It is no accident that the high pressure "strong steam" engine was developed in Cornwall. Oop north, the mines produced coal, so fuel was plentiful and cheap, and making a more efficient pumping engine was a low priority. In the Cornish tin mines, any fuel for the engines had to be imported from eg south Wales, so fuel cost was much more important to the economics of the mining operation. Hence, this drove the move to more efficient engines, which allowed engines to become compact enough to become mobile.
They weren't. Trevithick was from Cornwall, but he had to go to Wales to find backers for his strong steam work. Cornwall needed single- acting stationary engines for pumping, so they found the best way to efficiency (not being constrained by size) was by working at the low end of the cycle (compounding and condensing) and developing things like the Woolf and Bull engines.
The last Cornish engine was built in the 20th century, in North Wales (Dorothea quarry). They certainly weren't made obsolete overnight by Watt or Trevithick
Utter bollocks. Cavitation is a problem of _low_ pressure in pumps, sufficient to form bubbles (i.e. "cavities"). The one thing we know about a boiler feedwater pump of any form is that it has to work at around boiler pressure. Whilst the Giffard injector stops working with hot water, heat isn't a big problem for pumps.
Um.. Cavitation occurs when water boils against a surface, and the collapsing bubbles can lead to pitting and damage to that surface. The cause is localised boiling as a result of low pressures in those regions. In marine propellors, of course, it occurs at ambient temperatures and very low pressures, but there's no reason at all why it couldn't occur in a pump if the feed temperature were hot enough and the small drop in pressure produced at some point in the pump were large enough for the water to begin to boil.
Cavitation occurs at any place within a pump where the liquid pressure falls below the vapour pressure of the liquid being pumped. This could be at a surface, but doesn't have to be.
It typically occurs where the fluid enters the pump, where it has been accelerated, but before energy has been imparted to the liquid by the pump mechanism. According to Bernoulli's law, as the liquid is accelerated, its pressure falls.
It's a particular problem where the liquid is close to its boiling point. As the problem occurs at the entry to the pump, the pressure that the pump is delivering to is irrelevant, so it can occur with a boiler feed pump working with condensate.
If you were pumping condensate that was not sub-cooled (i.e. still at its boiling point and condensation pressure at the pump inlet), then cavitation is very likely.
If you ever look at an oil refinery or petrochemical plant, you will notice that the distillation columns do not sit on the floor, but are raised several metres above ground level on "skirts". This is so there is a hydrostatic head above the bottom product pumps, so that cavitation doesn't occur.
True - though the problems arise when it comes close to a surface (generally speaking - in submarine propulsors non-contact cavitation noise was an issue at one stage, I believe..).
But yes, it doesn't need to be on a surface. My bad.
/chomp/
Yep..
Agree.
So.. it does happen in pumps (sorry, Andy D.), and it might have occured in locomotive boiler pumps under some circumstances (e.g. early locomotives just after the tender tank had been refilled with boiling water from the lineside kettle, hot-feed under some circumstances e.g. early locomotives on the underground lines...).
Ahem.. the theory of cavitation is that localised boiling can occur anywhere that pressure is reduced enough to allow the fluid to boil. It's all about the conditions under which the fluid undergoes a phase change, after all.. (any other interpretation is a result of mis-understanding the theory) I've not worked with boiler feed pumps, nor in propellor design or use, but a simple consideration of thermal physics (plus, I'll admit, some seminars loooooong ago with Dai Trevena, who was one of the great experts on understanding cavitation...) tells me that if you drop pressure enough locally then the fluid is likely to boil, with bad consequences for any surface it comes in contact with. That's obvious from the theory..
To come back to where the discussion of pumps began - in early locomotives with primitive boilers and small heating surface, the choice between pumping near-boiler or cold water into the boiler was likely to make the difference between the machine being able to do useful work and not - hence the near
-universal use of lineside "kettles" to heat water for the locomotives.
... but which requires cold water, since the latent heat of the steam provides most of the pumping power, so the steam has to condense in the injector for it to work.
There's a thought.. could the restriction of the Giffard injector to delivering cold water only be the reason that so many locomotives were built through the 1860s and 1870s with a single injector and a feed pump on the other side, with arrangements to heat the tender water via a steam c*ck? Because with the small boilers of the day (imposed by civil engineering limits) using cold feed only would have had too much of an impact on performance? The Stirling brothers, certainly, both used single injector plus feed pumps on a lot of their locomotives - was this part of how the Stirlings got away with the combination of small, slender boiler (admittedly with a much better arrangement of tubes than most designers of the time) and relatively large cylinders? Later on, of course, the feed pumps were stripped off, leaving but the single injector. That must have made life difficult.
