Rapid Steam Pressure Loss

[daves1459]

You may recall a few weeks ago I asked for discussion on what would cause the excess laboring of my Disney 4-4-0 going up a hill. Thank you for all of your suggestions. Turns out it was a matter of break in and adequate cylinder oiling. Now that the stiffness has been worked out another problem has raised it's ugly head. The problem now is rapid steam pressure lose on a grade. I'm operating at the Illinois Live Steamers track that is built basically on the side of a hill. Grades are kept mostly to 2%. But, you are either going up hill or down. If tried several firing techniques with little success. Today I tried filling the boiler to a full glass then ascend the hill. The idea being with the boiler full at 120 PSI I have the most stored energy as I attack the hill. The results were with still 3/4 of a glass the steam pressure dropped 50 PSI half way up the hill.


I'm burning propane using 15 Solarflo rose bud burners rated at 3,000 btu each at 13 inches of water gas pressure. Attached is a photo of the burner assembly with the crow sheet fitted. The boiler is 1/8" thick copper with a front flu sheet baffle and "turbolators" in the tubes. Attached are photos of the boiler and baffle. The boiler steams rapidly in a stationary mode. At 13 inches of gas pressure it will lift both safeties at 125 psi in less than 5 minutes from cold and steam continuously against the safeties. The boiler will do the same thing at 8 inches of gas pressure in about 9-10 minutes.


The boiler seems to steam freely so why does it loose pressure so rapidly on a hill? I had an epiphany, maybe with the increased exhaust draft during hard working I'm cooling the boiler? So I installed a thermocouple in the space between the rear of the arch and the rear of the fire box. Sitting still with a little blower on and 13 inches of gas the fire box temperature is 2,200 degrees F. Put the valve gear is full forward position and move at a moderate speed with a light load and the temperature drops to 1,800 to 1,900 F. Climb a slight grade at the same ground speed with the throttle opened further and the fire box temperature drops to 1,400 F. Climb a steeper hill at 2 to 2 1/2 % again at about the same ground speed and the firebox temperature drops as low as 1,100 F. It's obvious that I sucking cold air through the fire box and cooling the boiler. Being 1/8" thick copper the cooling is rather fast. So the challenge is what to do about it. Some of the suggestions so far are:

1. Raise the exhaust nozzle up into the stack.
2. Put a variable damper in the smoke box.
3. Put variable dampers on the ash pan.
4. Remove the perferated baffle from between the burner inlet and head, replace with a solid baffle under the burner inlet.
5. Allow a small amount of air to enter the fire box through the fire door.
6. Put a monometer in the smoke box to measure vacuum during all operating conditions and add a damper for constant results.
7. Put a gas analyzer in the smoke box to determine is all of the propane is being burnt.

I'm interested in if any of you have had similar experiences and how you cured them.

Dave

[Bob D.]

If your exhaust nozzle in the smokebox was designed for a coal burner I'm sure you have to much draft working hard burning propane. Increase the hole diameter a bit. You will hear the difference in the "stack talk". Work your way up.in size.

[James Powell]

15x3=45 000 btu/hr, 2544 btu/h = 1 hp, so you should be able to generate around ( (0 .03x45 000) /2544=.5 hp, so if your train weighs 1000 lb, then you should be able to move uphill at (0.5*550)/1000 lb = 0.275 ft vert/sec, assuming your train weighs about 1000 lb (that's everything, loco, tender, fuel car, you, water, kitchen sink you are carrying...everything weighs approx 1000 lb)

2" grade = 5280*.02=104 ft/mile. Assume desired speed of 8 mph, that means (104*8)/3600=.23 ft/second. So, you should be able to make 8 MPH uphill, based on HP available.

I would start with #7 if you have access to one. It will give answers as to what is happening, whereas everything else is just a guess.

I'd be suspicious that you are drawing WAY too much air into the firebox, because your drafting arrangement is setup for coal not propane. Remember you have to heat that air, and it is not helping in any way to the whole deal.

James

[Fred_V]

I have read here that with propane you can extend the exhaust all the way to the top of the stack. You need no draft. How much draft do you have sitting still where you have the highest temperature of gas flow? You want that same thing to happen when pulling a load.
Fred V

[cbrew]

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?

[Marty_Knox]

I would suggest rather than raise the blast nozzle up, lower the petticoat so it is down around the nozzle and leave a gap at the top. This is the way Baldwin and Vulcan set up the front end in oil burners.
Also, open up the blast nozzle.

[ccvstmr]

Interesting write up. Sorry to read loco operating performance is suffering...at least the little tea kettle steams well. Maybe, too well. You listed various types of correction action as follows...

