Super heated Steam Throttle Lag

[Fred_V]

Dave, I used flare fittings. I've used compression also and I think those fittings are a bit smaller but I'm not positive about that.

[ccvstmr]

Dave...you might consider stainless Swagelok fittings. These use ferrules that compress around the outside of the tubes...with the ferrule outside having a tapered surface that seals against the mating fitting. Would steer clear of brass compression fittings due to softness. Carl B.

[daves1459]

I have enjoyed following this thread, I just got my Heavy Mikado fired up a few months ago, it has a superheater. I am in the process of decreasing the amount of superheat because I am destroying throttle valves. Originally my superheater elements ran all the way to the back of the firebox, I am going to shorten them by 6” and see what happens. The way you have yours piped you should not have any trouble with your throttle. As has been mentioned when welding the tubes you should get the oxygen out of them, I have a T on my shielding gas and run a low flow to the tube, you will also find that this helps a little with burn through.
Good luck.
Brent

Brent,

At last we have someone who has too much super heat! With your average boiler pressure, % stroke cut off, and what it is in your throttle valve that is being destroyed with a lot of assumptions I may be able to estimate the amount of super heat you are achieving. I have a similar loco to the one I'm building that is about 2/3s the size and no super heater. Using a thermocouple stuffed down the exhaust nozzle at 80% cut off I read 290 degrees F exhaust steam temperature. At 100 psi gauge, which about 114 psi absolute, the boiler steam temperature is about 340 degree F. Therefore I have a 50 degree F steam temperature reduction and all I need is 50 degrees F super heat to eliminate cylinder condensation. Any additional super heating will go towards increasing the steam volume. I suppose 200 degree F (212 degrees F is approx. 100 degree C) is enough as that will give 500 degree F total steam temp. which is safe for PTFE as stated earlier by another contributor, and other higher temp. sealants such as Viton and RTV.

Then the question is HOW to achieve 200 degrees F super heat. The general opinion in models is the majority of super heating occurs in the fire box. If 6 inches of super heater in the fire box equals 200 degrees is it safe to assume 3 inches would equal 100 degrees? I may end up just building the super heater and measuring the results then make adjustments if necessary.

Please forgive my ramblings, Dave

[Fender]

Why is ptfe a concern? The only place I can see where it would be used would be if a ball valve is used for the throttle valve. Why not use an all-metal gate valve if the throttle is in the smokebox? I’ve seen such ball valves get cooked in the smokebox, even without superheaters.

[daves1459]

Why is ptfe a concern? The only place I can see where it would be used would be if a ball valve is used for the throttle valve. Why not use an all-metal gate valve if the throttle is in the smokebox? I’ve seen such ball valves get cooked in the smokebox, even without superheaters.

The concern is not just ball valve throttles. Although I agree with you that the won't survived in a smoke box of a coal fired loco. It is everything down stream from the super heater. Some modelers us PTFE for cylinder packing and rod seals. Some use RTV sealants on gaskets or to replace gaskets and so on.

Dave