Author Topic: Steam water feed injectors  (Read 20117 times)

Offline sbwhart

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Steam water feed injectors
« on: June 16, 2015, 07:38:48 AM »
I've just completed making some steam injectors for my 5" gauge locomotive.

Injectors are a very cleaver little device that uses the flow of steam through a series of cones to pick up water and to feed it into the boiler against the boiler pressure, it's all to do with changes of velocity and pressure I won't even start to explain the physics of it as its way beyond me it enough just to know they work.

This is what they look like.



They are notoriously difficult to make to work as they have critical features that are difficult to actually measure, you have to get to these features in a round about way, and as I found out there are other pit falls just waiting to catch you. I did have a great deal of help from a friend and very talented model engineer (Pete) who gave me a great deal of advice as to the best way to go about making them, he also has a injector boiler test rig, that greatly help.

The first part to make is the body:- this is fabricated from some 3/8" thick brass bar.

First thing is to square the bar up and centre for the 5.5mm through hole postion



Then in the four jaw set up on this centre with a wobble bar.



Turn up and thread 5/16" * 32 ME and ream through 5.5mm



Then drill through for the overflow chamber and clack valve and tap again 5/16" * 32 ME.



Drill through for the water feed and the overflow and make some 5/16" * 32 ME. connectors



Silver solder the connectors and overflow to the body and drill through.



Make and fit a cap for the clack valve



And that's the body made.

Next up I'll show you how to make the taper D bits and how to make and fit the cones.

Cheers

Stew








« Last Edit: June 16, 2015, 07:45:07 AM by sbwhart »
A little bit of clearance never got in the way

Offline jadge

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Re: Steam water feed injectors
« Reply #1 on: June 16, 2015, 11:51:49 AM »
I've been looking at the theory of steam injectors, with a view to possibly designing one for my traction engines.

The isentropic expansion of the steam in the steam nozzle and the pick up of water by the resultant low pressure, but high speed, steam jet seem fairly straightforward. What I haven't completely got my head round yet is the conversion of the high speed jet back to a sufficiently high pressure to feed into the boiler. How can a flow of steam from the boiler, at a given pressure, inject a larger mass flow of water into the boiler at, or slightly above, the same pressure? I think it has to do with the enthalpy of the incoming steam. When the steam jet is condensed by the feed water this heat energy is given up to the water and it is this extra energy that allows the water to be injected into the boiler. So by this argument an injector wouldn't work using compressed air, as the air would not contain much heat energy.

Andrew

Online Jo

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Re: Steam water feed injectors
« Reply #2 on: June 16, 2015, 12:18:46 PM »
How can a flow of steam from the boiler, at a given pressure, inject a larger mass flow of water into the boiler at, or slightly above, the same pressure?

Don't forget that the water is cold, when the hot steam draws in and mixes with the cold water it forms a vacumme, which in turn sucks in more cold water the combination of which are at a higher pressure than the original steam, this higher pressure mixture can then force its way into the boiler  ;).

Which is why they don't work with warm water  :ShakeHead:

Jo
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Offline Stuart

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Re: Steam water feed injectors
« Reply #3 on: June 16, 2015, 12:26:08 PM »
Good work there Stew

Made a few in the past all but one worked , I could not solve it so it went in th bin

A good read on the subject by DAG Brown   
http://www.teepublishing.co.uk/books/operation-valve-gears-injectors/miniature-injectors-inside-and-out/

LBSC though he knew how they worked but a lot of his info was flawed , DAG Brown has it sorted , but you do need a injector rig to test/prove the flow rate

A major fault is faulty soldering ,pin holes and flux inclusions that are ok at first but fail in service

The last set I did was built for my class 4 but built into a scale body with the injector itself pushed in and sealed with o rings very small section ones , sorry I have no pics as they went to a new home but not on the loco when I sold it

Have fun but remember the full sized ones with the crocodile mouth work very well but do not scale , as the size gets smaller they get harder to make at 7 oz a min it's tricky

The other problem is the steam pressure range as you do not always have full steam that's why you need to build a test rig

They do need a clean regularly with very dilute citric acid NO poking the cone with a wire please
And a water filter is a must as is a good clack with a larger lift than used with ram pump
The sound of a injector feeding is music to my ears ( if they still worked)

Got the popcorn out for the long haul

Stuart
My aim is for a accurate part with a good finish

Offline gbritnell

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Re: Steam water feed injectors
« Reply #4 on: June 16, 2015, 12:33:33 PM »
Hi Stu,
Back when I was into steam and built my traction engine I studied the operation and making of injectors. At that time I didn't know enough to even try and with the intricacies of the jets I don't know if I'd still want to try. I'll be following along to see your procedures.
gbritnell
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Offline jschoenly

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Re: Steam water feed injectors
« Reply #5 on: June 16, 2015, 01:46:26 PM »
Wanting to build some small injectors myself, I'll certainly be keeping an eye on the next steps.  Thanks!
Jared
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Offline kvom

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Re: Steam water feed injectors
« Reply #6 on: June 16, 2015, 02:13:14 PM »
Interesting project.  On my loco I bought injectors from Superscale, as I doubted I'd succeed making my own.