1. Raise the exhaust nozzle up into the stack.
2. Put a variable damper in the smoke box.
3. Put variable dampers on the ash pan.
4. Remove the perferated baffle from between the burner inlet and head, replace with a solid baffle under the burner inlet.
5. Allow a small amount of air to enter the fire box through the fire door.
6. Put a monometer in the smoke box to measure vacuum during all operating conditions and add a damper for constant results.
7. Put a gas analyzer in the smoke box to determine is all of the propane is being burnt.

Sounds like the loco does not "breath" easily. You're pulling such a strong draft, you might be sucking the heat out of the locomotive. Might even be pulling unburned fuel out the stack as well. While a strong draft might be desired for burning coal, this has the opposite affect for a propane firing. Might have to sacrifice a sharp sounding "coal burning" exhaust...and accept a more muffled exhaust resulting from a continuously drafted propane burner. Did you try running the loco with the firebox door open (without the flames jumping out). Would seem this is the easiest way to produce a vacuum breaker while relieving a hard drafting boiler.

If you're firing the burners "harder" to compensate, you might actually produce the opposite affect. Too much fuel and not enough air, the resulting temperatures can drop...and you'll most likely be blowing fuel gas out the stack. If this is the case...DO NOT open the firebox door.

Before any design changes are made...stick that thermo-couple (T/C) over the stack to measure exhaust temperatures...standing still and under load. OR, is there a way to get the T/C into the smoke box? Then again, if a space is created to get the wire in...that's another way to produce a vacuum breaker. Another option, do you have a non-contact, handheld IR temperature "gun"...measure the smoke box exterior temperature standing still. This is NOT the exact exhaust temperature, but you will be looking for a drop in smoke box side temps with any design change made. Using the T/C or IR...if the front end temps are dropping, the heat is not getting sucked/blown out the stack.

Other instrumentation...if there's a way to measure the static pressure in the firebox under different operating conditions...that should tell you if there's a hard draft problem. The challenge (again) figuring out how to get a pressure reading from the firebox (change the door or snake a tube up from the underside?). Soon enough, sounds like you'll need an instrumentation car behind the tender to carry the extra equipment! Gas analyzer? You're kidding right? Who in this hobby has something like that in their tool box? Would think if the suction is "relieved", you'll get better and more thorough combustion of your fuel gases.

Other suggestions...as noted...you can change the nozzle and/or petticoat arrangement. Steer clear of more dampers or movable assemblies. Why make this more complicated. Keep things simple. Leave the firebox burner baffle in place for now...consider a baffle with greater percentage opening (larger holes) to reduce the partial vacuum being created later on. If it were me (and it's not) think you should 1st remove your front end smoke box baffle. Lastly, you mentioned before you had a non-steam consultant helping...what are his thoughts?

Good luck and keep us posted. Carl B.

[jcbrock]

Sitting still with a little blower on and 13 inches of gas the fire box temperature is 2,200 degrees F. Put the valve gear is full forward position and move at a moderate speed with a light load and the temperature drops to 1,800 to 1,900 F.

Are you turning the blower off once you start moving? Do you need to be in full forward position? Getting out of the corner doesn't save you as much on small steamers as large, but it does save you something. Enjoy the experimenting, you'll get it dialed in. When I first started running a steam engine I had similar problems, but once I had some experience, it was far easier to make steam going uphill than anywhere else. I had all that draft for free, I just had to put in a fire to match.

[daves1459]

Interesting write up. Sorry to read loco operating performance is suffering...at least the little tea kettle steams well. Maybe, too well. You listed various types of correction action as follows...

1. Raise the exhaust nozzle up into the stack.
2. Put a variable damper in the smoke box.
3. Put variable dampers on the ash pan.
4. Remove the perferated baffle from between the burner inlet and head, replace with a solid baffle under the burner inlet.
5. Allow a small amount of air to enter the fire box through the fire door.
6. Put a monometer in the smoke box to measure vacuum during all operating conditions and add a damper for constant results.
7. Put a gas analyzer in the smoke box to determine is all of the propane is being burnt.

Sounds like the loco does not "breath" easily. You're pulling such a strong draft, you might be sucking the heat out of the locomotive. Might even be pulling unburned fuel out the stack as well. While a strong draft might be desired for burning coal, this has the opposite affect for a propane firing. Might have to sacrifice a sharp sounding "coal burning" exhaust...and accept a more muffled exhaust resulting from a continuously drafted propane burner. Did you try running the loco with the firebox door open (without the flames jumping out). Would seem this is the easiest way to produce a vacuum breaker while relieving a hard drafting boiler.

