Re: We be injections (was AIR BRAKES 101.... was Re: RGS #20. Was: [HOn3] Division Point K-27 question)

John Stutz


I cannot claim much knowledge on the subject, just the basics and a few details.  Injectors came in three basic types: the familiar lifting injector placed high on the boiler, and less known non-lifting injector placed under the cab, which can powered by either full boiler pressure steam or exhaust steam. 

Lifting injectors lift water from the tender or tanks by generating a partial vacuum using a steam jet.  If the water or the injector is too hot, the water will boil and the injector cannot lift it.  Internally the injector is a precise arrangement of concentric converging and diverging cones: A steam jet and venturi for drawing in water and accelerating it, a mixing cone where the steam is condensed, and a pressure cone where the jet's impact builds the pressure needed to force the water into the boiler.  Don't ask me how or why it works - I'm just reporting on what I find in drawings.  But if the feed water contains any suspended grit, or is unusually hard, abrasion or deposition that reshapes the cones will degrade the injector's performance, to possible failure.

With non-lifting injectors, the feed problem is the possibility of drowning the injector with too much water.  They are also subject to the same degradation of the cones.  The exhaust steam version required auxiliary steam from the boiler when the engine was not working, and perhaps on starting, making their construction and operation more complicated than full pressure injectors. These were a fairly late development, say late 1920s, and the only North American narrow gauge use that I am aware of was on the Uintah and NdeM Mallets, and the White Pass 70 & 71.

Probably the most common source of injector failure was operator error, due largely to the multiplicity of designs, each with their unique details and often idiosyncratic operating quirks.

John Stutz

On October 11, 2020 at 12:46 PM "Lee Gustafson via" <bagustaf@...> wrote:


Thank you for the injector information. A better question is what causes injectors to fail?

Lee Gustafson 

On Oct 11, 2020, at 1:40 PM, John Stutz <john.stutz@...> wrote:


It is not clear what you are asking.  There are a number of reasons why an injector can fail to inject water into the boiler, most of which I have forgotten.  But I cannot recall any that I would describe as "stuck". 

None have anything to do with air brakes.

If an injector fails, you can hope to finish your run on the other one.  If both fail, you can try to make it into a siding with the water already in the boiler, but you had damm well better drop the fire before the crownsheet is uncovered.

John Stutz
On October 10, 2020 at 11:32 AM "Lee Gustafson via" <bagustaf@...> wrote:

What happens when an injector is sticking? What do you do?

Lee Gustafson 

On Oct 10, 2020, at 12:41 PM, Mike Conder <vulturenest1@...> wrote:

Sort of, I think.  The injector uses internal venturis powered by steam.  The venturis create a high velocity and low pressure (via conservation of energy) and then the mix is going fast enough and is now dense enough to get into the boiler.  Seems like magic, but it's just brilliant mechanical engineering! (and the triple valve is ALSO brilliant mechanical engineering!)

Mike Conder, mechanical engineer

On Sat, Oct 10, 2020 at 10:54 AM Nolan Hinshaw via <cearnog=> wrote:
On Oct 10, 2020, at 09:41, johnny graybeal < johnnyg@...> wrote:

> Reading the old books on this can be very confusing, but less so than talking about how a check valve lets water of less pressure go into a boiler of higher pressure. That still gives me headaches.

What forces water from an injector through a check valve is impulse, not pressure, It's the energy water (density higher than steam's) gets passing through an injector and gaining velocity (momentum is mass times the square of velocity, wherefrom impulse, which depends on compressing that momentum into very small intervals of time,...). Simple Newtonian mechanics at work - nothing to see here - move along...

From a pump, there's the force multiplier between the steam cylinder(s) and the water cylinder(s) that produces a positive pressure differential at the check.



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