Offline sbwhart

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Re: Steam water feed injectors
« Reply #7 on: June 17, 2015, 07:11:06 AM »
Wow I didn't think this would have so much interest.

I'm afraid I'm going to have to keep you all in suspenders until next week as I'm  a little tied up on domestic duties at the moment , and I didn't photographs making the D bits or the cones at the time so I'll make some spare and walk you guys through the procedure.


How can a flow of steam from the boiler, at a given pressure, inject a larger mass flow of water into the boiler at, or slightly above, the same pressure?

Don't forget that the water is cold, when the hot steam draws in and mixes with the cold water it forms a vacumme, which in turn sucks in more cold water the combination of which are at a higher pressure than the original steam, this higher pressure mixture can then force its way into the boiler  ;).

Which is why they don't work with warm water  :ShakeHead:

Jo

My loco is a tank engine as a result the feed water is warm to the touch, but my injectors work with it, I put this down to my friend Pete advice to have a slightly larger annulus (0.002"/0.05mm) between the steam cone and the combining cone, the theory being is that you need a greater volume of warm water to condense the steam and form the vacuum. I suppose those of you who studied the drawing are wondering how the hell doe's he control the annulus to that level well I'm afraid you'll just have to wait until next week to find out  :disappointed:

 :naughty:

Stew
A little bit of clearance never got in the way

Offline jadge

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Re: Steam water feed injectors
« Reply #8 on: June 18, 2015, 10:34:55 PM »

Don't forget that the water is cold, when the hot steam draws in and mixes with the cold water it forms a vacumme, which in turn sucks in more cold water the combination of which are at a higher pressure than the original steam, this higher pressure mixture can then force its way into the boiler  ;).

Which is why they don't work with warm water  :ShakeHead:

Jo

I'm not sure I follow that. As far as I can see from my calculations the jet of steam from the steam cone is already below atmospheric pressure. If it wasn't the injector would never pick up in the first place. When the steam and water mix then the steam is condensed, and gives up its heat of evaporation. This heat warms the water. The steam jet is also moving extremely fast, by conservation of momentum the mass of water is accelerated, but not to the same speed as the original steam jet. I think that the output cone then converts this momentum into pressure to feed the boiler. I don't know what happens to the pressure in the combining cone, but I'm not sure why it should rise? I thought that the problem with warmer water was that the steam would not then be properly condensed in the time taken to traverse the combining cones, and operation of the injector falls apart.

Andrew

Online Jo

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Re: Steam water feed injectors
« Reply #9 on: June 19, 2015, 09:07:24 AM »

I'm not sure I follow that. As far as I can see from my calculations the jet of steam from the steam cone is already below atmospheric pressure.

And it is HOT for Stew's boiler at 90Psi on the gauge, taking into account atmospheric pressure, that would be about 166 degrees C.

Try filling a suitable small glass bottle with water out of a just boiled kettle, then once the glass is hot to the touch pour out the water put your finger/hand over the open neck and put the container in some icy water and you will find that it forms an impressive vacuum without the need for any pressure being involved.

Warmer feed water just forms less of a vacuum so then it is down to how good the design/construction of the injector is.

Jo
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Offline sbwhart

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Re: Steam water feed injectors
« Reply #10 on: June 19, 2015, 01:12:11 PM »
Thanks for the explanation Jo I guess its the old gas law PV=MRT

I'm afraid its beyond me to give a full explanation about the ins and outs of how these things work.

All I know is that its not easy to make a working one, I'll now try to explain the method I used, I must admit that I had a lot of coaching from friend Pete on this which I would like to thank him for.

So making the cones:-

I'll post this in instalments as the method is rather long winded.

First Job is to make some tooling.

D Bits you will need two tapered D bit whilst you set up make a few spares to allow for breakages.



9 deg and 13 deg made from 3/16" dia silver steel (drill rod) to turn the taper you will need a razor sharp tool set perfectly on centre, to set you compound at the exact angle turn up a test length and run a DTI over a set length and not the deflection of the pointer for the 13 deg over a 10mm length the deflection will be 1.132mm




With the compound set gently turn the bar down until you have a sharp point, and give it a polish.

Part it off and centre drill the back and whilst your at it put a groove into it.



Then on the milling machine using a sharp milling cutter to reduce deflection of the point setion the D bit so that it is 0.003" 0.1mm above the centre line.



Use the groove to hang the D bit from a bit of wire , this is so you have something to hold onto, heat to cherry red and quench vertically into water, so that it stays straight.



When you make the 9 deg bit with the compound set make the assembly tool for the combination cones, this has to be made as accurately as you can.



To get the 0.101"(2.565) run out correct first turn a parallel diameter to this size, then with the compound gently turn the point down until it just runs out at the shoulder this shoule give you the correct size. then accurately turn the 0.431" (10.95mm) length.

