Rapid Steam Pressure Loss

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daves1459
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Re: Rapid Syeam Pressure Loss

Post by daves1459 » Tue Jul 10, 2018 3:15 pm

cbrew wrote:
Mon Jul 09, 2018 8:04 am
Morning Dave,
interesting write up. I have a few years of propane burning under my belt. the design and setup sounds good with the exception of the excessive draft chilling the fire box, as your testing shows.
the ways i have seen this addressed is, extending the blast nozzle up into the petty coat. this will go a long way to disrupt the vacuum, although this will require a good blower, but it sounds like you have this covered.


couple more questions. do you have any pictures showing how the burner assembly fits into the firebox?
have you measured the temp difference between the firebox point and the smoke box?
Attached is a photo of the burner assembly attached to the mud ring during fabrication. Also is a rear end picture of the burner with the "ash pan" mocked up. The ash pan can not be made any deeper as it already is only about an inch above the rails. What are you looking for?

Dave
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little giant
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Re: Rapid Syeam Pressure Loss

Post by little giant » Tue Jul 10, 2018 4:44 pm

What size are the holes for your blower? They almost look like the same size as your exhaust. I have 6 x .0625 diameter holes for my blower, (print dimensions from Gene Allen's consolidation drawings). Best way to determine blower/draft is to open fire box door and adjust until no flames come out of the door when you are ready to leaving the steaming bay.

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NP317
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Re: Rapid Syeam Pressure Loss

Post by NP317 » Tue Jul 10, 2018 8:01 pm

I expect there will be little vacuum in the firebox. The fire produces some combustion pressure.
Smokebox, yes.
~RN

Bob D.
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Re: Rapid Syeam Pressure Loss

Post by Bob D. » Tue Jul 10, 2018 11:09 pm

Your exhaust nozzle has been modified a few times..... In my opinion your current two ports are small enough to be causing excessive back pressure to the cylinders and that is part of the fight. The original, plugged center hole looks about right for coal. I think removing the plate is to extreme in the other direction but an easy test. You probably won't get much of any draft under way but you can make up that cracking the blower. I'd silver solder the two small holes and drill the center out just a bit larger.
Also think your blower holes are to large. They're using alot of steam. Smaller holes ups the velocity while using much less steam. Solder them up and redrill.
Tweaking that plate should really change things up.

Bob
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RET
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Re: Rapid Steam Pressure Loss

Post by RET » Wed Jul 11, 2018 8:31 am

Hi,

From what you say, it sounds as though you are getting way too much draft when running. "Stack talk" may sound nice, but even in a coal burner it wastes power. Opening up the blast nozzle until the ""stack talk" almost disappears is the way to go.

I haven't run propane myself (just coal), but everything I've read says you don't need much draft at all with propane. When its ready, Big Boy will be propane fired so then I'll learn what works in that world. From my coal burning experience, I've found that front end drafting is very important for a good running locomotive and is worth all the time you devote to it.

The best blower setup is four small nozzles around the blast pipe with the nozzles pointing up the stack. What works even better is when you combine the blast nozzle with the blower nozzles. Small blower nozzles running between 1/4 and 1/2 boiler pressure work best.

Finally, you need a baffle plate around the burners to limit the outside air that flows into the firebox. Remember with the burner design you are using, at least 90% of the necessary combustion air is supplied through the burner itself so as James said, all that excess air has to be heated and contributes nothing to the successful operation of the locomotive.

Hope this helps.

Richard Trounce.

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ChuckHackett-844
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Re: Rapid Steam Pressure Loss

Post by ChuckHackett-844 » Fri Jul 13, 2018 2:48 pm

Hi Dave,

Driving cross country at the moment (wife currently driving :-) ) so I'll try to be brief (difficult for the engineer in me :-) ) ...

Getting propane burners setup correctly can take a lot of fiddling or trial and error so patience is required.

I am returning from Train Mountain where my 4-8-4 Northern fired very well on propane (safeties going while climbing the serpentine). Her firebox is about 12 x 18 and I burn about 9 pounds of propane per hour of Train Mountain type running.

Your problem is very typical of propane-fired locomotives caused by the draft path being fine for normal firing but allows too much air when working up hill. The excess air (over and above that required for complete combustion) adds nothing to the energy produced and the extra (low temperature) air just serves to cool the boiler - exactly what you don't want.

I had the same problem but did not want to interfere with the great "stack talk" by altering the blast nozzle.

