Is this a pop-off valve (i.e. safety valve)?

[mmagliaro]

After last week's discussion of the steam injector check valve, I have another one.
I *think* this is the pop-off valve.  It looks like there are two of them, mounted on a little turret.

Is that right?

[wazzou]

The pop (pressure) valve would ordinarily be located right on the steam dome.  I don't know what those are but I can check.

[mmagliaro]

I wondered about that.

You can see some things inserted in the top of the steam dome.  One is the throttle control, but at least one of the others could be pop-offs.

Here are other views of the steam dome.  These are the best I have been able to find.  Top shots of steam locomotives are not common!  One if the opposite side of the steam dome.  One is a brass model.



What are 1, 2, and 3 in this view?

[wazzou]

It's a duplex air pump governor. 
One is a primary governor and the 2nd raises the pressure quickly from 90psi to 110psi in the event of heavy brake pressure to maintain an air level until things get back to normal.

This is from a 92 year old mans recollection from my description of Max's photo.

[Chris333]

Pop off are usually at the highest point of the boiler. I have no idea what is in the original photo.

[sd45elect2000]

Yes, those are the safety pops.

I have three in my collection.

Randy

[Chris333]

Looks like Bryan got it right:
http://www.basiccarpentrytechniques.com/Technology%204/The%20Traveling%20Engineers%27%20Association/images/r165.jpg

[sd45elect2000]

Ooops, now I see the circled devices, I missed them at first glance. Its an air compressor governor.

RSS

[mmagliaro]

Amazing!  THANKS, you guys!
And it does indeed appear that the small pipe coming out of the left side of that thing bends downward and goes to the compressor (the top half of which is shown in my photo).

Other supporting data.  This engine does indeed use a New York Air Brake "Duplex" pump, not a Westinghouse "cross compound" compressor.  They actually look very similar from the outside, except that the NY Duplex device looks like a cross-compound that is "upside down".

The access to this information through people who have really worked on steam locomotives is simple wonderful.

wazzou, is that 92 year old man your dad?

[wazzou]

wazzou, is that 92 year old man your dad?

@mmagliaro

Uncle.

[mmagliaro]

Well, now that I know what it is, I was able to find it in the ICS steam locomotive books.

wazzou, your uncle was dead-on correct.  It is a duplex compressor regulator correction: governer, and indeed, the two pressure settings are 90 lbs and 110 lbs.  Reading the book, the principle is this (if anybody cares)...

Normally, a single compressor regulator governor is placed in the air line between the compressor and the air reservoir tank.  It has a pipe going back to a gauge in the cab.  It's job is to cut off the compressor when the pressure in the tank reaches 90 lbs.  So it maintains a constant 90 lbs of air brake pressure.

So... why a duplex regulator governor?  Because right after you hit the brakes, the tank is depleted and it would be very advantageous to recover full pressure as fast as possible.  In addition, you might need to hit the brakes hard again in an emergency.  So under normal conditions, when the brakes are not being applied, the regulator limits the tank pressure to 90 lbs.  When the brakes are on, the other side of the regulator is used, whose spring valve is calibrated to 110 lbs.  So it allows the tank to pump up to 110, which permits faster recovery and also gives you some extra "oomph" of brake power for the subsequent brake application.

Here's an illustation from the ICS book: