What is your experience and practice with drifting on long descending grades? I've been taught, and I feel correctly, that using steam to cushion the cylinders and atomize lubrication is the best practice. However, keeping the vacuum of either smokebox gasses or air from being pulled requires more steam at higher speed cycles which leads to another question. How does velocity and volume come into play in this instance, and at what point does evenly wearing the valve at full stroke versus a shorter cutoff become a factor?

To help further illustrate my question, take a moment to read the page at this link:

http://5at.co.uk/index.php/definitions/terrms-and-definitions/snifting-and-bypass-valves.html

At what point in time did it become common at some railroads to simply fully shut the throttle off and allow the snifting valves to open?

The attached pix of superheater units illustrates damage done by drifting with throttle closed.

Robert Yuill

http://www.historicmachineryservices.com

I'll bite.

Engine? Date?

I'm not so much interested as to who, where and when as I am to why...

What do you feel drifting did to cause this problem? The cold air drawn in by the snifter valves? Some other factor?

I'll bite. My guess, this is a superheated engine with a dome throttle, and with the throttle closed, the units are empty, devoid of steam, and thus getting no cooling.

I agree. I'm most interested in the why. My take on the matter is these tubes definitely look overheated. Superheater units need a constant supply of steam moving through them to keep them cool. When one shuts off the throttle, such as a with a dome-style throttle, this effectively shuts off steam from running through the units. This causes them to overheat from the intense heat of the flue gases being emitted from the fire. This prompted the design of the superheater damper, that used a counterweight to direct the flow of flue gases away from the sensitive superheater units when the throttle was shut such as on a dome throttle setup. This particular device saw use on such roads as the PRR and even the EBT on their superheated engines. Locomotive builders discovered that this device was largely unneeded if the crews kept a drifting throttle, and thus, a constant flow of steam moving through the units to keep them cool. The two design specifications fail my brain currently, but between the type A and E style superheaters, one design kept steam continuously within the superheater i.e smokebox throttles, thus eliminating the need for the damper. This allowed the ability to run certain appliances off superheated steam like the whistle etc.

Best,
DC

Why would having the throttle closed on a drifting locomotive be any different on the superheater units than having the throttle closed while sitting at the terminal? Either way, you still have hot combustion gasses with no steam flow for cooling.

If I understand water brakes correctly, the throttle would be closed while the locomotive was 'drifting' down hill with brakes engaged, a common occurance on mountain railroads.

Eric

Well illustrated. The late Harry Wright told me about 45 years ago "you can't
drift a Superheated locomotive". He meant, of course, without some sort of
drifting throttle.

For one thing, the fire is minimal while sitting. Especially on oil burners.

And, not all locomotives were equipped with "snifter valves."

No steam flow = no flow of lubrication to valves and cylinders.

The fire should be minimal while drifting as well.

Lubrication to the cylinders is a separate issue which shouldn't affect the superheaters.

Eric

I don't know how others do it, but I would keep the throttle just cracked while driftng and from time to time open the cylinder cocks to check and see if there was a very small steam emission on each stroke. Kept steam in the Type A units and kept a little lubrication going and kept the relief valves closed. Valves and pistons/rings always looked beautiful. On a Type E, you should probably do it just for a little lubrication if you're drifting a long way. Can't hurt.

Dennis, you got it correct.

This particular operation drifts with throttle closed, for miles. What you see in the pix occurred with very little service time on units (less than a year), that were new at the time. We were able to straighten the units making them usable, which was cheaper for the customer than making all new units. Which we too can do.


Robert Yuill

http://www.historicmachineryservices.com

What should be and what actually happens is something you need to take into consideration. I'm not sure if you are aware, but when you shut down a locomotive after you've had it loaded such as cresting a grade, your flue gas temperature is going to still be as it was (freaking hot) when the locomotive had a load on it and a heavy demand for steam. It's not an on/off type of situation, especially with coal. It's something I like to refer to as thermal momentum. There is a time period in which the locomotive and fire cool down to an idle state when steam demand sharply decreases. Said cooling has been proven to be one of the most stressful situations any boiler will go through in its lifespan, but that's another story in an of itself. Even if you've transitioned from loaded to drifting, your superheaters units still need the cooling effect of the steam to prevent them from being heat damaged as Mr. Yuill's photograph so appropriately shows.

Remember that not all Type A-equipped locomotives have dome throttles; those with multiple-valve throttles should be able to drift with throttle closed, as the units will have steam in them. Still a good idea to drift with a bit of steam for lubrication, yes?

Howard P.

Right on the backhead of "Madame Queen" Santa Fe 2-10-4 #5000