Does the engineer have direct control over how much steam is directed to the blast pipe?

[Ulrich]

When you see a steam locomotive coming into a station you often see large clouds of steam billowing from the cylinders and enveloping the engine. To my understanding this is because the steam is no longer directed to the smokebox but exhausted to the atmosphere directly after use in the cylinders. My question... how does the engineer control this? Is there a linkage to the throttle whereby closing the throttle automatically reduces the amount of steam in to the smokebox, or is there some other mechanism and control that the engineer uses that works  independently of what happens with the throttle or brakes?  

[Overmod]

There are two likely reason for 'large clouds of steam coming into a station'.  First is that the blower (steam-nozzle assist for draft) is actuated, probably to minimize smoke while in the station or to keep steam pressure up during the stop.  Second is that the cylinder cocks have been opened before the stop, to ensure that no water carries over or condenses in the cylinder; the chief use of this is, of course, after the restart, but ensuring steam and water is cleared from both ends of the cylinder, and the cocks left unfrozen in the open position, may be a wise idea especially for long stops in cold weather.

There are many pictures and explanations of blowers on the Web, but (in light of the Giesl discussion currently going on in another thread) here is a picture of a Giesl ejector nozzle (from the British Pacific 'Fighter Command' that shows the blower arrangement remarkably clearly.

Note the small holes between the large exhaust nozzles, and the side manifold that feeds steam to them. It is harder to see the arrangement (other than the holes) in most conventional nozzles, as the manifold is mostly or wholly incorporated into the nozzle casting.

There is usually no 'external' exhaust of cylinder steam to the environment; it's all directed through the front end to the stack.  I have designed a couple of locomotives that do bypass some steam at very high cyclic rpm, but this has nothing to do with what you are describing.

There have been adjustable blastpipes and caps over the years, but these are intended to make use of a given amount of steam from the cylinders in producing different degrees of draft vacuum.  The exhaust steam itself remains regulated only by the throttle and valve gear.

Open cylinder cocks do reduce MEP somewhat, but this has nothing to do with 'venting cylinder pressure' for some other reason.

[selector]

Perhaps one could mention the snifter valves as well...

To the OP, yes there is a linkage to open the cylinder cocks, one of which is at each end of the cylinders, low on the curve of the cylinder walls, and usually linked by a bar with a bell-crank type of mechanism so that they both open and close concurrently in concert with the hogger's actuation of their linkage.  They are not linked to the throttle, but experienced hoggers automatically open them prior to reaching for the throttle when they want to move a steamer that has sat for more than a few minutes, and as overmod says, most especially when the outside air is cool or cold.

In the old analog clunk that is the steam locomotive, no linkages that I know of have multiple actuations.  The hogger sets the brake of a type with one lever, actuates the injectors with another, opens the cylinder cocks with yet another, opens the throttle with another, and sets the reverser position with another.

Crandell

[Overmod]

selector

Perhaps one could mention the snifter valves as well...

Yes, but these are bypass/drifting valves, and only slight amounts of steam would be normally present in the cylinders with the engine drifting or decelerating to a stop (depending on operating method).  (We had quite an amusing discussion over on steam_tech regarding whether it was 'better' to drift with admission steam to break vacuum, or just put the gear at mid and let the vacuum 'braking' (at a maximum of no more than about 14.7psi on the effective area of the piston face) fend for itself.

Meanwhile ... the 'usual' function of snifters is to admit air, not vent steam (air being wrong, of course, to admit into the cylinders of modern locomotives for a variety of reasons, but perceived as better than the alternative, combustion gas, cinders, etc. drawn back through the exhaust tract from the front end...)

Wagner and Trofimov valves handle the 'snifting' action entirely internally, directing the compression from end to end of a common tract incorporated in the valve passage.

(I suggest the same spelling convention I used for future reference to the latter man's name in English, as that is the spelling adopted by Meiningen, and they are responsible for most of the actual development and promulgation of this type of valve (including an attempt to provide damping pistons integral with the bypass heads to prevent the dreaded 'clink').  If people can attempt to standardize on "Walschaert" and get away with it, surely we can agree on this even if we differ on Cyrillic-to-Roman orthography...) 

Another potential source of 'clouds of steam' would be the use of a le Chatelier-style counterpressure brake ... but the exhaust of these systems, in my experience, is never down at 'cylinder level'; it is carried via special valves either into the front end or ( as in the case of the Winterthur system in particular) a separate "'scape-pipe" just behind the stack.

Another reason, admittedly a stretch, why steam might be seen in clouds would be if the cylinders had Okadee or similar relief valves, and these were lifting due to excessive compression as the engine came to a stop.  This would not produce 'clouds' of steam, but relatively short puffs...

[selector]

Thanks, Overmod.

 

Crandell