You will also need to make a 0.037" (0.94mm) thick washer and a setting ring drilled with a No 38 drill, and some stop sleeves for the D bits

The cones have to be made in the correct sequence as you use one to make the other.

Start first with the combination cones:- turn the dia down to be a push fit into the body and part off two slugs one 0.162" (4.11mm) long the other 0.213" (5.4mm) long.



These little slugs are not easy to hold true, I solved this by using soft jaws bored out to a step to grip the slugs



You will also have to set depth a stop on the 9 deg D bit using the setting ring.



Start with the shorter of the two slugs 0.162" check that it is running true, and centre drill and drill through No 56 then its best that use use the D bit floating so with ut held in a carrier and pushing with the tail stop cut the taper , position the d Bit flat side is underneath or on its side if you have it on top the swarf will build up and probably break it.





Then using a triangular scraper radius the mouth of the cone.



Remove the cone from the chuck, now your going to use it along with the washer to cut the next cone



Put the thicker slug in the chuck checj that its running true centre drill and drill No56 and using the D bit with the first cone and washer to cut the taper to the correct depth, with the triangular scraper radius the mouth of the cone, its important you do this things won't work without it.



With the tapers correctly cut in the cones all that required now is to finish off the stepped diameters





To assembly the cones into the body you know use the assembly tool to get them to the correct depth check that you have a gap between the two cones of 0.037" (0.94mm) by using a No 63 drill as a gauge through the over flow chamber. As a belt and brasses action secure with a tiny drop of High temp loctite 640.



 

Phew

That took some explaining.

Stew

The Steam cone is next but I'm going for a brew first.






A little bit of clearance never got in the way

Offline sbwhart

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Re: Steam water feed injectors
« Reply #11 on: June 19, 2015, 03:25:47 PM »
OK on with the Steam Cone



This cone and its location is the most critical part of the injector so its worth taking your time and if you have any doubts start again.

First job is to turn up the blank make it a nice slide fit in the body for a length of 1/2" (13mm) there are a number of pits falls her, so slightly undercut the shoulder so that it will but up flat against the body and won't be held off by any radius in the corner, also with a a radius tool just skim a short shoulder this is to avoid a false assembly from any residue of adhesive from the combining cone.



Face the length down so that the gap between the body is 0.094" measured with a No 63 drill, and set the saddle dial at zero, this is important.





Set the 9 deg D bit depth with a setting ring drilled with a No 40 drill, and centre drill, drill No 56  and cut the cone.
This D bit operation will swell the diameter so its best to do this before the next operation, I fell into this trap.



Now I will say this only once this is important. taking small cuts .001" reduce the diameter of the end for a short length with the radius tool until the body just buts up flat with the cone, then face off by a further 0.0015" (0.38) if you set you're dial at zero this is quite easy. This gives the steam cone the correct insertion into the combination cone with the correct annulus.





You should have a cone with an end that looks like this.



Part off

Set back up in the collet face the head off to length  centre drill.



13 deg D bit  depth set with a 1/8" diameter setting ring.



And finally radius the mouth with a triangular scraper.



Only one part left now.

Stew



A little bit of clearance never got in the way

Offline sbwhart

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Re: Steam water feed injectors
« Reply #12 on: June 19, 2015, 03:50:08 PM »
Delivery Cone

Turn up the blank again making it a nice slide fit in the body, and face the length off so that it just buts up against the body, zero the dial and reduce the length by 0.024" (0.61mm) this form the gap for the over flow.



With the radius tool turn the end down for a short length 3.5mm to a diameter of 3.2mm, just touch the face with the centre drill to give the No 68 drill a start and drill about 8mm deep, keep drawing the drill back and clean away any build up of swarf from the flutes or you'll end up breaking it.





And radius the mouth with the triangular scraper this is important.



Part off

Face the collar off to length centre drill and 13 deg D bit and radius the edge with the scraper



That's it with a bit of luck you should have a Injector that works.



These are the tools.



Injectors are a bit of a challenge to make you have to follow the procedure carefully and there are lots of little pitfalls that can catch you out, so you have to work very methodically, I got paranoid about getting the drills mixed up so I threaded them through a bit of paper.



And I found it paid to keep the D bits sharp on an oil stone.



But its a great sence of achievement when you get them working, and it gives you the bragging right at the track to say of course I make my own.

 :Jester: :Jester: :Jester: :Jester:

Stew

I'll try and take a Video of the injector working the next time I steam my loco.

« Last Edit: June 20, 2015, 08:33:06 AM by sbwhart »
A little bit of clearance never got in the way

Online BillTodd

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Re: Steam water feed injectors
« Reply #13 on: June 19, 2015, 05:35:36 PM »
Nice job StewartB-)

Thanks for explanation Jo , I saw someone demo'ing one on youtube vthe other day and was wondering how the thing could work! :-)
Bill
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Online Jo

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Re: Steam water feed injectors
« Reply #14 on: June 19, 2015, 05:43:24 PM »
8) looking forward to seeing it work Stew.

I recommend that you make a batch before you loose those reamers or they have their edges damaged in the drawer  ;)

Jo
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