So where in a boiler feedwater pump is such boiling going to occur? Where is there a drop in pressure (required) where either (one of which is also required) such a pressure drop approaches below atmospheric pressures, or else the temperature is approaching the steam temperature of the boiler? Even with feedwater heating, pump temperatures are nowhere near this high.
In many cases of cavitation, it's a highly localised situation, where a high speed pump can dynamically produce a localised low pressure. This isn't the case for boiler feedwater pumps.
What was it Harry? Was it the drugs that finally did for you?
After your thirty years as the Flying Boilerman, in between your fellowship at Harvard, you must have been one smart cookie. So what happened? Because you're a right feckin eejit these days.
There is a drop in pressure where the liquid accelerates on entering the pump. According to Bernoulli's law, when a fluid speeds up its pressure drops.
On Wed, 20 Jul 2011 14:55:08 -0700, Andy Dingley wrote:
The problem with pumps on early locomotives seems to have been boiler water seeping back past the valves to fill the pump with steam (in the lower pressure regime there). This appears (from contemporary accounts of how locomotives were operated) to have prevented boiler feed pumps from being used effectively, so that locomotive boiler capacity had to be based not on steam raising power but on water capacity for the intended length of run - the locomotive had to be stopped for a significant period for the boiler to be refilled (e.g. the suggestion from Matthew Murray that for Kenton & Coxlodge a locomotive with a single cylinder and a larger boiler might be more suitable than the Middleton type, as the run was longer and more water needed[1]). Initial methods of re-filling involved blowing down the boiler and refilling, very quickly a number of engineers (Murray, Chapman..) adopted raised cisterns which allowed the boiler to be filled while remaining pressurised. I have to confess that the idea of coupling a cistern of boiling water to a pressurised boiler - with 1810s valves and couplings - does not leave me with a warm fuzzy comfortable feeling! By 1814-15 the 'pet-c*ck', allowing steam to be bled from the pump, was in use - this has been repeatedly credited to George Stephenson, and if true probably represents his great contribution to the development of a practical steam engine - one that could top up its boiler on the move! Boiling water feed continued in use to tender tanks/barrels for many years, of course - even in the 1840s gauge trials there was controversy caused by some of the SG locomotives starting their runs with pre-heated water in the tenders, though by this time it was a matter of getting a bit more efficiency, rather than making the difference between the engine working and not. (summarised from Guy, ER4, 'The elusive railway kettle')
[1] In the end they went for close copies of the Middleton engines.
This set me off thinking and doing some reading up, and that's prompted a couple of ideas..
Pre-1815 locomotives seem to divide pretty evenly between those with cast iron and wrought iron boilers (with quite a few undetermined..).
denotes locomotives built by well-established foundaries or engine-builders with foundaries, # engines built by local workshops (e.g. colliery workshops)
1813 Brunton engine at Crich*
1814 Blenkinsop/Murray machines at Wigan (built by Daglish, Haigh Foundary)*?
1814-16 Chapman Whitehaven locomotive#
1814 Chapman Wallsend locomotive#
1814-16 Hedley 2-cyl locomotives at Wylam#
1815 Stephenson locomotives (chain-coupled)#
1814-15 Brunton locomotive at Newbottle #?
Plus a lot of 'uncatagorised', though the only one of those built by a major foundary seems to be the 1813 Chapman chain engine for Heaton, built by Butterley.
With the exception of the Brunton engine at Crich and the Wigan Blenkinsops (by Butterley and Haigh Foundary respectively), the wrought iron boilers seem to mainly be the products of local workshops. The only country-built machine that used a cast boiler was the first Wylam engine ('Black Billy')
- and that boiler was bought in (along with much of the machinery).
Hypothesis: in the early days of locomotive building cast iron was the preferred material for boilers, but only a limited number of companies could manufacture such large and complex items. As larger wrought iron plates became available it became easier for colliery workshops and smaller local foundaries to build boilers from wrought iron, avoiding buying in large and expensive items from outside. The emergence of George Stephenson as the dominant figure in railway practice from 1816 established the use of wrought iron boilers (as in the Stephenson standard locomotive) as the norm.
The hypothesis seems to fit available evidence, and oddly I've not seen it suggested before. Have I missed anything obvious (e.g. actual costings..). Thoughts/comments welcome..
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