Injector Sizing

[NarrowGauge]

Hi

I have just started work, my first project in 'proper' ME, to refurbish a 'Midge', believed to be to the George Gentry design, c1935. The model was bought in a disassembled condition and no devices for water delivery were included. My thoughts are to provide one injector and a hand pump.

The boiler has a capacity of approx. 16 pints and I have assumed a working pressure to be 80psi? The bits of plumbing in the box that appear to have been used for the water supply are formed from 1/4" copper tube, including the steam supply available for an injector.

Can I ask if fitting an injector and a hand pump is the right way to go and if so, what size of injector should I be looking for. Should I be looking for a hand pump with a 1/2" or 5/8" ram?

Lastly, are there any suppliers/manufacturers of such fittings that are considered to be 'preferred' sources.

Thanks for any help offered.

Dave.

[SteveW]

Dave,

As I understand it the new regs. require two methods of boiler filling when running, e.g. injector and hand pump.

As for sizing the simple method I've seen used is referenced by gauge. The wider the gauge, the bigger the boiler, the more water you have to replace when it's chuffing.

As for hand pump bore it depends on how patient you are. At say 10/12 pints running capacity you can get an idea of how many strokes it'll take to fill from empty. Is your wrist up to it? Many will suggest that model engineer's wrists are generally more than adequate to the task.

[NarrowGauge]

Hi Steve

Thanks for your comments.

Having glanced at some injectors on ME suppliers web pages, especially the 1/4" pipe sized options, my fear with the injector was to feed too much water and to loose too much pressure in the boiler. Can I ask, is it good practice to have injectors on for a short period with a greater delivery? or is it best practice to have them working on a slow delivery for a longer time? If using a larger capacity injector and similarly sized pipework, i.e. 1/4", is it acceptable to restrict the water feed to the injector by only part opening the water feed valve?

I take your point about the hand pump, RSI and all that! Given any choice, I would only plan to use the hand pump by exception. Perhaps a 5/8" bore pump would be preferable?, I will check the physical size, but would think that there is room to accommodate one. Sounds like 'horses for courses'.

Does any one happen to know what was specified in the original design?

Thanks for any suggestions.

Dave.

[SteveW]

Dave,

My expertise on injectors is limited to a bit of reading and sitting in on a DAG Brown lecture on them at the last MEX. However, from what I've worked out I would guess that it's not a good thing to go starving them of either steam or water. Each one is sized for water and steam flow. When they go you need to leave them to it with only minor fiddling on the steam and/or water to get them on-song.

Maybe if you quote the stroke and bores the old guys on the list could suggest how much steam and then water you need to replace at full chuff but this'll only give you an absolute minimum. I've found it far easier to have a quick boiler top-up option rather than have to leave it going and end up with another thread to monitor while driving/stoking/hanging on.

If you have too much feed then worste case is you'll drown the boiler and shut it down. This has to better than runnning out of water and having to start that frantic and ungainly action with the wrist while hunched at the driving position.

[NarrowGauge]

Thanks Steve for your further thoughts.

The cylinder size, so far as I can ascertain, is a 1 5/8" bore with a 2 7/8" stroke. There are just two of these, no fancy internal cylinders.

Are there any published reference sources/tables for such design requirements? Nothing too complex, but basic information that could be used as a guide.

Any information appreciated.

Thanks
Dave.

[gwrfan]

Hi All,

Just my twopenn'orth, but water into the boiler is better 'little and often', and probably by means of an axle or other form of driven pump. This can be regulated so that on the move a constant supply to water is supplied into the boiler. If the pump is effective, then all that's required is a a little tweaking of a water valve, which will allow surplus water back into a tank or tender. Although two water supplies are required under our 'Regulations', most locos usually have some form of pump in addition to handpump and injector.

For what it's worth, when I use the injector on my Pansy, it's water full on, and steam full on. Fills the boiler up in a jiffy!

Geoff

[SteveW]

Dave,

GWRFAN is absolutely correct with his suggestion of an axle/mechanical pump. Basically a hand pump is a pain in the wrist so to speak.

The injector is an ideal way of keeping things topped up while you're waiting to go. When running small adjustments to the mechanical pump flow bypass can deliver the correct amount to replace the water lost to steam. Once set everything is in balance and you get to sit back and enjoy the ride.

If you do the math that makes three forms of boiler feed. Each one has it's place and you sort of need them all, i.e.:

empty start = hand pump unless you can open up and get water in there some other way,
waiting = injector,
running = mechanical.

[NarrowGauge]

Thanks Gwrfan and Steve for the advice on the axle driven pump. Sounds as though I may have to consider this, although originally I was hoping that one would not be required.

At least for the initial fill I can easily unscrew the safety valve and use that which will save a bit on the wrist.

Thanks

Dave.

[gwrfan]

Hi Dave,

If you can fit such a mechanical pump without too much difficulty, I would highly recommend doing so. Injectors are notorious for failing to deliver, especially when you want them to do so in a hurry. Even if well made, if they get too warm they seem to fail. Obviously many are absolutely perfect, but others aren't. I would consider a hand pump as a last resort, and even they can fail, especially if the balls stick through lack of use. Both of my completed locos have all three methods of supply, and any others will also. You could also even fit a second injector if you have the time and money, LOL!

