Verifying Blast nozzle height

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: Harold_V, WJH, cbrew

Glenn Brooks
Posts: 1867
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Verifying Blast nozzle height

Post by Glenn Brooks » Tue Nov 19, 2019 1:05 am

Hello All,

This winter, I’ve decided to see if I can improve the steaming characteristics of my 12” ga Ottaway (coal fired, 12” boiler)

I’ve have now two years experience firing the loco, around a very short 430’ backyard track, having a 130 LF downgrade. So perhaps not ideal for maintaining steam pressure - yet it is what it is, with no more land available for long distance, high speed runs.

My immediate problem is the loco almost never produces excess steam capacity and rarely if ever pops the safeties while running.

So, Iam hoping to improve front end drafting efficiency to see if I can achieve more steam generation.

Iam starting to do a technical analysis of the blast pipe/nozzle placement and effectiveness of my smokestack and ‘aftermarket’petticoat added by a prior owner. The petticoat and stack extension are 5.75” long - reaching almost to the center midpoint of the smoke box shell. The stack itself extends 6.5” ABOVE THE SHELL- making the whole assembly 11.5” long. The current position of the nozzle is up inside the stack extension, 2.5” below the top of shell - a long way up there!

My immediate question is: when determining nozzle height, should I measure from the smoke stack choke point just below the top of the shell, or should I measure from the bottom of the petticoat?

Here’s what it looks like now.
9001BBC4-42EF-47F0-B315-BE3FD1FCC81D.jpeg
The twin blast nozzles are installed just 2.5” below the top of the smoke box shell.

Thanks much for clarifying!

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

rkcarguy
Posts: 1342
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Verifying Blast nozzle height

Post by rkcarguy » Tue Nov 19, 2019 2:05 am

There is a fair amount of room for more tubes, but it's a bit late for that haha.
Having looked at a bunch of different smoke boxes, the petticoat seams long/low. I would think that the nozzle just needs to be inside the stack/petticoat enough that it helps pull a vacuum on the stack so maybe you could trim 2" off the petticoat and try it, or make it adjustable somehow?

Rwilliams
Posts: 936
Joined: Thu Dec 22, 2005 2:45 pm
Location: Central California

Re: Verifying Blast nozzle height

Post by Rwilliams » Tue Nov 19, 2019 5:48 am

Front end relationships are important for decent operation and ease of steam production. There are the textbook dimensions that some try to adhere to and then there are dimensions that although not textbook in nature, seem to get the job done. The big issue seems to be the correct blast nozzle diameter in relation to the piston bore and the location of the blast nozzle opening in relation to the bottom of the stack extension. You already have a nice long stack extension with the bottom being at the level of the top of the tubes which will not need any modification. The bigger issue is to reduce the level of the nozzle opening to about 1/2 inch to 3/4 inch below the level of the stack extension base flare. This will give a more free flow of exhaust gas up the stack. Your stack is plenty long enough to generate a decent draft if the nozzle diameter is well matched to the cylinder bore diameter. Usually a nozzle bore diameter 1/7 of the piston bore diameter is a good starting point. It almost sounds like the nozzle diameter could be a bit large for the RPM of the drivers. I have seen small changes in nozzle bore diameter and location below the stack extension bring some much improved changes in operation. What boiler pressure are you running with?

Always beware of what the previous owner may have done to the front end. What he did may not have been for the best and experimentation is often the best direction for improvement in operation.

Pontiacguy1
Posts: 981
Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Verifying Blast nozzle height

Post by Pontiacguy1 » Tue Nov 19, 2019 7:38 am

One way to get some quick references on petticoat pipe and nozzle heights and such is to look at the Allen Models smoke box for a reference. There are literally hundreds of them out there and they all seem to steam really good unless there is something made wrong or some other non-design problem. It might be a good starting point to just look at a drawing of one of those and start from there, scaling up as necessary.

User avatar
Marty_Knox
Posts: 1418
Joined: Thu Jan 09, 2003 6:50 pm
Location: Michigan, USA

Re: Verifying Blast nozzle height

Post by Marty_Knox » Tue Nov 19, 2019 7:55 am

Stand over it and look down the stack. The most important thing is that the blast nozzle is centered under the stack.

User avatar
Fred_V
Posts: 4358
Joined: Sun Jan 12, 2003 3:26 pm

Re: Verifying Blast nozzle height

Post by Fred_V » Tue Nov 19, 2019 9:02 am

Should he connect the 2 exhaust tubes together into one blast nozzle?
Fred V
Pensacola, Fl.

