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Injectors.

Discussion in 'Locomotive M.I.C.' started by Eightpot, Aug 19, 2017.

  1. Eightpot

    Eightpot Part of the furniture

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    I have seen it stated that one can vary the feeding rate of injectors on locos. My experience is that they are either working at a constant rate, or not working at all. Is there any truth in this?
     
  2. Steve

    Steve Resident of Nat Pres Friend

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    Once the injector has picked up you can vary the feed rate within limits by varying the water feed but this does depend on the quality of the water control valve. The amount of variation is quite small, though, probably no more than 20%. I've never seen any actual figures for this.
     
  3. Big Al

    Big Al Resident of Nat Pres Staff Member Moderator Friend

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    I think that the above description is exactly right but this opinion is based on what I've been told and seen rather than actually experienced. Perhaps of more interest is the occasional inefficiency of use that can be observed when steam is seen blowing by on injectors.
     
  4. Jamessquared

    Jamessquared Resident of Nat Pres

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    If a lot of steam is blowing by (i.e. a rush of steam rather than a wisp), that would be a sign that the injector hasn't picked up (or has knocked off), so has to be stopped and restarted. More common is more than a dribble of water while running; that is a sign that the water feed is not quite adjusted right, or maybe of wear and tear.

    My experience is that they are an item that definitely was improved in usability over the years, as anyone who has tried those on a BR standard and a pre-grouping loco in close proximity will know!

    Tom
     
  5. Big Al

    Big Al Resident of Nat Pres Staff Member Moderator Friend

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    When Clan Line was having injector problems some time back it eventually became a distinct possibility that injectors can become 'life expired' and do not necessarily go on for ever. At that time quite a lot of work was undertaken comparing temperature variations across each injector. I mention this not to conclude anything seminal about the working of an injector but simply to suggest that whilst they are a fairly straightforward (and clever) piece of kit they can be tricky to regulate and need a subtle touch. It appears that @Jamessquared is implying that as well.
     
  6. jma1009

    jma1009 Active Member

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    On an ordinary injector (without adjustable cones) the rate of feed is determined by the size of the delivery cone throat.

    There will be an 'working range' or 'dry range' for a given pressure range of steam. The better the design the greater will be this pressure range.

    This range should be towards the working pressure of the loco and extend beyond it otherwise the injectors wont work when most needed when the safeties are blowing off.

    There will be an optimum range of steam pressure where delivery corresponds to the designed output of the injector.

    However it is a feature of injectors that when working at lower steam pressures than the above there is quite a considerable falling off in delivery output. There are lots of old books with graphs and tables and the maths that explain all this in much more detail.

    At some point as steam pressure reduces there is too large an imbalance between water and available steam, and regulation of the water supply via the injector water valve will help to reduce this imbalance. However there is a limit to this as the injector is no longer working at optimum and will be overly sensitive and at some point will cease to work all together.

    Cheers,
    Julian
     
  7. Steve B

    Steve B Active Member

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  8. Steve

    Steve Resident of Nat Pres Friend

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    I think that Clan Line has Monitor injectors and these may not have moving parts but they do wear out. The passage of steam and water through the cones of all injectors eventually erodes them, leading to loss of shape and loss of performance. The shape of the cones in injectors is critical in the modern(?) injector.
    The live steam injector used on the BR standards was essentially a GWR injector (which was essentially a Davies & Metcalfe injector) and is pretty reliable and easy to work, especially when you compare it with the LMS moving cone injectors (essentially a Gresham & Craven injector) and those used by the LNER where the clack boxes are the Achilles heel. Tom has compared the BR standard injector with some of the earlier injectors fitted to Bluebell locos and the experience he has is probably the same as that of most loco crews. I find that firemen brought up on a diet of BR /GWR injectors assume that they will work just like that and don't know what to do when confronted with one that needs to be coaxed into life. Anyone who has done battle with a Giffard injector will know that it takes patience and a lot of delicate adjustment of water, steam and cones to get them started.
    A good injector is worth its weight in gold.
     
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  9. Grashopper

    Grashopper Member

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    I believe that the issue with Clan Lines' injector was an historic and undetected flaw in the casting, which opened up a passage between two parts when hot. It was only discovered by cutting the injector casting apart.
     
  10. Eightpot

    Eightpot Part of the furniture

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    This sums it up nicely. My experience is mainly with Sentinel injectors (actually made by White's, Holden & Brook, possible Gresham & Craven too) which have a 3 mm delivery cone. They have a very good working pressure range from in excess of 275 psi right down to 30 or so. However, if running them for a period of time with the boiler pressure dropping one does have to cut the water supply back to have them working 'dry' at the overflow. To me this is the only way the delivery quantity will be lowered. With steam pressure kept up I just cannot see how the feed rate/quantity can be regulated, unless I've missed something.
     
  11. Robin

    Robin Member

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    The not-so-good variety appeared in Colonel Pringle's report into the 1913 Midland Railway collision at Ais Gill:

    "Driver Caudle gave the following account of his journey after passing Kirkby Stephen. He left the footplate when approaching Birkett Tunnel, on the near side, to oil the left driving auxiliary box, and was on the framing of the locomotive while the train passed through the tunnel. He then went round the smoke-box on to the off side, oiled the corresponding box, and got back to his place at the "front". Mallerstang up distant signal is situated on the near side of the railway, about 1,060 yards from the south end of Birkett Tunnel. A good view can be obtained of this signal as soon as the cutting at the south end of the tunnel is cleared -say for 500 yards. Caudle states that he got the impression, when he was outside the engine, that the distant signal was in the clear position. The wind also was stronger than he had expected to find it. He was consequently a longer time than usual going round the framing, and was outside when the engine passed the distant signal. On his return to the cab he found the water low in the glass, and his fireman (Follows) engaged in trying to get the right-hand injector to work. He immediately applied himself to the refractory injector, and forgot he had not assured himself of the position of the distant signal. He was preoccupied with his work on the water-feed, and, when he had succeeded in getting it to work satisfactorily, found that he had run past the home and starting signals at Mallerstang without observing their position. He kept the regulator and reversing lever in the same position and continued to run forward, being engaged in watching Follows put some coal on the fire. Suddenly he heard the engine whistle sounded, and shortly afterwards heard Follows say, " Look out, Sam, there's a red light in front of us." At once he pulled over the brake handle and closed the regulator. He had time to do no more before the collision occurred."

