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Post by Deleted on Dec 23, 2018 20:23:18 GMT
evening all
I suspect that I'm going to be asking a few questions over the next few months, most of which will be boiler related. To begin with, can I have suggestions for what size injectors to use for 4472's boiler.
design pressure is 110 PSI, recommended operating pressure is 90 PSI
The boiler will be fed by two injectors, nothing else so I need to get this right, I assume that I need two different sizes to allow for differing boiler pressure levels.
Don's drawing shows Number 10 drill (4.9 mm) size holes for both steam and water outlets for the injector steam valves, Derek Brown (DAG Brown) states 3/16 (4.7MM) so they are more or less in agreement.
I can't recall if Don recommends injector sizes, I'll try to re-read his notes again over xmas. Derek (who owns and drives a 5" A4) recommends a 12 oz as being a perfect size, I don't know if his loco has just one fitted or if this is the smaller or larger if he has two. So guys can anyone chip in with their opinions/opinions here please, I'm not expecting everyone to agree but would be interested to hear your views, more so if you have a similar loco running at these pressures..
Cheers
Pete
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mbrown
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Post by mbrown on Dec 23, 2018 21:03:12 GMT
Hi Pete,
As I understand it, the rationale for having two different sized injectors is that one is chosen to feed at a relatively slow rate that can just about keep up with the boiler when pulling hard, and the other one is larger so that you can fill the boiler more quickly if the water has dropped and needs bringing back up fairly fast, even at the expense of a drop in pressure. In an emergency, of course, you could use both together...
Given the size of your boiler, my hunch is that a 12 oz injector (the size I have on a 2.5"g loco, although it fills that boiler pretty fast) is sized to be left on most of the time when running, just maintaining a fairly steady water level. You'd need a bigger one to fill the boiler faster when the level had dropped.
Malcolm
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Post by Deleted on Dec 23, 2018 21:59:45 GMT
hi Malcolm Yes, sorry I was aware of why it's wise to have two different sized injectors, I think you are right that 12 oz is best suited for normal running, Derek says as much in his book. He also states the pipework needs to be 3/16. So assuming that the 12 oz is the smaller of the two I then need to look at a larger injector for higher pressures, 18 or 26oz perhaps? I'm not schooled in fluid principles, Derek gives a chart for the required pipework for various sized injectors, he states that the liquid velocity should be no more than 2 to 3 ft/sec? And that for the higher output end of the scale this can be relaxed a little but at the small end it must not be exceeded. Right now that's 'double dutch' to me. Now if the larger injector chosen is 18oz, that still fits within the 2 to 3 ft/sec using 3/16 OD pipe. However, if I wanted to go up to 26 oz it shows that I need 1/4" pipe, (3/16 gives a figure of 3 to 3.5 ft/sec which is more than Derek advises) I don't want to have different sized pipes going to the injector valves. I guess I could just go for a 12 and 18 oz.. the question is will the 18 oz be good enough in the higher operating range. I have another thing to consider, Derek states that the bore size must remain the same along the length of the pipework, including through the fittings and with any bends being no more than 3 x the pipe diameter. Is this just from the injector into the boiler or does it include the tender feed which IIRC is 1/4"? Lot's of things to learn and decide, hence why I'm asking the wise old sages of the forum... Kind regards Pete
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Post by Jim on Dec 23, 2018 22:29:55 GMT
Hi Pete,
On the Britannia I have two 3/16" 18oz/min injectors, one high pressure (max 125psi, min 43psi cut out 24psi) and one low pressure (Max 125psi min36psi cut out 20psi) They were chosen on the recommendation of our club boiler inspector who apart from being a top bloke has built more than 50 fine scale 5" gauge locos on commission. As always I believe it comes down to what you feel comfortable with for your particular loco and needs.
With Santa on the way all the very best for Christmas and the New Year Pete and may Flying Scotsman continue to eveolve into one of the finest fine scale models ever.
Jim
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Post by Deleted on Dec 23, 2018 22:37:11 GMT
Merry Xmas to you too Jim and thanks for your input, can I ask what the operating pressure of your boiler is, please...I would be very happy to use one 12oz and one 18 oz if the 18oz is capable of getting water in under high pressure, going by what you show in your figures it seems that 18oz is more than capable of doing the job. I can then happily use the 3/16 pipework for both, fitting under the 2 to 3 ft/sec velocity. That just leaves the question, is it a problem having 1/4" pipe from tender to injector feed? I'm thinking not as it's not under any pressure but always worth asking.
