Injectors, a Blessing or a Curse

[Jack LovelockParticipant @jacklovelock88377]

I am finishing off a 12 year build of a 71/4" 2-8-2 loco and have run into a last- minute frustration with injectors that refuse to work.

My boiler is 50 litres and i have used 3/8" fittings throughout. This includes the copper tubing.

The 2 injectors are 3/8" No8 (160 oz/min) and likewise the clack valves.

One injector sometime works, but the other refuses.

My boiler inspector tells me that the injectors are much too large for the boiler, But there is an arbitrary formular that says the correct injector size should by 4 times the boiler capacity in litres? i.e. 4 x litre capacity = required flow rate in oz/minute. That is for a fast fill. So my No8 injectors should be about right.

However, I changed down to 1/4" No 6 injectors (80 oz/min) which means a change in copper tubing from 3/8" to 1/4". These being placed just a few inches from injectors. Yes, bad practice but where else?

Both injectors refuse to function.

I am currently waiting on a friend to test these injectors on a test rig that he has, but they are unused new injectors and I suspect they will be OK.

I did not appreciate the importance of keeping injector feed and delivery lines free of sharp bends, but now suspect this may be my problem.

I propose changing back to 3/8" tubing and the No8 injectors, but re-pipe eliminating any 90-degree elbows with 30mm radius bends.

I am new to all this and would be grateful for any suggestions or advice.

I'm getting desperate, but not yet to the point of using electric pumps!

[Jack LovelockParticipant @jacklovelock88377]

[Werner SchleidtParticipant @wernerschleidt45161]

Hi Jack,

one of the faults can be that the water valves are water thight ,but not air thight. If the injector try to start it can be possible that air could be sucked in via the shaft to the water . Then the injector fails immediatly. This was a finding i had after some use of my injectors.

[Paul LousickParticipant @paullousick59116]

Injectors are excellent when they work But are trouble when they don't.

The boiler on my Ruston Proctor traction engine is slightly smaller than yours and holds about 45 litres. A good, free flow of water is important and lots of sharp bends should be avoided, especially from the tank to the injector.

I use No 6 horizontal injectors, rated at 4 pints/minute which are a good size for my engine. Filling the boiler with too big a volume of cold water can cause thermal shock or a quick drop in pressure.

I originally ordered 2 injectors from the company that supplied the castings but had problems with them. Replaced them with another brand and have had no problems.

The water supply to the injector and the outlet to the clack valve uses 5/16" OD tube (not pipe that is sized by its nominal bore). Check that the injector connections are correct as I had to exchange those that I had ordered because the supplier did not know the different standard between pipes and tubes.

[duncan webster 1Participant @duncanwebster1]

I can't see any reason why having the wrong capacity injector would stop it working. As long as you supply enough water and steam and have a good route from the injector to the boiler it should work. Having too large a capacity will bring down the pressure too fast, but not prevent it starting. Temporarily rig up a water supply with no bends perhaps with plastic pipe and see if that helps. Jusr because injectors are new doesn't mean they work. There used to be a supplier who provided a test cert but he has retired.

[Nigel Graham 2Participant @nigelgraham2]

Possible problems:

The flow restrictions caused by acute bends – you have mentioned that one.

From that though, are the clacks appropriate for the injectors? If too small they might create enough back-pressure to upset the injector.

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Air-leaks on the suction side – the very devil to locate because by their nature they don't show unless in, or upstream of, the water valve, and sufficient for water to ooze out. One clue that this is happening, is the injector feeding the boiler, but chirruping like an excited guinea-pig in the process, and occasionally spitting hot water from the overflow. They should run quietly, with a fairly steady sound of rushing water.

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Is the injector too large for the steam supply available? That might be what the boiler inspector had in mind. The pipework may be right but is the steam-valve and the passage to it through the manifold, of comparable size?

Injectors need be matched not only to steam and water supply but also to working pressure-range. They can be fussy about these, and though this is not intuitive it seems they can also have too much water! More accurately, they want plenty of water at the inlet, but then the water flows through a very tiny annulus between the steam and combining-cones, and the geometry there is not normally adjustable but is critical to the device's behaviour for a given steam flow and pressure. (My own club's 'Wren' has an injector that needs the water choking back almost to the point of closed, to make it start – that should not be necessary.)

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To answer Paul's point about water temperature, injectors want cold water into them, but they do not feed cold water to the boiler. They are actually moderate feed-water heaters! They rely on the heat energy in the steam for their operation, and although much of that is converted to the kinetic energy in the moving water, the rest stays as heat absorbed by the water. The water is not so hot that it maintains the water and steam conditions, but is still hot enough not to worry the boiler's structure unduly, especially if the feed-clack is away from the firebox – as is normal on locomotives.

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It's worth buying a copy of Doug Hewson's book on injectors. Though mainly describing how to make ones that work (many of us do seem to think injectors are among the Black Arts) he covers areas like servicing and fault-finding. He also makes, and explains, the interesting point that a miniature locomotive should not need an axle-driven pump. Model traction-engines do, because their prototypes do; but almost all full-size locomotives relied on injectors. Though making a working, BR Standard pattern, exhaust-steam injector for a 5"g locomotive, might be off-limits!

[julian atkinsParticipant @julianatkins58923]

Posted by Nigel Graham 2 on 10/07/2022 00:39:20:

It's worth buying a copy of Doug Hewson's book on injectors.

I think you mean the book by Derek A G Brown "Miniature Injectors inside and out" published by TEE

[Jack LovelockParticipant @jacklovelock88377]

I much appreciate the injector experiences you have shared which have been very helpful.

I was dismayed to find the bench test on the new No 6 injectors revealed that one had never been completed, with the steam nozzle non-existent. Just a straight drilled 1mm hole! I expressed my frustration with my well-known supplier, which helped to reduce my own embarrassment at not spotting the problem myself.

I am now waiting hopefully for a replacement.

Meanwhile I have ordered D.A.G. Brown's book on miniature injectors.

My interest in electric pumps continues to grow.

Thanks Guys

Regards

Jack

[Nigel Graham 2Participant @nigelgraham2]

Julian –

Yes, D.A.G Brown.

Not Doug Hewson.

Sorry about that – Thank you for the correction!

I should have gone and looked.

.

Jack –

Glad we could all help, and don't worry that you did not see the manufacturing fault. No-one would or should expect that of a bought injector, and I think we'd all be chasing all sorts of other possibilities and questioning our own wokmanship before wondering if the thing had actually been finished!

Don't give up on injectors though. Assuming they have been made properly and the plumbing is as it should be, they should be no less reliable than any other feed system. Electric pumps are bit less troubled by obscure faults like hidden air leaks or over-hot water, but they can fail, too!

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