Running a British Injector above its rated pressure?

[Howard Gorin]

In short an injector works on velocity, not pressure. At any given pressure the velocity of steam is much greater the velocity of water. An injector uses the velocity of steam to raise the velocity of water. If you can raise the velocity of water entering the boiler to a level greater then the velocity of water trying to leave the boiler then the injector will fill the boiler. If the velocity of the water trying to enter the boiler is less then the velocity of water trying to leave the boiler then the boiler check will (hopefully) keep the water from leaving the boiler and the injector discharges through the overflow.
Even low pressure steam has more velocity then high pressure water (with in limits) Elesco made injectors that started on high pressure steam and then cut over to exhaust steam, more complicated but they worked.

[k36no4862002]

Gents,
I did not know Double B Designs was still going.
I bought 2 custom made injectors off him years ago for my K36 - they were 1/2" pipe feed. I have never had an issue with them and so when it comes to getting a pair for the Challenger I will go straight back to him.

Not sure about your pressure ranges but if you were to fit 2 injectors then you could have a low pressure and a high pressure one which should cover every situation.

Paul-

[JJG Koopmans]

In short an injector works on velocity, not pressure.......

That might suffice for the explanation, however, what is described is momentum,density times velocity squared
which has a dimension of a pressure!
kind regards
Jos Koopmans

[Fred_V]

Jos, one thing I found with British injectors is that they don't like warm water so saddle tanks are bad news and even side tanks. Injectors are a real serious problem right now as there are none being made in the USA any more. Hopefully that will change this year sometime.

[JJG Koopmans]

Fred,
Hot water is a problem for any injector, even for the 1:1 locomotives. The problem being that steam contains a lot of heat and has to be totally condensed for any effect. So above a certain water temperature there is insufficient condensation and the device will not work.
kind regards
Jos Koopmans

[Howard Gorin]

This is also why here is a high limit for working pressure. As the steam pressure and temperature increases it becomes impossible to condense the steam.
There were special injectors made that could deliver more pressure then the pressure of the incoming steam. These special injectors were used to hydrotest boilers.

[Greg_Lewis]

Another good book on model injectors is The Model Injector, by Ted Crawford. The first half is on theory and design, the second half on making one. He includes information on modifying the pressure range. Even if you aren't interested in making injectors, the education is worth the modest price. You can get a copy here:
http://www.ameng.com.au/81_the_model_injector.htm

[blff cty lcmtv wrks]

one more funny story, and then i will quit.

many years ago the late r.f wilson, charter member of the midsouth live steamers and myself were running his i.c. 2614 locomotive over at the late austin barr's track. the injectors dribbled real bad, so we came up with this real cool idea, we thought. we took some clear plastic tubing and hooked it to the overflows on the injectors and ran the tubing back to the tender and let it dump the water back into the tank. that kept the water from running all over the ground. problem solved, ha!! the next thing we know, both injectors quit. we had heated the water tank up so hot i believe you could have steamed hotdogs in the tender. hot water, bad. we had a good laugh out of it.

big c

[NP317]

I recall reading (long ago) that early British tank steam locomotives used in the London Underground returned exhaust steam to the side (?) water tanks. This required frequent replacement of that water as it heated above condensing temperatures. I supposed they used axle or crosshead pumps for boiler water supply, because injectors would not work with the heated water?
~RN

[apm]

I sent over a few emails and here's what I heard back form the suppliers so far;

Maidstone Engineering;
http://www.maidstone-engineering.com/
#8 horizontal =$98USD
#6 =$50
They say that their injectors should work ok up to approximately 130psi but can make ones up to 150psi. I have an inquiry back to find out of those are special order and how to do so, as reading between the lines it wasn't clear if their "Should work up to 130psi" was guaranteed or not.

GSS Model Engineering Supplies;
http://www.gssmodelengineers.com/rw_sho ... a53c78ada9
They tell me that they can get injectors up to 125 but their standard offerings are only good to 100psi, so I am still awaiting clarification on that one.

Pollymodelengineering.co.uk
says that theirs are all tested by their suppliers up to 120psi

From the emails I received from all suppliers queried so far I am almost getting the impression that it may just be something as simple as how they are setting these things up to get them to hit the higher pressures. Maybe an added shim or something as they seem to indicate they can all get higher pressure injectors. Given the responses I also sent back an email over to http://www.livesteammodels.co.uk/ to see if they too can do a higher pressure one on request. I am no longer sure if they are all the same manufacturers or not based on the responses I am getting back.

At this point given that these injectors in the scheme of things are so low cost I may just buy an extra one or two of them from different vendors.

I am still not sure if I am in the 4, 6, or 8pint per minute range on my locomotive. Anyone care to guess what a RMI Forney would use for an injector? I may go for one of the 4pints and one of the 8pints per min as the 6pint per minute seem to be harder to obtain, and just have one large in case the water level needs to be brought up fast and a smaller one for more regular use.

[James Powell]

To figure out water consumption, figure out steam consumption. Multiply cylinder volume times 4 times RPM times .5 or .75 (depending on your pessimism) and divide by the volume of steam:water ratio at your operating pressure. This will give you the volume of water you need to put in every minute. (note- higher pressures = lower ratio of steam generated, it is a gas and is compressible, whereas water is a liquid and is for our purposes, not compressible. .75 is low if anything for the correction, and you could use as high as 1 (full swept volume), but it depends on how you drive and if the boiler can steam the engine at the chosen speed & power anyway- you can come up with a range and as usual, GIGO)

James

[apm]

Hi James,

So that makes things somewhat easier here's my math;
Cyl Dia=3"
Stroke=4"
Cyl vol=7.07in3
Vol/rev=50in3

Now the wheels are 10" dia, that means I go 31.4inches/full wheel revolution
That gets 2017revs/mile=100769cubic in=58.3cubic feet, for the moment I will assume your correction factor is a simple 1, which I believe would translate to 0 expansion essentially which we know is false, but I can run with that for estimation purposes. Playing around with the numbers a little more and the 58.3cubic feet assuming 100psi saturated steam average the steam table shows me that is 3.89cubicft/lb of steam bringing me all the way to 15lbs/mile of steam consumption, or 1.796 gal/mile.

What is the average speed of a live steam locomotive? I am guessing 5mph, with a top speed of 8?

So worst case I am using up 7-14gals/hr, since there are 6.66UK pints/US gallon, (not 4 US ones) that is 95.7pints/hr or 1.6pints/min at top speed does this sound about right for those of you with a similarly sized locomotive?

Now the next two considerations to make, is what % of the time would one want their injector running during normal operation maybe 50% max, so go with two of the 4pint/hr ones and be done with it?

Shy of a clishay I built years back this will be the first serious locomotive I have made and since it won't have any axle pump on it I will only have two injectors and an emergency hand pump on board to get water in the boiler, so I want to make sure I get it right.

Thanks for the help,

Adam