Regards

Geoff

[jgb7573]

Given the choice, I'd have two injectors for normal use and a hand pump for emergencies. I'd have them set for different pressure (one higher than the other). With injectors properly maintained and well plumbed in, they are very reliable these days.

With injectors, you get water in the boiler whenever you want it. No need for minute adjustments. Being used to driving locos with this arrangement, I always get caught out by mechanical pumps. I always have to double think about whether to open or close a bypass to get more or less water into the boiler. It's also much easier to control blowing off with injectors. Pressure getting a bit high? Well put on the squirt!

[gwrfan]

jgb7573 said:

Pressure getting a bit high? Well put on the squirt!

Hi John,

Yes, that's always such a reassuring sound, listening to the water being injected into the boiler, when the valves are lifting! At least you know the boiler water level will be rising, and quickly!

Regards from freezing Cornwall

Geoff

[andrewtoplis]

Just to throw in a random spanner...

in full size practice, locos always have two injectors, and have done since 1900! LBSCR Stroudley engines which had all pumps oftn had to run up and down the platform to fill the boiler, if this wasnt possible they would oil the rails so the wheels span on the spot! I think one jack and one pump is a good compromise. The downfall of injectors is they use steam to put cold water in, hitting the boiler twice if you see what I mean. A good full size injector CAN be operated with slightly differing amounts of steam and water, so it can run slower or faster to a certain extent (which you asked about earlier) , though i am not sure about model ones.

Andy

[gwrfan]

Hi Andy and Dave,

I think it might be difficult in our smaller gauges to adjust the water and steam supplies in order to control the amount of water injected into the boiler. That's why some people have two injectors, which will operate at different steam pressures. Yes, the difficulty with one injector is that you usually have to turn everything on full blast, and then you lose steam pressure. It's a case of having enough steam pressure, and fire, and having the blower on at the same time (when stationery). As I said before, I always turn the water on fully first, then open the injector valve, slowly at first, then open it up. I suppose the larger gauge lads can adjust flows accordingly. I haven't as yet, used my injector 'on the run', only whilst stationary.

Geoff

[andrewtoplis]

I can only speak from standard gauge experience, my Friend. We have the advantage of a nice big control to play with, not a tiny scale one!! When I finish the day on a standard gauge loco, i fill the boiler up by putting on the injector usually around 140psi, then just close the water valve when it starts to run by as the pressure drops. Its not unusual for the jack to carry on down to 40psi. It certainly is possible to run it with only half water supply in that scenario.

I will shut up now and stop clouding the issue...

Andy

[gwrfan]

Andy,

You're a lucky so-and-so, haha! I think I need to pay the I-O-W a visit next summer.

I think the main problem is that our tiny injectors get warm, and are sometimes fed by warmed water, especially tank locos, and then they start to play up. That's one reason that I put the water on full, and then turn on the steam slowly at first, until the water picks up, then open the steam valve more. Seems to work for me.

Geoff

[andrewtoplis]

"You're a lucky so-and-so, haha! I think I need to pay the I-O-W a visit next summer."

I will let you know when I will be there...If you like? We could hopefully sort you out with a ride (if I have the right sort of driver)

[andrewtoplis]

ps you have now joined the seasoned member ranks I see.....kept that one quiet didnt you LOL

[gwrfan]

Andy,

Like all good things, it crept up on me, haha.

At one of our club meetings in the summer, there were about a dozen of us there doing some maintenance. I was quite please to find out that I was actually the YOUNGEST member there, LOL! ;D

[chris vine]

Can I take up a point with Andy on injectors?

It isn't really fair to say that they are putting cold water into the boiler and using steam as a double hit. The thing is that the water is going in hot compared to the water which is going in stone cold from a pump. It doesn't really matter how inefficient an injector is (in the sense of how much steam it takes to inject a given amount of water) because the water going into the boiler is just a bit hotter. In fact the only energy loss from the Injector is heat loss from the pipes and injector body itself.

By comparison a pump, say operating off the main engine cylinders via an eccentric or whatever will be rather inefficient because, even if the pump itself is 100%, it is being driven by the cylinders which are probably less than 10% efficient (steam energy to mechanical energy) all the extra exhaust from the cylinders (or from a pump's own steam cylinder) is just waste heat going up the chimney.

You could do what some people do and that is to fit two injectors with different capacities, say one twice the size of the other. The small one could be left running almost all the time and the large one used to make adjustments to the water level. Did they do this in full size practice? I expect so....??!!!

Cheers
Chris.

[andrewtoplis]

Hi Chris,

I happily take your point about pumps and injectors, I actually came out in favour of one of each a few posts ago!

I dont think that injectors were set differently in full size practice. I cannot remember working on any engine with a major difference between the two injectors, although some engine do have slow or fast injectors. Most engines have two injectors that are basically the same, unless one is an exhaust steam injector which works on a lower pressure as it uses exhaust steam...although one only finds these on engines which would be doing long periods with the regulator open or the extra complexity isnt worth it. Remember that injector failures were not that uncommon, so each injector must be able to suffice in an emergency.

I am sure someone with more experience such as Spurley could give us a more definitive answer...

Andy