Super heated Steam Throttle Lag

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daves1459
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Joined: Mon Jun 07, 2010 7:58 pm

Super heated Steam Throttle Lag

Post by daves1459 » Wed May 08, 2019 8:27 pm

I'm installing a hair pin type super heater in my current project. It is a 7 1/2" gauge NYC 999 with a 8" barrel diameter wagon top copper boiler with steam dome throttle and is propane fired. There are five elements of .500" O.D. X .410" I.D. 304 stainless tube about 30" long. I was discussing the project with some fellow live steamers the other day. Two said they knew of locomotives with similar sets ups where the owners removed the super heater assemblies due to poor throttle response or excessive lag. My first loco was a 7 1/2" 2-6-0 with out a super heater and with a smoke box throttle. Throttle response was immediate in all circumstances. My current ride a Disney 7 1/2" 4-4-0 and has a steam dome throttle. I have noticed a slight lag in throttle response at starting from still. But, immediate response once under way. I had credited the lag to the filling of the dry pipe and etc. before enough pressure on the pistons occurred to launch the locomotive. Again, once underway the lag seems to disappear. Can I expect similar behavior with a super heater just more extreme lag? Uncomfortable lag? Any one out there with experience with a similar super heater and throttle set up that can give me a heads up on what to expect? Any remedies? Or is the lag insignificant once I get used to it and/or learn to deal with it?

I've wanted a super heated locomotive for some time. The 999 project is probably the last loco that I'll build so thought I'd give the super heater a try.

Thanks, Dave

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Carrdo
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Re: Super heated Steam Throttle Lag

Post by Carrdo » Wed May 08, 2019 10:19 pm

It could be the location of the throttle in the steam dome as with a smokebox front end throttle, the length of the piping between the throttle and the cylinders is much shorter. Also with superheating, the throttle should be located after the superheater elements.

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NP317
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Re: Super heated Steam Throttle Lag

Post by NP317 » Wed May 08, 2019 11:18 pm

Carrdo wrote:
Wed May 08, 2019 10:19 pm
It could be the location of the throttle in the steam dome as with a smokebox front end throttle, the length of the piping between the throttle and the cylinders is much shorter. Also with superheating, the throttle should be located after the superheater elements.
I have fired and engineered 5 full-sized logging steam locomotives, 4 with superheaters, by 3 different manufacturers.
They all had dry superheaters, i.e. after the throttle. And they all had damper systems in the smokebox that opened from throttle steam to an external cylinder opening the internal damper, then allowing combustion gases to pass over the superheater elements into the smokebox. This prevented the dry elements from burning out when not passing steam.
I modeled this damper control unit on mr Mikado. Pics below. It is a dummy unit.
~RN
SmokeboxDetails1small.jpg
SmokeboxDetails2.jpg

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Fred_V
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Re: Super heated Steam Throttle Lag

Post by Fred_V » Thu May 09, 2019 8:01 am

I have a radiant SH in my Hunslet with throttle before the SH and in the dome. It works great but I really get little, if any, superheat. It does dry the steam very well and eliminates the wet stack that is so common with Hunslet engines. I don't notice any lag but sometimes after sitting for a few minutes the SH gets hot and I do get a surge when opening the throttle.

Once I had some priming and that was interesting. The slug of foam would hit the SH and the engine would take off and accelerate after the throttle was shut off. The foam kept getting turned into usable steam.
Fred V
Pensacola, Fl.

RET
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Re: Super heated Steam Throttle Lag

Post by RET » Thu May 09, 2019 10:34 am

Hi Dave,

The 3 1/2" gauge Boston & Albany I have has three "hairpin" style stainless superheaters in it with a copper boiler. The firebox is 5" long, but the superheaters only extend 1 1/2" into the firebox. The locomotive is coal fired which gives a lot of radiant heat.

Even with the little firebox exposure I have in the present setup, I find that as soon as I rake the fire so that it is bright, the locomotive immediately picks up speed which tells me the superheaters are working. One of my projects is to extend the superheaters at least another two inches into the firebox and maybe 3 inches to go almost all the way to the back of the firebox. That should really increase the contribution they make to the locomotive's performance. The throttle is in the smokebox and has two poppets which seal on Viton "O" rings. The smaller poppet is too big so fine control is a little difficult, but quite doable. Making that poppet smaller is also on my "to do list."

From what I've seen, superheaters work, but for a model, they need to be stainless and extend into the firebox to be effective. If they don't, it isn't worth the extra work. They are common in England, but for some reason they aren't used much over here. Since you plan to fire with propane, it would likely help if you had a stainless screen over the burners, or some other way of creating the radiant equivalent of the bright coal bed that you get with a coal fire.

I would also suggest that you do the "Superpower" thing by opening up the steam passages between the throttle and the valve chests to minimize any pressure drops that may occur. If the throttle is correctly designed and made, you should be able to crack it open just a little bit and after a few seconds delay, the locomotive will slowly move off and gather speed (just like the real thing).

Please let us know how all this turns out.

Richard Trounce.

Berkman
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Re: Super heated Steam Throttle Lag

Post by Berkman » Thu May 09, 2019 10:52 am

Most big engines had the dampers removed pretty quickly on the superheaters. A lot of USRA power was built with dampers then they were removed.
Almost all modern steam had front end throttles.

