, and it sucks the steam out.
In practice of course it isn't that good because (a) you have to pump water out of the condensor (b)there's always a bit of air in there, and you have to pump that out too (c)the pressure isn't actually zero...
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