Pete
NB: I wasn't aware that you can have two injectors of the same size with different pressure ratings?
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jma1009
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Post by jma1009 on Dec 23, 2018 22:52:39 GMT
Hi Pete,
Off the top of my head I would state DAG Brown specified a 0.110" dia passageway through injector steam valve for the standard medium size injector ie around 22 to 24 oz per minute delivery. It might be that this was Laurie Lawrence's figures, so apologies at this late hour, without checking.
There are lots of issues to consider in my view; primarily of standardisation of fitting connections so that things can be swapped around if necessary, and how many spares you want to have.
A 12 oz per minute injector will have typically 1/4" x 40 TPI connections. An 18 oz per minute injector might have 5/16 x 32 TPI connections, or 9/32" x 32 TPI connections. A 22-24 oz per min injector will typically have 5/16" x 32 TPI connections.
Personally, I would standardise the connections on both sides. I would use the standard medium size of 22-24 oz per min for Doncaster because they are very reliable and less prone to failure. Alternatively you could obtain 3 or 4 off 18 oz per min injectors of the same connection size.
I wouldn't go down to 12 oz per minute for a big 5"g loco such as Doncaster.
Cheers,
Julian
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Post by Deleted on Dec 23, 2018 23:27:09 GMT
Hi Julian
Thanks for your input, much appreciated. As I said I don't currently understand fluid velocity etc, never needed it before so it's something new to get my head around...I've read what Derek has written a few times and consulted his chart of page 92 of his 'Miniature Injectors inside and out' book...From what I understand the fluid velocity must not be above a given rate, Derek gives this as 2 to 3 ft/sec, please understand I do not know the reasons for this, just trusting in Derek. To keep in this range a 12 oz injector could use 0.110 as you say, it could go larger to approx 0.140 keeping it between the 2 to 3 ft/sec range, go larger and this drops, smaller and it raises. Derek states that this can be relaxed a little at the high output range but must not be exceeded at the small end. Some clarification would help me with this. Looking again at the chart, 26 oz needs bore size of 0.16 to 0.20 to keep within the 2 to 3 ft/sec... I think thin walled 3/16" copper pipe is 1.59 which may be ok for 26 oz...I don't have figures in the chart for 22 or 24oz but suppose it can be worked out, as they are slightly smaller then perhaps they are more suitable to 3/16".
Standardisation of injector connections I can probably do if injectors are close in size to each other as I plan/hope to make my own. I said 12 oz as that's what Derek has advised being a good size to keep the boiler topped up, I'm thinking that perhaps he's thinking of it more or less being left on, I'll need to read more of his book to learn more.
Cheers
Pete
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Post by Jim on Dec 23, 2018 23:50:25 GMT
Hi Pete, The maximum working pressure for the Britannia is set at 100psi.
Jim
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Lisa
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Post by Lisa on Dec 24, 2018 3:37:43 GMT
A simple formula we used to go by is the volume of the boiler in litres multiplied by 4 for the maximum oz/min you want for a 'quick fill' injector, half that for one that's left on to keep it topped up, or pick something about half way between those values for a good general purpose injector.
examples: Dad's sweet pea had a 7 litre boiler, x4 = 28oz/min, half that is 14oz/min, we had a 20oz injector The heidi I built had a 20 litre boiler, x4 = 80oz/min, half that is 40oz/min, I had 60oz and 72oz injectors.
You can get roughly the right volume with: the square of half the diameter (in mm), times pi, times the length of the fire tubes (in mm), divided by a million (πr²*h/1000000).
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kipford
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Building a Don Young 5" Gauge Aspinall Class 27
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Post by kipford on Dec 24, 2018 7:51:34 GMT
Pete Every time you do work on a fluid, expand it, contract it, turn it etc, you get a pressure drop. Pressure drop in a pipe is proportional to square of the fluid velocity. Hence changes in the fluid flow geometry can have a significant effect on the pressure in the system and why DAG is keeping the changes in geometry to a minimum. Hope this makes sense. Dave
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Post by Deleted on Dec 24, 2018 8:55:33 GMT
Thanks, Lisa and Dave, I'm digesting everything here and all that I can find in Derek's book and Don's words....lots of reading, re-reading ahead...keep the comments coming please guys...
