Sunday, 15 November 2015

Another wet Sunday

Today and last Sunday the weather was awful so I haven't managed to let Alfred stretch his legs for a couple of weeks (though today turned out to be better than forecast). Here are a couple of pictures from a few weeks ago, the first time we took Alfred down to Urmston Model Engineers' track at Abbotsfield Park.

Me on Alfred, Malc in the background. On the right is Dave, a club member who was allocated to 'keep an eye on us' as new boys. Alfred's clearly 'all set to go', blowing off vigorously. By my second week at the track I'd learned to keep the steam pressure high, but just below blowing-off point. 

Just as on a full size locomotive, if the pressure is approaching the red line on the gauge you can use the injectors to put more cold water into the boiler (if the level isn't already high - a good fireman keeps a bit of boiler water capacity in reserve for just this purpose), and / or open the firebox door to allow cold air to be drawn through the boiler tubes. Both these tactics will halt a pressure rise, though if the fire is vigorous you might need to do both! If the pressure does continue to rise, the safety valves rise and the excess steam 'blows off', but no fireman wants his engine to do that. Not only is it a waste of coal and water, it also indicates poor firing technique (unless circumstances - such as an unexpected delay before starting - are beyond the fireman's control). A big main line locomotive can loose up to 20 gallons a minute through the safety valves if it blows off.

However, letting the boiler water level get too high brings its own problems - the loco will 'prime'; water as well as steam will be drawn into the cylinders. On locomotives with slide valves this just results in a lot of steam and dirty water coming out of the chimney giving the driver and first few passengers a dirty shower. Opening the cylinder drain cocks will help reduce this as much of the water will be ejected through the cocks, but not all. Almost all model locomotives, and smaller full sized locomotives, have slide valves and these have the advantage that any over-pressure in the cylinders caused by hydraulicing (incompressible water getting in) will simply cause the slides to lift off their seats, relieving the pressure. 

Larger locos have piston valves as these are more efficient in fast running, and with these hydraulicing can lead to serious damage as the driving piston hits the incompressible water at the end of its stroke. This can blow out the cylinder end and bend the connecting rods. It's always good practice to open the drain cocks before moving off, as the cylinders may be relatively cold and steam might condense in them. We do this with slide valve engines as well as piston valve ones simply so it becomes an ingrained habit and one is less likely to forget when driving a piston valve loco. Soon after moving off the cylinders will be up to temperature and the cocks can be closed. This explains why steam locomotives often disappear in clouds of steam from their front end as they start off.

To reduce the chance of steam condensing in the cylinders, a careful driver will 'pre-warm' them immediately before departure. This involves putting the loco in full forward (valve) gear, brakes on, drain cocks open, and gently opening the regulator. Initially water, then steam, will issue from the drain cocks. This is repeated with the loco in full backwards gear, and repeated again in forward and backward until steam only (no water) reliably issues from the drain cocks. 

Malc on 'The Beast'

'The Beast' is a free-lance 2-4-0 locomotive loosely based on an American 'Baldwin'. It's quite powerful, capable of hauling several loaded passenger carriages, and both Malc and myself had a go at driving it. It's easier to drive than Alfred because it has a larger boiler and firebox, so there is more 'reserve' of fire size and steam pressure. Full size locomotives are of course even easier in this respect as they have comparatively massive boiler and fire capacity. However, they are physically much more demanding, especially to fire as several tons of coal may need to be manually shoveled into the firebox during the course of a journey.

'The Beast' is owned by the club, but its custodian is Keith, the father of James from whom I purchased Alfred. Keith was giving passenger rides, and allowed Malc and me to drive these passenger trains under his supervision (he sat behind us).

Let's hope for better weather next Sunday so I can give Alfred another run. That'd be his first in three weeks!

Alfred's footplate


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