The process of being a good Grampa to my grandkids includes showing them lots of steam locomotive movies on Youtube. I think I have them trained now. They walk in the door and want to see trains! Anyway, I was watching a clip the other day that featured the Blue Mountain & Reading 425. One thing I noticed right away was the front end throttle, not something seen often on a Pacific. When I learned that the locomotive was built in 1928 I was surprised as I was under the impression that front end throttles were a later "superpower" development. What can anyone tell me about the development of front end throttles and when the began appearing on locomotives?

I don't have anything handy that would show this particular locomotive "as built", so I don't know if this is how she came from the factory. There were a number of locomotives that were retrofitted with front-end throttles though.

This type was located after the Superheater and were a little quicker to respond to a slip. They were also sequential, using not one throttle valve, but several smaller ones actuated by a cam. The big power plant that I worked in for years had this type on the turbine generators and it was nice when picking them up or cutting load to actually see them work.

425 was built new with a front end throttle.

Front end throttles were actually a fairly early development in steam locomotive technology. According to John White's A History of the American Steam Locomotive, "Beginning in the 1850's throttle valves were mounted in the smokebox. By the 1860s it was reported that nearly all American locomotives were equipped with front end throttles."
This arrangement had advantages, notably the ease of inspecting the valve in that location, but it also had the chief disadvantage of malfunctioning and sticking at high smokebox temperatures. The front end throttle was replaced by the poppet valve throttle located in the steam dome, which maintained popularity until the resurgence of the front end throttle, in improved form, in the 1920s.

Steve Piwowarski

A major advantage of a front end throttle with a superheated locomotive is that it allows superheated steam to be tapped off upstream of the throttle valves and supplied to auxiliaries like the air compressors and blower. This increases the efficiency of these items in the same way it increases the efficiency of the locomotive's cylinders.

Here is a link to a patent for a smoke box throttle dating to 1917:
http://www.google.com/patents/US1279132 ... le&f=false

To piggyback on the last post, another advantage of a front end throttle located after the superheater elements in the steam circuit is that the superheater elements always contain steam, and are therefore less suceptible to damage.

SP

It wasn't that hard to handle the throttle on little 2-8-0s running 180 psi. A big locomotive with a requirement to flow a large volume of steam might require a single dome-type throttle approaching the size of a man hole cover. Try to operate that with 300 psi on one side.

Several smaller valves (with a pilot) made possible control of the steam with reasonable effort. Additionaly, larger diameter boiler shells necessitated shallower domes due to fixed clearances along the ROW. The American Multiple Valve Throttle fit the bill.

Running appertinances with superheated steam not only kept steam in the superheater units but kept that steam moving through the units and transferring heat vice burning.

I know using superheated steam in the cylinders caused some problems related to the break down of lubrication in the cylinders. In the case of the front end throttle, does passing superheated steam through the throttle have any negative effect on the workings of the throttle valve?

The American Multiple Valve Throttle was designed with superheated steam in mind and, to my knowledge, the fact that the steam was superheated resulted in no ill effects to the unit.

Adam,

There are cases of extreme super-heat that caused the steam cylinder oil to fail, which in turn caused rings to fail, which in turn lead to road failures. One most popular locomotive of the east coast suffered such a failure a number of years ago.

Since I have you, you may want to re-read the thread on the series of Southern Locomotives. One of the two of us may be mistaken in our presentation of information. It could very well be me.....?

There are some great threads on RYPN about super heat and failures that can and have resulted.

Kindly yours,

JohnE.

Donnell,

Yes, steam, either saturated (direct from the boiler to the unit before going through the super heater elements) and or super heated steam which has passed the units direct from the boiler before it enters the cylinders can cause damage to the individual valves. This is commonly known as steam cutting. Steam cutting can be caused by a variety of reasons. In some cases, dirty water that passes through the units, in other cases by over treating a boiler with chemical that passes through the system, in other cases, a result of poor material at construction. All the same, front end throttles and even dome throttles, super heated or not, can, do and will steam cut over time.

Good annual maintenance (if not more often as needed) will encourage inspection and addressing of steam cuts in throttles and valves.

Great question!

JohnE.

Mr. Rimmasch,

He wasn't asking about lubrication. He was asking if the throttle valves had problems due to superheated steam.

In my opinion, steam cutting in locomotive throttles doesn't really have anything to do with whether the steam is saturated or superheated. As you stated, there are a number of reasons steam cutting happens but I still contend that the throttle valves had no issues due to the steam being superheated.