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injector design

operating range.

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nipper30/03/2011 13:29:29
31 forum posts
I have made a successful injector but it will only work up to 70psi. I really need it to operate up to 100psi. It runs dry between 40 and 70 but as the pressure builds up it starts to drip and at 90psi it has full overflow. How can I fix it.
John Baguley30/03/2011 18:41:22
460 forum posts
49 photos
You need a steam cone with a slightly smaller bore, say 0.002" for a start. That will increase the operating pressure. It will also raise the minimum pressure as well though and it may only work down to 50 or 60psi.
You could try shimming the steam cone further out from the body to increase the annular gap slightly but that is not an ideal solution and may mess it up completely!
mgj30/03/2011 19:05:48
1008 forum posts
14 photos
I have just bought that book Miniature Injectors Inside and Out by DAG Brown. It will tell you exactly how to size an injector and sort its pressure range, and how to stop all lthe dribbles and runny noses etc.
According to Mr Brown, if in operation you get an overflow, the strong likelihood is that the annnular gap is too large. The thing can suck more water than it can cope with, and this results in a transfer of energy into the overflow water which obviously is not being recovered into the delivered stream, so delivery pressure is reduced.
To alter the operating range you will need to alter the size of the steam cone slightly. He says if it works at high but not low pressure, the steam cone is too small. If it won't operate at high pressure the steam cone bore is too big.
I claim no expertise, having bought the book to discover how to keep injectors working well, but Mr Brown clearly does, and he tells one exactly how to test and diagnose many faults. So it might be a good investment.
I have an injector to sort and now I know how to do it, so with a bit of luck it wil be sorted by the weekend.

nipper30/03/2011 19:32:59
31 forum posts
Thanks John and MGJ. I made the injector to the DAG Brown dimensions, or so I thought, so I will check the size. At least it sounds as if shims are not going to sort it out. I may try machining the inner face of the cone just to see what happens but it seems a new cone is required. I will post the results but not for a few weeks.
mgj30/03/2011 22:59:33
1008 forum posts
14 photos
Well good luck - hes talking about corrections of around .005" for movement of the steam cone, so perhaps you could start with that amount of forwards movement to reduce the annular space and stop/reduce the overflow. If that doesn't work his suggestion is to slim down the nose of the steam cone and that will allow some more forwards movement. Even so, he reckons that .005 or so should be all thats needed if the thing has been made accurately.
Steam cone dia, I don't have the book to hand, but I think one is talking of a smidgette as it were. Just urge the reamer in a tiny bit by hand?
Reading his book, I doubt that you are out by much, because his injectors are mostly done with locos in mind, and he's always referring back to about 80psi, presumably for copper boilers.
I know my injector on the TE is not very happy over 80psi either, but then it came with a certificate that said it wasn't so I have no complaint when its attached to a 125psi boiler!

Edited By mgj on 30/03/2011 23:00:23

DMB31/03/2011 18:14:03
1003 forum posts
I remember a fellow club member, many years ago, demonstrated his experimental injector where the steam cone could be moved in and out as it was on a screw thread with a hexagon, to fit a spanner. It worked, at various positions according to the gauge pressure.
nipper31/03/2011 19:54:48
31 forum posts
Thanks for the comments. I have measured the steam cone bore and while it should be .041 (59 drill) a 58 will pass while a 57 will not so I am only about .002" out. I will skim the inner face of the cone and see how it goes. I have made 7 of these injectors and have 2 that work OK at 100psi, 4 untested and the one I am fiddling with so we will see. As the bore is too large, putting the reamer back in is not an option.
mgj01/04/2011 18:12:13
1008 forum posts
14 photos
When you say skim the inner face- its the locating flange you want to skim by say .005 to move the cone inwards and close up the annular gap to stop overflow and dribbles.
Steam cone for higher pressures - I confirm having looked it up - smaller steam cone bore for higher pressure and adjustment of about .002 on dia are the order of the day. (But increasing operating pressure will also mean you need more pressure to start it, so hte whole envelope moves upwards - and vice versa)
As I said, I claim no expertise - you are getting the book second hand.

nipper01/04/2011 19:02:29
31 forum posts
Sorry about some misunderstanding but I meant skim the inside of the steam cone flange to increase penetration. We will see.
nipper26/08/2011 10:52:22
31 forum posts
thanks for the comments they were very helpful. I have made a new steam cone with a 61 drill hole as opposed to a 59 so its a couple of thou smaller. It now works well up to 100 psi but is not much good below 50. I will keep one injector as a 30-80 and one as a 50-100. I don't have a pump so reliable injectors are a must. These are reliable, need little or no trimming of the water but do need to have the steam reduced as the pressure increases. Of my seven injectors I have 1 26oz and 4 18oz working well, some without any alteration at all and two untested. My challange now is to get the jublees working well as that are not reliable, they need a lot of water trimming and then run wet. I will recone one and see if I can make it better.
ady26/08/2011 11:32:44
612 forum posts
50 photos
I don't know if it will help but at sea when we wore a breathing apparatus set I was always impressed how well a thing called a "demand valve" worked.
On one side was an open high pressure air cylinder(around 180psi) and on the other side was me.
It only gave me air when I breathed in and I often wondered how it worked and how it was so reliable.
Scuba gear probably has something along the same lines.
Might be worth researching, as I say, it was very reliable.
Clive Hartland26/08/2011 14:45:24
2569 forum posts
40 photos
Can someone explain the relationship of Injectors and the 'Ounce' rating?
They are quoted as so many ounces etc. How do you determine the rate for a certain boiler?
nipper29/08/2011 10:31:19
31 forum posts
Hi Clive. If you have back copies of ME look at 11 feb 2000 where Mr brown tells us how to do it. I worked it out for my 56XX and it came out as a measly 6.72 OZ. This is at a constant speed, perfect valve events and 35% cut off etc. You will have to cater for real life such as starting and playing catch up if the water level is low and possibly a poor injector. Hope this helps.
John Baguley29/08/2011 11:01:07
460 forum posts
49 photos
I took Derek's formula and converted it to a spreadsheet for convenience. You can download it from here:

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