Talk:Priming (steam locomotive)

Cause and effect

The term priming applies to the carry-over from the boiler, and the water in the cylinder is the effect of this. The more common cause of water in the cylinder, however, is condensation when steam is first admitted to a cold cylinder: thus the cocks are always open when the loco is starting. When they are opened at the end of a run it's because it's good practice to have them so while the loco is stationary: it prevents any build-up of pressure in the cylinders if there's a bit of a leak past the regulator—many a loco has run away with itself for this reason. This comes from personal knowledge (no, not the running away!) which isn't allowed here of course, but I'm confident that I have sound sources to insert when I can find a couple of minutes. --Old Moonraker (talk) 20:31, 3 October 2008 (UTC)

Text expanded slightly. --Old Moonraker (talk) 16:22, 7 October 2008 (UTC)

Um, ever heard of pressure relief valves?

The whole subject of engine damage due to water in the cylinder seems ridiculous, and the solution obvious. Install two pressure relief valves on opposite ends of the cylinder head, which only trigger at pressures slightly beyond normal operation... such as if liquid is being compressed in the cylinder head. Problem solved. This would also solve hydrolock on gas and diesel engines. DMahalko (talk) 02:05, 12 December 2011 (UTC)

Yes: see Cylinder cocks, lower down on the page you mentioned. But: don't add that here as they can only be made large enough to deal with the small amounts of water caused by condensation in cold cylinders. --Old Moonraker (talk) 07:09, 12 December 2011 (UTC)

Ah, Wikipedia. Where a problem that has afflicted steam engines for 150+ years can be solved in moments and the whole problem described as "ridiculous".

Drain cocks have existed since the beginning. Mostly manual, some spring loaded and automatic. They're used to release condensation in cold cylinders, not carry-over when running. Manual cocks in particular stay open through the cylinder cycle, so they have to remain fairly small. Largely as a solution to that, larger automatic valves were used in later years, that could be larger in capacity. The problem is twofold: water volume from serious priming is far greater than that from condensation. Secondly the pressure delivering carry-over water to the cylinder is far greater than the pressure available to clear it through the drain valves, thus needing a ridiculously huge drain valve if it was to clear the water as it might be needed. The exhaust side of the cylinder is vented to atmosphere through a large bore pipe to the blastpipe, which is always open - in cases of bad priming, this is where most of the water emerges, as rain from the chimney. The pressure on the exhaust side of the piston thus doesn't rise significantly until the piston almost reaches dead centre. This gives a very short time when a drain cock would be able (and would have sufficient pressure across it) to clear a large volume of water. Andy Dingley (talk) 10:37, 12 December 2011 (UTC)

The worst possible hydrolock/priming case should be calculable. Max piston speed the engine can ever be expected to be running normally, total size of the compression volume, maximum expected liquid expulsion rate required to safely clear the hydrolock/priming.

So let's say a steam engine needs a huge 4 or 8 inch diameter check valve on the end cap of the cylinder head, which almost never activates -- except in the worst possible situation where the cylinder head / piston / crank will be destroyed if it isn't there, leading to a potentially stranded engine.

I still don't see what the big problem is implementing this. The cost of having it is known as "insurance", like air brakes that fail in the braking condition. Not having air brake mechanicals would also likely be cheaper..

So to follow up, there could be a sensor on the check valve so that if it opens, it triggers a warning signal on the engine control panel to alert the operator of the condition and maybe they can do something to stop it to save boiler water, or just not worry about it since the check valve is doing its protective job. DMahalko (talk) 01:08, 14 December 2011 (UTC)

Rename to "steam locomotive"

So non-locomotives never suffer from priming? Why is it that discussing page moves beforehand is never done these days? 8-( Andy Dingley (talk) 00:23, 10 November 2013 (UTC)

Apologies for not discussing here first. Sadly I've pretty much given up on that as 99% of the time the response is dead air. Be bold and all that. Here's the reasoning for the move. A "steam engine" is the cylinders and their associated components that drive the machinery (e.g. connecting rods, valve gear etc.). The term "steam engine" does not include the boiler and associated gubbins. Since priming is an effect found in the boiler and dome/regulator mechanism, "steam engine" is the wrong term to use. I agree, on reflection, that "steam locomotive" is also unsatisfactory. So what about titling the article "Priming (steam boiler)" instead, since it's the boiler where priming occurs and that is independent of the type of larger mechanism that the boiler is part of. Laplorfill (talk) 09:02, 10 November 2013 (UTC)