Steam engines usually start off with the cylinder drain-cocks open. I'd think that air from the boiler would be ejected during that time, but I don't think it would be visible separately from the mixture of steam and condensate that emerges.

I have seen a loco that was raising steam, shunted up and down a stretch of shed road. The gear was in forward when the loco was shunted backwards and it was in reverse gear when shunted forward. Thus the cylinders were acting as air compressors. The object of the exercise was to get the boiler pressure up as soon as possible, because the loco was needed urgently.

I was told this by the shed master in charge of the depot. I am not sure if the pumping up of the boiler with air, did any good, however the shed master was adamant that it was a worthwhile exercise. Make what you will of this!

Andrew.

Industrial boilers have a vent cock on the top of the drum.When raising steam from cold it is left open until all the air is expelled and steam is coming out. When shutting the boiler down for the weekend etc, a check valve is screwed into the vent cock and the cock opened so that as the boiler cools, air can be sucked in so the boiler does not end up under a partial vacuum, which could strain the tubes etc.

Mentioning shutting down at the weekend prompts a slight digression. Where I was drug up in West Yorkshire we got Easter Monday and Tuesday off, not Good Friday. It never occurred to me to question this until I moved away. Suggestion is that mill owners weren't going to shut down Friday, open up Saturday then shut down again, more economical to have 3 days straight. Might be tosh of course. Friend of my dad's was an engine minder, actually quite a skilled job, responsible for keeping the mill engine running and the mill producing. It's all gone now. 

Edited By duncan webster on 16/06/2023 00:04:55

My book doesn't mention that, but it's strong on mechanicals and barely mentions operating. Loads of information on boilers and nothing about how to raise steam.

Two mentions of air being allowing in deliberately to avoid damage caused by a vacuum.

• When the boiler loses pressure cooling down after work, a valve opens to let air in. This stops the structure from being pressured in reverse, and prevents maintenance accidents. I guess a cold locomotive with a vacuum in the boiler would move unexpectedly if the regulator was opened.
• When a locomotive is drifting, a valve opens to prevent a vacuum forming in the cylinder and pipework. If one forms, muck is drawn from the firebox into the works. The book says this is particularly bad for superheaters.

Could be wrong, but I don't think a bit of air in a boiler would make any difference to starting and running. If an air filled boiler were heated to 10bar, the steam would be diluted by 10%. However, the air would be at the same temperature and pressure as the steam – much the same energy content -, and the regulator, valves, piston and blast would all still work. (Testing and running model steam engines with compressed air is common. )

Once the engine is running the percentage of air would soon drop, and the boiler output would be pure steam.

However, air in steam could be bad news chemically. Compressed hot oxygen corrodes metal vigorously and I'd expect it to play havoc with the lubrication too. Short boiler life, and maybe a brief burst of rapid wear on the piston and valves every time the engine is started from cold, forcing more maintenance.

Any nastiness would be much worse in full-size than a model – much less air in small boiler than a big one, and models do very little work. A full-size loco would be heated and cooled at least once per day, and they steamed something like a million miles per year. Bad things happen in full-size that don't worry models and vice-versa.

Dave

Any air contained in the boiler while raising steam will be blown out when the safety valve opens.

The air in a boiler can be a serious consideration on some full-size plant or special equipment, but it does not seem to have been regarded a problem on locomotives and traction-engines. It would be expelled on steam-raising via the blower, cylinder drains and safety-valves; maybe too through auxiliaries like the ejector on first use.

The real problem is that of boiler corrosion by dissolved oxygen in the water, including along the water-line in a boiler laid up part-full.

I have known a miniature side-tank locomotive draw in water as it cooled, and we traced that to the hand-pump in one of the tanks. Not the injector because its water-valve would be closed. Nor would the injector be a way in for air because the overflow valve closes to partial vacuum.

.

One installation that does need air bleeding from it, is a water main

These, both raw-water ones from the reservoirs to treatment-works, and local distribution lines; are fitted with automatic air-bleed valves at strategic points.