Steve Goodbody
Posts: 155
Joined: Thu May 25, 2006 7:16 am

Re: Verifying Blast nozzle height

Post by Steve Goodbody » Tue Nov 19, 2019 9:10 am

Hi Glenn,

As Marty says, the nozzle must be aligned with the chimney centerline as a starting point. I also wonder about the apparent shortage of tubes which won't help with drafting I'm afraid as, ultimately, this is all about pulling air through the fire and the tubes are the conduit for that airflow.

However, the above aside and in short answer to your question, one key height is from the choke point to the blast nozzle. However there's more to it than just that. I went through a similar exercise to yours a few years ago and put together a spreadsheet to quickly calculate the various key drafting dimensions based upon what seemed to be certain consensus points that have been published over the years in various sources. I can send you a copy of the spreadsheet if that would help (or just sketch out the calculations), please pm me if so.

In my case, after playing around with the numbers, and then testing a few configurations with a manometer hooked up to the smokebox, I found that, as long as the various dimensions and proportions were about right, there's some margin for error in the exact diameters, heights and ratios. Ultimately the choice of coal and firing technique is a major factor also.

Best regards
Steve

User avatar
Fred_V
Posts: 4358
Joined: Sun Jan 12, 2003 3:26 pm

Re: Verifying Blast nozzle height

Post by Fred_V » Tue Nov 19, 2019 9:35 am

Steve, what kind of manometer readings were you getting?
Fred V
Pensacola, Fl.

Steve Goodbody
Posts: 155
Joined: Thu May 25, 2006 7:16 am

Re: Verifying Blast nozzle height

Post by Steve Goodbody » Tue Nov 19, 2019 10:16 am

Hi Fred -
Wish I could remember, and this is going back a couple of years I'm afraid, but I think I have some pictures and records somewhere. The manometer tests were with the engine running on air and not steam. The main point was that my original starting point was not ideal and changing to closer-to-ideal proportions (choke height above nozzle, choke diameter, chimney internal diameter, nozzle diameter) saw a marked increase in smokebox vacuum which was pleasing. Once the main proportions were corrected I could then mess around experimenting with various blast nozzle diameters under steam to find something that suited the coal.
Best regards
Steve

User avatar
cbrew
Posts: 2845
Joined: Sat Jan 04, 2003 12:17 pm
Location: Vancouver Wa

Re: Verifying Blast nozzle height

Post by cbrew » Tue Nov 19, 2019 10:25 am

Morning Glenn,
just looking at the attached picture, the first order of business would be to pop the three unions on the steam feed to gain a clear shot of the blast nozzle/s .
it looks at the two exhaust pipes have a 45 degree elbow, is that accurate?
if this is true, this will need to be replaced.
also the exhaust is too high.
and it would be a good idea to add a blower ring.
If it is not live steam. its not worth it.

Steve Goodbody
Posts: 155
Joined: Thu May 25, 2006 7:16 am

Re: Verifying Blast nozzle height

Post by Steve Goodbody » Tue Nov 19, 2019 1:57 pm

Hi Glenn - have forwarded the spreadsheet by Email.
Hope things work out and let us know how you get on.
Best regards
Steve

Glenn Brooks
Posts: 1867
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Verifying Blast nozzle height

Post by Glenn Brooks » Tue Nov 19, 2019 2:02 pm

Hello All, thanks for your comments on this.

In answer to your questions, here are a couple of data points:
- my working pressure is 143 PSI.
- existing oriface size is .238” for each nozzle, .476” total; (1/7th Cylinder dia rule should be .358” )
- nozzles are offset to the rear and left of stack center - by a significant amount

Re: fire tubes. I think my cellphone camera has foreshortened the front end boiler view a bit. The boiler has 4 rows of fire tubes, around 22 flues 1” diameter for all. They could have put 2 more on each side, I think. But overall it seems to be a standard code boiler design...

Definitely the blast nozzles are misaligned with the center of stack. - both are offset to the rear and left side of the stack.
I plan to correct the nozzle placement this winter. I would like to machine a single blast plenum connecting to the two exhaust ports, then put a four hole “pepper box “ design nozzle in the plenum. I tried to nudge the existing nozzles back into alignment, but no movement. The blast tubes are likely 65 year old black iron pipe, so Iam hesitant to try a bend the pipe as an interim measure. (Committed to one last steam up in December]

Also, in the photo, the elbows are part of the throttle plumbing. Behind them, two straight blast pipes come out of the saddle and head up into the stack. Iam thinking about replacing these old iron throttle fittings with stainless or copper pipe, curved around the outside of the firebox to each steam chest inlet. This would free up access to the flues for cleaning. Any recommendations on type of tubing to use? And does it matter the run to the left side is longer than the run to the right steam chest??


Thanks much for your recommendations!

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

Post Reply