If you're firing the burners "harder" to compensate, you might actually produce the opposite affect. Too much fuel and not enough air, the resulting temperatures can drop...and you'll most likely be blowing fuel gas out the stack. If this is the case...DO NOT open the firebox door.

Before any design changes are made...stick that thermo-couple (T/C) over the stack to measure exhaust temperatures...standing still and under load. OR, is there a way to get the T/C into the smoke box? Then again, if a space is created to get the wire in...that's another way to produce a vacuum breaker. Another option, do you have a non-contact, handheld IR temperature "gun"...measure the smoke box exterior temperature standing still. This is NOT the exact exhaust temperature, but you will be looking for a drop in smoke box side temps with any design change made. Using the T/C or IR...if the front end temps are dropping, the heat is not getting sucked/blown out the stack.

Other instrumentation...if there's a way to measure the static pressure in the firebox under different operating conditions...that should tell you if there's a hard draft problem. The challenge (again) figuring out how to get a pressure reading from the firebox (change the door or snake a tube up from the underside?). Soon enough, sounds like you'll need an instrumentation car behind the tender to carry the extra equipment! Gas analyzer? You're kidding right? Who in this hobby has something like that in their tool box? Would think if the suction is "relieved", you'll get better and more thorough combustion of your fuel gases.

Other suggestions...as noted...you can change the nozzle and/or petticoat arrangement. Steer clear of more dampers or movable assemblies. Why make this more complicated. Keep things simple. Leave the firebox burner baffle in place for now...consider a baffle with greater percentage opening (larger holes) to reduce the partial vacuum being created later on. If it were me (and it's not) think you should 1st remove your front end smoke box baffle. Lastly, you mentioned before you had a non-steam consultant helping...what are his thoughts?

Good luck and keep us posted. Carl B.

A couple of bystanders reported that they could smell propane as the loco climbed a hill. Although I did not catch the odor. Possibly the raw propane was simply blown over my head. Anyway, since a few did smell propane it is obvious the complete combustion is no occurring in a hill climbing environment. Maybe to much air rushing into the fire box and up the stack or not enough air lingering in the firebox for the amount of gas.

The smoke box baffle is a common and valuable appliance in the smoke box design. More often it is called a diaphragm. It's purpose and the reason for the angle is to even out the smoke box vacumm accross all of the flu outlets to balance the cumbustion gas flow evenly through all of the flues. With out it, particularly when a fire box arch is present and the height of the bottom edge of the stack is near the top row of flues, most of the cumbustion gasses simply pass through the top row or rows of the flues.

I did consult with my non-steam oriented friend. He actually operated the loco with the thermocouple. Sneeky, cunning, and sly that he is he did not offer any particular solution.

Dave

[tom stamey]

Sounds like to much air. Also, from the picture you gave us a question is does any flame reach the water legs sides of the firebox? It seems like all the burners point straight up to crown sheet.

As others have said very little blower should be needed.

[daves1459]

Sounds like to much air. Also, from the picture you gave us a question is does any flame reach the water legs sides of the firebox? It seems like all the burners point straight up to crown sheet.

As others have said very little blower should be needed.

The flames actually radiate sideways through slots as can be seen in the attached photo. There is a plug in the end of the burner so no flame goes straight up. At 13 inches of gas pressure the flame diameter is about 2 1/2". Burners are oriented so the flame from one burner is in between the flames of the adjacent burner. All burners have one or two flames impinging on the firebox wall. At full gas pressure the bottom of the fire box is a uninterrupted carpet of flame.

Dave

[daves1459]

It appears the general opinion is too much draft. I imagine the next step is to cause a reduction in draft and determine if there is a measurable reduction in the amount of steam pressure loss for the same amount of work. It is very easy for me to remove the exhaust nozzle as shown in the picture and leave the blower. Admittedly I set up the nozzle for lots of stack talk. After all part of the fun is listening to exhaust bark as the loco works. Unfortunately in doing so I apparently am over drafting the fire. With the nozzle out I'll have a 1/2" diameter hole like a shot gun barrel shooting straight up the center of the stack. That should drastically reduce the draft.

I also have a hand held electronic monometer that I use to balance carburetors that I can rig up to a plug in the firebox door and measure how many inches of vacuum are created. That with thermocouples in the firebox and smokebox should give a reasonable indication of what changes do what.

I'll be able to do the exhaust nozzle removal test this Saturday, although another muggy heat wave is forecast for this week end and I'm definitely a fair weather modeler. Making the firebox door plug and setting up the monometer will take longer.

Dave