First thing to do is arrange a plate at the bottom of the burner to close off the air flow as much as possible while still allowing you to sustain full fire with the blower on. This can be done with a plate below the burner or a plate with holes in it placed over a "bunsen burner" type of burner. You will be surprised how little air area is needed - remember: extra airflow is just cooling the boiler.

Next you have to do one of two things: a) a variable damper in the smoke-box to "dump" excess vacuum or b) a variable damper under the burner to cut down draft when working hard.

"b" is difficult because there is no easy way to arrange things so that the damper is automatic - closed for less air while working hard and open for more air when needed.

On my engine I used a water manometer to measure the smoke-box vacuum required to support "full fire" (max usable gas pressure). In all modes of operation more vacuum than this is wasting energy. Mine was 2" water.

My locomotive has two stacks - the rear was a dummy. I designed a "poppet valve" having a large diameter aluminum seat and an aluminum (light) poppet valve held closed by two ball-point pen springs on a 1/8" stem. I adjusted it so that it opened at 2" water vacuum. It works well, as I climb a hill I can see the valve opening in time with the exhaust blasts.

For people without the extra stack I recommend that they machine a large (depends on locomotive but in general as large as practical) rectangular hole in the bottom of the smoke-box, install a rectangular skirt that extends up into the smoke-box to form a flat surface and fit a damper door on top held closed by a light spring adjusted to whatever vacuum you need for full fire.

Some additional tips I find useful:

Arch - An arch is absolutely required for propane firing. Propane is a slow burning fuel. It is still "burning" after you can't see the flame. This burning is "quenched" as soon as the gasses enter the tubes because its temperature falls below that required to support combustion. The longer the gas path before it passes into the tubes the better. If at any time you smell an 'acid' smell from a propane locomotive you are smelling un-burned hydrocarbons (i.e.: wasted fuel).

My arch starts just below the lower row of tubes, extends back about 1.5" and then bends up to within about 1" of the crown sheet at a steep angle (maybe 30 degrees from vertical) and then extends to the rear to within about 3-4 inches of the rear of the firebox. The arch should fit as close as possible to the firebox sides to prevent gasses from getting around the sides of the arch. My choice of arch shape was designed to expose as much of the firebox as possible to the radiant heat from the burner while providing the longest gas path.

Flames on firebox - you should not allow the flames to directly impinge on any firebox surface - again, the flame is quenched as soon as it contacts a metal surface below the combustion temperature. This is sometimes difficult to accomplish while trying to get as much flame as possible in the firebox. On my "furnace type" burner (9 tubes fed by one orifice & mixing tube) I relocated the side row of holes to the top of the two side burner tubes to prevent the flames from hitting the firebox sides.

Another approach that I have not yet tried is to arrange a stainless steel strip between the flame and the firebox sides. The idea here is that the SS strip will quickly rise to combustion temp (red hot). It will then not quench the flame because it is red hot and it will radiate the heat energy to the firebox sides.

Flames too high in the firebox - I see this a lot with the "bunsen burner" type burners like you have. The bottom of the flame should be level with the bottom of the water in the water leg (not the leg itself). Too much higher and you are not exposing all the water to direct heat radiation.

Additional Ideas

I find that, to support full fire, I do not need the blower if I can hear any stack talk at all so I will sometimes shut off the blower if I am on a long uphill climb. If one does this they must be absolutely sure to remember to turn the blower back on (or burner off) if the throttle is closed at all (stopping, derailment, etc.) or you'll have to repaint the cab sides after the burner flames strip the paint!

Someday I hope to design an automatic blower control that is controlled by smoke-box vacuum.

(I have not tried any of these)

You can add twisted metal strips in the tubes ("turbulators"). This has two effects. They promote heat transfer to the tubes (cooler smoke-box temps) and they add air resistance to the gas path to slow down the gasses and increase smoke-box vacuum (increasing the effectiveness of a smoke-box damper).

Adding a stainless steel grid above the burner - this will glow red hot and increase the radiant energy available to the firebox.

Hope this is of some help ...
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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ChuckHackett-844
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Re: Rapid Steam Pressure Loss

Post by ChuckHackett-844 » Fri Jul 13, 2018 2:56 pm

... forgot to add: I have a gauge on the blower feed line in the cab. This gauge has a piece of tape on it which indicates the blower pressure that supports full fire (about 10 psi in my case). Excess pressure is just wasting steam and - again - bringing in too much air. If I let someone run my engine I tell them to be sure that the blower pressure is at, or above, the tape at all times.
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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