Pete
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Post by delaplume on Dec 24, 2018 10:13:12 GMT
Pete Every time you do work on a fluid, expand it, contract it, turn it etc, you get a pressure drop. Pressure drop in a pipe is proportional to square of the fluid velocity. Hence changes in the fluid flow geometry can have a significant effect on the pressure in the system and why DAG is keeping the changes in geometry to a minimum. Hope this makes sense. Dave Hello Dave, If I might add to that ??------- Hi Peter}---"Fluids" and "Fluid flow" in this case can also include Gasses such as Air and Steam.....Any pipework, trunking etc needs to stay consistent in}--- 1) material type ( surface drag ), 2)--cross-sectional shape ( Changing from round to square can have potential energy losses. ) and -- 3)--cross-sectional area plus 4)--routing in general eg}--- too sharp a turn through 180 degrees might be restrictive to a certain degree..... If we now take a closer look at 1).......Just for example's sake imagine a circular - section pipe with a gas or fluid passing through it......Due to surface friction the gas / fluid in contact with the pipe's inner surface will be held back to a certain degree by friction whereas the "core" part will be travelling faster....So it's at that central point where we introduce the steam oil to be atomised and mixed before entering the cylinder block.... Sorry---going back to my RN Engine Room days -- alas. In miniature injectors all this is still valid but the impact, size for size, is much more pronounced...... Alan
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mbrown
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Post by mbrown on Dec 24, 2018 12:48:38 GMT
Hi Lisa - that's a wonderfully simple and helpful formula. Thanks - one ot keep in the file for the future.
Julian - when I had problems with my 11 oz. Chiverton injectors (which you very kindly sorted out for me) I looked around for replacements and found that no commercial company now makes injectors with 1/4" x 40 connections - even the smallest injectors on the market seem to be fitted into grossly oversized bodies with 5/16" x 32 connections as standard. Worth remembering if anyone is following older designs that specify smaller unions - or, like me, wants to replace their injectors with new ones.....
Maybe someone, some time, will produce decent, neat bodied, small injectors for commercial sale again.....
Malcolm
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Post by delaplume on Dec 24, 2018 14:11:03 GMT
Hello everyone, Yes, I'd second that request for Gordon's long-lost quality within a commercial injector...so I did a 'tinternet trawl and found this}----- www.paviersteam.com/fittings-accessories/chiverton-injectors/ ---------- I don't know how old this link is but after the Christmas Hols. it might be worth a phone call ?? Alan
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Post by delaplume on Dec 24, 2018 14:19:06 GMT
Hello all, Just a bit more regarding Fluid Dynamics}---- A simple car, motorbike or motor-lawnmower carburetor employs a VENTURI effect to make it work.... and so do our injectors.... Have a look at this}---- en.wikipedia.org/wiki/InjectorNotice that reference is also made to the Smokebox and the venturi effect of exhaust gasses passing from the Blast pipe up to the Petticoat pipe and the chimney liner........ Smooth, big diameter passageways with minimal changes etc are at the heart of Sam Ell's draughting arrangements at Swindon, ( Among other things, of course )..
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mbrown
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Post by mbrown on Dec 24, 2018 15:57:26 GMT
Hello everyone, Yes, I'd second that request for Gordon's long-lost quality within a commercial injector...so I did a 'tinternet trawl and found this}----- www.paviersteam.com/fittings-accessories/chiverton-injectors/ ---------- I don't know how old this link is but after the Christmas Hols. it might be worth a phone call ?? Alan Indeed - but Pavier only make the medium and large sizes of the Chiverton range.... Malcolm
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Post by Deleted on Dec 24, 2018 16:15:09 GMT
I have a small Chiverton on 4470.. it's always picked up for me, although, to be honest, has had little use.... lovely little injector
Pete
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Post by Roger on Dec 24, 2018 18:01:22 GMT
Hi Pete, Bob Bramson's book 'all you need to know about miniature injectors and ejectors' has a table from which the following comes..
5" 4-6-0 18oz & 24oz 5" Pacific 22oz & 28oz
He states that a general rule is that for working pressures up to 150psi, one square inch of grate area requires one fluid ounce per minute feed.
It's a very good book and well worth getting, even though I can't make head nor tail of some of the diagrams and descriptions. I did email the publisher, but they didn't bother answering. If anyone has the book and can translate what it means, I'd be most grateful.
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Post by Deleted on Dec 24, 2018 18:15:48 GMT
thanks, Roger.. I've just done a quick search but can't find a copy listed at the usual suspects.... I'll keep my eye open for it though...
Cheers
Pete
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Post by Deleted on Dec 24, 2018 18:39:07 GMT
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