    Regarding the injectors, Col. Pringle stated "... it appears probable that the contrariness of the right-hand injector was not likely to have been caused altogether by low steam pressure. Injectors are sometimes peculiar in action and require humouring. It must also be noted that the fireman (Follows) was new to this engine, though not to the class."

    http://www.railwaysarchive.co.uk/documents/BoT_AisGill1913.pdf
     
  12. Steve

    Steve Resident of Nat Pres Friend

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    Col. Pringle's words about injectors are quite close to the reality of many injectors of yesteryear. When I first started on the Talyllyn too many years ago, most of the locos injectors required patience to get them to pick up. It was the same when I became involved with the NYMR, especially with the combination injectors fitted to the Lambton tanks, and the injectors on 62005, 44767 & 63395 were not of the best, either. If you had one working injector, you took your loco out, even if that was temperamental. You were expected to cope with it. How times have changed. A loco without two reliable injectors is now regarded as a failure. Quite rightly, I might add, but years of humouring reluctant injectors means that I can usually get them to pick up when others have given up at the second attempt.
     
  13. Steve

    Steve Resident of Nat Pres Friend

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    For a given steam supply condition of pressure and flow there are limits beyond which an injector will not work. If there is too much water the energy in the steam is insufficient to create sufficient pressure to overcome the clack valve held on its seat by boiler pressure. If there is too little feed water there is not enough to condense all the steam to water, in which case you get an unstable jet . Between these two limits, the injector should work.
     
  14. Dag Bonnedal

    Dag Bonnedal New Member

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    Exactly, but this also translates to a pressure range, min. to max., over which the injector is designed to work and to a max. temperature for the feed water. The closer you are to one of these limitations, the smaller the range for one of the other parameters gets. And further, the more the cones and the moving parts get worn the operating ranges close in, until it only works at a very narrow pressure range and really cool water.
    As an example, we have Swedish built Gresham & Craven lifting injectors on 3 of our locos and we have a few of those of the right size, brand new, as spares. But the spares come from train heating boilers and are thus designed for less than 100 psi. Thus they are useless until they are rebuilt with new cones. But still, just the housings with valves represent a significant value, well worth to have for the future.

    By the way, is there any one you can recommend that is good on overhauling this type of injector?
     
    Last edited: Aug 22, 2017
  15. Steve

    Steve Resident of Nat Pres Friend

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    A good few years ago I sent a pair of Davies & Metcalfe combination injectors to Metcalfe Engineering (successors to D & M) for overhaul. They charged a small fortune and they were no better when they came back. I then sent them to South Coast Steam who overhauled them for a slightly smaller fortune but they did work when they came back. Neither firm had the facilities to test them then and probably still don't. Both firms are still in business and I think South Coast Steam have the original G & C patterns. Be warned that you need to check them over when you get them back; such things as glands may not have any packing in.
     
  16. Dag Bonnedal

    Dag Bonnedal New Member

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    Thanks, just read that the Lyn 762 team (Alan Keef) had to send the brand new injectors back to Metcalfe to be reworked as they did not work properly at full pressure.
    We sent 2 Gresham & Craven injectors to Hugh Phillips Eng., Abergavenny 40 years ago for full overhaul. This was very expensive and the work was not first class. No smooth transitions between cylindrical and conical parts of the cones. (This co. later went out of business.)

    One fascinating thing about injectors is that the well known Pullen book on the subject was written in 1893 and he states the injectors have an upper pressure limit that only can be surpassed by using "Compound Injectors", i.e. two injectors in tandem. This is due to the fact that the steam in a convergent nozzle can only reach the speed of sound, which is enough only for lower pressures. Still, the illustrations in the book show many injectors with convergent/divergent supersonic steam nozzles, aka. Laval Nozzles, an invention that had been done 5 years before the book was published. Professor Pullen was obviously totally unaware of this whole concept and theory, which makes the book a bit outdated.
     
  17. jma1009

    jma1009 Active Member

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    Instead of Pullen (my copy is the second edition from 1900) try

    https://archive.org/stream/practicetheoryof00knearich#page/n9/mode/2up

    I hope the link works - I am not very good at these things.

    The early Gifford injector has a converging steam cone nozzle. All later injectors have a diverging steam cone nozzle.

    Compound injectors (a sort of injector within an injector) have a number of useful features for certain applications namely as stated above higher delivery pressure range, and particularly feeding hot water (very important for the UK locomotive export market to SAR etc). Develop the theme a bit and you can get an injector to work on very low steam pressure and hence the exhaust injector.

    Cheers,
    Julian
     
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  18. W.Williams

    W.Williams Member

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    How often do injectors go for overhaul?
     
  19. The Saggin' Dragon

    The Saggin' Dragon Nat Pres stalwart Staff Member Moderator Friend

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    I would imagine that most are only overhauled when the performance drops off noticeably. Not quite RTF, but along those lines.
     

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