These are very simple and effective: a chamber slightly above the pipe carries a spherical float that drops away from a sealing-ring if air reduces the chamber water-level. The air escapes past it and through a vent, until the float reaches the seal and is held there by what is now just water pressure.

An isolating valve allows the air-valve to be serviced without taking the pipe out of use.

Yes it would do. Steam inherently contains more energy than compressed air at the same pressure because the steam contains the latent heat used to convert the water from liquid to gas (steam) form in the boiler. That latent heat is then available to convert into kinetic energy (power) as it is expanded through the engine.

Edited By Hopper on 17/06/2023 00:33:53

Hi Dave, ( My book doesn't mention that, but it's strong on mechanicals and barely mentions operating. Loads of information on boilers and nothing about how to raise steam).

Which book?

I have a commercial boiler operator's ticket and this is one of the procedures that was taught to us which probably applies to large factory installations with lots of pipe work at higher pressure than used on model engines.

I also drive a full size traction engine, operating at 180 psi and we do purge the boiler of air at start up (only because we were taught that way) and always start the engine, out of gear with the drains open to warm everything before driving. Any air left in the boiler would be purged at this time.

If the reverser is used to slow the engine when going down a big hill, the engine acts like a compressor and pumps air into the boiler, often exceeding the working pressure and blowing-off thru the relief valve with no problem taking-off again when the throttle is opened. (Note: some traction engines do not have brakes and the only way to stop them is to go into reverse)

Edited By Paul Lousick on 17/06/2023 05:25:31

Same here. Licensed to run steam boilers on three continents. In industry you always vent the air out of the boiler as it warms up from cold. Air in the steam is corrosive, both in the boiler, the steam pipework, process machinery and the condensate return pipework. So you get rid of it ASAP. A lot of money gets spent on chemicals to help remove air from water and steam throughout the cycle, and on de-aerators to physically remove air from feedwater. So venting the air out of a cold boiler drum as it warms up is important.

Steam locomotives were a relatively primitive piece of equipment, often run on relatively untreated water, without a condenser or condensate return, so they probably did not worry about it quite as much. Just let it go up the stack or out the cylinder drain cocks and called it good.

On model engines it probably matters even less.

The book is Poultney, Edgar Cecil. (1edn September 1951) Steam Locomotion. London: Caxton

Although the book claims to be written from 'the point of view of locomotive engine drivers and firemen, with operating in mind', I disagree. For example, the pros and cons of round and square top fireboxes in both narrow and wide configurations are explained, but not how to actually fire one. More focus on why than how, as when book explains that the design depth of a firebox is limited by the maximum length that can be fired, stated to be about 10' 6", which is why grates may be sloped so that coal falls to the back. (A bit further than the fireman can shovel it!)

The book tells the fireman why his firebox is arranged just so, but not how to stoke it. I guess the 'how to' part was taught separately. I imagine a callow youth first employed as an engine cleaner being shown how to build and start a fire in the shed so that the engine was ready for the crew when they arrived hours later. Then he might be promoted to learn how to fire a moving shunting engine effectively, gradually gaining the experience needed to stoke main-line goods, passenger and express engines.

The 1951 date is relevant: written when UK railway steam was at its peak, so good coverage of the end-game. I don't think the book would have been published a couple of years later by which time it was obvious steam was on the way out. Officially so in 1955 and the last British Railways steam locomotive was made in 1960. Although I recommend it wholeheartedly, there was never a second edition. Excellent book for someone like me – comprehensive technical detail, well explained, many drawings, and mostly without the heavy maths a university trained engineer is expected to grip.

Dave

Yes filthy work. I worked around a couple of coal fired boilers and did not miss the glamour when I left. Wait until you have to climb down into the ash pit to retrieve and repair the chain-grate stoker (a metal chain conveyor belt) that has broken and fallen into the pit. No thanks. Then with the loco guys, there was all that shovelling. They would have been fit.

Edited By Hopper on 17/06/2023 11:01:10