I'd still recommend putting the throttle in the smokebox after the superheater elements. This will guarantee they dont burn out too fast, especially if you are extending them all the way to the back of the firebox. Thinking of using something like inconel for them ?

Berkman
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Re: Super heated Steam Throttle Lag

Post by Berkman » Thu May 09, 2019 10:52 am

Most big engines had the dampers removed pretty quickly on the superheaters. A lot of USRA power was built with dampers then they were removed.
Almost all modern steam had front end throttles.

I'd still recommend putting the throttle in the smokebox after the superheater elements. This will guarantee they dont burn out too fast, especially if you are extending them all the way to the back of the firebox. Thinking of using something like inconel for them ?

daves1459
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Re: Super heated Steam Throttle Lag

Post by daves1459 » Wed May 15, 2019 8:22 pm

I'm proceeding with the installation of a super heater. My dry pipe connection in the smoke box allows me to remove the super heater and convert to saturated steam with out a horrible amount of work in the event the super heater becomes a problem.

A question: Since one of the main efficiency improvements with super heated steam is at any steam pressure the steam volume increases as the temperature increases while the pressure remain the same. If the throttle is down stream from the super heater how is the increased volume realize in the cylinders? If the throttle is up stream and since there is a pressure drop through the throttle I can rationalize in my mind how the increased volume of the higher temperature steam makes it way to the cylinders and reduces the amount of steam needed to be passed through the throttle.

Anyway, my motivation for the super heater element construction came from "Builder01" in the thread he started on Oct 08, 2016 titled "Super Heater Return Bends" and in particular his comments on Dec 06,2016.

Attached are photos of my process test samples. In the first photo the sample on the left was my first try, on the right my second. I used a 30 degrees included angle. The most difficult part of the process was welding the "crotch" between the tubes. My tubes are 316 stainless steel (The 304 I wrote above is incorrect.)with a .049" thick wall. With air inside and no cooling the metal heats up quick. I do my own TIG welding and getting the crotch filleted with out burn through or sag was a real trick. I ended up using short stitch welds with cooling periods between stitches. I used .045" diameter 316 filler rod and only 35 to 40 amps straight current DC and a 1/16" diameter electrode.

You'll notice the sample on the left has a slight collapsing of the left tube and the point of the assembly slightly cocked to the right. This is because during the closing weld of the crotch at 45 amps I over heated the left tube and the tube sagged to it's inside. The right hand sample was not over heated at lower amperage and both tubes bent/formed evenly. Since 100% weld penetration is very desirable for a super heater I decided to section and inspect the left hand sample. It can be seen that I was getting good penetration and filet form along the tube seem. But, the sag area caused an interior problem. You can see the "stalactites" in the sag area were forming a choke in the left tube which would obviously cause reduced flow or a hot spot.

I hope these notes offer some help to those who wish to fabricate their own super heaters.

Dave
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daves1459
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Re: Super heated Steam Throttle Lag

Post by daves1459 » Wed May 15, 2019 8:42 pm

With the process development out of the way I proceeded to make the five elements needed for my loco. At present each element will be 29" long depending upon how far they protrude into the fire box area. The photo looking straight down into the fire box shows my thought on how far that protrusion should be. Comments? You'll notice on each side of the fired box is a 1/4" square copper rail that starts below the bottom row of tubes and rises to the top of the fire door about 3" from the back head. The rails are the shelf for the 1/4" thick 316 stainless steel arch. I show the super heater elements about 1 1/2" in from the back edge of the arch. Comments? I have another loco with a similar, but shorter, fire box also on propane with a 1/4" thick 316 arch but no super heater. At full burner the temperature at the top of the fire door between the back head and arch is 1,400 - 1,500 degrees F. So far the arch gets to a very dull red heat but has not sagged or distorted.

Dave
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Builder01
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Re: Super heated Steam Throttle Lag

Post by Builder01 » Thu May 16, 2019 5:22 am

Looks great! The return bends are the only part of my boiler that I did not make myself. I had a professional welder do that for me. To keep the inside of the return bend free from weld, I think he flooded the tube with an inert gas, maybe nitrogen, I'm not sure though. My fire box is pretty short and I have run the super heater tubes almost all the way to the firebox back head. I think they actually go into the fire box a little farther that what the photo shows.
DSCN1297 - reduced 4.JPG

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Fred_V
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Re: Super heated Steam Throttle Lag

Post by Fred_V » Thu May 16, 2019 7:34 am

Dave, with that many tubes you might actually get some superheat. Most guys say they only get dry steam.
Here is what I have; made from 1/4" SS pipe. Short firebox so not much exposure.
Attachments
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Fred V
Pensacola, Fl.

Berkman
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Re: Super heated Steam Throttle Lag

Post by Berkman » Thu May 16, 2019 8:06 am

Just an idea but would it be possible for the dry pipe to run directly from the steam dome to the back of the firebox as per Fred's pic then have a few back and forth loops across the top of the firebox, then two pipes, again per Fred's pic back to the smokebox, then combine at the throttle valve, then to the cylinders?

Seems that would provide a good amount of superheat area above the firebed but reduce the amount of plumbing needed and also eliminate the possibility of creating flue/tube situation that would be hard to clean with coal.

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